One new compound from Borreria alata (Aubl.) DC (Rubiaceae) in Vietnam

Borreria is a genus of Rubiaceae widespread in tropical and subtropical America, Africa, Asia, and Europe. Studies have confirmed that extracts as well as some isolated compounds of species of Borreria genus possess diverse biological activities, including anti-inflammatory, antitumor, antimicrobial, antioxidant, anti-ulcer... In this paper, we present the chemical structures of four compounds isolated from Borreria alata, collected at Di Linh district, Lam Dong province, Vietnam. 3β, 6β, 23-trihydroxyurs12-en-28-oic acid (1), sodium deacetylasperulosidate (2), 7β-hydroxy-11-methylforsythide (3) and sodium loganate (4). Among them, three compounds (1), (2), (3) were known for the first time in Borreria genus to our best knowledge and (4) is a new compound. The chemical structures of these compounds wereelucidated by analysis of 1D and 2D NMR and HR-MS spectroscopic data, as well as by comparison with those reported in the literature.


INTRODUCTION
Borreria alata (Aubl.)DC. (synonym: Spermacoce alata Aubl., B. latifolia K. Schum.)belongs to the Rubiaceae family [2].In Nepal, the roots juice of Borreria alata is used to treat malaria [3].There was only one paper that reported the isolation of eight compounds from B. alata growing in Indonesia [4].In Vietnam, B. alata is a wide weed in coffee gardens and there has not yet been chemically studied.
Because phytochemicals depends on phenotypic and genotypic factors, the aim of this study was to investigate the chemical constituents of Borreria alata growing in Vietnam.In this paper, we described the isolation and structural elucidation of a new compound (4), together with three known ones (1 -3).

General
NMR spectra were recorded on a Bruker Avance 500 (500 MHz for 1 H NMR and 125 MHz for 13 C NMR) and HR-ESI-MS spectra were recorded on Bruker micrOTOF Q-IImass spectrometer.All the instruments are in the Center of Analysis, University of Science, VNU-HCM.

Plant materials
The whole plant of Borreria alata was collected at Lam Dong province, Viet Nam in November 2012.The scientific name was authenticated by the botanist Vo Van Chi.A voucher specimen (No US-C031) was deposited at the herbarium of the Department of Organic Chemistry, University of Science, VNU-HCM.
Compound (4) was isolated as a colorless amorphous powder.The molecular formula of (4) was determined as C16H23O10Na from the HR-ESI-MS with the pseudomolecular ion peak at m/z 399.1260 [M+H] + (Calcd.for C17H23O11Na+H, 399.1266), and with the sodiated molecular ion peak at m/z 421.1076 [M+Na] + (Calcd.for C17H23O11Na+Na, 421.1087).The 13 CNMR data (Table 1) of (4) revealed 17 signals of a -Dglucopyranosyliridoid.The -glucopyranosyl moiety was proved by the anomeric proton signal at H 4.66 (1H, d, J = 8.0 Hz, H-1') corresponding to C-1' (C 99.8), as well as two signals at H 3.91 (1H, dd, J = 12.0, 1.0 Hz, H-6'a) and 3.70 (1H, m, H-6'b) corresponding to C-6' (C 62.7).The COSY along with HSQC and HMBC experiments supported the assignments of the protons and carbons belonging to the glucopyranosyl moiety.The presence of a iridoid skeleton was supported by the appearance of two olefinic carbon signals at C 146.5 (C-3) and 121.9 (C-4) together with an acetal carbon signal at C 96.7 (C-1).The complete assignments of all proton and carbon resonances were relied on the results of COSY, HSQC and HMBC experiments.The chemical shift values of C 75.3 and 42.5 were assigned for two methine carbons C-7 and C-8, respectively.The HMBC correlations between the methyl proton signal at H 1. 10 (3H,d,7.0 Hz, with carbon signals at C 75.3 (C-7), 42.5 (C-8), 47.0 (C-9) confirmed the position of the methyl group at C-8.The position of the carborxyl group at C-4 was confirmed by the cross-peak of the methyl proton signal at H 7.06 (1H, brs, H-3) and carbon signal at C 176.0 (C-11) in the HMBC spectrum.Based on the HMBC correlation between the anomeric proton signal at H 4.66 (1H, d, J = 8.0 Hz, H-1') with the acetal carbon C-1 at C 96.7 the glucopyranosyl moiety attached to the was aglycon at C-1.
To confirm the configuration of C-1, compound (4) was acid hydrolyzed in order to measure the 1 H NMR spectrum of the aglycone and to compare this with literature data.However, the obtained amount of compound (4) was too little to hydrolyze.Therefore, based on the rule of 1,1 '-disaccaride [8-10] by comparing the chemical shift values of the isolated iridoid glycoside (δC-1' 99.8 in CD3OD) with that of the β-D-glucopyranose (δC-1 98.9 in pyridine-d5) with ΔδC = 0.9 ppm less than 3.5 ppm, carbon C-1 of compound (4) was proposed to possess the S-configuration or the glycosidic linkage had a orientation.On the basis of above data of compound (4) and in comparison with published data of loganic acid [11] and of two diastereoisomers of loganin in the literature [12], we noticed that the chemical structure of ( 4) could be consistent with that of loganic acid.However, a careful comparison of the NMR data of compound (4) with those of loganic acid [11] showed complete differences at C-11 (+4.6 ppm), C-4 (+7.7 ppm), C-3 (-5.5 ppm), C-5 (+0.1 ppm), C-1 (-0.9 ppm) and H-3 (-0.4 ppm).The anionization effect was reported to cause the deshielding of the 13 C NMR chemical shift values of these carbons [13].Literature reported that the chemical shift values of sodium salt of monotropein [13]were different from the corresponding ones of monotropein [13] at C-11 (+4.4 ppm), C-4 (+5.5 ppm), C-3 (-5.6 ppm), C-5 (+1.4 ppm), C-1 (-0.5 ppm) and H-3 (-0.3 ppm).These altenations were also observed in spectral data of compound (4) and loganic acid [11].This is an important proof to identify that (4) existed in the sodium salt.Therefore, the structure of compound (4) was determined as sodium loganate.To the best of our knowledge, ( 4) is a new compound.Từ khóa: Borreria alata,sodium deacetylasperulosidate,natri loganate,3β,6β,