HEMISYNTHESIS OF SOME BIOGENETICALLY ANOMALOUS 17-BETA-NEOCLERODANE DITERPENOIDS

The reactivity of several natural neoclerodane diterpenoids (eriocepha lin, isoeriocephalin, 7,8-didehydroeriocephalin, picropolin and picrop olinone) has been investigated. The transformations catalysed by basic reagents (epimerisations at C-8, transacetylations from the C-7 alpha to C-6 alpha positions, formation of stable 7 alpha,19-hemiacetals an d tautomeric mixtures of 7 alpha,19-hemiacetal and 19-hydroxy-7-keto f orms) were rationalised in each case by the influence of steric and st rain effects caused by the functionality and stereochemistry at the C- 20 position of the neoclerodane framework. The acid catalysed nucleoph ilic substitution of a 20-O-acetyl group in 20,12-hemiacetals by a 20- O-methyl group was also studied. Other reactions (alpha-ketol rearrang ements, formation of enol eaters and reduction of diosphenol groups) a s well as chemical correlations between several neoclerodanes were als o carried out, providing useful data for the chemistry of these compou nds. Finally, the hemisynthesis of teuvincentin C, starting from 6-O-a cetylisoeriocephalin, supports our previous hypothesis on the formatio n of the biogenetically anomalous 17 beta-neoclerodane diterpenoids. ( C) 1998 Elsevier Science Ltd. All rights reserved.