Tebello Nyokong
- Language: English
- Type: Article
- Identifier: vital:7213 , http://hdl.handle.net/10962/d1006423
- Description: For most people, coming up with a research project that will lead to the introduction of a new treatment for cancer is a romantic aspiration that never really gets off the ground. But for Professor Nyokong, it is a goal in the process of being realised. DST /NRF professor of medicinal chemistry and nanotechnology at Rhodes's Department of Chemistry she is at the forefront of the introduction of advanced cancer-fighting drugs into the country.
- Full Text:
- Language: English
- Type: Article
- Identifier: vital:7213 , http://hdl.handle.net/10962/d1006423
- Description: For most people, coming up with a research project that will lead to the introduction of a new treatment for cancer is a romantic aspiration that never really gets off the ground. But for Professor Nyokong, it is a goal in the process of being realised. DST /NRF professor of medicinal chemistry and nanotechnology at Rhodes's Department of Chemistry she is at the forefront of the introduction of advanced cancer-fighting drugs into the country.
- Full Text:
Teaching reading in grade 4 Namibian classrooms : a case study
- Authors: Mutenda, Josephine
- Date: 2008
- Subjects: Literacy -- Study and teaching (Elementary) -- Namibia English language -- Study and teaching (Elementary) -- Namibia Reading (Elementary) -- Namibia Second language acquisition
- Language: English
- Type: Thesis , Masters , MEd
- Identifier: vital:1797 , http://hdl.handle.net/10962/d1003682
- Description: Literacy is currently a topic of great concern in Namibia. Learners in this country have difficulty in reading and writing, and are often functionally illiterate. This study focuses on the beliefs, experiences and practices of teachers of reading in English to second language learners in Grade 4. Grade 4 is the transitional grade from mother tongue to English as the medium of instruction. The switch to English makes teaching reading in that language especially challenging. The study is structured according to the case study mode of enquiry, with the target respondents comprising Grade 4 teachers. Classroom observation, interviews and document analysis were used as means of collecting data. The main findings revealed that beliefs and experience had an impact on the way in which reading was taught in this study. It emerged that the teachers’ childhood experiences of literacy and learning to read are perpetuated in their beliefs, their attitudes, their basic conceptualization of reading and their current practices, all of which directly affect the children in their classes. Although the teachers’ professional training also had some influence on their teaching methods, neither of the two teachers interviewed had received much in-service support on how to teach reading, and they depended to a large extent on their recall of how they had learned to read themselves. Both teachers were taught in a traditional, teacher-centered way, involving phonics, rote-learning and drilling. Because they see themselves as competent readers, they believe that these approaches were effective and worth sustaining. Tentative recommendations arising from analysis of the data indicate possible areas for improvement in the teaching of reading, and offer guidelines to help teachers cope with the challenge.
- Full Text:
- Date Issued: 2008
- Authors: Mutenda, Josephine
- Date: 2008
- Subjects: Literacy -- Study and teaching (Elementary) -- Namibia English language -- Study and teaching (Elementary) -- Namibia Reading (Elementary) -- Namibia Second language acquisition
- Language: English
- Type: Thesis , Masters , MEd
- Identifier: vital:1797 , http://hdl.handle.net/10962/d1003682
- Description: Literacy is currently a topic of great concern in Namibia. Learners in this country have difficulty in reading and writing, and are often functionally illiterate. This study focuses on the beliefs, experiences and practices of teachers of reading in English to second language learners in Grade 4. Grade 4 is the transitional grade from mother tongue to English as the medium of instruction. The switch to English makes teaching reading in that language especially challenging. The study is structured according to the case study mode of enquiry, with the target respondents comprising Grade 4 teachers. Classroom observation, interviews and document analysis were used as means of collecting data. The main findings revealed that beliefs and experience had an impact on the way in which reading was taught in this study. It emerged that the teachers’ childhood experiences of literacy and learning to read are perpetuated in their beliefs, their attitudes, their basic conceptualization of reading and their current practices, all of which directly affect the children in their classes. Although the teachers’ professional training also had some influence on their teaching methods, neither of the two teachers interviewed had received much in-service support on how to teach reading, and they depended to a large extent on their recall of how they had learned to read themselves. Both teachers were taught in a traditional, teacher-centered way, involving phonics, rote-learning and drilling. Because they see themselves as competent readers, they believe that these approaches were effective and worth sustaining. Tentative recommendations arising from analysis of the data indicate possible areas for improvement in the teaching of reading, and offer guidelines to help teachers cope with the challenge.
- Full Text:
- Date Issued: 2008
Teachers' perceptions of participative management in a primary school in Namibia
- Authors: Kambonde, Samuel Angaleni
- Date: 2008
- Subjects: Education, Elementary -- Namibia Teachers -- Namibia School management and organization -- Namibia Educational leadership -- Namibia
- Language: English
- Type: Thesis , Masters , MEd
- Identifier: vital:1795 , http://hdl.handle.net/10962/d1003680
- Description: Participative management, a key theme in contemporary management literature, is one of the central aims of the decentralization policy in Namibian education. Current perceptions of participative management on the part of teachers in Namibia are therefore of topical interest. This study set out to explore such perceptions among teachers in a Namibian primary school. The study is an interpretive case study focusing on six individual teachers’ understanding and experience of participative management within their place of work, a primary school in the Oshikoto region of Namibia. Semi-structured interviews, observation and document analysis were used to gather data. The study revealed a strong sense of commitment among the respondents to participative management and its accompanying practices, such as shared decision making and broad stakeholder involvement. Participative management was implemented chiefly through a committee structure for school organisation and government. Respondents also highlighted challenges facing participative management, such as the persistence of autocratic leadership, conflicts of interest in decision making, laziness and unwillingness among staff members, and a lack of understanding among parents concerning their role in the schooling of their children. HIV/AIDS was also seen to pose challenges to the free and frequent participation of stakeholders. The chief recommendation arising from these findings is that school leaders and managers’ understanding of tenets of participative management, such as site-based management and democratic management approaches, requires improvement and development.
- Full Text:
- Date Issued: 2008
- Authors: Kambonde, Samuel Angaleni
- Date: 2008
- Subjects: Education, Elementary -- Namibia Teachers -- Namibia School management and organization -- Namibia Educational leadership -- Namibia
- Language: English
- Type: Thesis , Masters , MEd
- Identifier: vital:1795 , http://hdl.handle.net/10962/d1003680
- Description: Participative management, a key theme in contemporary management literature, is one of the central aims of the decentralization policy in Namibian education. Current perceptions of participative management on the part of teachers in Namibia are therefore of topical interest. This study set out to explore such perceptions among teachers in a Namibian primary school. The study is an interpretive case study focusing on six individual teachers’ understanding and experience of participative management within their place of work, a primary school in the Oshikoto region of Namibia. Semi-structured interviews, observation and document analysis were used to gather data. The study revealed a strong sense of commitment among the respondents to participative management and its accompanying practices, such as shared decision making and broad stakeholder involvement. Participative management was implemented chiefly through a committee structure for school organisation and government. Respondents also highlighted challenges facing participative management, such as the persistence of autocratic leadership, conflicts of interest in decision making, laziness and unwillingness among staff members, and a lack of understanding among parents concerning their role in the schooling of their children. HIV/AIDS was also seen to pose challenges to the free and frequent participation of stakeholders. The chief recommendation arising from these findings is that school leaders and managers’ understanding of tenets of participative management, such as site-based management and democratic management approaches, requires improvement and development.
- Full Text:
- Date Issued: 2008
Taxation and electronic commerce
- Stelloh, Marcus M, Stack, Elizabeth M
- Authors: Stelloh, Marcus M , Stack, Elizabeth M
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: Conference paper
- Identifier: vital:6066 , http://hdl.handle.net/10962/d1004611
- Description: Transactions conducted using the Internet have expanded dramatically in the past few years and countries and their governments have become concerned about the consequences that electronic commerce may have on their tax revenues. Because of this many organisations and inter-governmental agencies have met to try to design a solution that will be compatible with the systems of the various countries and achieve tax neutrality. A number of proposals were made and discussed to try to design a fair and efficient e-tax system. The proposed system that is ultimately adopted must consider different tax bases and systems in order to achieve this. In this research the impact of e-commerce on the imposition of income tax was briefly referred to and four different proposals for levying value-added tax or sales tax were analysed in order to compare the advantages and disadvantages of each and to determine which system would most adequately address the needs of e-commerce. Certain modifications and additions to the proposed systems have been suggested in order to satisfy the specific needs of the South African tax system, while still taking other countries’ tax systems into account. Using Amazon.com Inc. and Skype Technologies South Africa Limited as examples, it is demonstrated how the new amended system will work. It was found that the proposed systems and the system adapted to meet the South African needs would, with a few relatively minor changes to the Value-Added Tax legislation, be suitable for the purposes of imposing value-added tax on e-commerce transactions.
- Full Text:
- Date Issued: 2008
- Authors: Stelloh, Marcus M , Stack, Elizabeth M
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: Conference paper
- Identifier: vital:6066 , http://hdl.handle.net/10962/d1004611
- Description: Transactions conducted using the Internet have expanded dramatically in the past few years and countries and their governments have become concerned about the consequences that electronic commerce may have on their tax revenues. Because of this many organisations and inter-governmental agencies have met to try to design a solution that will be compatible with the systems of the various countries and achieve tax neutrality. A number of proposals were made and discussed to try to design a fair and efficient e-tax system. The proposed system that is ultimately adopted must consider different tax bases and systems in order to achieve this. In this research the impact of e-commerce on the imposition of income tax was briefly referred to and four different proposals for levying value-added tax or sales tax were analysed in order to compare the advantages and disadvantages of each and to determine which system would most adequately address the needs of e-commerce. Certain modifications and additions to the proposed systems have been suggested in order to satisfy the specific needs of the South African tax system, while still taking other countries’ tax systems into account. Using Amazon.com Inc. and Skype Technologies South Africa Limited as examples, it is demonstrated how the new amended system will work. It was found that the proposed systems and the system adapted to meet the South African needs would, with a few relatively minor changes to the Value-Added Tax legislation, be suitable for the purposes of imposing value-added tax on e-commerce transactions.
- Full Text:
- Date Issued: 2008
Synthetic, spectrometric and computer modelling studies of novel ATP analogues
- Gxoyiya, Babalwa Siliziwe Blossom
- Authors: Gxoyiya, Babalwa Siliziwe Blossom
- Date: 2008
- Subjects: Spectrum analysis Tuberculosis -- Treatment Chemotherapy Adenosine triphosphate Adenosine triphosphate -- Synthesis Adenosine triphosphate -- Computer simulation Adenosine triphosphate -- Spectrometric imaging Glutamine synthetase Tuberculosis -- Chemotherapy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4386 , http://hdl.handle.net/10962/d1005051
- Description: This study has been concerned with the design and synthesis of A TP analogues with the potential to act as inhibitors of glutamine synthetase - a novel target for therapeutic intervention in the treatment of tuberculosis. Using a structural -analogy approach, various 3-indolylalkanoic acid, benzimidazole and pyrazolo[3,4-dJpyrimidine derivatives have been prepared and characterized. Alkylation of the heterocyclic bases using 4-(bromomethyl)-2,2-dimethyl-1 ,3-d ioxolane, 2-(bromomethoxy)ethyl acetate and 2-(chloroethoxy)ethanol in the presence of either NaH or BulOK afforded the corresponding N-alkylated derivatives of benzimidazole and 4-aminopyrazolo[3,4-dJpyrimidine (4-APP). Similar reactions with 3-indo lylalkanoic esters resulted in O-alkyl cleavage with the formation of new esters. Alkylation of benzimidazole with allyl bromide, 4-bromobutene and 2-methylbut-2-ene has also been shown to afford the corresponding l-alkenylbenzimidazoles in moderate to excellent yield (43-96%). Subsequent oxidation of these products using CTAP, gave the dihydroxy derivatives in poor to good yields (26-77%). Phosphorylation of various hydroxy derivatives of benzimidazole and 4-APP has been achieved using diethyl chlorophosphate to afford the corresponding monophosphate and 1,2-diphosphate esters. Glycosylation of each of the heterocyclic bases has been successfully achieved using 1,2,3,4,6-penta-O-acetyl-D-glucopyranose and SnCl4 in acetonitri le, while methanolysis of the resulting tetraacetates, using methanolic NaOMe, afforded the hydroxy derivatives in good yield (50-70%). Various 1- and 2-dimensional NMR spectroscopic methods (e.g., IH, 13C, lip, COSY, HSQC and HMBC) have been used to confirm the structures of the synthesized A IP analogues. The application of NMR prediction programmes has been explored, permitting assessment of their agreement with the experimental data and confirmation of assigned structures. High-resolution electron impact (EI) mass spectrometric data have been used to explore the mass fragmentation pathways exhibited by selected derivatives, and certain common fragmentations have been identified. Molecular modelling of selected products as potential glutamine synthetase ligands has been performed on the Accelrys Cerius2 platform, and interactive receptor-ligand docking studies have been conducted using the Ligand-Fit module. These studies have revealed possible hydrogen-boding interactions between the selected analogues and various amino acid residues in the glutamine synthetase active site.
- Full Text:
- Date Issued: 2008
- Authors: Gxoyiya, Babalwa Siliziwe Blossom
- Date: 2008
- Subjects: Spectrum analysis Tuberculosis -- Treatment Chemotherapy Adenosine triphosphate Adenosine triphosphate -- Synthesis Adenosine triphosphate -- Computer simulation Adenosine triphosphate -- Spectrometric imaging Glutamine synthetase Tuberculosis -- Chemotherapy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4386 , http://hdl.handle.net/10962/d1005051
- Description: This study has been concerned with the design and synthesis of A TP analogues with the potential to act as inhibitors of glutamine synthetase - a novel target for therapeutic intervention in the treatment of tuberculosis. Using a structural -analogy approach, various 3-indolylalkanoic acid, benzimidazole and pyrazolo[3,4-dJpyrimidine derivatives have been prepared and characterized. Alkylation of the heterocyclic bases using 4-(bromomethyl)-2,2-dimethyl-1 ,3-d ioxolane, 2-(bromomethoxy)ethyl acetate and 2-(chloroethoxy)ethanol in the presence of either NaH or BulOK afforded the corresponding N-alkylated derivatives of benzimidazole and 4-aminopyrazolo[3,4-dJpyrimidine (4-APP). Similar reactions with 3-indo lylalkanoic esters resulted in O-alkyl cleavage with the formation of new esters. Alkylation of benzimidazole with allyl bromide, 4-bromobutene and 2-methylbut-2-ene has also been shown to afford the corresponding l-alkenylbenzimidazoles in moderate to excellent yield (43-96%). Subsequent oxidation of these products using CTAP, gave the dihydroxy derivatives in poor to good yields (26-77%). Phosphorylation of various hydroxy derivatives of benzimidazole and 4-APP has been achieved using diethyl chlorophosphate to afford the corresponding monophosphate and 1,2-diphosphate esters. Glycosylation of each of the heterocyclic bases has been successfully achieved using 1,2,3,4,6-penta-O-acetyl-D-glucopyranose and SnCl4 in acetonitri le, while methanolysis of the resulting tetraacetates, using methanolic NaOMe, afforded the hydroxy derivatives in good yield (50-70%). Various 1- and 2-dimensional NMR spectroscopic methods (e.g., IH, 13C, lip, COSY, HSQC and HMBC) have been used to confirm the structures of the synthesized A IP analogues. The application of NMR prediction programmes has been explored, permitting assessment of their agreement with the experimental data and confirmation of assigned structures. High-resolution electron impact (EI) mass spectrometric data have been used to explore the mass fragmentation pathways exhibited by selected derivatives, and certain common fragmentations have been identified. Molecular modelling of selected products as potential glutamine synthetase ligands has been performed on the Accelrys Cerius2 platform, and interactive receptor-ligand docking studies have been conducted using the Ligand-Fit module. These studies have revealed possible hydrogen-boding interactions between the selected analogues and various amino acid residues in the glutamine synthetase active site.
- Full Text:
- Date Issued: 2008
Synthesis, photophysics and electrochemical study of tin macrocycles
- Authors: Khene, Mielie Samson
- Date: 2008
- Subjects: Electrochemistry , Photochemistry , Phthalocyanines , Macrocyclic compounds , Spectrum analysis
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4376 , http://hdl.handle.net/10962/d1005041 , Electrochemistry , Photochemistry , Phthalocyanines , Macrocyclic compounds , Spectrum analysis
- Description: Three non-peripherally substituted tin(IV) macrocylic compounds, octahexylphthalocyaninato dichlorotin(IV) (35a), octahexyltetrabenzo-5,10,15-triazaporphyrinato dichlorotin(IV) (35b) and octadecylphthalocyaninato dichlorotin(IV) (35c) were synthesized and their photophysical and electrochemical behaviour studied. Complex (35b), containing a CH group in place of one of the aza nitrogen atom of the phthalocyanine core, shows a split Q band due to its lower symmetry. The triplet state quantum yields were found to be lower than would be expected on the basis of the heavy atom effect of tin as the central metal for phthalocyanine derivatives (35a and 35c). In contrast, (35b) shows a triplet quantum yield ΦT = 0.78. The triplet state lifetimes were solvent dependent, and were higher in THF than in toluene. Cyclic voltammetry and spectroelectrochemistry of the complexes revealed only ring based redox processes. This thesis also reports on the microwave syntheses of tetrasulphonated tin phthalocyanine and tetrasulphonated tin α,β,γ-tetrabenzcorrole. The latter was only formed at low ratios (< 1:8) of 4-sulfophthalic acid to urea. Both complexes are aggregated in aqueous media, but can be partly or fully disaggregated by the addition of Triton X-100. The SnTSTBC complex has lower triplet life times and yields, while binding constant and quenching (of bovine serum albumin) constant are lower for SnTSTBC, compared to SnTSPc. Finally Non-peripherally (α) tetra- (40) and octa-(38a) substituted dodecyl-mercapto tin(IV) phthalocyanines where synthesized and the electrochemical behavior studied. Cyclic voltammetry and spectroelectrochemistry show ring-based reductions for (38a) and (40); the former shows two ring oxidations, while the latter shows only one ring based oxidation. The adsorption kinetics of (38a) and (40) on a gold electrode have been investigated by electrochemical impedance spectroscopy (EIS). The equilibrium constant (K) for the adsorption and the Gibbs free energy ΔG(ads) of the self-assembled monolayer (SAMs) were evaluated based on the Frumkin isotherm. The interaction factor between adsorbate –adsorbate molecules is also discussed.
- Full Text:
- Date Issued: 2008
- Authors: Khene, Mielie Samson
- Date: 2008
- Subjects: Electrochemistry , Photochemistry , Phthalocyanines , Macrocyclic compounds , Spectrum analysis
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4376 , http://hdl.handle.net/10962/d1005041 , Electrochemistry , Photochemistry , Phthalocyanines , Macrocyclic compounds , Spectrum analysis
- Description: Three non-peripherally substituted tin(IV) macrocylic compounds, octahexylphthalocyaninato dichlorotin(IV) (35a), octahexyltetrabenzo-5,10,15-triazaporphyrinato dichlorotin(IV) (35b) and octadecylphthalocyaninato dichlorotin(IV) (35c) were synthesized and their photophysical and electrochemical behaviour studied. Complex (35b), containing a CH group in place of one of the aza nitrogen atom of the phthalocyanine core, shows a split Q band due to its lower symmetry. The triplet state quantum yields were found to be lower than would be expected on the basis of the heavy atom effect of tin as the central metal for phthalocyanine derivatives (35a and 35c). In contrast, (35b) shows a triplet quantum yield ΦT = 0.78. The triplet state lifetimes were solvent dependent, and were higher in THF than in toluene. Cyclic voltammetry and spectroelectrochemistry of the complexes revealed only ring based redox processes. This thesis also reports on the microwave syntheses of tetrasulphonated tin phthalocyanine and tetrasulphonated tin α,β,γ-tetrabenzcorrole. The latter was only formed at low ratios (< 1:8) of 4-sulfophthalic acid to urea. Both complexes are aggregated in aqueous media, but can be partly or fully disaggregated by the addition of Triton X-100. The SnTSTBC complex has lower triplet life times and yields, while binding constant and quenching (of bovine serum albumin) constant are lower for SnTSTBC, compared to SnTSPc. Finally Non-peripherally (α) tetra- (40) and octa-(38a) substituted dodecyl-mercapto tin(IV) phthalocyanines where synthesized and the electrochemical behavior studied. Cyclic voltammetry and spectroelectrochemistry show ring-based reductions for (38a) and (40); the former shows two ring oxidations, while the latter shows only one ring based oxidation. The adsorption kinetics of (38a) and (40) on a gold electrode have been investigated by electrochemical impedance spectroscopy (EIS). The equilibrium constant (K) for the adsorption and the Gibbs free energy ΔG(ads) of the self-assembled monolayer (SAMs) were evaluated based on the Frumkin isotherm. The interaction factor between adsorbate –adsorbate molecules is also discussed.
- Full Text:
- Date Issued: 2008
Synthesis, photophysical and photochemical studies on long chain zinc phthalocyanine derivatives
- Ogunsipe, Abimbola, Durmus, Mahmut, Atilla, Devrim, Gürek, Ayşe Gül, Ahsen, Vefa, Nyokong, Tebello
- Authors: Ogunsipe, Abimbola , Durmus, Mahmut , Atilla, Devrim , Gürek, Ayşe Gül , Ahsen, Vefa , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/266019 , vital:53910 , xlink:href="https://doi.org/10.1016/j.synthmet.2008.06.007"
- Description: The synthesis and characterization of 2,9,16,23-chloro-3,10,17,24-triethyleneoxysulphanylphthalocyaninato zinc(II) (CTESZnPc) is described. The photophysics and photochemistry of CTESZnPc and those of tetrakis(triethyleneoxysulphanyl)zinc phthalocyanine (TTESZnPc) and octakis(triethyleneoxysulphanyl)zinc phthalocyanine (OTESZnPc), are presented and compared with those of unsubstituted zinc phthalocyanine (ZnPc). The presence of triethyleneoxysulphanyl substituents on the ZnPc ring gave rise to higher values of singlet oxygen (ΦΔ) and photodegradation (ΦPd) quantum yields in DMF. However, TTESZnPc, OTESZnPc and CTESZnPc are less fluorescent than ZnPc, judging from their fluorescence quantum yield (ΦF) values. Fluorescence was lower in toluene than in DMF due to aggregation in the former solvent. Triplet quantum yield (ΦT) values were found to increase with the presence of substituents on the ZnPc ring, while triplet lifetimes (τT) were found to vary linearly with the logarithms of solvent viscosities. The fluorescences of the substituted ZnPc complexes were effectively quenched by benzoquinone (BQ), and the quenching data analyzed by the Stern–Volmer equation. The Stern–Volmer constants and the diffusion-controlled bimolecular rate constants were calculated.
- Full Text:
- Date Issued: 2008
- Authors: Ogunsipe, Abimbola , Durmus, Mahmut , Atilla, Devrim , Gürek, Ayşe Gül , Ahsen, Vefa , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/266019 , vital:53910 , xlink:href="https://doi.org/10.1016/j.synthmet.2008.06.007"
- Description: The synthesis and characterization of 2,9,16,23-chloro-3,10,17,24-triethyleneoxysulphanylphthalocyaninato zinc(II) (CTESZnPc) is described. The photophysics and photochemistry of CTESZnPc and those of tetrakis(triethyleneoxysulphanyl)zinc phthalocyanine (TTESZnPc) and octakis(triethyleneoxysulphanyl)zinc phthalocyanine (OTESZnPc), are presented and compared with those of unsubstituted zinc phthalocyanine (ZnPc). The presence of triethyleneoxysulphanyl substituents on the ZnPc ring gave rise to higher values of singlet oxygen (ΦΔ) and photodegradation (ΦPd) quantum yields in DMF. However, TTESZnPc, OTESZnPc and CTESZnPc are less fluorescent than ZnPc, judging from their fluorescence quantum yield (ΦF) values. Fluorescence was lower in toluene than in DMF due to aggregation in the former solvent. Triplet quantum yield (ΦT) values were found to increase with the presence of substituents on the ZnPc ring, while triplet lifetimes (τT) were found to vary linearly with the logarithms of solvent viscosities. The fluorescences of the substituted ZnPc complexes were effectively quenched by benzoquinone (BQ), and the quenching data analyzed by the Stern–Volmer equation. The Stern–Volmer constants and the diffusion-controlled bimolecular rate constants were calculated.
- Full Text:
- Date Issued: 2008
Synthesis, photochemical and photophysical properties of gallium and indium phthalocyanine derivatives
- Authors: Chauke, Vongani Portia
- Date: 2008
- Subjects: Phthalocyanines , Photochemotherapy , Electrochemistry , Gallium , Indium
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4375 , http://hdl.handle.net/10962/d1005040 , Phthalocyanines , Photochemotherapy , Electrochemistry , Gallium , Indium
- Description: The syntheses of octasubstituted and unsusbstitituted Gallium(III) chloride and indium(III) chloride phthalocyanines (GaPc and InPc), their photophysical, photochemical and nonlinear optical parameters are hereby presented. The photocatalytic oxidation of 1-hexene using the synthesized GaPc and InPc complexes as well as electrochemical characterization is also presented in this thesis. Fluorescence quantum yields do not vary much among the four Ga complexes, except for complex 21c; therefore it was concluded that the effect of substituents is not significant among them. Solvents however, had an effect on the results. Lower Φ[subscript F] values were obtained in low viscosity solvents like toluene, relative to highly viscous solvents, such as DMSO. The triplet quantum yields were found to be lower in DMSO than in DMF and toluene. The rate constants for fluorescence, intersystem crossing and internal conversion as well as fluorescence and triplet lifetimes are reported. Photodegradation and singlet oxygen quantum yields have also been reported. There was no clear correlation between the latter parameters. It was however established that the four gallium MPcs were stable, within the allowed stability range for phthalocyanines. High quantum yields of triplet state (Φ[subscript T] ranging from 0.70 to 0.91 in dimethysulfoxide, DMSO) and singlet oxygen generation (Φ[subscript greek capital letter delta], ranging from 0.61 to 0.79 in DMSO) were obtained. Short triplet lifetimes 50 to 60 μs were obtained in DMSO). Calculated non-linear parameters of these complexes are compared with those of the corresponding GaPc derivatives and tetrasubstituted GaPc and InPc complexes. The optical limiting threshold intensity (I[subscript lim]) values for the InPc and GaPc derivatives were calculated and compared with those of corresponding tetrasubstituted InPc and GaPc complexes. The octasubstituted were found to be better optical limiters. Photocatalytic oxidation of 1-hexene by GaPc (21a-c) and InPc (22a-c) derivatives is also presented. The photocatalytic oxidation products for 1-hexene were 1,2- epoxyhexane and 1-hexen-3-ol. The % conversion values of 1-hexene and % selectivity of 1,2-epoxyhexane were generally higher for InPc derivatives. Even though InPc derivatives showed better photocatalytic results than GaPc derivatives, the former were less stable relative to the latter. Both type I and type II mechanism were implicated in the photocatalysis mechanism.
- Full Text:
- Date Issued: 2008
- Authors: Chauke, Vongani Portia
- Date: 2008
- Subjects: Phthalocyanines , Photochemotherapy , Electrochemistry , Gallium , Indium
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4375 , http://hdl.handle.net/10962/d1005040 , Phthalocyanines , Photochemotherapy , Electrochemistry , Gallium , Indium
- Description: The syntheses of octasubstituted and unsusbstitituted Gallium(III) chloride and indium(III) chloride phthalocyanines (GaPc and InPc), their photophysical, photochemical and nonlinear optical parameters are hereby presented. The photocatalytic oxidation of 1-hexene using the synthesized GaPc and InPc complexes as well as electrochemical characterization is also presented in this thesis. Fluorescence quantum yields do not vary much among the four Ga complexes, except for complex 21c; therefore it was concluded that the effect of substituents is not significant among them. Solvents however, had an effect on the results. Lower Φ[subscript F] values were obtained in low viscosity solvents like toluene, relative to highly viscous solvents, such as DMSO. The triplet quantum yields were found to be lower in DMSO than in DMF and toluene. The rate constants for fluorescence, intersystem crossing and internal conversion as well as fluorescence and triplet lifetimes are reported. Photodegradation and singlet oxygen quantum yields have also been reported. There was no clear correlation between the latter parameters. It was however established that the four gallium MPcs were stable, within the allowed stability range for phthalocyanines. High quantum yields of triplet state (Φ[subscript T] ranging from 0.70 to 0.91 in dimethysulfoxide, DMSO) and singlet oxygen generation (Φ[subscript greek capital letter delta], ranging from 0.61 to 0.79 in DMSO) were obtained. Short triplet lifetimes 50 to 60 μs were obtained in DMSO). Calculated non-linear parameters of these complexes are compared with those of the corresponding GaPc derivatives and tetrasubstituted GaPc and InPc complexes. The optical limiting threshold intensity (I[subscript lim]) values for the InPc and GaPc derivatives were calculated and compared with those of corresponding tetrasubstituted InPc and GaPc complexes. The octasubstituted were found to be better optical limiters. Photocatalytic oxidation of 1-hexene by GaPc (21a-c) and InPc (22a-c) derivatives is also presented. The photocatalytic oxidation products for 1-hexene were 1,2- epoxyhexane and 1-hexen-3-ol. The % conversion values of 1-hexene and % selectivity of 1,2-epoxyhexane were generally higher for InPc derivatives. Even though InPc derivatives showed better photocatalytic results than GaPc derivatives, the former were less stable relative to the latter. Both type I and type II mechanism were implicated in the photocatalysis mechanism.
- Full Text:
- Date Issued: 2008
Synthesis, electrochemical characterization of tetra-and octa-substituted dodecyl-mercapto tin phthalocyanines in solution and as self-assembled monolayers
- Khene, Samson M, Geraldo, Daniela A, Togo, Chamunorwa A, Limson, Janice L, Nyokong, Tebello
- Authors: Khene, Samson M , Geraldo, Daniela A , Togo, Chamunorwa A , Limson, Janice L , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/264541 , vital:53743 , xlink:href="https://doi.org/10.1016/j.electacta.2008.08.018"
- Description: Non-peripherally (α) tetra- (9) and octa-(7) substituted dodecyl-mercapto tin(IV) phthalocyanines were synthesized and their electrochemical behavior studied. Cyclic voltammetry and spectroelectrochemistry show ring-based reductions for 7 and 9, the former shows two ring oxidations, while the latter shows only one ring-based oxidation. The adsorption kinetics of (7) and (9) on gold electrode were investigated by electrochemical impedance spectroscopy (EIS). The equilibrium constant (K) for the adsorption and the Gibbs free energy (ΔGads) of the SAMs were evaluated based on the Frumkin isotherm.
- Full Text:
- Date Issued: 2008
- Authors: Khene, Samson M , Geraldo, Daniela A , Togo, Chamunorwa A , Limson, Janice L , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/264541 , vital:53743 , xlink:href="https://doi.org/10.1016/j.electacta.2008.08.018"
- Description: Non-peripherally (α) tetra- (9) and octa-(7) substituted dodecyl-mercapto tin(IV) phthalocyanines were synthesized and their electrochemical behavior studied. Cyclic voltammetry and spectroelectrochemistry show ring-based reductions for 7 and 9, the former shows two ring oxidations, while the latter shows only one ring-based oxidation. The adsorption kinetics of (7) and (9) on gold electrode were investigated by electrochemical impedance spectroscopy (EIS). The equilibrium constant (K) for the adsorption and the Gibbs free energy (ΔGads) of the SAMs were evaluated based on the Frumkin isotherm.
- Full Text:
- Date Issued: 2008
Synthesis and solvent effects on the photophysicochemical properties of novel cadmium phenoxy phthalocyanines
- Chidawanyika, Wadzanai J U, Antunes, Edith M, Nyokong, Tebello
- Authors: Chidawanyika, Wadzanai J U , Antunes, Edith M , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/268649 , vital:54217 , xlink:href="https://doi.org/10.1016/j.jphotochem.2007.09.020"
- Description: The syntheses of novel cadmium phenoxy complexes; tetrakis{1,(4)-tert-butylphenoxyphthalocyaninato}cadmium(II) (5a), tetrakis{1,(4)-phenoxyphthalocyaninato}cadmium(II) (5b), tetrakis{2,(3)-tert-butylphenoxyphthalocyaninato}cadmium(II) (6a) and tetrakis{2,(3)-phenoxyphthalocyaninato}cadmium(II) (6b) are presented herein. The spectroscopic and photophysicochemical properties have also been carried out and discussed together with the influence of various organic solvents on these properties. Spectroscopic properties, i.e. ground state electronic absorption and fluorescence spectra have been found to vary as a function of substituent position on the phthalocyanine macrocycle, i.e. α-substitution versus β-substitution. The photophysical parameters are reported as well as the photodegradation and singlet oxygen quantum yields, where the complexes were found to exhibit good photostability with the production of appreciable amounts of singlet oxygen.
- Full Text:
- Date Issued: 2008
- Authors: Chidawanyika, Wadzanai J U , Antunes, Edith M , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/268649 , vital:54217 , xlink:href="https://doi.org/10.1016/j.jphotochem.2007.09.020"
- Description: The syntheses of novel cadmium phenoxy complexes; tetrakis{1,(4)-tert-butylphenoxyphthalocyaninato}cadmium(II) (5a), tetrakis{1,(4)-phenoxyphthalocyaninato}cadmium(II) (5b), tetrakis{2,(3)-tert-butylphenoxyphthalocyaninato}cadmium(II) (6a) and tetrakis{2,(3)-phenoxyphthalocyaninato}cadmium(II) (6b) are presented herein. The spectroscopic and photophysicochemical properties have also been carried out and discussed together with the influence of various organic solvents on these properties. Spectroscopic properties, i.e. ground state electronic absorption and fluorescence spectra have been found to vary as a function of substituent position on the phthalocyanine macrocycle, i.e. α-substitution versus β-substitution. The photophysical parameters are reported as well as the photodegradation and singlet oxygen quantum yields, where the complexes were found to exhibit good photostability with the production of appreciable amounts of singlet oxygen.
- Full Text:
- Date Issued: 2008
Synthesis and photophysical properties of lead phthalocyanines
- Modibane, D.K., Nyokong, Tebello
- Authors: Modibane, D.K. , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/268636 , vital:54216 , xlink:href="https://doi.org/10.1016/j.poly.2007.12.004"
- Description: This work reports on the synthesis and photophysical parameters of tetra-and octa-substituted new lead phthalocyanines. The complexes synthesized are: 1,4-(tetraphenoxyphthalocyaninato)lead (7a), 1,4-(tetra-tert-butylphenoxyphthalocyaninato)lead (7b), 2,3-(tetraphenoxyphthalocyaninato)lead (8a), 2,3-(tetra-tert-butylphenoxyphthalocyaninato)lead (8b), 2,3-octaphenoxyphthalocyaninatolead (9a) 2,3-[octakis(4-t-butylphenoxyphthalocyaninato)]lead (9b). Photophysical properties were studied for these complexes in a dimethylsulfoxide, dimethylformamide, toluene, tetrahydrofuran and chloroform. The fluorescence spectra were different from excitation spectra due to demetallation upon excitation. High triplet quantum yields ranging from 0.70 to 0.88 (in DMSO, DMF and toluene) and low triplet lifetimes (20–50 μs in DMSO, and more than 10 μs in the rest of the solvents) were observed due to the presence of heavy atom.
- Full Text:
- Date Issued: 2008
- Authors: Modibane, D.K. , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/268636 , vital:54216 , xlink:href="https://doi.org/10.1016/j.poly.2007.12.004"
- Description: This work reports on the synthesis and photophysical parameters of tetra-and octa-substituted new lead phthalocyanines. The complexes synthesized are: 1,4-(tetraphenoxyphthalocyaninato)lead (7a), 1,4-(tetra-tert-butylphenoxyphthalocyaninato)lead (7b), 2,3-(tetraphenoxyphthalocyaninato)lead (8a), 2,3-(tetra-tert-butylphenoxyphthalocyaninato)lead (8b), 2,3-octaphenoxyphthalocyaninatolead (9a) 2,3-[octakis(4-t-butylphenoxyphthalocyaninato)]lead (9b). Photophysical properties were studied for these complexes in a dimethylsulfoxide, dimethylformamide, toluene, tetrahydrofuran and chloroform. The fluorescence spectra were different from excitation spectra due to demetallation upon excitation. High triplet quantum yields ranging from 0.70 to 0.88 (in DMSO, DMF and toluene) and low triplet lifetimes (20–50 μs in DMSO, and more than 10 μs in the rest of the solvents) were observed due to the presence of heavy atom.
- Full Text:
- Date Issued: 2008
Synthesis and electrochemical characterisation of a near infrared absorbing oxo vanadium (IV) octapentylthio-phthalocyanine
- Mbambisa, Gcineka, Nyokong, Tebello
- Authors: Mbambisa, Gcineka , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/266008 , vital:53909 , xlink:href="https://doi.org/10.1016/j.poly.2008.06.004"
- Description: The synthesis of an α-substituted phthalocyanine oxo vanadium(IV) 1,4,8,11,15,18,22,25-octapentathiophthalocyanine (4) which absorbs at 850 nm in dichloromethane is reported. The complex is purple in colour and becomes green on reduction. The cyclic and square wave voltammetries of the complex show five redox couples. The spectroelectrochemical data showed only ring based processes. The ring reduced species is observed at wavelengths greater than 680 nm rather than the usual 500–600 nm range typical of ring reduced phthalocyanine complexes.
- Full Text:
- Date Issued: 2008
- Authors: Mbambisa, Gcineka , Nyokong, Tebello
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/266008 , vital:53909 , xlink:href="https://doi.org/10.1016/j.poly.2008.06.004"
- Description: The synthesis of an α-substituted phthalocyanine oxo vanadium(IV) 1,4,8,11,15,18,22,25-octapentathiophthalocyanine (4) which absorbs at 850 nm in dichloromethane is reported. The complex is purple in colour and becomes green on reduction. The cyclic and square wave voltammetries of the complex show five redox couples. The spectroelectrochemical data showed only ring based processes. The ring reduced species is observed at wavelengths greater than 680 nm rather than the usual 500–600 nm range typical of ring reduced phthalocyanine complexes.
- Full Text:
- Date Issued: 2008
Sustainability assessment using a unit-based sustainability assessment tool: The case of three teaching departments at Rhodes University, South Africa
- Authors: Togo, Muchaiteyi
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/386863 , vital:68182 , xlink:href="https://www.ajol.info/index.php/sajee/article/view/122776"
- Description: A sustainability assessment study was performed with three teaching departments at Rhodes University – Ichthyology and Fisheries Science, Anthropology, and Accounting. The assessment used a Unit-based Sustainability Assessment Tool (USAT) and was guided by systems thinking and the ontological framework provided by critical realism. Results of the study showed that the Department of Ichthyology and Fisheries Science had a higher integration of sustainability issues in its activities than the other departments sampled, with Accounting having the lowest integration. Interviews conducted with departmental heads and content analyses of documents revealed differences in sustainability issues addressed and in approaches used in tackling them among these departments. The study is intended to inform the Mainstreaming of Environment and Sustainability in African (MESA) Universities Partnership, which promotes mainstreaming environment and sustainability in universities during the United Nations Decade of Education for Sustainable Development. The study does not provide answers to mainstreaming activities, but opens up space to debate and deliberate how to deal with the mainstreaming of sustainability in universities. It identified some of the challenges to be addressed in university-wide mainstreaming work, and affirmed the need for systems thinking in bringing about change at institutional level to extend changes taking place in individual teaching contexts.
- Full Text:
- Date Issued: 2008
- Authors: Togo, Muchaiteyi
- Date: 2008
- Subjects: To be catalogued
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/386863 , vital:68182 , xlink:href="https://www.ajol.info/index.php/sajee/article/view/122776"
- Description: A sustainability assessment study was performed with three teaching departments at Rhodes University – Ichthyology and Fisheries Science, Anthropology, and Accounting. The assessment used a Unit-based Sustainability Assessment Tool (USAT) and was guided by systems thinking and the ontological framework provided by critical realism. Results of the study showed that the Department of Ichthyology and Fisheries Science had a higher integration of sustainability issues in its activities than the other departments sampled, with Accounting having the lowest integration. Interviews conducted with departmental heads and content analyses of documents revealed differences in sustainability issues addressed and in approaches used in tackling them among these departments. The study is intended to inform the Mainstreaming of Environment and Sustainability in African (MESA) Universities Partnership, which promotes mainstreaming environment and sustainability in universities during the United Nations Decade of Education for Sustainable Development. The study does not provide answers to mainstreaming activities, but opens up space to debate and deliberate how to deal with the mainstreaming of sustainability in universities. It identified some of the challenges to be addressed in university-wide mainstreaming work, and affirmed the need for systems thinking in bringing about change at institutional level to extend changes taking place in individual teaching contexts.
- Full Text:
- Date Issued: 2008
Suicide ideation among police members in the Eastern Cape
- De Bruin, Carmelitia Elmarie
- Authors: De Bruin, Carmelitia Elmarie
- Date: 2008
- Subjects: Police -- South Africa -- Eastern Cape , Suicide , Suicide -- Psychological aspects , Police -- South Africa -- Job stress
- Language: English
- Type: Thesis , Masters , M Soc Sc (C Psy)
- Identifier: vital:11852 , http://hdl.handle.net/10353/146 , Police -- South Africa -- Eastern Cape , Suicide , Suicide -- Psychological aspects , Police -- South Africa -- Job stress
- Description: Suicide, the endpoint of the continuum that begins with suicide ideation, is known to be an important precursor of attempted suicides. The purpose of this study was to determine the level of suicide ideation of police members and to determine the differences between suicide ideation of various demographic variables, in particular, the gender, rank and race groups of police members. The convenience sample (N = 111) was drawn from police stations in the Nelson Mandela Metropole in the Eastern Cape Province. The Adult Suicide Ideation Questionnaire (ASIQ) and a Biographical Questionnaire were administered. The results indicated that 1.4 % of the sample showed significant levels of suicidal ideation scoring a high score above 31 (i.e., above the 97th percentile by ASIQ). The T-test, One-way (ANOVA) and the Pearson Chi-Square analysis showed no significant differences between suicide ideation and the demographic variables based on the police station, age, gender, race, rank, years of service, years of current position, educational qualifications, marital status, presence of medical conditions, number of alcoholic drinks consumed per week, smoking behaviour and exercise behaviour. However, participants with previous suicide attempts had significant high scores on ASIQ.
- Full Text:
- Date Issued: 2008
- Authors: De Bruin, Carmelitia Elmarie
- Date: 2008
- Subjects: Police -- South Africa -- Eastern Cape , Suicide , Suicide -- Psychological aspects , Police -- South Africa -- Job stress
- Language: English
- Type: Thesis , Masters , M Soc Sc (C Psy)
- Identifier: vital:11852 , http://hdl.handle.net/10353/146 , Police -- South Africa -- Eastern Cape , Suicide , Suicide -- Psychological aspects , Police -- South Africa -- Job stress
- Description: Suicide, the endpoint of the continuum that begins with suicide ideation, is known to be an important precursor of attempted suicides. The purpose of this study was to determine the level of suicide ideation of police members and to determine the differences between suicide ideation of various demographic variables, in particular, the gender, rank and race groups of police members. The convenience sample (N = 111) was drawn from police stations in the Nelson Mandela Metropole in the Eastern Cape Province. The Adult Suicide Ideation Questionnaire (ASIQ) and a Biographical Questionnaire were administered. The results indicated that 1.4 % of the sample showed significant levels of suicidal ideation scoring a high score above 31 (i.e., above the 97th percentile by ASIQ). The T-test, One-way (ANOVA) and the Pearson Chi-Square analysis showed no significant differences between suicide ideation and the demographic variables based on the police station, age, gender, race, rank, years of service, years of current position, educational qualifications, marital status, presence of medical conditions, number of alcoholic drinks consumed per week, smoking behaviour and exercise behaviour. However, participants with previous suicide attempts had significant high scores on ASIQ.
- Full Text:
- Date Issued: 2008
Study South Africa
- International Education Association of South Africa (IEASA), Higher Education South Africa (Organization), Kishun, Roshen
- Authors: International Education Association of South Africa (IEASA) , Higher Education South Africa (Organization) , Kishun, Roshen
- Date: 2008
- Subjects: Education, Higher -- South Africa , Universities and colleges -- South Africa , Technical Institutes -- South Africa , Vocational guidance -- South Africa
- Language: English
- Type: text , book
- Identifier: http://hdl.handle.net/10962/65010 , vital:28653 , ISBN 9780620415521
- Description: [Extract from message from Minister of Higher Education, Hon. Naledi Pandor]: The International Education Association of South Africa (IEASA) has produced yet another magnificent publication on higher education in South Africa It is an excellent source of information for everyone interested in universities and research in this country. It is especially useful for international students who plan to study in South Africa, and for people involved in developing international programmes. The theme of this edition, “The role of internationalisation in South Africa’s knowledge environment, is opportune at this juncture. It provides role-players in higher education with the opportunity to examine critical issues such as the place, purposes, benefits and limitations of internationalisation in the sector and the contribution internationalisation makes to the knowledge economy of our country. To participate effectively in the knowledge economy, South Africa has to grow its research base. For this, we need a pool of vibrant young researchers. South Africa is able to provide opportunities for groundbreaking research, and internationalisation provides students, academics and researchers with prospects to broaden the scope of their research. Exchange programmes, bilateral research connections, collaborative partnerships and other international links lend themselves to pathways through which local and international staff and students can expand their horizons and skills. In this way internationalisation can assist South Africa in realising its objectives. The South African higher education sector has identified priority areas in which it needs to develop human capital. They include science, technology and engineering. We can use international programmes to accelerate our capacity building plans and in particular the training and development of postgraduate students. Through research collaborations, for example, postgraduate students can be jointly trained and co-supervised with partners. This has beneficial outcomes for students, research partners and academics, as well as for the system as a whole. Aside from benefiting from sending our students abroad, South Africa also gains by receiving foreign students. International students bring with them different viewpoints, technologies and skills, which assist in developing new perspectives and techniques in South Africa Furthermore, cultural interaction enhances our own students' experiences. By studying together students come to understand and accept cultural differences and are enriched. These interactions, in turn, build positive relations which have positive long term benefits in developing economic and social links between people and countries, and which will help overcome newly surfaced problems of xenophobia in South Africa South Africa is currently host to more than 60,000 international students and many academics. The greatest proportion of international students and academics are from Africa particularly from the Southern African Development Community. As a host country we ensure high quality courses at universities, in order for our qualifications to be recognisable worldwide. South African institutions produce professionals who are highly sought- after around the world. This publication supports South Africa's education sector by providing information and by publicising our institutions and the sector as a whole. I would like to thank IEASA for the important role it plays in higher education. , 8th Edition
- Full Text:
- Date Issued: 2008
- Authors: International Education Association of South Africa (IEASA) , Higher Education South Africa (Organization) , Kishun, Roshen
- Date: 2008
- Subjects: Education, Higher -- South Africa , Universities and colleges -- South Africa , Technical Institutes -- South Africa , Vocational guidance -- South Africa
- Language: English
- Type: text , book
- Identifier: http://hdl.handle.net/10962/65010 , vital:28653 , ISBN 9780620415521
- Description: [Extract from message from Minister of Higher Education, Hon. Naledi Pandor]: The International Education Association of South Africa (IEASA) has produced yet another magnificent publication on higher education in South Africa It is an excellent source of information for everyone interested in universities and research in this country. It is especially useful for international students who plan to study in South Africa, and for people involved in developing international programmes. The theme of this edition, “The role of internationalisation in South Africa’s knowledge environment, is opportune at this juncture. It provides role-players in higher education with the opportunity to examine critical issues such as the place, purposes, benefits and limitations of internationalisation in the sector and the contribution internationalisation makes to the knowledge economy of our country. To participate effectively in the knowledge economy, South Africa has to grow its research base. For this, we need a pool of vibrant young researchers. South Africa is able to provide opportunities for groundbreaking research, and internationalisation provides students, academics and researchers with prospects to broaden the scope of their research. Exchange programmes, bilateral research connections, collaborative partnerships and other international links lend themselves to pathways through which local and international staff and students can expand their horizons and skills. In this way internationalisation can assist South Africa in realising its objectives. The South African higher education sector has identified priority areas in which it needs to develop human capital. They include science, technology and engineering. We can use international programmes to accelerate our capacity building plans and in particular the training and development of postgraduate students. Through research collaborations, for example, postgraduate students can be jointly trained and co-supervised with partners. This has beneficial outcomes for students, research partners and academics, as well as for the system as a whole. Aside from benefiting from sending our students abroad, South Africa also gains by receiving foreign students. International students bring with them different viewpoints, technologies and skills, which assist in developing new perspectives and techniques in South Africa Furthermore, cultural interaction enhances our own students' experiences. By studying together students come to understand and accept cultural differences and are enriched. These interactions, in turn, build positive relations which have positive long term benefits in developing economic and social links between people and countries, and which will help overcome newly surfaced problems of xenophobia in South Africa South Africa is currently host to more than 60,000 international students and many academics. The greatest proportion of international students and academics are from Africa particularly from the Southern African Development Community. As a host country we ensure high quality courses at universities, in order for our qualifications to be recognisable worldwide. South African institutions produce professionals who are highly sought- after around the world. This publication supports South Africa's education sector by providing information and by publicising our institutions and the sector as a whole. I would like to thank IEASA for the important role it plays in higher education. , 8th Edition
- Full Text:
- Date Issued: 2008
Study of the formation of artifacts following Dichloromethane reaction with some nitrogenous drugs
- Mohammadi, Ali, Amini, Mohsen, Hamedani, Moteza P, Torkabadi, Hossein H, Walker, Roderick B
- Authors: Mohammadi, Ali , Amini, Mohsen , Hamedani, Moteza P , Torkabadi, Hossein H , Walker, Roderick B
- Date: 2008
- Language: English
- Type: Article
- Identifier: vital:6409 , http://hdl.handle.net/10962/d1006495
- Description: In this work, the quaternization reaction of some nitrogenous drugs in dichloromethane under stress condition and room temperature at different times are studied. Under these conditions, drug-chloromethochloride adducts or artifacts were found to be formed for clozapine, ofloxacin and olanzapine. The structures of the resultant adducts were elucidated using 1H NMR spectroscopy. In addition, the amount of intact drug was determined using in-house validated HPLC methods with UV detection.
- Full Text:
- Date Issued: 2008
- Authors: Mohammadi, Ali , Amini, Mohsen , Hamedani, Moteza P , Torkabadi, Hossein H , Walker, Roderick B
- Date: 2008
- Language: English
- Type: Article
- Identifier: vital:6409 , http://hdl.handle.net/10962/d1006495
- Description: In this work, the quaternization reaction of some nitrogenous drugs in dichloromethane under stress condition and room temperature at different times are studied. Under these conditions, drug-chloromethochloride adducts or artifacts were found to be formed for clozapine, ofloxacin and olanzapine. The structures of the resultant adducts were elucidated using 1H NMR spectroscopy. In addition, the amount of intact drug was determined using in-house validated HPLC methods with UV detection.
- Full Text:
- Date Issued: 2008
Studies towards the synthesis of novel, coumarin-based HIV-1 protease inhibitors
- Authors: Rashamuse, Thompho Jason
- Date: 2008
- Subjects: Coumarins , Protease Inhibitors , HIV infections -- Treatment , HIV (Viruses) , AIDS (Disease) -- Treatment , Heterocyclic compounds -- Derivatives
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4359 , http://hdl.handle.net/10962/d1005024 , Coumarins , Protease Inhibitors , HIV infections -- Treatment , HIV (Viruses) , AIDS (Disease) -- Treatment , Heterocyclic compounds -- Derivatives
- Description: A series of the Baylis-Hillman adducts have been obtained by reacting protected O-benzylated and unprotected substituted salicylaldehydes with methyl acrylate or tertbutyl acrylate, respectively, using DABCO as catalyst. Treatment of the Baylis-Hillman adducts with HCl in a mixture of acetic acid and acetic anhydride afforded the corresponding 3-(chloromethyl)coumarin derivatives with yields of up to 94%. Similar use of HI afforded the corresponding 3-(iodomethyl)coumarins but, depending on the reaction time, the reduced 3-methyl analogues could also be obtained. Arbuzov reactions of the 3-(halomethyl)coumarin derivatives have been undertaken to afford 4-phosphorylated and 1’-phosphorylated derivatives, regioselectivity being dependent on the halide-leaving group. The 3-(chloromethyl)coumarin derivatives have been subjected to nucleophilic (SN) attack by benzylamine to give the corresponding 3- [(benzylamino)methyl]coumarin derivatives in yields of up to 74%. Further treatment of the 3-[(benzylamino)methyl]coumarin derivatives with chloroacetyl chloride afforded the chloroacetamide derivatives, which exhibit hindered rotation about the amine C(O)-N bond. The acetamide derivatives have also been subjected to Arbuzov reaction conditions to afford the phosphorylated derivatives in yields of up to 86%. In a preliminary modelling study, hydrolysed analogues of the synthesized phosphorylated derivatives have been docked into the active site of the HIV-1 protease enzyme using the Cerius-2 Ligandfit software module to provide an insight into potential receptor-ligand hydrogen bonding interactions.
- Full Text:
- Date Issued: 2008
- Authors: Rashamuse, Thompho Jason
- Date: 2008
- Subjects: Coumarins , Protease Inhibitors , HIV infections -- Treatment , HIV (Viruses) , AIDS (Disease) -- Treatment , Heterocyclic compounds -- Derivatives
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4359 , http://hdl.handle.net/10962/d1005024 , Coumarins , Protease Inhibitors , HIV infections -- Treatment , HIV (Viruses) , AIDS (Disease) -- Treatment , Heterocyclic compounds -- Derivatives
- Description: A series of the Baylis-Hillman adducts have been obtained by reacting protected O-benzylated and unprotected substituted salicylaldehydes with methyl acrylate or tertbutyl acrylate, respectively, using DABCO as catalyst. Treatment of the Baylis-Hillman adducts with HCl in a mixture of acetic acid and acetic anhydride afforded the corresponding 3-(chloromethyl)coumarin derivatives with yields of up to 94%. Similar use of HI afforded the corresponding 3-(iodomethyl)coumarins but, depending on the reaction time, the reduced 3-methyl analogues could also be obtained. Arbuzov reactions of the 3-(halomethyl)coumarin derivatives have been undertaken to afford 4-phosphorylated and 1’-phosphorylated derivatives, regioselectivity being dependent on the halide-leaving group. The 3-(chloromethyl)coumarin derivatives have been subjected to nucleophilic (SN) attack by benzylamine to give the corresponding 3- [(benzylamino)methyl]coumarin derivatives in yields of up to 74%. Further treatment of the 3-[(benzylamino)methyl]coumarin derivatives with chloroacetyl chloride afforded the chloroacetamide derivatives, which exhibit hindered rotation about the amine C(O)-N bond. The acetamide derivatives have also been subjected to Arbuzov reaction conditions to afford the phosphorylated derivatives in yields of up to 86%. In a preliminary modelling study, hydrolysed analogues of the synthesized phosphorylated derivatives have been docked into the active site of the HIV-1 protease enzyme using the Cerius-2 Ligandfit software module to provide an insight into potential receptor-ligand hydrogen bonding interactions.
- Full Text:
- Date Issued: 2008
Studies on the use of essential oils for the control of Sitophilus Zeamais (Motschulsky) (Coleoptera; Curculionidae): a pest of stored maize grains
- Odeyemi, Oluwakemi Oluwaseyi
- Authors: Odeyemi, Oluwakemi Oluwaseyi
- Date: 2008
- Subjects: Curculionidae , Essences and essential oils , Corn -- Diseases and pests -- South Africa , Biological pest control agents -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD (Botany)
- Identifier: vital:11301 , http://hdl.handle.net/10353/168 , Curculionidae , Essences and essential oils , Corn -- Diseases and pests -- South Africa , Biological pest control agents -- South Africa
- Description: The common maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), a well known pest of stored-maize grain in most parts of the world, was identified as one of the major constraints of harvested maize grains in the Eastern Cape Province, South Africa. The use of plants or their products is one of the recent methods being investigated for insect pest control worldwide. Therefore, the main aim of the present study was to identify available plants in the Eastern Cape that could be used to combat the problem of Sitophilus zeamais in stored-maize grains. For the realization of the aims of this research, the following studies were carried out; a preliminary survey was conducted to obtain baseline information on the farmers’ knowledge and experience of indigenous insect pest control methods in the Eastern Cape. Also, studies on the insecticidal potential of the essential oils of some plants were investigated against the maize weevil. The quality parameters of maize grains treated with the essential oils was also studied and, using a rat model, the toxicity of the essential oils was investigated. The outcome from this study revealed that there is awareness amongst the farmers in the Eastern Cape on the use of plants or their products to control insect pests. Unfortunately, such methods are currently being neglected and the knowledge of their application was found to be eroding. Among the various essential oils screened were those from Mentha longifolia L. and Tagetes minuta L. which evoked an appreciable level of contact, fumigant and repellent toxicity on the maize weevil. Further work done to determine the effects of the oils on maize stored over a period of three months revealed that the two oils had no adverse effect on the proximate compositions and some quality parameters of the stored maize. However, the toxicological study conducted on rats showed that the oils at tested concentrations exhibited some level of toxicity. It is, therefore, suggested that the essential oils of M. longifolia and T. minuta should not be used to treat maize grains intended for human consumption.
- Full Text:
- Date Issued: 2008
- Authors: Odeyemi, Oluwakemi Oluwaseyi
- Date: 2008
- Subjects: Curculionidae , Essences and essential oils , Corn -- Diseases and pests -- South Africa , Biological pest control agents -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD (Botany)
- Identifier: vital:11301 , http://hdl.handle.net/10353/168 , Curculionidae , Essences and essential oils , Corn -- Diseases and pests -- South Africa , Biological pest control agents -- South Africa
- Description: The common maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), a well known pest of stored-maize grain in most parts of the world, was identified as one of the major constraints of harvested maize grains in the Eastern Cape Province, South Africa. The use of plants or their products is one of the recent methods being investigated for insect pest control worldwide. Therefore, the main aim of the present study was to identify available plants in the Eastern Cape that could be used to combat the problem of Sitophilus zeamais in stored-maize grains. For the realization of the aims of this research, the following studies were carried out; a preliminary survey was conducted to obtain baseline information on the farmers’ knowledge and experience of indigenous insect pest control methods in the Eastern Cape. Also, studies on the insecticidal potential of the essential oils of some plants were investigated against the maize weevil. The quality parameters of maize grains treated with the essential oils was also studied and, using a rat model, the toxicity of the essential oils was investigated. The outcome from this study revealed that there is awareness amongst the farmers in the Eastern Cape on the use of plants or their products to control insect pests. Unfortunately, such methods are currently being neglected and the knowledge of their application was found to be eroding. Among the various essential oils screened were those from Mentha longifolia L. and Tagetes minuta L. which evoked an appreciable level of contact, fumigant and repellent toxicity on the maize weevil. Further work done to determine the effects of the oils on maize stored over a period of three months revealed that the two oils had no adverse effect on the proximate compositions and some quality parameters of the stored maize. However, the toxicological study conducted on rats showed that the oils at tested concentrations exhibited some level of toxicity. It is, therefore, suggested that the essential oils of M. longifolia and T. minuta should not be used to treat maize grains intended for human consumption.
- Full Text:
- Date Issued: 2008
Studies in marine diterpene chemistry
- Van Wyk, Albert Wynand Wincke
- Authors: Van Wyk, Albert Wynand Wincke
- Date: 2008
- Subjects: Natural products Diterpenes Mollusks Marine metabolites Chemical oceanography
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4354 , http://hdl.handle.net/10962/d1005019
- Description: This thesis comprises both a natural product investigation and a synthetic component. The natural product investigations are presented in Chapters Two and Three. In Chapter Two the isolation and spectroscopic identification of the new isocopalane diterpene 12S,13R,14Sisocopalan- 13-ol-12,14-diacetate (2.1) and two known 3-(14S)-isocopal-12-ene-15-oyl-1- acetyl-sn-glycerol (2.2) and 3-(14S)-isocopal-12-ene-15-oyl-2-acetyl-sn-glycerol (2.3) from a single, large, unidentified sub-Antarctic nudibranch, collected near Marion Island, approximately 2000 km south of Cape Town are described. Chapter Three discusses the isolation, spectroscopic structure elucidation and anti-oesophageal cancer activity (3.1-3.4 only) of two known labdane diterpenes 6β,7α-diacetoxylabda-8,13E-dien-15-ol (3.1) and 2α,6β,7α-triacetoxylabda-8,13E-dien-15-ol (3.2) and one new 6β,7α,15-triacetoxylabda 8,13E-diene (3.3), as well as new 3α,11-dihydroxy-9,11-seco-cholest-4,7-dien-6,9-dione (3.4) and cholest 7-en-3,5,7-triol (3.5) from the endemic pulmonate mollusc, Trimusculus costatus. The absolute configuration of 3.2, and hence 3.1 and 3.3 (from biogenetic arguments) was determined through X-ray diffraction of a single crystal of the camphanate ester of 3.2. The absolute configuration of the secondary hydroxyl at C-3 of 3.4 was established using the Modified Mosher’s method. The synthetic component of the thesis commences in Chapter Four with the semi-synthesis of labdane diterpene nitriles 9α-cyano-15,16-epoxy-7β-hydroxylabda-13(16),14-dien-6-one (4.1), 9α-cyano-15,16-epoxy-7-hydroxylabda-7,13(16),14-trien-6-one (4.2) and 9α-cyano-15,16- epoxy-6β,7β dihydroxylabda-13(16),14-diene (4.3) from the terrestrial labdane diterpene, hispanolone (4.4). This work is an extension of previous synthetic studies directed towards the synthesis of T. costatus metabolites. Diterpenes 4.1-4.3 exhibited in planta activity against the economically important crop pathogens, Magnaporthea grisea and Puccinia recondita. Chapter Five describes the successful semi-synthesis of two isomeric marine molluscan labdane diterpene aldehyde metabolites, labd-13E-ene-8β-ol-15-al (5.1) and labd-13Z-ene- 8β-ol-15-al (5.2) from the commercially available, terrestrial plant derived, labdane diterpene manool (5.3). Diterpenes 5.1 and 5.2, originally isolated from the Mediterranean nudibranch,Pleurobranchaea meckelii and selected diterpenes arising from this synthesis were evaluated for their activity against an oesophageal cancer cell line (WHCO1). Chapter Six further develops the research discussed in Chapter Five, where ethyl 17-norabiet-13(15)-E-en-8β-ol- 16-oate (5.49) and ethyl 17-norabiet-13(15)-Z-en-8β-ol-16-oate (5.50) were first semisynthesized serendipitously. Based on their structural relationship to naturally occurring tricyclic diterpenes with anti-plasmodial activity, tricyclic diterpenes, 17-norpimaran-13α- ethoxy-8,16-olactone (6.6), 17-norisopimar-15-ene-8β,13β-diol (6.7), 17-norisopimarane- 8β,16-diol (6.8) and 17-norabiet-13(15)-ene-8β,16-diol (6.9) were semi-synthesized from the terrestrial labdane diterpene, 5.3, and critically evaluated for their antimalarial potential from parasite inhibition and haemolytic studies.
- Full Text:
- Date Issued: 2008
- Authors: Van Wyk, Albert Wynand Wincke
- Date: 2008
- Subjects: Natural products Diterpenes Mollusks Marine metabolites Chemical oceanography
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4354 , http://hdl.handle.net/10962/d1005019
- Description: This thesis comprises both a natural product investigation and a synthetic component. The natural product investigations are presented in Chapters Two and Three. In Chapter Two the isolation and spectroscopic identification of the new isocopalane diterpene 12S,13R,14Sisocopalan- 13-ol-12,14-diacetate (2.1) and two known 3-(14S)-isocopal-12-ene-15-oyl-1- acetyl-sn-glycerol (2.2) and 3-(14S)-isocopal-12-ene-15-oyl-2-acetyl-sn-glycerol (2.3) from a single, large, unidentified sub-Antarctic nudibranch, collected near Marion Island, approximately 2000 km south of Cape Town are described. Chapter Three discusses the isolation, spectroscopic structure elucidation and anti-oesophageal cancer activity (3.1-3.4 only) of two known labdane diterpenes 6β,7α-diacetoxylabda-8,13E-dien-15-ol (3.1) and 2α,6β,7α-triacetoxylabda-8,13E-dien-15-ol (3.2) and one new 6β,7α,15-triacetoxylabda 8,13E-diene (3.3), as well as new 3α,11-dihydroxy-9,11-seco-cholest-4,7-dien-6,9-dione (3.4) and cholest 7-en-3,5,7-triol (3.5) from the endemic pulmonate mollusc, Trimusculus costatus. The absolute configuration of 3.2, and hence 3.1 and 3.3 (from biogenetic arguments) was determined through X-ray diffraction of a single crystal of the camphanate ester of 3.2. The absolute configuration of the secondary hydroxyl at C-3 of 3.4 was established using the Modified Mosher’s method. The synthetic component of the thesis commences in Chapter Four with the semi-synthesis of labdane diterpene nitriles 9α-cyano-15,16-epoxy-7β-hydroxylabda-13(16),14-dien-6-one (4.1), 9α-cyano-15,16-epoxy-7-hydroxylabda-7,13(16),14-trien-6-one (4.2) and 9α-cyano-15,16- epoxy-6β,7β dihydroxylabda-13(16),14-diene (4.3) from the terrestrial labdane diterpene, hispanolone (4.4). This work is an extension of previous synthetic studies directed towards the synthesis of T. costatus metabolites. Diterpenes 4.1-4.3 exhibited in planta activity against the economically important crop pathogens, Magnaporthea grisea and Puccinia recondita. Chapter Five describes the successful semi-synthesis of two isomeric marine molluscan labdane diterpene aldehyde metabolites, labd-13E-ene-8β-ol-15-al (5.1) and labd-13Z-ene- 8β-ol-15-al (5.2) from the commercially available, terrestrial plant derived, labdane diterpene manool (5.3). Diterpenes 5.1 and 5.2, originally isolated from the Mediterranean nudibranch,Pleurobranchaea meckelii and selected diterpenes arising from this synthesis were evaluated for their activity against an oesophageal cancer cell line (WHCO1). Chapter Six further develops the research discussed in Chapter Five, where ethyl 17-norabiet-13(15)-E-en-8β-ol- 16-oate (5.49) and ethyl 17-norabiet-13(15)-Z-en-8β-ol-16-oate (5.50) were first semisynthesized serendipitously. Based on their structural relationship to naturally occurring tricyclic diterpenes with anti-plasmodial activity, tricyclic diterpenes, 17-norpimaran-13α- ethoxy-8,16-olactone (6.6), 17-norisopimar-15-ene-8β,13β-diol (6.7), 17-norisopimarane- 8β,16-diol (6.8) and 17-norabiet-13(15)-ene-8β,16-diol (6.9) were semi-synthesized from the terrestrial labdane diterpene, 5.3, and critically evaluated for their antimalarial potential from parasite inhibition and haemolytic studies.
- Full Text:
- Date Issued: 2008
Studies directed towards the synthesis of chromone carbaldehyde-derived HIV-1 protease inhibitors
- Authors: Molefe, Duduzile Mabel
- Date: 2008
- Subjects: Protease Inhibitors , HIV infections , HIV (Viruses) , AIDS (Disease) , Proteolytic enzymes , Heterocyclic compounds -- Derivatives , Chemical kinetics , Nuclear magnetic resonance spectroscopy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4526 , http://hdl.handle.net/10962/d1015542
- Description: A series of chromone-3-carbaldehydes have been prepared using Vilsmeier-Haack methodology while a corresponding series of chromone-2-carbaldeydes have been synthesized via the Kostanecki-Robinson reaction. Baylis-Hillman reactions have been conducted on both series of chromone carbaldehydes using three different catalysts, viz., 1,4-diazabicyclo(2.2.2]octane (DABCO), 1,8-diazabicyclo[5.4.0]undec- 7-ene (DBU) and 3-hydroxyquinuclidine (3HQ), and acrylonitrile, methyl acrylate and methyl vinyl ketone as the activated alkenes. These reactions have typically (but not always!) afforded both normal Baylis-Hillman and dimeric products. Attention has also been given to the use of 1-methyl-2-pyrrolidine (1-NMP), an ionic liquid, to replace normal organic solvents, and it has been found that, in the presence of DABCO, chromone-3-carbaldehydes afford the dimeric products alone. Reactions of chromone-3-carbaldehydes with methyl vinyl ketone have yielded unexpected, novel adducts, which appear to arise from preferential attack at C(2) in the chromone nucleus. Research on chromone-2-carbaldeydes under Baylis-Hillman conditions has also resulted in the formation of some interesting products instead of the expected Baylis-Hillman adducts. The Baylis-Hillman products have been explored as substrates for aza-Michael reactions using various amino derivatives including protected amino acids in the presence of the tetrabutylammonium bromide (TBAB) and the ionic liquid, 3-butyl-1- methylimidazoleboranetetrafluoride (BmimBF₄), as catalysts. The aza-Michael products have been targeted as truncated ritonavir analogues for investigation as potential HIV -1 protease inhibitors, and representative compounds have been subjected to enzyme inhibition assays to explore the extent and type of inhibition. Lineweaver-Burk and Dixon plots have indicated competitive inhibition in one case as well as non-competitive inhibition in another, and the inhibition constants (Ki) have been compared with that of the ritonavir. Computer modelling studies have also been conducted on selected chromonecontaining derivatives, using the ACCELRYS Cerius² platform. Interactive docking of the chromone-containing ligands into the HIV -1 protease receptor site, using the Ligandfit module, has indicated the importance of hydrogen-bonding interactions mediated by bridging water molecules situated in the receptor cavity. NMR spectroscopy has been used to elucidate complex and competing mechanistic pathways involved in the Baylis-Hillman reactions of selected 2-nitrobenzaldehydes with MVK in the presence of DABCO - reactions which afford the normal BaylisHillman product, the MVK dimer and syn- and anti-Baylis-Hillman type diadducts. The kinetic data confirm the concomitant operation of two pathways and reveal that, in the initial stage of the reaction, the product distribution is kinetically controlled, whereas in the latter stage, thermodynamic control results in the consumption of the normal Baylis-Hillman product and predominance of the anti-diadduct.
- Full Text:
- Date Issued: 2008
- Authors: Molefe, Duduzile Mabel
- Date: 2008
- Subjects: Protease Inhibitors , HIV infections , HIV (Viruses) , AIDS (Disease) , Proteolytic enzymes , Heterocyclic compounds -- Derivatives , Chemical kinetics , Nuclear magnetic resonance spectroscopy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4526 , http://hdl.handle.net/10962/d1015542
- Description: A series of chromone-3-carbaldehydes have been prepared using Vilsmeier-Haack methodology while a corresponding series of chromone-2-carbaldeydes have been synthesized via the Kostanecki-Robinson reaction. Baylis-Hillman reactions have been conducted on both series of chromone carbaldehydes using three different catalysts, viz., 1,4-diazabicyclo(2.2.2]octane (DABCO), 1,8-diazabicyclo[5.4.0]undec- 7-ene (DBU) and 3-hydroxyquinuclidine (3HQ), and acrylonitrile, methyl acrylate and methyl vinyl ketone as the activated alkenes. These reactions have typically (but not always!) afforded both normal Baylis-Hillman and dimeric products. Attention has also been given to the use of 1-methyl-2-pyrrolidine (1-NMP), an ionic liquid, to replace normal organic solvents, and it has been found that, in the presence of DABCO, chromone-3-carbaldehydes afford the dimeric products alone. Reactions of chromone-3-carbaldehydes with methyl vinyl ketone have yielded unexpected, novel adducts, which appear to arise from preferential attack at C(2) in the chromone nucleus. Research on chromone-2-carbaldeydes under Baylis-Hillman conditions has also resulted in the formation of some interesting products instead of the expected Baylis-Hillman adducts. The Baylis-Hillman products have been explored as substrates for aza-Michael reactions using various amino derivatives including protected amino acids in the presence of the tetrabutylammonium bromide (TBAB) and the ionic liquid, 3-butyl-1- methylimidazoleboranetetrafluoride (BmimBF₄), as catalysts. The aza-Michael products have been targeted as truncated ritonavir analogues for investigation as potential HIV -1 protease inhibitors, and representative compounds have been subjected to enzyme inhibition assays to explore the extent and type of inhibition. Lineweaver-Burk and Dixon plots have indicated competitive inhibition in one case as well as non-competitive inhibition in another, and the inhibition constants (Ki) have been compared with that of the ritonavir. Computer modelling studies have also been conducted on selected chromonecontaining derivatives, using the ACCELRYS Cerius² platform. Interactive docking of the chromone-containing ligands into the HIV -1 protease receptor site, using the Ligandfit module, has indicated the importance of hydrogen-bonding interactions mediated by bridging water molecules situated in the receptor cavity. NMR spectroscopy has been used to elucidate complex and competing mechanistic pathways involved in the Baylis-Hillman reactions of selected 2-nitrobenzaldehydes with MVK in the presence of DABCO - reactions which afford the normal BaylisHillman product, the MVK dimer and syn- and anti-Baylis-Hillman type diadducts. The kinetic data confirm the concomitant operation of two pathways and reveal that, in the initial stage of the reaction, the product distribution is kinetically controlled, whereas in the latter stage, thermodynamic control results in the consumption of the normal Baylis-Hillman product and predominance of the anti-diadduct.
- Full Text:
- Date Issued: 2008