Abstract
Three new tannin-related compounds (I, II and III), along with lindleyin (IV), (+)-catechin, 3-O-galloyl-(-)-epicatechin, gallic acid, 3, 5, 4'-trihydroxystilbene 4'-O-β-D-(6"-O-galloyl)-glucopyranoside, 3, 5, 4'-trihydroxystilbene 4'-O-β-D-glucopyranoside and 4-(4'-hydroxyphenyl)-2-butanone 4'-O-β-D-glucopyranoside, have been isolated from commercial rhubarb (Rhei Rhizoma). On the basis of spectral and chemical evidence, I, II and III were characterized as 3, 3'-di-O-galloylprocyanidin B-2, 3-O-galloylprocyanidin B-1 and 1, 2, 6-tri-O-galloylglucose, respectively. The occurrence of IV in rhubarb is of great significance, since IV has been reported to have analgesic and anti-inflammatory activities almost equal to those of aspirin and phenylbutanone. Tannins in rhubarb have been partially purified (designated as rhatannin (V)). Thiolysis degradation and enzymatic hydrolysis have shown that V is mainly composed of C4 to C8 linked 3-O-galloyl-(-)-epicatechin units in the extension part (upper part) with either 3-O-galloyl-(-)-epicatechin or (+)-catechin unit in the lower terminal part.
