The Diels–Alder route to drimane related sesquiterpenes; synthesis of cinnamolide, polygodial, isodrimeninol, drimenin and warburganal
The stereospecific preparation of various 1-vinyl-2,6,6-trimethylcyclohex-1-enes (6) as potential diene precursors in the Diels–Alder reaction with dimethyl acetylenedicarboxylate have been investigated. The reaction of the parent diene (6a) with dimethyl acetylenedicarboxylate affords an adduct (18) in 94% yield. This species was reductively isomerised using 10% Pd/C/H2 and a mineral acid to give a trans-fused decalin diester (19). Reduction of (19) with lithium aluminium hydride afforded 1,4,4a,5,6,8,8a-octahydro-5,8,8a-trimethyl-1 β,4a α,8aβ-naphthalene-1,2-dimethanol (24) a key starting material for the highly efficient syntheses of five drimane sesquiterpene natural products, cinnamolide (1), polygodial (2), isodrimeninol (3), drimenin (4), and warburganal (5). Microbial oxidation reactions using C. elegans or A. niger of (2), (24), and (1) gave good yields of the corresponding 3β-hydroxy derivatives, (30), (31), and (32). Several other unusually substituted drimane derivatives are reported.