Totally stereoselective synthesis of 1,3-disaccharides through Diels-Alder reactions

A nonclassical, totally stereoselective synthesis of orthogonally protected 1,3-disaccharides is reported. Enantiomerically pure beta-keto-delta-lactones, efficiently obtained from glucal and galactal, are transformed into electron-poor heterodienes and chemo-, regio-, and stereoselectively cycloadded to glycals as electron-rich dienophiles, to directly afford 2-thiodisaccharides. The reductive desulfurization of the latter smoothly gave the corresponding 2,2'-dideoxydisaccharides.