ASYMMETRIC-SYNTHESIS OF TANSHINDIOL-A

The ultrasound-promoted Diels-Alder reaction of 3-methyl-4,5-benzofurandione (10) with appropriately substituted vinylcyclohexenes has led to the synthesis of tanshindiol A (9), a minor component of the Chinese traditional medicine, Dan Shen, prepared from the roots of Salvia miltiorrhiza Bunge. The regioselectivity of the cycloaddition was enhanced by the ultrasonication and was also strongly influenced by the solvent. The asymmetric synthesis of (+)-(4R)-tanshindiol A [(+)-(4R)-91, the naturally occurring enantiomer, proceeded from the optically enriched methyl 6-(benzoyloxy)-1-vinylcyclohexene-6-carboxylate [(-)-(R)-15] or 6-hydroxy-6-(hydroxymethyl)-l-vinylcyclohexene acetonide [(+)-(R)-291 (both 92% ee). Enantioselectivity in the preparation of these diene systems was accomplished by the benzoyl peroxide quench of the lithioenamine derived from methyl 1-oxocyclohexane-2-carboxylate and (S)-valine tert-butyl ester. This procedure, adapted from Koga's asymmetric alkylation of alpha-alkyl beta-keto esters, proved highly effective in preparing the necessary tertiary alcohol in protected form and with high optical purity.