Some aspects of natural product chemistry

• Main Author: Comer, Frederick William
• Language: English
• Published: University of British Columbia 2011
• Subjects: Aromatic compounds; Stereo-chemistry; Steroids
• Online Access: http://hdl.handle.net/2429/36727

In Part I, the C.D. spectra of a number of 3-keto and 20-keto steroids are reported. The 3-keto steroids were comprised of two series of compounds. For one series, the conformationally transmitted effects of olefinic centers located in, or exocyclic to, rings B and C were investigated. It was found that these effects were reflected in the C.D. spectra when the double bond was located in ring B, but not when the double bond was located in ring C. Further, no correlation could be made between the relative rates of alkali-catalyzed benzaldehyde condensation and the C.D. data. For the other series, the conformations of ring A in a number of steroids substituted at the 2- and 4- positions are discussed. The C.D. results are in agreement with other evidence on this subject, and suggest that a 1,3-diaxial methyl interaction leads to flattening of ring A. For the Δ⁵- 4,4-dimethyl-3-keto steroids, a non-chair conformation is indicated, but a distinction between a boat and a flat chair conformation cannot be made . For 2 α -bromo-4,4-dimethylcholest-5-en-3-one, a conformational equilibrium is suggested. The 20-keto steroids were comprised of a large number of 16-substituted pregnanone and 17 α -pregna-none derivatives. The C.D. spectra of the 16,17 trans compounds and the 16α , 17α -cis-compounds were similar to those of the 16-unsubstituted parent compounds. Modifications in rings A and B had little effect upon the dichroism of the 20-keto group. The C.D. spectra of the 16β, 17β-compounds were very sensitive to the nature of the 16β -substituent, These results are interpreted in terms of the preferred conformation of the 17β -acetyl group. Finally, the C.D. spectra of a number of 16, 17-epoxy-20-keto steroids are reported and discussed with reference to the "reversed octant rule". In Part II, the structural determination of the mould metabolite, hirsutic acid C, is reported. The functional groups were established from chemical information; however the X-ray analysis of the p-bromo-phenacyl ester was required to reveal the novel ring system. During the X-ray irradiation an unusual solid-phase rearrangement occurred, transforming the α-epoxy hydroxyl system of p-bromophenacyl hirsutate to a β -hydroxy ketone system without disrupting the crystal structure. The X-ray analysis revealed a 50:50 mixture of starting material and rearrangement product. A combination of the X-ray and the chemical data was required to complete the structural determinations of both products. The generality of the rearrangement process was investigated. It occurred with methyl hirsutate and dihydromethylhirsutate, but not with hirsutic acid. It could not be induced thermally. It did not occur with the steroids 3 α - and 3 β -hydroxy-4 β, 5-epoxy-5β-cholestane. The rearrangement process is probably controlled by the nature of the molecular packing and hydrogen-bond formation. In Part III, the biogenetic-type syntheses of a number of acetate-derived aromatic compounds are reported. A discussion of the preparation of the condensed polypyrone intermediates is given, and in particular the synthesis of the tetrapyrone, 4-hydroxy-9-methyl-2,5,7,12-tetraketo 1,6,8,11-tetra-oxachrysene from the condensation of bis (2,4-dichlorophenyl) malonate and the tripyrone, 7-methyl-1-hydroxy-3,5,10-triketo 4,6,9-trioxaphenanthrene is reported. Treatment of the tripyrone with methanolic potassium hydroxide solution resulted in ring opening to form a poly- β-keto chain, and subsequent aldol-type condensation to give aromatic compounds representative of the naturally-occurring 6,8 dihydroxy-isocoumarins and C-acetylorsellinic acid. Treatment with methanolic magnesium methoxide solution gave aromatic compounds derivable from different cyclization modes of the poly- β-keto chain. Among the compounds isolated were two representative of the naturally-occurring curvulinic acid and the 5,7-dihydroxychromones respectively. Structural assignments were made largely on the basis of the characteristic spectral properties of the compounds. === Science, Faculty of === Chemistry, Department of === Graduate