Isolation and Identification of Steroidal Saponins in Taiwanese Yam Cultivar (Dioscorea pseudojaponica Yamamoto)

A new furostanol pentaoligoside and spirostanol tetraoligoside were isolated for the first time from yam tubers (Dioscorea pseudojaponica Yamamoto) from Taiwan, together with four known yam saponins, methyl protodioscin, methyl protogracillin, dioscin, and gracillin. Their structures were characterized as 26-O-β-D-glucopyranosyl-22α-methoxyl-(25R)-furost-5-en-3β, 26-diol, 3-O-α-L-rhamnopyranosyl-(1→2)-O-{[α -L-rhamnopyranosyl-(1→4)]-O-[α-L-rhamnopyranosyl-(1→4)]} -β-D-glucopyranoside, and (25R)-spirost-5-en-3β-ol 3-O-α-L-rhamnopyranosyl-(1→2)-O-{[α-L-rhamnopyranosyl-(1→ 4)]-O-[α-L-rhamnopyranosyl-(1→4)]}-β-D-glucopyranoside. The structural identification was performed using LC-MS and ¹H and ¹³C NMR. The methanol extract of yam tubers was fractionated by XAD-2 column chromatography using a methanol/water gradient elution system to yield furostanol and spirostanol glycoside fractions. Preparative high-performance liquid chromatography, employing a C18 column and a mobile phase of methanol/water (69:31, v/v), was used to separate each furostanol glycoside, whereas a mobile phase of methanol/water (79:21, v/v) was used to resolve the individual spirostanol glycosides. The conversions from steroid saponins to diosgenin after acid hydrolysis were around 68 and 90% for furostanol and spirostanol glycosides, respectively.