CHEMISTRY OF ZAMORANIC ACID .9. HOMOCHIRAL SYNTHESIS OF POLYGODIAL AND WARBURGANAL FROM 17-ACETOXY-7-LABDEN-15-OL

14,15-dinor-17-acetoxy-7-labden-13-one, 4 was prepared from zamoranic acid methyl ester 5. Photochemical cleavage of 4 gave 12-acetoxy-7,9(11)-drimadiene, 3, in 75% overall yield. The chemo- and diastereoselective epoxidation of 3 afforded 12-acetoxy-9 alpha and 9 beta(11)-epoxy-7-drimene, 17 and 18 in 4:1 ratio. Ring-opening of 17 (or the mixture 17/18) using BF3.Et(2)O or ring-opening of the mixture 17/18 lead to the synthetic precursor of polygodial: (9R)-12-acetoxy-drimen-11-al, 20, with a 90% diastereoisomeric excess. The chemo- and diastereoselective cis-hydroxylation of diene 3 led to the synthetic precursor of warburganal 9 alpha,11,l2-trihydroxy-7-drimene, 29 with 47% yield. Polygodial and warburganal were prepared from zamoranic acid methyl ester in 55 and 27% overall yield, respectively.