Cowania mexicana D. DON (Rosaceae) is distributed in western North America, and its aerial parts have been used as a cough suppressant and a remedy for respiratory disease by America Indians. As chemical constituents of this plant, the presence of triterpenoids, such as ursolic acid 1) and cucurbitan-type triterpenes 2) as inhibitors of Epstein-Barr virus early antigen (EBV-EA) activation were reported. We reported the isolation and characterization of nine tannins and related polyphenols [casuarinin (2), stachyurin (3), pterocarinin A, stenophyllanin A (4), alienanin B, casuglaunin A, (Ϫ)-epicatechin, procyanidin B-4 and ellagic acid] along with two nitrile glucosides (menisdaurin and lithospermoside) from the leaves and stems of C. mexicana. Their inhibitory effects on in vitro EBV-EA activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) and on in vivo two-stage carcinogenesis on mouse back-skin were also reported.3) Further chemical investigation on a polar fraction of the plant extract has resulted in the isolation of a new complex tannin dimer named cowaniin (1) and a known nitrile glucoside. This paper describes the isolation and structure elucidation of the new tannin, along with five known polyphenols from a non-polar fraction.A concentrated 70% aqueous acetone homogenate of the dried aerial parts of C. mexicana was extracted with Et 2 O, EtOAc and water-saturated n-BuOH to give the respective extracts and the water-soluble portion. The Et 2 O extract was chromatographed over polyvinyl gels to afford five known compounds, quercetin 3-O-a-L-arabinofuranoside, 4) and pcoumaric, p-hydroxybenzoic, protocatechuic and gallic acids. The separation and purification of the n-BuOH and watersoluble portion by polystylene and/or polyvinyl gels gave cowaniin (1) and a known nitrile glucoside, purshianin.5) The known compounds were identified by direct comparison with authentic samples and/or by their spectral comparisons with data reported in the literature.Cowaniin (1) was obtained as an off-white amorphous powder and exhibited an [MϩNH 4 ] ϩ ion peak at m/z 2162 in electrospray ionization (ESI)-MS. The molecular formula was determined to be C 97 H 68 O 57 by elemental analysis and NMR spectra along with MS data. The 1 H-NMR spectrum of 1 exhibited two 2H singlets (d 7.09, 7.06) and seven 1H singlets (d 6.83, 6.72, 6.62, 6.55, 6.53, 6.51, 6.25), which were assignable to two galloyl and four hexahydroxydiphenoyl (HHDP) groups. These acyl groups were consistent with the appearance of ten ester carbonyl carbon signals in C-NMR resonances of glucose and catechin residues of 1, which were assigned by the heteronuclear multiple quantum connectivity (HMQC) spectrum, indicated a close resemblance to those of stachyuranin A (6) 6) and stenophyllanin B (5) 7) rather than stenophyllanin A (4) as shown in Table 1. These NMR spectral features and MS data, taking the absence of A-ring protons of catechin into consideration, suggested the dimeric structure of cowaniin, in which the 6-and 8-positions of the catechin u...
