GLYCOSYLIDENE CARBENES .17. GLYCOSIDATION OF BENZYL BETA-D-RIBOPYRANOSIDES AND BETA-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 i 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 (alpha/beta ca. 4:1), while glycosidation in THF leads mostly to the 1,2- and 1,4-linked regioisomers (beta > alpha). 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.