Trang 20 Some compounds isolated from leaves of Lumnitzera racemosa growing in Vietnam

From L. racemosa leaves eight compounds were isolated: myricetin (1), quercetin (2), myricetin 3-O-α-L-rhamnopyranoside (3), myricetin 3-O-(2-O-galloyl-α-Lrhamnopyranoside) (4), myricetin 3-O-(3-Ogalloyl-α-L-rhamnopyranoside) (5), 3-Omethylellagic acid (6), (3S,5R,6S,7E)-3,5,6trihydroxy-7-megastigmen-9-one (7) and gallic acid (8). Their chemical structures were unambiguously elucidated by analysis of 1D and 2D NMR and high resolution ESI mass spectroscopic data, as well as by comparison with those reported in the literature. The αglucosidase inhibition was evaluated on isolated compounds. Among them, 1, 4, 5, 6 and 8 exhibited good activities with the IC50 values in the range of 1.319.3 M.


INTRODUCTION
Lumnitzera racemosa, an Indo-West Pacific mangrove plant, wildly grows in many mangrove forests in Vietnam.Some extracts from Lumnitzera racemosa leaves were reported to possess bioactivities, e.g.antimicrobial, hepatoprotective and antioxidant.This species was traditionally used to treat asthma, diabetes and snake bite.Some reports on the chemical constituents of Lumnitzera racemosa have been reported and there had one study on the antioxidant and cytotoxic activities of this plant growing in Vietnam [3]

General
The NMR spectra were measured on a Bruker Avance spectrometer, at 500 MHz for 1 H and 125 MHz for 13 C; the HR-ESI-MS were recorded on a HR-ESI-MS MicrOTOF-Q mass spectrometer in the University of Science, National University -HCM City.

Bioassay
The inhibitory activity of α-glucosidase was determined according to the modified method of Kim et al. (625 μL) to start the reaction.Each reaction was performed at 37 °C for 30 min and stopped by adding 0.1 M Na2CO3 (375 μL).Enzymatic activity was quantified by measuring the absorbance at 401 nm.One unit of α-glucosidase activity was defined as the amount of enzyme liberating p-nitrophenol (1.0 μM) per min.The IC50 value was defined as the concentration of αglucosidase inhibitor that inhibited 50 % of αglucosidase activity.Acarbose, a known αglucosidase inhibitor, was used as a positive control.The result was presented in Table 2.

RESULTS AND DISCUSSION
Isolation and purification of compounds from Lumnitzera racemosa leaves were performed using combinations of chromatographic fractionation of some ethyl acetate extracts to afford eight compounds (1-8) (Fig. 3).Their structures were elucidated as the following.
The HR-ESI-MS spectrum of (1) gave a quasimolecular ion peak at m/z 317.0315 [MH]  corresponding to the molecular formula of C15H10O8.The 1 H-NMR spectrum of (1) in DMSO-d6 showed a down field signal at δH 12.49 (1H, s) indicating the presence of a chelated hydroxyl at C-5 position.Two metacoupled doublet proton signals at δH 6.18 (1H, d, 2.0 Hz) and 6.36 (1H, d, 2.0 Hz) were assigned to H-6 and H-8, respectively, of ring A of the 5,7dihydroxyflavonoid.Moreover, a singlet signal at δH 7.24 (2H, s) was characteristic of a symmetric B ring.These spectral data revealed the presence of a myricetin skeleton.The good compatibility between these NMR data of (1) and those reported in the literature [6] confirmed its structure to be myricetin.However, instead of a signal integrating for two protons, the presence of an ABX system at δH 7.67 (1H, d, 2.5 Hz), 7.53 (1H, dd, 2.0 Hz) and 6.88 (1H, d, 8.5 Hz) corresponded to protons of the 1,3,4-trisubstituted phenyl group.The spectral data were compatible with those of quercetin [6].
Compound (3) and (1) showed similar spectral pattern but the former possessed a rhamnose moiety with proton signals at δH 5.18 (1H, d, 1.5 Hz, H-1) of an anomeric proton, 0.84 (3H, d, 6.0 Hz, H-6), and signals from δH 3.0 to 4.0 of an α-L-rhamnose moiety.In addition, this anomeric proton showed the HMBC cross-peak with carbon C-3 (δC 134.3) proving that the sugar moiety linking to the aglycone at its C-3.The good compatibility between these NMR data of (3) and those reported in the literature [5] confirmed its structure to be myricetin 3-O--Lrhamnopyranoside.
The spectral data of (5) were closely related to those of (4) with signals of a rhamnose unit, a myricetin skeleton and a galloyl group.The comparison of the 1 H NMR spectrum of (5) with that of (4) (Table 1) showed that the H-3 was downfield shifted suggesting the galloyl group was located at the rhamnose C-3 position, which was further confirmed by the HMBC cross-peak of the rhamnose proton signal H-3 at δH 5.04