Enantioselective syntheses of the assigned structures of the helibisabonols A and B

The enantioselective syntheses of the compounds with the assigned structures of the helibisabonols A and B have been accomplished. Using an enzymatic desymmetrization of the sigma-symmetrical diol (route a) and a diastereoselective conjugate addition of the methyl to the enone with a chiral auxiliary (route b) we constructed the key tertiary stereogenic center at the benzylic position (C7) and then used an asymmetric dihydroxylation for assembling the C10 stereogenic center. In addition, possible diastereoisomers of the natural products were prepared and detailed comparisons of the H-1 and C-13 NMR spectra were conducted. As a result, the structure originally assigned to helibisabonol A may be revised to (7R,10R)-1. In the case of helibisabonol B, the (7R,10R)-2 would be reasonable based on a comparison of the NMR data and the biogenetic parallelism with helibisabonol A. (C) 2011 Elsevier Ltd. All rights reserved.