STUDIES DIRECTED TOWARD THE SYNTHESIS OF (+)-SESBANIMIDE-A - CONSTRUCTION OF THE AB-RING SYSTEM (A FORMAL TOTAL SYNTHESIS)

Complete details of a study leading to the asymmetric synthesis of the AB-ring system of (+)-sesbanimide A (1), a potent cytotoxic agent belonging to the class of sesbania alkaloids, are described. Key features of the synthesis include (a) the stereoselective introduction of the C8 and C9 hydroxyl groups through the use of a diastereoselective catalytic osmylation of enantiomerically enriched alpha-alkoxy allylsilanes and (b) a chelation-controlled nucleophilic addition of a vinyl Grignard reagent to a syn alpha-alkoxy beta-(silyloxy) aldehyde for the introduction of the C7 stereocenter required for assembly of the B-ring.