Flavan derivatives. XXVIII. Stereochemistry of products synthesized from flav-3-enes and flavanones

Abstract

Further applications of flav-3-enes for synthesis of flavan derivatives are reported, with discussion of stereochemical aspects. Hydrogenation of three 3-sub-stituted flav-3-enes gave the corresponding 2,3-cis-flavans, viz. (�)-epiafzelechin tetramethyl ether, and (�)-epicatechin pentamethyl ether and pentaacetate. Hydroxylation of 5,7,3',4'-tetramethoxyflav-3-ene with osmium tetroxide gave 5,7,3',4'-tetramethoxy-2,3-trans-flavan-3,4-cis-diol. The trans-trans isomer of this diol proved configurationally unstable towards hot acetic anhydride and sodium acetate, and gave the 2,3-trans-3,4-cis diacetate in addition to the normal trans-trans diacetate. Two 2,4-cis-flavan-4-01s were found to give 2,4-trans-4-ethoxyflavans, and thus resembled flavan-3,4-diols in their solvolytic reaction with ethanol. Meerwein-Ponndorf reduction of flavanone gave 2,4-cis-flavan-4-ol (i.e. the quasi-equatorial alcohol).