Diels-Alder reactions of dieno-pyranosides. Anomeric versus allylic stereoselection

A series of carbohydrate-derived dienes with different patterns of substitution on the pyranose ring were synthesized and their Diels-Alder reactions investigated. The diene moiety was incorporated into the pyranose ring by oxidation of 4-O-methanesulfonate esters of sugar derivatives to enals, followed by Witting alkenation. This new class of dienes underwent cycloaddition with maleimide or its N-phenyl derivative to give annulated pyranosides. The Diels-Alder reactions were highly stereoselective, giving single products in some cases. Structural analysis of the reaction products was carried out by NMR spectroscopy and X-ray crystallography. The results indicated a strong preference for the formation of the products resulting from addition of the dienophile to the face of the diene opposite the anomeric center. In cases where the anomeric and allylic substituents on the diene occupied opposite faces, addition of the dienophile occurred predominantly from the face opposite the more remote anomeric center