SYNTHETIC STUDIES ON THE GINKGOLIDES - TOTAL SYNTHESIS OF (+/-)-BILOBALIDE

Two syntheses of the C15 ginkgolide, bilobalide, are presented. One approach results in a formal synthesis by intersecting an intermediate in the Corey synthesis, while a second approach results in a completed total synthesis which is considerably shorter than the first. Both approaches rely on a stereoselective intramolecular [2 + 2] photocycloaddition to control much of the stereochemistry. A diastereoselective aldol condensation serves to establish the relative stereochemical relationship between the secondary and tertiary alcohols in the second approach. This circumvents problems of incorporation of the tertiary carbinol which were encountered in both previous approaches.