The first total syntheses of enantiomerically pure naturally occurring ellagitannins gemin D and its regioisomer hippomanin A

The total syntheses of naturally occuring ellagitannins gemin D (1) and its regioisomer hippomanin A (2) are reported. In addition, the phase-transfer catalyzed benzylation reaction of the 2,3-glucopyranoside diols 3-7 is des cribed. Our studies have illustrated the influence of the structure of 2,3- glucopyranoside diols on the regioselectivity of the phase-transfer catalyz ed benzylation at their free 2,3-OH groups. We could show, that both phase- transfer catalyzed benzylations of 2,3-glucopyranoside diols using tetrabut ylammonium hydrogensulfate (Bu4NHSO4) or using tetrabutylammonium iodide (B u4NI) disfavour the formation of the corresponding 3-O-monobenzylated produ cts and preferentially give the 2-O-monobenzylated products. However, the r atio of the generated 2- versus 3-O-mono- and 2,3-dibenzylated products fro m these reactions also strongly depends upon the nature of the starting mat erials. The glucopyranosides 3 and 4 are the first examples, which allow th e completely regioselective monobenzyIation at the 2-OH positions by a phas e-transfer catalyzed reaction.