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Studies in natural products

University of British Columbia

In part I of this thesis is described the structural determination of thamnosin, a minor component obtained from Thamnosma montana Torr. and Frem. Thamnosin, C₃₀H₂₈O₆, was shown by NMR and IR spectra to posess a coumarin chromophore and the mass spectrum suggested that this substance was cleaved under electron impact into two equal halves. Catalytic hydrogenation of thamnosin gave its dihydro-derivative and thereby indicated the presence of one easily reduced olefinic linkage. The UV spectrum of the latter suggested that two 6-alkyl-7-methoxycoumarin chromophores were present. The cleavage of thamnosin into lower molecular weight fragments was achieved by osmium tetroxide hydroxylation of the double bond followed by periodic acid oxidation of the resulting diol. The two aldehydic products (C₁₁H₈O₄ and C₁₉H₂₀O₄) which were obtained from this reaction were subsequently characterized. The smaller fragment was identified as 7-methoxycoumarin-6-aldehyde by direct comparison with an authentic sample. The other fragment, C₁₉H₂₀O₄ was initially shown by spectroscopic evidence, to possess a 6-substituted 7-methoxycoumarin system. The nature of the substituent at C-6 was subsequently identified as cyclohexene derivative bearing two methyl groups and a tertiary formyl functions. Summation of the above and other evidence allowed a structural assignment to thamnosin. It is seen that this substance represents a novel system which has not been previously obtained in nature. A detailed discussion of the mass spectra of thamnosin and its derivatives is presented. Part II describes a possible synthetic route to the vobasine- and sarpagine-type alkaloids. Three different approaches to the preparation of 5-dehydro salts and to ring closure of corynanthenoid bases via transannular cyclizations were attempted. Firstly, oxidation of sitsirikine (121) and dihydro-corynantheic acid ethyl ester (126) by mercuric acetate predominantly gave 3-dehydro salts (122) and (127), respectively, while the formation of 5-dehydro salts was not significant to form the bridge between C-16 and C-5 via transannular cyclization. Secondly, mercuric acetate, oxidations of 3-benzyl-derivatives of corynanthenoid bases followed by transannular cyclizations were attempted. Preparation of the isomeric 3ɤ- and 3β-benzylyohimbines was accomplished by the reaction of benzyl magnesium bromide with 3-dehydro-yohimbine perchlorate. The stereochemistry of these compounds was established by NMR and mass spectra. Accordingly, 3ɤ-benzyl-derivattves of dihydrocorynantheine, dihydrocorynantheic acid ethyl and methyl ester and the tetracyclic methyl ketone (157) were prepared. Oxidation of these derivatives by mercuric acetate proceeded but transannular cyclization was not successful. Thirdly, oxidation of 3,4-seco-corynantheinoid bases by mercuric acetate was attempted. Dihydrocorynantheal ethylene aectal methiodide (173) was treated with sodium in liquid ammonia to give 3,4-seco-N[subscript: b]-methyldihydro-corynantheal ethylene acetal (174), which could be oxidized by mercuric acetate to the dehydro salt. However subsequent hydrolysis of the ethylene acetal was not successful.

Text

2011-07-19

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http://hdl.handle.net/2429/36126

Chemistry

University of British Columbia

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