SMI(2)-PROMOTED CHEMISTRY AT THE ANOMERIC CENTER OF CARBOHYDRATES - REDUCTIVE FORMATION AND REACTION OF GLYCOSYL SAMARIUM-(III) REAGENTS

Reaction of tri-O-benzyl-2-deoxy-alpha-D-arabino-hexopyranosyl chlorid e at room temperature with the efficient single electron transfer syst em of SmI2-THF-HMPA, in the presence of cyclopentanone (Barbier condit ions), gave selectively the corresponding alpha-C-glycoside in 70% yie ld. It is postulated that the reactive transient intermediate is a chi ral anomeric organosamarium (III) species. Lower yields were obtained in the presence of aldehydes, such as n-butanal or isobutanal. It is a lso demonstrated that glycosyl phenyl sulfones undergo reductive samar iation. 2,3,4,6-Tetra-O-benzyl-beta-D-glucopyranosyl phenyl sulfone re acted with cyclopentanone, in the presence Of SMI2-THF-HMPA, to give t he corresponding beta-C-glycoside selectively in 25% yield, together w ith the elimination product (40%). When the protecting group at C-2 wa s an acetate, a very fast beta-elimination of the organosamariurn inte rmediate occurred to give a practically quantitative yield of a substi tuted glycal. This work constitutes the first application of samarium (II) iodide to the reductive metallation of the anomeric center of car bohydrates.