Glycosylidene Carbenes. Part 17. Glycosidation of benzyl β-D- and β-L-ribopyranosides. Further evidence for the effect of stereoelectronic control on the regioselectivity of glycosidation

The H-bonds of the enantiomeric ribosides 4 and 5 and their glycosidation by the diazirine 1 are described. HO–C(2) and HO? C(4) of 4 and 5 form a ‘flip-flop’ H-bonding system, with HO? C(3) acting as a H-bond donor to O? C(2) or O? C(4). HO? C(2) and HO? C(4) of monomeric 4 and 5 are thus the most strongly acidic OH groups. Glycosidation of 4 and 5 by 1 depends on the solvent, the temperature, and the concentration. It yields up to 91% of a mixture of anomeric pairs of the 1,2-, 1,3-, and 1,4-linked disaccharides 8–13 and 20–25 , respectively, which were characterized as their diacetates 14–19 and 26–31 (Scheme). Glycosidation in CH₂Cl₂ and in dioxane yielded mostly the 1,3-linked disaccharides 10/11 and 22/23 (α/β ca. 4:1), while glycosidation in THF leads mostly to the 1,2- and 1,4-linked regioisomers (β>α). There are small, but significant differences in the glycosidation of 4 and 5 . These, the regio-, and the stereoselectivities are rationalized as the consequences of the stereoelectronic control of both the H-transfer from HO? C(2) or HO? C(4) to the intermediate carbene and of the formation of the glycosidic C? O bond, and of the coordination of the intermediate oxycarbenium ion with THF.