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

Abstract

AbstractThe 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.