Mechanistic studies on the stereoselective formation of glycosyl iodides: first characterization of ¢l-D-glycosyl iodides

Treatment of glycosyl acetates with one equivalent of iodotrimethylsilane at low temperature results in the quantitative formation of glycosyl iodides. Carbohydrates that possess a participating group at the C-2 position initially form /3-o-glycosyl iodides, which quickly equilibrate to the o~-iodo anomers. The /3 anomer of peracetylated glucose reacts faster than the ce anomer, presumably because the C-2 acetate can assist in displacing the silylated anomeric acetate. In contrast, the o~ anomer reacts faster than the /3 anomer in substrates lacking a participating group at C-2. For example, activation of 1-O-acetyl-2,3,4,6-tetra-Obenzyl-o~-D-glucopyranose leads to formation of the /3 iodide, while the corresponding /3 acetate produces the ce iodide. Although the /3 iodides quickly equilibrate to the o~ anomers, they can be prepared in sizable quantities at low temperatures where equilibration is slow. This report describes the first stereoselective formation and characterization of /3-D-glycosyl iodides.