Flavan derivatives. XXVII. Synthesis of a new racemate of leucocyanidin tetramethyl ether (2,3-cis-3,4-trans-isomer); N.M.R. spectra of the four racemates of leucocyanidin tetramethyl ether (5,7,3',4'-Tetramethoxyflavan-3,4-diol)

Abstract

The previously unknown 2,3-cis-3,4-trans racemate of 5,7,3?,4?- tetramethoxy-flavan-3,4-diol has been prepared by indirect epimerization of the 4-hydroxyl group in the cis-cis-diol. Tabulated N.M.R. data for the four possible racemic leucocyanidin tetramethyl ethers (and their diacetates) provide the basis for unequivocal assignment of configuration to the methyl ethers of naturally occurring monomeric flavan-3,4-diol leucocyanidins. The n.m.r, data are presented to assist in clarification of the structures of these natural products; this clarification is especially desirable because more than the theoretically possible number of dextrorotatory 5,7,3?,4?- tetramethoxyflavan-3,4-diols have already been reported.