Facile Synthesis of Oleanolic Acid Monoglycosides and Diglycosides

Oleanolic acid and its glycosides are important natural products, possessing various attractive biological activities such as antitumor, antivirus and anti-inflammatory properties. In the present work, fifteen oleanolic acid saponins bearing various saccharide moieties, including 3-monoglycoside, 28-monoglycoside and 3,28-diglycoside, were easily synthesized in high yields. Benzyl was chosen as the protective group for the COOH(28) group, instead of commonly used methyl and allyl, to avoid difficulties in the final deprotection. Alkali-promoted condensation of the carboxylic acid with bromo-glycosides was found to be more efficient in the synthesis of 28-glycosides. Two approaches were investigated and proved practicable in the preparation of 3,28-diglycosides. This method is suitable for preparing oleanolic acid glycosides with structural diversity for extensive biological evaluation and structure-activity relationship study, and it also apply new idea for the corresponding synthetic methods to the glycoside derivatives of other triterpenoid.