Facile synthesis of four natural triterpene saponins with important antitumor activity

The first synthesis of four natural triterpene saponins, which exhibit significant antitumor activities, was concisely achieved by adopting a stepwise glycosylation. The key intermediate 13 was afforded via Bu2SnO-mediated regioseletive benzoylation. During the preparation of the target compounds, it was found that the α-L-arabinopyranosyl unit in intermediates 17 and 20 existed in the unusual ¹C₄ conformation, and after removing the benzoyl groups, the α-L-arabinopyranosyl unit was normal in a typical ⁴C₁ form.