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

Two syntheses of the C15 ginkgolide, bilobalide, are presented. One ap proach results in a formal synthesis by intersecting an intermediate i n the Corey synthesis, while a second approach results in a completed total synthesis which is considerably shorter than the first. Both app roaches rely on a stereoselective intramolecular [2 + 2] photocycloadd ition to control much of the stereochemistry. A diastereoselective ald ol condensation serves to establish the relative stereochemical relati onship between the secondary and tertiary alcohols in the second appro ach. This circumvents problems of incorporation of the tertiary carbin ol which were encountered in both previous approaches.