Functionalization of trans-Decalin. III. A Stereospecific Preparation of Vicinal cis Two Methyl Groups of Eremophilane Skeleton, Leading to dl-Dehydrofukinone

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Abstract
A procedure for the preparation of dl-dehydrofukinone (1), an eremophilane type sesquiterpene, from the diene adducts (3), prepared from the reaction of 4-methyl-3-methoxycarbonyl-2-cyclohexen-1-one with butadiene, is described. Acetalization-reduction (LiAlH4) of 3 followed by treatment of the corresponding mesylate with base provided 5,6-cis-5-methyltricyclo[4.4.1.01,6]undec-8-en-2-one (7) in 69% overall yields. Reductive cleavage of 7 with lithium metal afforded trans-4β,4aβ-dimethyl-Δ6,7-octalin-1-one (8), bearing a set of vicinal cis two methyl groups on the C-4 and C-4a carbons in an 83% yield. Functionalization of the double bond of 8 involves (1) reduction of carbonyl group and following tetrahydropyranylation, (2) epoxidation followed by regiospecific reduction of the oxirane ring at the C-6 position, and (3) subsequent oxidation of the hydroxyl group, giving trans-4aβ,5β-dimethyl-8β-tetrahydropyranyloxydecalin-2-one (14b) in good yields. The conversion of 14b into the desired 1 was achieved smoothly by (1) hydrolysis of tetrahydropyranyl ether, (2) pyrolysis of its mesylate, and (3) subsequent aldol reaction with acetone followed with dehydration and isomerization of the double bond.