TETRAOXYGENATED XANTHONES FROM THE LATEX OF GARCINIA COWA GROWING IN VIET NAM Nguyen

Seven tetraoxygenated xanthones, namely fuscaxanthone A, 7-O-methylgarcinone E, cowagarcinone A, cowaxanthone, rubraxanthone, cowanin and cowanol, were isolated from the dichloromethane extract of the latex of Garcinia cowa Roxb. ex Choisy. Their structures were elucidated on the basis of 1D, 2D NMR spectroscopic data and compared with reported data. This is the first time the chemical constituents of Garcinia cowa Roxb. ex Choisy have been investigated in Vietnam.


General
NMR spectra were recorded on a Bruker Advance 500 spectrometer at 500 and 125 MHz for 1 H and 13 C, respectively, at Institute of Chemistry -Vietnam Academy of Science and Technology.Chemical shifts are shown in δ (ppm) in CDCl 3 with tetramethylsilane (TMS) as an internal reference.Melting points were measured on Buchi B545 apparatus (no correction).Column chromatography (CC) were carried out on silica gel 60 (Merck, 5-40 μm), silica gel 100 (Merck, 63-200 μm), and/or sephadex LH-20 (GE Healthcare).Visualization of thin layer chromatography (TLC) plates was performed using UV light (254 and 365 nm), staining with H 2 SO 4 10 % solution.Commercial solvents were purified and dried, when necessary, by standard methods just prior to use.

Plant materials
The latex of G. cowa was collected in Phu Quoc island -Kien Giang province, in December 2015.The plant materials were identified by Dr. Nguyen Quoc Binh, Vietnam National Museum of Nature.The herbarium specimen has been deposited at Institute of Natural Products Chemistry -Vietnam Academy of Science and Technology with the plant specimen number GC2015128.

Extraction and isolation
The latex of G. cowa (3.0 kg) existing in a form of a brown solid, was crushed into small pieces and then was dried in the oven at the temperature of 45 o C in three days to achieve 2.8 kg dried latex.The dried latex was extracted with methanol (MeOH) (3 L × 3) at room temperature using conventional ultrasound-assisted technique.The solvent was evaporated under reduced pressure to give a dark brown residue (500.0 g).The residue was further extracted with DCM (500 mL × 3) to yield DCM extract (96.7 g).The left residue was then extracted with ethylacetate (EtOAc) (500 mL × 3) to afford EtOAc extract (145.1 g).
The NMR spectrum of compound 3 exhibited 2xCH 2 , 1xCH 3 , 1xCH= and 1xCq sp 2 more than compound 2. Analysing the structure of 3 in comparison with reported value [9], 3 was determined to be cowagarcinone A.
Compound 4 or 5 contains only one geranyl group substituting on xanthonoid skeleton.The HMBC correlations of H-1' to C-1, -2, -3 in compound 4 and the correlation of H-1', -2' to C-8 in compound 5 proved that the geranyl group bonds to C-2 for 4 and to C-8 for 5. Therefore, the structure of 4 and 5 were elucidated as cowaxanthone [5] and rubraxanthone [17,18], respectively.The 1 H and 13 C-NMR spectrum of compound 6 exhibited the same resonance signals with those of compound 5 except for the replacement of aromatic proton H-2 by a prenyl group.The HMBC spectrum showed the correlation of H-1' to C-1, -2, -3 suggesting the appearance of one