A facile approach to diosgenin and furostan type saponins bearing a 3β-chacotriose moiety Original Research Article

Combination of a one-pot coupling technique and the use of benzyl ethers as permanent protecting groups offered a short and simple route to dioscin-type saponins. This strategy in combination with a mild reductive opening procedure of the spiroketal function in diosgenin also offered a convenient approach to bidesmosidic furostan type saponins. Me3N·BH3/AlCl3 promoted acetal opening of 3-O-TBDMS-protected diosgenin gave the 26-OH acceptor 9 into which a benzylated β-glucose moiety was introduced by a SN2-type imidate coupling. After cleavage of the silyl ether, the 3β-O-glucose and the 4-O-linked rhamnose of the chacotriose unit were introduced by a NIS/AgOTf-promoted one-pot coupling sequence utilising thioglycoside donors and their different reactivity in different solvents. After removal of a benzoyl group, the same coupling conditions were also used for the coupling of the second 2-O-linked rhamnose unit. The target substance was obtained after cleavage of the protecting benzyl ethers under Birch-type conditions, which did not affect the double bond in the steroid skeleton.