New Factors Governing Stereoselectivity in Borohydride Reductions ofβ‐D‐Glycoside‐2‐uloses − The Peculiar Effect of “Activated” DMSO
Comparative evaluation of the manno/gluco ratios obtained in the conventional reductions of β-D-glucoside-2-uloses (1−4, 13 and 14) reveals the influence of the substitution pattern: the presence of a 4,6-O-acetal function results in lower stereoselectivity in the monosaccharide-uloside cases and low stereoselectivity in the disaccharide-uloside cases, while the absence 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 peculiar effect of acetoxydimethylsulfonium acetate (“activated” DMSO) was observed. In all cases, its presence strongly increased the manno-selectivity of the reduction. A simple, preparatively expedient, commonly suitable protocol has been elaborated for achieving high manno-selectivities and, hence, satisfactory yields.