Echinocystic acid 3,16- O-bisglycosides from Albizia procera

Three echinocystic acid 3,16- O-bisglycosides were isolated from the bark of Albizia procera. Their structures were elucidated by 1D and 2D NMR experiments, FABMS as well as chemical means. In contrast to other cytotoxic echinocystic acid glycosides with N-acetyl glucosamine unit, the glycosides were found inactive when assayed by MTT method for their cytotoxicities against the human tumor cell lines HEPG2, A549, HT29 and MCF7. The results showed that the free hydroxyl group at C-16 of echinocystic acid, is important for exhibiting cytotoxic properties. Three triterpene glycosides and two known ones were isolated from the bark of Albizia procera by using chromatographic techniques. The structures of the compounds were determined to be 3- O-β- d-xylopyranosyl-(1 → 2)-β- d-galactopyranosyl-(1 → 6)-2-acetamido-2-deoxy-β- d-glucopyranosyl echinocystic acid 16- O-β- d-glucopyranoside, 3- O-β- d-xylopyranosyl-(1 → 2)-α- l-arabinopyranosyl-(1 → 6)-2-acetamido-2-deoxy-β- d-glucopyranosyl echinocystic acid 16- O-β- d-glucopyranoside and 3- O-α- l-arabinopyranosyl-(1 → 2)-α- l-arabinopyranosyl-(1 → 6)-2-acetamido-2-deoxy-β- d-glucopyranosyl echinocystic acid 16- O-β- d-glucopyranoside. Their structures were determined by NMR techniques including HOHAHA, 1H- 1H COSY, ROE, HMQC and HMBC experiments together with FABMS as well as acid hydrolysis. To the best of our knowledge, the new compounds are considered the first examples of echinocystic acid 3,16- O-bisglycosides. In contrast to other cytotoxic echinocystic acid glycosides with N-acetyl glucosamine unit, the new glycosides were found inactive when assayed by MTT method for their cytotoxicities against the human tumor cell lines HEPG2, A549, HT29 and MCF7. The results showed the importance of the free hydroxyl group at the aglycone C-16 for exhibiting cytotoxic properties.