Glycosylidene carbenes - Part 25 - Glycosidation of ginkgolides B and A

Ginkgolide B (1b) has been glucosylated in THF with the glucosylidene-derived diazirine 2 under thermal or photochemical conditions. Depending on the amount of 2, we obtained either monoglucosides (5-8), diglucosides (13-17), or triglucosides (21-23). In keeping with earlier results, the use of THF as solvent led mostly to beta-D-glucosides. The modest regioselectivity in the formation of the monoglucosides, glucosylated either at O-C(1) or O-C(10), is rationalized on the basis of the relative kinetic acidity of the intra-and intermolecularly H-bonded OH groups of 1b. The tertiary HO-C(3) of the monoglucosides was more readily glucosylated than the secondary HO-C(1) or HO-C(10) (H-bonded). Glucosidation with 3.5 equiv. of 2 led to triglucosides, with the tri-beta-D-glucoside 21 (42 %) as the major product. Catalytic hydrogenation afforded the free glucosides 9-12, 18-20, and 24. The di- and triglucosides are readily soluble in H2O. Glucosidation with 2 of the ginkgolide-A-derived tertiary alcohol 25 yielded 93 % of the beta-D-anomeric glucoside 26. Similarly, glycosidation of 25 with the lactosylidene-derived diazirine 34 proceeded with a very high stereoselectivity, yielding 92 % of the beta-D-lactoside 35, that was deprotected to the H2O soluble acetate 36.