Hemisynthesis of some biogenetically anomalous 17β-neoclerodane diterpenoids

Article abstract of DOI:10.1016/S0040-4020(98)00890-4

The reactivity of several natural neoclerodane diterpenoids (eriocephalin, isoeriocephalin, 7,8-didehydroeriocephalin, picropolin and picropolinone) has been investigated. The transformations catalysed by basic reagents (epimerisations at C-8, transacetylations from the C-7α to C-6α positions, formation of stable 7α,19-hemiacetals and tautomeric mixtures of 7α,19-hemiacetal and 19-hydroxy-7-keto forms) were rationalised in each case by the influence of steric and strain effects caused by the functionality and stereochemistry at the C20 position of the neoclerodane framework. The acid catalysed nucleophilic substitution of a 20-O-acetyl group in 20,12- hemiacetal by a 20-O-methyl group was also studied. Other reactions (α- ketol rearrangements, formation of enol esters and reduction of diosphenol groups) as well as chemical correlations between several neoclerodanes were also carried out, providing useful data for the chemistry of these compounds. Finally, the hemisynthesis of teuvincentin C, starting from 6-O- acetylisoeriocephalin, supports our previous hypothesis on the formation of the biogenetically anomalous 17β-neoclerodane diterpenoids.