On the Chemical Constituents of Dipsacus asper
Chemical and pharmaceutical bulletin
Dipsacus asper WALL., a perennial plant distributed in mountainous regions of southwestern China, has been used in Chinese traditional folk medicines as antibacterial, anti-inflammatory, and anti-complement agents, and as growth stimulants of osseous cells as well. 1,2) In a recent report, the extract of D. asper was found to reduce the cognitive deficits and overexpression of b-amyloid protein induced by aluminum exposure. 3) Due to its numerous medicinal uses, extensive chemical
... mical investigations have been conducted on this plant.      During the screening of plant extracts with cytotoxicity, the ethanolic extract of the roots of D. asper showed significant activity by the 3-[4,5-dimethylthiazol-2-yl]-2,5diphenyltetrazolium bromide (MTT) assay, so we began to investigate the chemical constituents responsible for its cytotoxicity. Bioassay-guided fractionation of the extract led to the isolation of main cytotoxic components, e.g., caffeic acid, 2,6-dihydroxycinnamic acid, vanillic acid, 2Ј-O-caffeoyl-Dglucopyranoside ester, and caffeoylquinic acid, together with five new bis-iridoid glucosides dipsanosides C-G (1-5), one new iridoid glucoside 3Ј-O-b-D-glucopyranosyl sweroside (6), and the five known iridoid glucosides, loganin, cantleyoside, triplostoside A, lisianthioside, and 6Ј-O-b-D-apiofuranosyl sweroside as well as the triterpenoids oleanic acid and akebiasaponin D. All of the iridoidal glycoside monomers and dimers were found to be inactive in cytotoxic assays (see Table 4 ). In a rapid communication, we reported the structure elucidation of two iridoid glycosides tetramers, the most complex iridoid glycosides hitherto known. 5) In this paper, we report the structural determination of the new iridoid glycosides 1-6 by various spectroscopic means, as well as the isolation and bioassay results of all above-mentioned compounds. Results and Discussion Many bis-iridoid glucosides have been isolated from Dipsacaceae, such as sylvestrosides I-IV from D. sylvestris, 9) and laciniatosides I-V from D. laciniatus. 10,11) Bis-iridoids have also been reported from D. asperoides, 12) D. ferox 13) and from D. japonica, 14) but none to date from D. asper. Bis-iridoids, composed of secologanin and loganin units, have so far been reported to have the same stereochemistry as the model compound cantleyoside, with a b-configuration at the C-1 position. Compound 1, white powder, [a] D 20 Ϫ34.8°(cϭ0.2095, MeOH), showed IR absorption bands due to the hydroxyl functions (3431 cm Ϫ1 ) and a,b-unsaturated ester carbonyl groups (1701, 1637 cm Ϫ1 ), as well as UV absorption maximum at 234.4 nm due to the a,b-unsaturated carbonyl groups. The molecular formula, C 36 H 52 O 21 , was confirmed by high resolution (HR)-ESI-MS. Acid hydrolysis of 1 gave D-glucose, which was identified by HPLC and TLC analysis according to the reported procedure in the literature. 15) The Bioassay-guided fractionation of 95% EtOH extract from the roots of Dipsacus asper lead to the isolation of some phenolic acids (caffeic acid, 2,6-dihydroxycinnamic acid, vanillic acid, 2-O-caffeoyl-D-glucopyranoside ester, and caffeoylquinic acid) as the major active components, and five new iridoid glucoside dimers (1-5) and one new iridoid glucoside monomer (6), other known iridoid glycosides loganin, cantleyoside, triplostoside A, lisianthioside, 6-O-b b-D-apiofuranosyl sweroside, as well as triterpenoids oleanic acid and akebiasaponin D. The structures of new compounds 1-6 were determined as dipsanosides C (1), D (2), E (3), F (4), G (5), and 3-O-b b-D-glucopyranosyl sweroside (6) by spectroscopic, including 1D and 2D NMR techniques, and chemical methods.