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

• Published in: Carbohydrate research 2002-11, Vol.337 (21), p.2153-2159
• Main Authors: Lahmann, Martina, Gybäck, Helena, Garegg, Per J., Oscarson, Stefan, Suhr, René, Thiem, Joachim
• Format: Article
• Language: English
• Subjects: Birch-type reduction; Chacotriose; Diosgenin; One-pot glycosylation; Reductive acetal opening; Saponins
• ISSN: 0008-6215; 1873-426X; 1873-426X
• DOI: 10.1016/S0008-6215(02)00275-6

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. Me 3N·BH 3/AlCl 3 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 S N2-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. Graphic