1,2-cis-C-glycoside synthesis by samarium diiodide-promoted radical cyclizations

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.