Jg. Urones et al., CHEMISTRY OF ZAMORANIC ACID .9. HOMOCHIRAL SYNTHESIS OF POLYGODIAL AND WARBURGANAL FROM 17-ACETOXY-7-LABDEN-15-OL, Tetrahedron, 51(6), 1995, pp. 1845-1860
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(1
1)-drimadiene, 3, in 75% overall yield. The chemo- and diastereoselect
ive 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 t
he synthetic precursor of polygodial: (9R)-12-acetoxy-drimen-11-al, 20
, with a 90% diastereoisomeric excess. The chemo- and diastereoselecti
ve cis-hydroxylation of diene 3 led to the synthetic precursor of warb
urganal 9 alpha,11,l2-trihydroxy-7-drimene, 29 with 47% yield. Polygod
ial and warburganal were prepared from zamoranic acid methyl ester in
55 and 27% overall yield, respectively.