GLYCOSYLIDENE CARBENES - PART-25 - GLYCOSIDATION OF GINKGOLIDE-B AND GINKGOLIDE-A

Ginkgolide B (1b) has been glucosylated in THF with the glucosylidene- derived diazirine 2 under thermal or photochemical conditions. Dependi ng on the amount of 2, we obtained either monoglucosides (5-8), digluc osides (13-17), or triglucosides (21-23). In keeping with earlier resu lts, the use of THF as solvent led mostly to beta-D-glucosides. The mo dest regioselectivity in the formation of the monoglucosides, glucosyl ated 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 rea dily glucosylated than the secondary HO-C(1) or HO-C(10) (H-bonded). G lucosidation with 3.5 equiv. of 2 led to triglucosides, with the tri-b eta-D-glucoside 21 (42 %) as the major product. Catalytic hydrogenatio n afforded the free glucosides 9-12, 18-20, and 24. The di- and triglu cosides are readily soluble in H2O. Glucosidation with 2 of the ginkgo lide-A-derived tertiary alcohol 25 yielded 93 % of the beta-D-anomeric glucoside 26. Similarly, glycosidation of 25 with the lactosylidene-d erived diazirine 34 proceeded with a very high stereoselectivity, yiel ding 92 % of the beta-D-lactoside 35, that was deprotected to the H2O soluble acetate 36.