Synthesis of β-D-Glucopyranosides of 12α-Epimers of 20S-Protopanaxadiol and Betulafolientriol

β-D-Glucopyranosides of 3β, 12α,20S-trihydroxydammar-24-ene (1) and 3α,12α,20S-trihydroxydammar-24-ene (2), close structural analogs of ginsenoside-Rh2 and a metabolite of Panax ginseng glycosides (compound K), were synthesized. Glycosylation of 20S-hydroxydammar-24-ene-3,12-dione (5) and 3α-acetoxy-20S-hydroxydammar-24-en-12-one (6) by 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide (8) under classical Koenig-Knorr reaction conditions followed by deacetylation by NaOMe in MeOH gave free 20-O-β-D-glucopyranosides 10 and 13, treatment of which with NaBH₄ in i-PrOH formed 20-O-β-D- glucopyranosides of the 12α-epimers of 20S-protopanaxadiol 11 and betulafolientriol 14. Condensation of 3β,12α,20S-trihydroxydammar-24-ene (1) and glycosyl donor 8 in the presence of Ag₂O in CH₂Cl₂ followed by removal of the protecting groups gave 3-O-β-D-glucopyranoside 16; of 3α,12α,20S-trihydroxydammar-24-ene (2) under the same conditions, a mixture of 3- and 12-O-β-D-glucopyranosides 19 and 20.