1,2‐<i>cis</i>‐<i>C</i>‐glycoside synthesis by samarium diiodide‐promoted radical cyclizations

Abstract

Abstract The samarium diiodide reduction of glycosyl pyridyl sulfones bearing a silicon‐tethered unsaturated group at the C2–OH position leads to the stereo‐specific synthesis of 1,2‐ cis‐C ‐glycosides in good yield after desilylation. These reactions proceed via an anomeric radical with subsequent 5‐ exo cyclization. Unlike the corresponding glycosyl phenyl sulfones, the pyridyl derivatives react instantaneously with samarium diiodide and do not require a cosolvent such as hexamethylphosphoramide (HMPA). Under these reaction conditions radical cyclization precedes the second reduction step. Examples of 5‐ exo‐trig and ‐ dig ring closures are given. The synthetic utility of this method was demonstrated by a short synthesis of methyl C ‐isomaltoside.