Thesis (M.Sc.) - Carleton University, 2007. / Includes bibliographical references (p. 130-139). Also available in electronic format on the Internet.

TA and TB toxins are host-specific phytotoxins produced by the fungus Alternaria alternata f. sp. Iycopersici, the causative agent of Alternaria stem canker disease in tomato. Both compounds are isolated as an equilibrium mixture of the esters formed by either the C(13) or C(14) hydroxy groups with the Re prochiral carboxy group of tricarballylic acid. The design and execution of syntheses for these toxins is necessary in order to study structure-function relationships for T A and TB toxins and their application as natural herbicides. The aim of the synthetic study presented in this thesis is to develop and implement a methodology for the synthesis of the C(1)-C(9) unit of the C17 amino¬pentol backbone of the TA and TB toxins with the required functional groups and appropriate stereochemistry using a chiral sulfoxide as an auxiliary to control the stereochemistry in key steps of the synthetic route. (2R,4S,5R,6R)-2,6-Dimethyloctane-1 ,4,5-triol, synthon B, and (2S,4R,5R)-1-aminononane-2,4,5,9-tetrol, synthon A were identified by retrosynthetic analysis of the C17 aminopentol backbone of TA toxin as key intermediates for a proposed synthesis. Further analysis of synthon B identified a C5 synthon that can be obtained from (2S)-malic acid by functional group transformations, chiral sulfoxide methodo¬logy and an appropriate protective group strategy. The work presented in the thesis shows that a protected intermediate corresponding to the abovementioned C5 synthon, (2S,4S)-2,4,5-trihydroxy:-pentanal can be prepared from (2S)-malic acid, but that using either Sharpless methodology or chiral sulfoxide methodology for the introduction of the third stereogenic centre and chain extension to a C9 unit, failed as a result of the steric crowding caused by the acetonide protecting group. As a result a different synthetic route is proposed. The results obtained in the work on TA toxin were applied to the synthesis of the C(1)-C(9) aminotetrol unit of the backbone of TB toxin. / Dissertation (MSc (Chemistry))--University of Pretoria, 2007. / Chemistry / unrestricted

Dessertation describes of the first reported syntheses of the natural products lairdinol A and depsilairdin. The key steps in the synthesis of depsilairdin were N-terminal extension (C←N) of the protected proline fragment, hydrolysis of the tetrapeptide fragment with free secondary alcohol in the proline moiety and esterification of the HOBt ester of tetrapeptide fragment with the bromomagnesium alkoxide of lairdinol A.

Dessertation describes of the first reported syntheses of the natural products lairdinol A and depsilairdin. The key steps in the synthesis of depsilairdin were N-terminal extension (C←N) of the protected proline fragment, hydrolysis of the tetrapeptide fragment with free secondary alcohol in the proline moiety and esterification of the HOBt ester of tetrapeptide fragment with the bromomagnesium alkoxide of lairdinol A.

Thesis (M.Sc.(Chemistry))--University of Pretoria, 2000. / Includes abstract in English. Includes bibliographical references. Also available online.

Orientador: Rosana Marta Kolb / Banca: Catarina dos Santos / Banca: Luis Octávio Regasini / Resumo: Atualmente, há esforços crescentes para o desenvolvimento de alternativas de manejo de plantas daninhas, devido ao aumento de espécies resistentes aos herbicidas e às preocupações ambientais. Mesmo que não se possa eliminar o uso de herbicidas convencionais, sua utilização pode ser reduzida pela descoberta de compostos bioativos, mais efetivos e/ou com menor toxicidade ao ambiente do que as moléculas que vem sendo usadas. O cinamaldeído e a curcumina são moléculas naturais que apresentam diversas atividades biológicas, e as chalconas são precursoras de flavonoides, que apresentam atividade alelopática, influenciando o desenvolvimento das plantas. Visando prospectar novos herbicidas, foi avaliado o potencial fitotóxico do cinamaldeído, da curcumina e de diferentes estruturas de chalconas metoxiladas. Inicialmente, foi feita triagem de 17 chalconas com bioensaio de desenvolvimento inicial com alface, para selecionar a mais promissora, e que seria testada com daninhas (seis espécies). Estas foram tratadas com 5 mL de solução 1 x 10-3 mol L-1 de cinamaldeído, curcumina ou 3,4-dimetoxichalcona (BC-18); como controle negativo e positivo usou-se água destilada e glifosato (componente ativo do Roundup®), respectivamente. Os comprimentos da raiz e parte aérea foram avaliados quando as plântulas dos controles negativos atingiram aproximadamente 5 cm. Todos os compostos induziram a inibição do crescimento quando comparados ao controle negativo, afetando a raiz e a parte aérea das plântu... (Resumo completo, clicar acesso eletrônico abaixo) / Absctract: Currently, there is increasing effort to develop weed management alternatives due to the increase of herbicide resistant species and environmental concerns. Even if the use of conventional herbicides cannot be eliminated, their use can be reduced by the discovery of bioactive compounds, more effective and/or with less toxicity to the environment than the molecules that are being used. Cinnamaldehyde and curcumin are natural molecules that exhibit various biological activities, and chalcones are precursors of flavonoids, which have allelopathic activity, influencing the development of plants. Aiming to prospect new herbicides, the phytotoxic potential of cinnamaldehyde, curcumin and different methoxylated chalcone structures was evaluated. Initially, 17 chalcones were screened in bioassay of initial development of lettuce, to select the most promising one, and tested with weeds (six species). These were treated with 5 mL of 1 x 10-3 mol L-1 solution of cinnamaldehyde, curcumin or 3,4-dimethoxychalcone (BC-18); as negative and positive control, distilled water and glyphosate (Roundup® active component) were used, respectively. The root and shoot lengths were evaluated when the seedlings of the negative controls reached approximately 5 cm. All the compounds induced growth inhibition when compared to the negative control, affecting the root and shoot of the seedlings. Cinnamaldehyde showed higher phytotoxic activity than glyphosate in five species, whereas curcumin showed greater inhibition in one species and glyphosate-like inhibition in two other species. BC-18 showed greater phytotoxicity than glyphosate in three species and glyphosate-like activity in other two species... (Complete abstract click electronic access below) / Mestre

No description available.

no abstract provided by author / Master of Science

The Best PhD Thesis in the Faculties of Dentistry, Engineering, Medicine and Science (University of Hong Kong), Li Ka Shing Prize,2007-2008 / published_or_final_version / Biological Sciences / Master / Master of Philosophy

Thesis (doctoral)--Swedish University of Agricultural Sciences, 2003. / Appendix consists of reprints and manuscripts of five papers co-authored with others. Includes bibliographical references. Also partially available online in PDF format; online version lacks appendix.