π-Facial selectivity in Diels–Alder reactions of C-2-vinyl glycals. Stereocontrolled route to annulated C-glycopyranosides

An allylic methoxy substituent at C-3 in C-2 vinyl glycals (1b,c) induces complete antiπ-facial selectivity in the thermal Diels–Alder reaction with malefic anhydride; in contrast, products resulting from anti and syn approaches are observed when linear acetylenic compounds are employed as dienophiles; the latter adducts readily aromatize to afford a mixture of products with the pyranose ring unchanged (8) and opened (9).