ENANTIOSELECTIVE SYNTHESIS OF CARBOHYDRATE PRECURSORS VIA 1,2-2,3-BIS-EPOXIDE INTERMEDIATES

Bis-epoxidation of the DPS-substituted allenylcarbinyl acetates 9, 13, 19, 22, and 30 afforded the enones 10, 36, 20, 23, and 31, respective ly, in 80-90 % yield with excellent stereoselectivity. Treatment with DBU effected C to O DPS transfer leading to the methyl-branched hexose precursors, enones 11, 37, 21, and 24. The higher homologue 33 gave t he branched 7-deoxyheptose precursor 33. Reduction of enones 11, 37, a nd 33 with NaBH4-CeCl3 yielded the alpha-(S) alcohols 12, 38, and 35 i n high yield. Alcohol 38 was converted to the 1-deoxy-4-methylpyranose tetraacetate 48 by epoxidation, base treatment, desilylation, and ace tylation. An acyclic analogue of 48, acetonide 53, could be prepared f rom epoxide 39 by treatment with PhSH and NaOH, followed by silyl ethe r cleavage, acetonide formation, and Pummerer rearrangement-reduction. On the other hand, hydroxylation of alcohol 38 with OsO4-NMO led to t he selectively protected branched hexitol 59, with high diastereoselec tivity. The allylic alcohol benzoate 63 was likewise converted to diol 64.