Stereoselective glycosylations of a family of 6-deoxy-1,2-glycals generated by catalytic alkynol cycloisomerization

Photolysis of 0.25 equiv of W(CO)(6) in the presence of tertiary amines (triethylamine or DABCO) and highly functionalized terminal alkynyl alcohols catalyzes single-step, high-yield cycloisomerization to endocyclic enol ethers. This transformation is general for each diastereomeric 3,4-bissilyl ether of 5-hydroxyl-hexyne, leading to enantio- and diastereoselective syntheses of each isomer of 6-deoxy-1,2-glycals. Stereoselective glycosylations have also been demonstrated for each glycal diastereomer, and have been applied in the preparation of D-digitoxose-beta-4-D-digitoxose glycal.