New factors governing stereoselectivity in borohydride reductions of beta-D-glycoside-2-uloses - The peculiar effect of "activated" DMSO

Comparative evaluation of the manno/gluco ratios obtained in the convention al reductions of beta -D-glucoside-2-uloses (1-4, 13 and 14) reveals the in fluence of the substitution pattern: the presence of a 4,6-O-acetal functio n results in lower stereoselectivity in the monosaccharide-uloside cases an d low stereoselectivity in the disaccharide-uloside cases, while the absenc e of a 4,6-O-acetal group provides distinctly higher stereoselectivity. The 3-O-benzyl and 3-O-allyl ethers vicinal to the carbonyl to be reduced have a similar influence on the steric outcome of the carbonyl reduction. A pec uliar effect of acetoxydimethylsulfonium acetate ("activated" DMSO) was obs erved. In all cases, its presence strongly increased the manno-selectivity of the reduction. A simple, preparatively expedient, commonly suitable prot ocol has been elaborated for achieving high manno-selectivities and, hence, satisfactory yields.