Chemical Semisynthesis and Biotransformation with Rhizopus nigricans of Several Sesquiterpenes: Obtention of New 1α- and 2α-Hydroxyselinane Derivatives

Starting with the natural product vulgarin, isolated from Artemisia canariensis, several acetylated, acetonated and oxidized polyhydroxylated eudesmanes and eudesmenes were semisynthesized. Some of these derivatives were biotransformed with the fungus Rhizopus nigricans and thus metabolites with new hydroxylation, reduction and/or deacetylation were isolated. Incubation of 1β,6α-diacetoxy-12-hydroxy-5α,11β-H-eudesm-4(15)-ene gave a 2-hydroxyselinane derivative in high yield (60%). Microbiological transformations of 1-oxo- and 6-oxoeudesmanes gave other useful hydroxyselinane derivatives in high proportions as the result of a stereoselective reduction of the carbonyl groups at these positions by the fungus on the β-face. Moreover, R. nigricans gave occasionally, regioselective deacetylated and/or hydrolyzed isopropylidene compounds.