Ring A Modifications of Podocarpic Acid: Towards the Synthesis of Quassinoids

Abstract

Efficient methods for the formation of the hydroxy enone moieties in (17), (18), (45) and (46) from podocarpic acid (1) have been developed. The exocyclic alkene (13) has been converted into the α-hydroxy enones (17) and (18) in six steps and high overall yield. Oxidation of the enones (7) and (44) with manganese(III) acetate gave high yields of the α-acetoxy enones (10) and (47) and (48), respectively. Hydrolysis of (10), (47) and (48) afforded the α-hydroxy enones (11), (45) and (46), respectively. Hydroboration-oxidation of the alkene mixture (38) and (41) provided the alcohol (53) (53%) which was readily oxidized to the ketone (21) (92%). This ketone (21) was converted in three steps into the hydroxy enone (57) in 39% overall yield. Hydroxylation of the enone (44) via the silyl dienol ether (58) gave the α- hydroxy enones (45) (16%) and (46) (33%). The stereochemistry of 12-methoxy-19-nor-5β-podocarpa-3,8,11,13-tetraen-2-one (44) was established by X-ray crystallography.