Identification of ellagic acid derivatives from stem bark of Syzygium guineense (Myrtaceae)

An LC-DAD-ESI-MSn technique was developed to investigate crude extracts of the stem bark of S. guineense for phenolic compounds. Ellagic acids were detected on the basis of their UV absorption spectra and ESI-MS fragmentation patterns. Six ellagic acid conjugates in the complex extract of S. guineense were identified. With the aim to confirm the structures, further purification of the extract afforded the known 3-O-ellagic acid-4'-O-alpha-rhampyranonosides (3), two new ellagic acid rhamnopyranosides, 3-O-methylellagic acid-4'-O-alpha-2"-O-acetylrhamnopyranoside (4) and 3-O-methylellagic acid-4'-O-alpha-3"0-acetylrhanmopyranoside (6). The structures were identified on the basis of ID- and 2D- NMR, IR, and UV spectroscopic and mass spectrometric data. The HMBC and NOESY data of these compounds were most useful for determination of their structures. Radical scavenging activities were evaluated in the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. Compounds 3, 4 and 6, with IC50 values of 6.1 +/- 0.5, 5.8 +/- 0.4, and 6.2 +/- 0.4 mu M, respectively were as active as ellagic acid (1) (IC50= 4.9 +/- 0.3 mu M) and more active than catechin (IC50=10.8 +/- 0.3 mu M).