A New Neolignan Glycoside from the Leaves of Acer truncatum

A new neolignan glycoside, (7R,8R)-7,8-dihydro-9'-hydroxyl-3'-methoxyl- 8-hydroxymethyl-7-(4-hydroxy-3-methoxyphenyl)-1'-benzofuranpropanol 9'-O-beta-D- glucopyranoside (1) was isolated from the leaves of Acer truncatum along with (7R,8R)-7,8-dihydro-9'-hydroxyl-3'-methoxyl-8-hydroxymethyl-7-(4-O-alpha-L-rhamno- pyranosyloxy-3-methoxyphenyl)-1'-benzofuranpropanol (2), schizandriside (3), lyoniresinol (4), berchemol (5), (-)-pinoresinol-4-O-beta-D-glucopyranoside (6), hecogenin (7), chlorogenic acid (8) and neochlorogenic acid (9). Their structures were elucidated on the basis of extensive spectroscopic data. The absolute configuration of compounds 1 was established by its CD spectrum. The antibacterial activities of compounds 1-7 were evaluated.


Biological activity
Compounds 1-7 were tested for their antibacterial activities against Escherichia coli, Staphylococcus aureus, Micrococcus luteus and Bacillus cereus using the paper disk method.All stock cultures were grown on tryptic soy agar plates.Test strains were transferred to fresh tryptic soy broth before use and a disk containing only DMSO was used as negative control.The compounds were found to be inactive at concentrations of up to 50 µg/disk, except for schizandriside (3), which showed moderate antibacterial activity, affording inhibitory zone sizes of 11 mm against Staphylococcus aureus at a concentration of 2 µg/disk.

Conclusions
Nine phenolic constituents including a new neolignan glycoside were isolated from the leaves of Acer truncatum.Their structures were established on the basis of 1D-and 2D-NMR experiments, CD data and comparison with literature values.The antibacterial activities of pure compounds 1-7 was tested against four microbial species.Only schizandriside (3) showed moderate antibacterial activity against S. aureus.

General
FAB mass spectra were obtained on a VG Auto spec-3000 spectrometer and high-resolution ESI mass spectra were recorded on an API Qstar Pulsar instrument.1D-and 2D-NMR experiments were performed on Bruker AM-400 and DRX-500 instruments with TMS as internal standard.Chemical shifts (δ ) were expressed in ppm with reference to the solvent signals; coupling constants (J) are given in Hertz (Hz).IR spectra were taken in KBr on a Bio-Rad FTS-135 infrared spectrophotometer.Optical rotations were measured in a JASCO DIP-370 digital polarimeter.UV spectra were measured using a Shimadzu UV-2401PC spectrophotometer.CD spectra were run on a JASCO J-810 instrument.Column chromatography (CC) was performed using 200-300 mesh silica gel (Qingdao Marine Chemical Inc., Qingdao, P.R. China), on silica gel H (10-40 µm, Qingdao Marine Chemical Inc.) and Lichroprep RP-18 (43-63 µm, Merck).

Plant material
Leaves of A. truncatum were collected in Kunming, Yunnan province, P. R. China, in August 2004.The plants were identified by Prof. Ting-Zhi Xu, Kunming Institute of Botany, Chinese Academy of Science.

Extraction and isolation
Air-dried leaves of A. truncatum (20 kg) were extracted with H 2 O.The extract was evaporated in vacuo to give a black-brown gum, which was applied to ADS-7 porous resin and divided into four fractions: H 2 O fraction, 30% EtOH fraction, 70% EtOH fraction, and 90% EtOH fraction.The 30% EtOH fraction (256 g) was subjected to CC (SiO 2 , CHCl 3 /MeOH 9:1→7:3) to afford fractions Fr.1-10, as judged by TLC.Fr. 2 (20 g) was further purified by CC (first SiO 2 , petroleum ether/AcOEt, then RP-18 gel) to afford 7 (135 mg).Fr. 3 (12 g) was subjected to CC (SiO 2 , petroleum ether/AcOEt) to afford 5 (20 mg).Fr. 4 (18 g) was subjected to CC (SiO 2 , petroleum ether/AcOEt ) to yield 4 (37 mg).Fr. 5 (19 g) was subjected to CC (SiO 2 , CHCl 3 /AcOEt) to yield 8 (13 mg) and 9 (10 mg).Fr. 7 (23 g) was subjected to CC (SiO 2, petroleum ether/AcOEt) to afford 6 (226 mg).Fr. 9 (20 g) was subjected to CC (first, SiO 2 ; CHCl 3 /MeOH, then RP-18 gel) to afford 1 (37 mg), 2 (65 mg) and 3 (171 mg).  1. [4] Antibacterial activity was tested by the disk diffusion method with minor modifications.E. coli, S. aureus, M. luteus and B. cereus were subcultured in tryptic soy broth (TSB), incubated for 18 h at 37 °C and then the bacterial cells were suspended, according to the McFarland protocol, in saline solution to produce a suspension of about 10 -5 CFU•mL -1 .An aliquot of this suspension (15 µL) was mixed with sterile tryptic soy agar (TSA, 15 mL) at 40 °C and poured onto an agar plate in a laminar flow cabinet.Each tested compound was dissolved in DMSO and added to a paper disk (6 mm diameter) that was dried and placed on the agar plate containing the bacterial cells (5 samples/disk plus control).A disk containing only DMSO was used as negative control.The susceptibility of the bacteria to the test compounds was determined by the formation of an inhibitory zone after 18 h of incubation at 37 °C.Experiments were run in triplicate, and the results were determined as mean values of the three measurements.