Heterocycles. XXIV: Synthesis of optically pure 2,3-trans-5,7,3',4',5'-pentahydroxyflavan-3,4-diols and comparison with naturally occurring leucodelphinidins

Two enantiometric chalcone epoxides 4a and 4b have been synthesized in the optically pure state by the phase-transfer asymmetric epoxidation of the chalcone 3 accompanied by hplc separation. Stereoselective cyclization of 4a and 4b and subsequent methylation afford pure (+)- and (−)-ampelopsin pentamethyl ethers 6a and 6b, respectively. Reduction of 6a and 6b with NaBH 4 /MeOH diastereoselectively gives the pure 2,3-trans-3,4-trans-flavan-3,4-diols 7a and 7b. On the other hand, reduction with NaBH 4 /dioxane diastereoselectively furnishes the pure 2,3-trans-3,4-cis-flavan-3,4-diols 8a and 8b. From the comparisons of the specific rotations and melting points, it is deduced that the natural leucodelphinidins isolated from Cleistanthus collinus and Eucalyptus pilularis may belong to the 2,3-cis-flavan-3,4-diols