Synthesis of Two Natural Oleanolic Acid Saponins

Abstract 3‐O‐[β‐D‐Glucopyranosyl‐(1→3)‐α‐L‐arabinopyranosyl]‐oleanolic acid‐28‐O‐[β‐D‐glucopyranosyl] ester 1 was synthesized concisely by a convergent strategy. Using stepwise fashion for the synthesis of saponin 2, 3‐O‐{[β‐D‐glucopyranosyl‐(1→2)]‐[α‐L‐arabinopyranosyl‐(1→3)]‐α‐L‐arabinopyranosyl}‐oleanolic acid‐28‐O‐(β‐ D‐glucopyranosyl) ester, an abnormal phenomenon, that the terminal arabinosyl residue took the 1C4 conformation instead of typical 4C1 form, was observed. Deprotection or heating could not resume the normal conformation, which resulted in the product of 2′ not 2.