Xanthones from the green branch of Garcinia dulcis

Abstract Two new prenylated xanthones, namely dulcisxanthone H and dulcisxanthone I along with garciniaxanthone C, were isolated from the dichloromethane extract of the green branch of Garcinia dulcis. Their structures were elucidated by the analysis of 1-D and 2-D NMR spectral data. Their antibacterial activities were also examined. Two new prenylated xanthones named dulcisxanthone H and dulcisxanthone I were isolated from the green branch of Garcinia dulcis.


Introduction
The genus Garcinia (Clusiaceae) is a rich source of phenolic compounds with exhibited biological activities such as, antioxidative, antibacterial, antifungal and cytotoxic activities (Chin et al. 2008;Ngoupayo et al. 2009;Chen et al. 2010;Feng et al. 2014;Fouotsa et al. 2014). Garcinia dulcis is an Asian plant used in traditional medicine against lymphatitis, parotitis, struma and other disease conditions (Kasahara & Henmi 1986). Some of the xanthones, flavonoids and biflavonoids of this plant species have shown antimalarial, antibacterial, antioxidative and antiandrogenic activities (Likhitwitayawuid et al. 1998;Deachathai et al. 2005;Shakui et al. 2014). We have previously reported the isolation of 12 new compounds and 64 known compounds from the fruits, flowers, seeds and leaves of the species collected in Thailand and tested them for antibacterial and antioxidative activities (Deachathai et al. 2005(Deachathai et al. , 2006(Deachathai et al. , 2008Saelee et al. 2015). As a continuation of our search for new natural products in relation to biological activity, we now report the isolation and structural elucidation of three prenylated xanthones from the green branch of G. dulcis and examined their antibacterial activity.

Results and discussion
The dichloromethane extract of an air-dried green branch of G. dulcis was purified by chromatography along with partitioning with 5% NaOH and an organic solvent to give two new prenylated xanthones named dulcisxanthone H (1) and dulcisxanthone I (2), along with known garciniaxanthone C (3) (Minami et al. 1994;Iinuma et al. 1996) (Figure 1). Their structures were elucidated by the analysis of spectroscopic data and comparison of the NMR spectral data with previous reports in the literature.
All the isolated xanthones and the crude dichloromethane extract were tested for their antibacterial activity against Staphylococcus aureus ATCC25923, methicillin-resistant S. aureus (MRSA) SK1, Escherichia coli ATCC25922 and Pseudomonas aeruginosa ATCC27853, with vancomycin and gentamicin as the standard drugs. They showed no antibacterial activity against those bacterial strains at the minimum inhibitory concentration (MIC) of 200 μg/mL, except for the crude extract, that displayed antibacterial activity with an MIC of 128 μg/mL.

General experimental procedures
Melting points were obtained on a Fisher-Johns Melting Point Apparatus. Infrared (IR) spectra were determined on a Perkin-elmer 783 FTS 165 FT-IR spectrometer and were recorded as wave number (cm −1 ). ultraviolet (uV) absorption spectra were determined by using MeOH on a Shimadzu uV-160A spectrophotometer and principle bands (λ max ) were recorded as wavelengths (nm) and log ε in a methanol solution. electrospray ionisation mass spectrometric (eSI-MS) data were recorded on a Waters Alliance 2690 and a Micromass LCT (united Kingdom) mass spectrometers at the Scientific equipment Center, Prince of Songkla university. Nuclear magnetic resonance (NMR) spectra were measured on a bruker FT-NMR ultra Shield TM 300 spectrometer at the Department of Chemistry, Faculty of Science, Prince of Songkla university. 1 H and 13 C NMR spectra were measured at 300 and 75 MHz, respectively. Chemical shifts (δ) were recorded in parts per million (ppm) in CDCl 3 and acetone-d 6 containing TMS as an internal standard (δ 0.00) and the coupling constant (J) was expressed in hertz (Hz). Column chromatography (CC) was performed with silica gel 100 (70-230 Mesh ASTM, Merck), silica gel RP-18 (40-63 μm, Merck) and Sephadex LH-20 (Amersham biosciences, Sweden). Preparative thin-layer chromatography (pTLC) and thin-layer chromatography (TLC) were performed on silica gel GF 254 (20 × 20 cm with a layer thickness of 0.2 mm, Merck) and compounds were detected under uV (254 nm) fluorescence. All solvents were of spectroscopic grade or distilled from glass prior to use.

Plant material
A green branch of G. dulcis was collected from Nakhon Si Thammarat in the southern part of Thailand, in April 2013. The voucher specimen (Coll. No. 02, Herbarium No. 0012652) has been deposited at the Herbarium of the biology Department, Faculty of Science, Prince of Songkla university, Thailand.

Antibacterial assay
This was carried out according to the previously reported procedure of Saelee et al. (2015).

Conclusion
Investigation of the chemical constituents of a dichloromethane extract of a G. dulcis green branch led to the isolation of three prenylated xanthones. Dulcisxanthone H and dulcisxanthone I are new naturally occurring compounds. Further investigation of antibacterial active compound from this plant species is ongoing.

Supplementary material
Supplementary material relating to this article is available online, alongside Tables S1-S3 and Figures S1-S14.