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-hydroxy-1-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-β-4-d-digitoxose glycal.