Rare biscoumarin derivatives and flavonoids from Hypericum riparium

Hypericum riparium A. Chev. is a Cameroonian medicinal plant belonging to the family Guttiferae. Chemical investigation of the methanol extract of the stem bark of H. riparium led to the isolation of four natural products, 7,7'-dihydroxy-6,6'-biscoumarin (1), 7,7'-dihydroxy-8,8'-biscoumarin (2), 7-methoxy-6,7'-dicoumarinyl ether (3), 2'-hydroxy-5'-(7″-methoxycoumarin-6″-yl)-4'-methoxyphenylpropanoic acid (4), together with one known 7,7'-dimethoxy-6,6'-biscoumarin (5), two flavones, 2'-methoxyflavone (6) and 3'-methoxy flavone (7), and two steroids, stigmast-4-en-3-one (8) and ergosta-4,6,8,22-tetraen-3-one (9). In addition, tetradecanoic acid (10), n-pentadecanoic acid (11), hexadecanoic acid (12), cis-10-heptadecenoic acid (13), octadecanoic acid (14) campesterol (15), stigmasterol (16), β-sitosterol (17), stigmastanol (18), β-eudesmol (19), 1-hexadecanol (20), and 1-octadecanol (21) were identified by GC-MS analysis. Compound 4 consists of a phenylpropanoic acid derivative fused with a coumarin unit, while compounds 2 and 3 are rare members of C8-C8' and C7-O-C6 linked biscoumarins. Their structures were elucidated by UV, IR, extensive 1D- and 2D-NMR experiments and electrospray (ESI) high resolution mass spectrometry (MS) including detailed MS/MS studies. This is the first report on the isolation of biscoumarins from the genus Hypericum, although simple coumarin derivatives have been reported from this genus in the literature. The cytotoxic activities of compounds 2-5 were evaluated against the human prostate cancer cell line PC-3 and the colon cancer cell line HT-29. They do not exhibit any significant cytotoxic activity.