New saponins from Sechium mexicanum.

The chemical study of Sechium mexicanum roots led to the isolation of the two new saponins {3-O-beta-D-glucopyranosyl (1 --> 3)-beta-D-glucopyranosyl-2beta,3beta,16alpha,23-tetrahydroxyolean-12-en-28-oic acid 28-O-alpha-L-rhamnopyranosyl-(1 --> 3)-beta-D-xylopyranosyl-(1 --> 4)-alpha-L-rhamnopyranosyl-(1 --> 2)-alpha-L-arabinopyranoside} (1) and {3-O-beta-D-glucopyranosyl (1 --> 3)-beta-D-glucopyranosyl-2beta,3beta,16alpha,23-tetrahydroxyolean-12-en-28-oic acid 28-O-alpha-L-rhamnopyranosyl-(1 --> 3)-beta-D-xylopyranosyl-(1 --> 4)-[beta-D-apiosyl-(1 --> 3)]-alpha-L-rhamnopyranosyl-(1 --> 2)-alpha-L-arabinopyranoside} (2), together with the known compounds {3-O-beta-D-glucopyranosyl-(1 --> 3)-beta-D-glucopyranosyl-2beta,3beta,6beta,16alpha,23-pentahydroxyolean-12-en-28-oic acid 28-O-alpha-L-rhamnopyranosyl-(1 --> 3)-beta-D-xylopyranosyl-(1 --> 4)-alpha-L-rhamnopyranosyl-(1 --> 2)-alpha-L-arabinopyranoside} (3), tacacosides A(1) (4) and B(3) (5). The structures of saponins 1 and 2 were elucidated using a combination of (1)H and (13)C 1D-NMR, COSY, TOCSY, gHMBC and gHSQC 2D-NMR, and FABMS of the natural compounds and their peracetylated derivates, as well as by chemical degradation. Compounds 1-3 are the first examples of saponins containing polygalacic and 16-hydroxyprotobasic acids found in the genus Sechium, while 4 and 5, which had been characterized partially by NMR, are now characterized in detail.