Measurements of the solubilities of some silver halides in water by electrical methods
- Authors: Malan, George McPherson
- Date: 1955
- Subjects: Silver halides -- Solubility , Silver halides -- Analysis , Conductometric analysis , Potentiometry , Silver halides
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4515 , http://hdl.handle.net/10962/d1013502
- Description: [Introduction, p. 3] The solubilities of the silver halides cannot be determined by the conventional methods of analytical chemistry because they are too sparingly soluble (of the order 1 x 10⁻⁵ to 1 x 10⁻⁶ g . equiv./1. at 25°C.) However, electrical, and to a lesser extent optical methods, are admirably suited because of their greater sensitivity. The conductometric and potentiometric methods are the two most important electrical techniques for the measurement of the solubilities of sparingly soluble salts, and are the ones employed in this research. There are large discrepancies between the published values for the solubility of silver chloride. Results obtained by the conductometric and potentiometric methods disagree. In addition, figures quoted by independent authors using the same method differ by as much as 15%.
- Full Text:
- Date Issued: 1955
- Authors: Malan, George McPherson
- Date: 1955
- Subjects: Silver halides -- Solubility , Silver halides -- Analysis , Conductometric analysis , Potentiometry , Silver halides
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4515 , http://hdl.handle.net/10962/d1013502
- Description: [Introduction, p. 3] The solubilities of the silver halides cannot be determined by the conventional methods of analytical chemistry because they are too sparingly soluble (of the order 1 x 10⁻⁵ to 1 x 10⁻⁶ g . equiv./1. at 25°C.) However, electrical, and to a lesser extent optical methods, are admirably suited because of their greater sensitivity. The conductometric and potentiometric methods are the two most important electrical techniques for the measurement of the solubilities of sparingly soluble salts, and are the ones employed in this research. There are large discrepancies between the published values for the solubility of silver chloride. Results obtained by the conductometric and potentiometric methods disagree. In addition, figures quoted by independent authors using the same method differ by as much as 15%.
- Full Text:
- Date Issued: 1955
Macro-prudential banking regulation, interest rate spread and the conduct of monetary policy in South Africa
- Shayanewako,Varaidzo Batsirai
- Authors: Shayanewako,Varaidzo Batsirai
- Date: 2018
- Subjects: Monetary policy Economic stabilization
- Language: English
- Type: Thesis , Doctoral , D.Com
- Identifier: http://hdl.handle.net/10353/14093 , vital:39843
- Description: The joint implementation of macro-prudential regulatory and monetary policies is necessary for modern banking systems not only to achieve optimal interest rate spread but to cushion the economy against the catastrophic effects of systemic risk. Therefore, the understanding of the interconnection between macroprudential regulation, interest rate spread and monetary policy, supported with empirical evidence, enables economies to build health and sound financial systems and achieve desirable growth rates. Thus, this study utilises the Vector Error Correction (VEC) mechanism to empirically investigate the interconnection between macro-prudential regulation, interest rate spread and monetary policy in South Africa. Quarterly time series data for the period from 1994 to 2016 was employed with the New Keynesian Dynamic Stochastic General Equilibrium (NKDSGE) model as the framework for analysis. The findings of this study suggest that a negative relationship exists between interest rate spread, macro-prudential regulation and monetary policy in South Africa. Moreover, the systems equation provided evidence that a short run causality running from macroprudential regulation to monetary policy is prevalent in South Africa. The empirical model of the study was found to be desirable as evidence of no serial correlation, no ARCH effect and non-normality in residuals was detected. Evidence from this study further suggest that interest rate spread has a dampening effect on monetary policy, but in the long-run this effect seems reversible in South Africa. As a result, this study recommends that caution should be taken on the appropriate selection of measures of macro-prudential regulation and its tool-kit as it can be used to disguise the symptoms of a lax monetary policy. This implies that the South African Reserve Bank (SARB) should sternly supervise and regulate the extension of credit by commercial banks in line with its inflation targeting monetary policy rule in order to achieve financial stability and ensure optimal interest rate spread that can stimulate the economy to growth.
- Full Text:
- Date Issued: 2018
- Authors: Shayanewako,Varaidzo Batsirai
- Date: 2018
- Subjects: Monetary policy Economic stabilization
- Language: English
- Type: Thesis , Doctoral , D.Com
- Identifier: http://hdl.handle.net/10353/14093 , vital:39843
- Description: The joint implementation of macro-prudential regulatory and monetary policies is necessary for modern banking systems not only to achieve optimal interest rate spread but to cushion the economy against the catastrophic effects of systemic risk. Therefore, the understanding of the interconnection between macroprudential regulation, interest rate spread and monetary policy, supported with empirical evidence, enables economies to build health and sound financial systems and achieve desirable growth rates. Thus, this study utilises the Vector Error Correction (VEC) mechanism to empirically investigate the interconnection between macro-prudential regulation, interest rate spread and monetary policy in South Africa. Quarterly time series data for the period from 1994 to 2016 was employed with the New Keynesian Dynamic Stochastic General Equilibrium (NKDSGE) model as the framework for analysis. The findings of this study suggest that a negative relationship exists between interest rate spread, macro-prudential regulation and monetary policy in South Africa. Moreover, the systems equation provided evidence that a short run causality running from macroprudential regulation to monetary policy is prevalent in South Africa. The empirical model of the study was found to be desirable as evidence of no serial correlation, no ARCH effect and non-normality in residuals was detected. Evidence from this study further suggest that interest rate spread has a dampening effect on monetary policy, but in the long-run this effect seems reversible in South Africa. As a result, this study recommends that caution should be taken on the appropriate selection of measures of macro-prudential regulation and its tool-kit as it can be used to disguise the symptoms of a lax monetary policy. This implies that the South African Reserve Bank (SARB) should sternly supervise and regulate the extension of credit by commercial banks in line with its inflation targeting monetary policy rule in order to achieve financial stability and ensure optimal interest rate spread that can stimulate the economy to growth.
- Full Text:
- Date Issued: 2018
Borderline hospitality: homestays as a commercial hospitality development project in Grahamstown, Eastern Cape
- Von Lengeling, Volkher Heinrich Christoph
- Authors: Von Lengeling, Volkher Heinrich Christoph
- Date: 2016
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/706 , vital:19983
- Description: This study started as an anthropological investigation of commercial hospitality from the point of view of the hands-on host. The chosen case study for this investigation was the Kwam eMakana Government Initiated Poverty Alleviation Project which offered homestays in the townships of Grahamstown East since 2004. Homestays are the most intimate form of commercial hospitality, one step removed from non-commercial or social hospitality. Even at the homestay level there is a conceptual conflict between poverty and (Westernized) commercial hospitality, however, Kwam homes are more middle class than poor. Later the investigation revealed the deeper-seated form of poverty of the Kwam participants being (almost) illiterate. Kwam was a development project like many others, in which huge amounts of money were spent in the name of the project but very little of the benefits reached the intended beneficiaries. Thus, as fieldwork ensued, the emphasis of research migrated from an empirical study of homestay hospitality, to actively assist with the struggle of the Kwam hostesses to maintain the project and gain autonomy for themselves. This study was from the outset reflexive, as the host’s point of view could technically only be presented by auto-ethnography. Then the investigation shifted to a form of engaged anthropology far exceeding advocacy as it is usually understood. The presentation of this can be called radical reflexivity, while it is simultaneously an ethnographical account in the sense of anthropology ‘at home’. It also implied, besides ethical concerns, revisiting literary sensibilities, such as the use of a third person narrative for the reflexive account. To conceptualize the development process of both Kwam and the research interventions Bourdieu’s ‘totality of capital’ (in which the strands of economic, symbolic, cultural and social capitals intertwine) proved most useful. By assessing the various capitals the development of the project and the power struggles central to it can be understood. This study confirms that long-term anthropological investigation is best suited to the study of development projects, if not necessary for real development to be effected. Reflexivity and ethnography are complementary methods to reveal truths which under certain research circumstances may have been very difficult or even impossible to research.
- Full Text:
- Date Issued: 2016
- Authors: Von Lengeling, Volkher Heinrich Christoph
- Date: 2016
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/706 , vital:19983
- Description: This study started as an anthropological investigation of commercial hospitality from the point of view of the hands-on host. The chosen case study for this investigation was the Kwam eMakana Government Initiated Poverty Alleviation Project which offered homestays in the townships of Grahamstown East since 2004. Homestays are the most intimate form of commercial hospitality, one step removed from non-commercial or social hospitality. Even at the homestay level there is a conceptual conflict between poverty and (Westernized) commercial hospitality, however, Kwam homes are more middle class than poor. Later the investigation revealed the deeper-seated form of poverty of the Kwam participants being (almost) illiterate. Kwam was a development project like many others, in which huge amounts of money were spent in the name of the project but very little of the benefits reached the intended beneficiaries. Thus, as fieldwork ensued, the emphasis of research migrated from an empirical study of homestay hospitality, to actively assist with the struggle of the Kwam hostesses to maintain the project and gain autonomy for themselves. This study was from the outset reflexive, as the host’s point of view could technically only be presented by auto-ethnography. Then the investigation shifted to a form of engaged anthropology far exceeding advocacy as it is usually understood. The presentation of this can be called radical reflexivity, while it is simultaneously an ethnographical account in the sense of anthropology ‘at home’. It also implied, besides ethical concerns, revisiting literary sensibilities, such as the use of a third person narrative for the reflexive account. To conceptualize the development process of both Kwam and the research interventions Bourdieu’s ‘totality of capital’ (in which the strands of economic, symbolic, cultural and social capitals intertwine) proved most useful. By assessing the various capitals the development of the project and the power struggles central to it can be understood. This study confirms that long-term anthropological investigation is best suited to the study of development projects, if not necessary for real development to be effected. Reflexivity and ethnography are complementary methods to reveal truths which under certain research circumstances may have been very difficult or even impossible to research.
- Full Text:
- Date Issued: 2016
Rural-urban migration and its impact on rural development in Nigeria
- Authors: Abizu, Odion Stanley
- Date: 2018
- Subjects: Rural development Rural-urban migration -- Nigeria Rural poor
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10353/13815 , vital:39714
- Description: This thesis focuses on the developmental impact of rural-urban migration in Nigeria. While, recognising the negative impact of rural-urban migration, the study argues that in order to benefit internal migrants as well as rural development, there must be a paradigm shift to focus on how the benefits of migration on development can be achieved. It is proposed that rural-urban migration occurs as a response to economic, education, health, social and environmental factors. The challenges confronting rural areas are urban bias development paradigm pursued by the successive governments resulting in rural underdevelopment that exacerbates the rural urban disparity. And the neglect of agriculture in rural areas resulting from the discovery of oil in Nigeria. The study revealed that flawed policies adopted by successive Nigerian governments led to joblessness, entrenched social inequality and opportunities. One of the outcomes of these short-sighted policies was rural underdevelopment, which accelerated rural-urban migration in Nigeria. However, the study also demonstrates that the income flows from the migrants to the various rural households has contributed to improvement in the living standard of the households. The fragmentary approaches to rural development that is currently the case in Nigeria would not generate the required level of development and growth needed in the rural communities to improve the living standard. Thus, there is need for a policy framework in Nigeria, which would encourage the private sector to play a vital role in contributing their part in the provision of infrastructural facilities in the rural communities. Although this thesis is a case study of the impact of rural-urban migration on rural development in Nigeria, it can serve to appreciate the role migration can play in promoting development both in rural and urban areas in Nigeria and other Saharan Africa countries.
- Full Text:
- Date Issued: 2018
- Authors: Abizu, Odion Stanley
- Date: 2018
- Subjects: Rural development Rural-urban migration -- Nigeria Rural poor
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10353/13815 , vital:39714
- Description: This thesis focuses on the developmental impact of rural-urban migration in Nigeria. While, recognising the negative impact of rural-urban migration, the study argues that in order to benefit internal migrants as well as rural development, there must be a paradigm shift to focus on how the benefits of migration on development can be achieved. It is proposed that rural-urban migration occurs as a response to economic, education, health, social and environmental factors. The challenges confronting rural areas are urban bias development paradigm pursued by the successive governments resulting in rural underdevelopment that exacerbates the rural urban disparity. And the neglect of agriculture in rural areas resulting from the discovery of oil in Nigeria. The study revealed that flawed policies adopted by successive Nigerian governments led to joblessness, entrenched social inequality and opportunities. One of the outcomes of these short-sighted policies was rural underdevelopment, which accelerated rural-urban migration in Nigeria. However, the study also demonstrates that the income flows from the migrants to the various rural households has contributed to improvement in the living standard of the households. The fragmentary approaches to rural development that is currently the case in Nigeria would not generate the required level of development and growth needed in the rural communities to improve the living standard. Thus, there is need for a policy framework in Nigeria, which would encourage the private sector to play a vital role in contributing their part in the provision of infrastructural facilities in the rural communities. Although this thesis is a case study of the impact of rural-urban migration on rural development in Nigeria, it can serve to appreciate the role migration can play in promoting development both in rural and urban areas in Nigeria and other Saharan Africa countries.
- Full Text:
- Date Issued: 2018
Consolidating democracy through integrating the chieftainship institution with elected councils in Lesotho: a case study of four community councils in Maseru
- Authors: Kapa, Motlamelle Anthony
- Date: 2010
- Subjects: Customary law -- Lesotho Constitutional law -- Lesotho Local government -- Lesotho -- Maseru Culture and law -- Lesotho -- Maseru Lesotho -- Politics and government Democracy -- Lesotho
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:2786 , http://hdl.handle.net/10962/d1002996
- Description: This study analyses the relationship between the chieftainship institution and the elected councils in Lesotho. Based on a qualitative case study method the study seeks to understand this relationship in four selected councils in the Maseru district and how this can be nurtured to achieve a consolidated democracy. Contrary to modernists‟ arguments (that indigenous African political institutions, of which the chieftainship is part, are incompatible with liberal democracy since they are, inter alia, hereditary, they compete with their elective counterparts for political power, they threaten the democratic consolidation process, and they are irrelevant to democratising African systems), this study finds that these arguments are misplaced. Instead, chieftainship is not incompatible with liberal democracy per se. It supports the democratisation process (if the governing parties pursue friendly and accommodative policies to it) but uses its political agency in reaction to the policies of ruling parties to protect its survival interests, whether or not this undermines democratic consolidation process. The chieftainship has also acted to defend democracy when the governing party abuses its political power to undermine democratic rule. It performs important functions in the country. Thus, it is still viewed by the country‟s political leadership, academics, civil society, and councillors as legitimate and highly relevant to the Lesotho‟s contemporary political system. Because of the inadequacies of the government policies and the ambiguous chieftainship-councils integration model, which tend to marginalise the chieftainship and threaten its survival, its relationship with the councils was initially characterised by conflict. However, this relationship has improved, due to the innovative actions taken not by the central government, but by the individual Councils and chiefs themselves, thus increasing the prospects for democratic consolidation. I argue for and recommend the adoption in Lesotho of appropriate variants of the mixed government model to integrate the chieftainship with the elected councils, based on the re-contextualised and re-territorialised conception and practice of democracy, which eschews its universalistic EuroAmerican version adopted by the LCD government, but recognises and preserves the chieftainship as an integral part of the Basotho society, the embodiment of its culture, history, national identity and nationhood.
- Full Text:
- Date Issued: 2010
- Authors: Kapa, Motlamelle Anthony
- Date: 2010
- Subjects: Customary law -- Lesotho Constitutional law -- Lesotho Local government -- Lesotho -- Maseru Culture and law -- Lesotho -- Maseru Lesotho -- Politics and government Democracy -- Lesotho
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:2786 , http://hdl.handle.net/10962/d1002996
- Description: This study analyses the relationship between the chieftainship institution and the elected councils in Lesotho. Based on a qualitative case study method the study seeks to understand this relationship in four selected councils in the Maseru district and how this can be nurtured to achieve a consolidated democracy. Contrary to modernists‟ arguments (that indigenous African political institutions, of which the chieftainship is part, are incompatible with liberal democracy since they are, inter alia, hereditary, they compete with their elective counterparts for political power, they threaten the democratic consolidation process, and they are irrelevant to democratising African systems), this study finds that these arguments are misplaced. Instead, chieftainship is not incompatible with liberal democracy per se. It supports the democratisation process (if the governing parties pursue friendly and accommodative policies to it) but uses its political agency in reaction to the policies of ruling parties to protect its survival interests, whether or not this undermines democratic consolidation process. The chieftainship has also acted to defend democracy when the governing party abuses its political power to undermine democratic rule. It performs important functions in the country. Thus, it is still viewed by the country‟s political leadership, academics, civil society, and councillors as legitimate and highly relevant to the Lesotho‟s contemporary political system. Because of the inadequacies of the government policies and the ambiguous chieftainship-councils integration model, which tend to marginalise the chieftainship and threaten its survival, its relationship with the councils was initially characterised by conflict. However, this relationship has improved, due to the innovative actions taken not by the central government, but by the individual Councils and chiefs themselves, thus increasing the prospects for democratic consolidation. I argue for and recommend the adoption in Lesotho of appropriate variants of the mixed government model to integrate the chieftainship with the elected councils, based on the re-contextualised and re-territorialised conception and practice of democracy, which eschews its universalistic EuroAmerican version adopted by the LCD government, but recognises and preserves the chieftainship as an integral part of the Basotho society, the embodiment of its culture, history, national identity and nationhood.
- Full Text:
- Date Issued: 2010
Effect of nanoparticles on the photophysicochemical behaviour of metallophthalocyanines
- Moeno, Sharon Keitumetse Gail Mpheletso
- Authors: Moeno, Sharon Keitumetse Gail Mpheletso
- Date: 2011 , 2011-03-30
- Subjects: Phthalocyanines Nanoparticles Photochemistry Quantum dots
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4310 , http://hdl.handle.net/10962/d1004968
- Description: The synthesis, spectroscopic characterization, and studies of the photophysicochemical behaviour of selective anionic, cationic and neutral metallophthallocyanine (MPc) complexes were carried out and the results are presented herein. Studies on the effect of the central metal ion, the solvent used and the presence of nanoparticles on the photophysicochemical properties were conducted. The findings showed that the photophysicochemical parameters were mostly enhanced in the presence of central metal ions of high atomic numbers and also in the presence of nanoparticles. It was also observed that solvents that encouraged the monomericity of the MPc complexes also lead to improved photophysical and photochemical behaviour. CdTe quantum dots (QDs) stabilized with mercaptocarbonic acids were also observed to cause stimulated emission of the MPcs through Förster resonance energy transfer (FRET) thus acting as energy donors while the respective MPc acted as energy acceptors in all the FRET studies. FRET was observed following the photoexcitation of QDs for all monomeric anionic MPcs but it was also shown to occur for some cationic MPcs in organic media. Both the substituent and solvent used were found to exert a strong influence on the occurrence of FRET. Other cationic MPcs however showed different behaviour in the presence of the meraptocarbonic stabilized CdTe QDs; with the cationic porphyrazine giving clear indications of Pc ring reduction. The rest of the cationic MPcs did not give clear evidence of Pc ring reduction, instead they showed signs of aggregate formation possibly from the assembly of electrostatic ion pair complexes which could result in reduction of the quaternized pyridinium ring of the substituent. Both the QDs and the MPc complex emission spectra were significantly quenched for each in the presence of the other. Stern-Volmer quenching studies indicated that both static and dynamic quenching of the QDs in the presence of MPcs took place. The fluorescence lifetimes of the mercaptopropionic acid (MPA) capped CdTe QDs in the presence of various MPc complexes showed quenching of mostly the longer lifetimes of the QDs in the presence of MPcs suggesting that the surface defects and states are involved in the interaction of the QDs and MPcs. An MPc complex terminating in thio tethers was employed in the conjugation to AuNPs. Spectroscopic and microscopic studies confirmed the formation of the MPc-AuNP conjugate which was also shown to exhibit improved photophysicochemical properties compared to the free MPc.
- Full Text:
- Date Issued: 2011
- Authors: Moeno, Sharon Keitumetse Gail Mpheletso
- Date: 2011 , 2011-03-30
- Subjects: Phthalocyanines Nanoparticles Photochemistry Quantum dots
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4310 , http://hdl.handle.net/10962/d1004968
- Description: The synthesis, spectroscopic characterization, and studies of the photophysicochemical behaviour of selective anionic, cationic and neutral metallophthallocyanine (MPc) complexes were carried out and the results are presented herein. Studies on the effect of the central metal ion, the solvent used and the presence of nanoparticles on the photophysicochemical properties were conducted. The findings showed that the photophysicochemical parameters were mostly enhanced in the presence of central metal ions of high atomic numbers and also in the presence of nanoparticles. It was also observed that solvents that encouraged the monomericity of the MPc complexes also lead to improved photophysical and photochemical behaviour. CdTe quantum dots (QDs) stabilized with mercaptocarbonic acids were also observed to cause stimulated emission of the MPcs through Förster resonance energy transfer (FRET) thus acting as energy donors while the respective MPc acted as energy acceptors in all the FRET studies. FRET was observed following the photoexcitation of QDs for all monomeric anionic MPcs but it was also shown to occur for some cationic MPcs in organic media. Both the substituent and solvent used were found to exert a strong influence on the occurrence of FRET. Other cationic MPcs however showed different behaviour in the presence of the meraptocarbonic stabilized CdTe QDs; with the cationic porphyrazine giving clear indications of Pc ring reduction. The rest of the cationic MPcs did not give clear evidence of Pc ring reduction, instead they showed signs of aggregate formation possibly from the assembly of electrostatic ion pair complexes which could result in reduction of the quaternized pyridinium ring of the substituent. Both the QDs and the MPc complex emission spectra were significantly quenched for each in the presence of the other. Stern-Volmer quenching studies indicated that both static and dynamic quenching of the QDs in the presence of MPcs took place. The fluorescence lifetimes of the mercaptopropionic acid (MPA) capped CdTe QDs in the presence of various MPc complexes showed quenching of mostly the longer lifetimes of the QDs in the presence of MPcs suggesting that the surface defects and states are involved in the interaction of the QDs and MPcs. An MPc complex terminating in thio tethers was employed in the conjugation to AuNPs. Spectroscopic and microscopic studies confirmed the formation of the MPc-AuNP conjugate which was also shown to exhibit improved photophysicochemical properties compared to the free MPc.
- Full Text:
- Date Issued: 2011
Structural and synthetic investigations of South African marine natural products
- Authors: Beukes, Denzil Ronwynne
- Date: 2000
- Subjects: Natural products -- South Africa Marine invertebrates -- South Africa Marine metabolites -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4350 , http://hdl.handle.net/10962/d1005015
- Description: A chemical investigation of six different marine invertebrates, collected along the South African coastline, resulted in the isolation and structural elucidation of fifteen previously undescribed secondary metabolites along with seven known compounds. The structures of the new metabolites were determined by a combination of spectroscopic and chemical methods. The endemic false limpet Siphonaria capensis was shown to contain two unusual polypropionate metabolites capensinone (162) and capensifuranone (163) as well as 2,4,6,8-tetramethyl-2-undecenoic acid (164) and the known polypropionates (E)- and (Z)siphonarienfuranone (149 and 161). Capensinone is the first example of a marine polypropionate containing a cyc1opentenone moiety. An investigation of the endemic South African soft coral Pieterfaurea unilobata yielded six new, highly oxygenated, pregnadiene sterols (180-185) and the known metabolite (169). Compounds 180-185 are the first pregnadienes obtained from the marine environment containing a C-7 substituent. An alternative procedure for the quick assignment of the absolute configuration at C-3 in this series of compounds was proposed. A companson of the pyrroloiminoquinone alkaloids of three undescribed l'}trunculid sponges resulted in the isolation of 3-dih¥drodiscorhabdin C (243), 3-dihydrodiscorhabdin B (244), discorhabdin H (197) and the previously reported alkaloids discorhabdin A (189) and discorhabdin D (192). While all three sponges were found to be morphologically different they all contained discorhabdin A as the major metabolite and discorhabdin H as one of their minor metabolites. It was found that a feature common to most of the South African latrunculid sponges is the reduction of the C-3 carbonyl gr,o up in some of the minor metabolites. The indole alkaloids, dilemmaones A-C (261-263), containing an unusual cyc1opentanone-indole skeleton, were isolated in trace amounts by bioassay guided fractionation of an extract obtained from a mixed collection of sponges collected near Cape Town. In an attempt to acquire more of these novel compounds for further investigation of their biological activity, several synthetic strategies towards their total synthesis were explored. A key feature of these approaches was the exploitation of the regioselective Gassman's artha-alkylation procedure for the introduction of an aromatic methyl substituent.
- Full Text:
- Date Issued: 2000
- Authors: Beukes, Denzil Ronwynne
- Date: 2000
- Subjects: Natural products -- South Africa Marine invertebrates -- South Africa Marine metabolites -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4350 , http://hdl.handle.net/10962/d1005015
- Description: A chemical investigation of six different marine invertebrates, collected along the South African coastline, resulted in the isolation and structural elucidation of fifteen previously undescribed secondary metabolites along with seven known compounds. The structures of the new metabolites were determined by a combination of spectroscopic and chemical methods. The endemic false limpet Siphonaria capensis was shown to contain two unusual polypropionate metabolites capensinone (162) and capensifuranone (163) as well as 2,4,6,8-tetramethyl-2-undecenoic acid (164) and the known polypropionates (E)- and (Z)siphonarienfuranone (149 and 161). Capensinone is the first example of a marine polypropionate containing a cyc1opentenone moiety. An investigation of the endemic South African soft coral Pieterfaurea unilobata yielded six new, highly oxygenated, pregnadiene sterols (180-185) and the known metabolite (169). Compounds 180-185 are the first pregnadienes obtained from the marine environment containing a C-7 substituent. An alternative procedure for the quick assignment of the absolute configuration at C-3 in this series of compounds was proposed. A companson of the pyrroloiminoquinone alkaloids of three undescribed l'}trunculid sponges resulted in the isolation of 3-dih¥drodiscorhabdin C (243), 3-dihydrodiscorhabdin B (244), discorhabdin H (197) and the previously reported alkaloids discorhabdin A (189) and discorhabdin D (192). While all three sponges were found to be morphologically different they all contained discorhabdin A as the major metabolite and discorhabdin H as one of their minor metabolites. It was found that a feature common to most of the South African latrunculid sponges is the reduction of the C-3 carbonyl gr,o up in some of the minor metabolites. The indole alkaloids, dilemmaones A-C (261-263), containing an unusual cyc1opentanone-indole skeleton, were isolated in trace amounts by bioassay guided fractionation of an extract obtained from a mixed collection of sponges collected near Cape Town. In an attempt to acquire more of these novel compounds for further investigation of their biological activity, several synthetic strategies towards their total synthesis were explored. A key feature of these approaches was the exploitation of the regioselective Gassman's artha-alkylation procedure for the introduction of an aromatic methyl substituent.
- Full Text:
- Date Issued: 2000
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