New Phenylpropanoid Glycosides from Eurya emarginata (Thunb.) Makino

Abstract

Eurya emarginata (Thunb.) Makino belonging to Theaceae is an evergreen branched shrub with coriaceous and emarginated leaves. These trees are distributed along the coastal regions in the southern parts of Korea including Jeju Island. The genus Eurya contains more than 100 species worldwide, but only two species have been described in Korea. The leaves of E. emarginata and E. japonica have been used in Korean traditional medicine to treat arthritis. Previous biological studies of the extract of E. emarginata reported the anti-cancer, anti-inflammation and radiation protection activities, and identified phenolic glycosides as the active constituents. As part of an ongoing study aimed at identifying bioactive compounds from plants in Jeju Island, this study conducted phytochemical investigations on this plant. From the ethanol extract of the leaves and twigs of E. emarginata, new phenylpropanoid glycosides, eutigosides D (1) and E (2), were isolated along with five known compounds, eutigoside B (3), eutigoside C (4), cinnamic acid (5), rengyolone (6) and cleroindicin B (7) (Fig. 1). We herein report the structure elucidation and antioxidative activities of isolates 1-7. Compound 1 was obtained as viscous liquid with molecular ion peak at m/z 446.1577 in HR-FAB-MS, indicating a molecular formula of C23H26O9 (11 unsaturations). The C NMR spectrum revealed only 20 carbon peaks suggesting the presence of a symmetric moiety, which was readily identified as a phenyl group based on the characteristic aromatic H and C NMR signals appearing in narrow δ ranges (Tables 1 and 2). The trans-olefinic structure was also clearly characterized by the large coupling constant (J = 16.0 Hz) of the sp peaks at δ 7.76 (H-7’’) and 6.52 (H-8’’) ppm. This olefin was determined to be connected to both the phenyl and a carbonyl (C-9’’) groups by an inspection of the heteronuclear multiple bond correlation (HMBC) spectrum, which established a cinnamoyl substructure. The other aromatic protons were characterized to have the ABX spin system in the H NMR spectrum (Table 1). For example, the signal for H-5 showed a coupling constant of J = 8.5 Hz, which confirmed the ortho coupling to H-6. The signal for H-6 showed ortho and meta coupling to H-2 with J = 2.8 Hz. The remaining substructure –OCH2CH2was confirmed by its H, C and COSY NMR data. The HMBC cross signal of H-8/C-1 indicated the linkage of this ethylene unit to C-1 of the aromatic ring (Fig. 2). The presence of a sugar moiety in compound 1 is also clearly recognized by the H and C NMR spectra. As protons of the sugar exhibited relatively large coupling constants due to their axial H/H interactions, this hexose was assumed to be glucose. This was confirmed by the hydrolysis of the compound 1 with aq. HCl followed by the comparative TLC analysis of the aqueous phase to the authentic D-glucose. As the anomeric This paper is dedicated to Professor Eun Lee on the occasion of his honourable retirement. Figure 1. Structures of compounds 1-7 isolated from E. emarginata.