Phenolic and Lignan Glycosides from the Butanol Extract of Averrhoa carambola L. Root

Fifteen compounds, which included six chiral lignans and nine phenolic glycosides, were separated from the butanol fraction of Averrhoa carambola L. root and identified. All of the compounds, namely 3,4,5-trimethoxyphenol-1-O-β-D-glucopyranoside (1), benzyl-1-O- β-D-glucopyranoside (2), (+)-5'-methoxyisolariciresinol 3α-O- β-D-gluco-pyranoside (3), (+)-isolariciresinol 3α-O- β-D-glucopyranoside (4), koaburaside (5), (+)-lyoniresinol 3α-O- β-D-glucopyranoside (6), (−)-lyoniresinol 3α-O- β-D-glucopyranoside (7), (−)-5'-methoxyisolariciresinol 3α-O- β-D-glucopyranoside (8), (−)-isolariciresinol 3α-O- β-D-glucopyranoside (9), 3,5-dimethoxy-4-hydroxyphenyl 1-O-β-apiofuranosyl (1''→6')-O-β-D-glucopyranoside (10), 3,4,5-trimethoxyphenyl 1-O-β-apiofuranosyl (1''→6')-β-gluco-pyranoside (11), methoxyhydroquinone-4- β -D-glucopyranoside (12), (2S)-2-O- β-D-gluco-pyranosyl-2-hydroxyphenylacetic acid (13), 3-hydroxy-4-methoxyphenol 1-O -β-D-apio-furanosyl-(1''→6')-O - β-D-glucopyranoside (14) and 4-hydroxy-3-methoxyphenol 1-O -β-D-apiofuranosyl-(1''→6')-O - β-D-glucopyranoside (15) were isolated from this plant for the first time.


Introduction
Nowdays, herbal medicine is accepted worldwide as an alternative therapy [1,2]. Averrhoa carambola L. (Oxalidaceae) is a perennial herb widely distributed in China, Taiwan, Malaysia, India, Brazil, America, etc. Its roots, have been used as a Traditional Chinese Medicine for thousands of years in the remedy of lithangiuria, arthralgia and chronic paroxysmal headache. In our previous study, both ethanol extract and polysaccharide from the roots showed hypoglycemic and antioxidant effects [3,4]. Many publications have also indicated that compounds from Averrhoa carambola leaves displayed hypoglycemic, hypotriglyceridemic, anti-lipid peroxidative and anti-atherogenic properties in streptozocin-induced diabetic rats [5,6].
Previous literature reports on Averrhoa carambola L. have only reported the isolation and identification of a few compounds such as β-sitosterol, lupeol and 1,5-dihydroxy-6,7-dimethoxy-2methyl-anthraquinone 3-O-β-glucopyranoside [7][8][9][10]. Therefore, this study involved separating more compounds from the herb aiming to offer better insight into its chemical constituents. In this research, the 60% aqueous ethanol (aq. EtOH) extract from Averrhoa carambola L. roots was suspended in H 2 O and further extracted with cyclohexane, ethyl acetate (EtOAc) and n-butanol (n-BuOH), respectively. Then, the butanol extract was successively purified by open silica gel, Sephadex LH-20, ODS column and P-HPLC to obtain 15 compounds that could be useful for investigating the hypoglycemic or antioxidant substances of the plant and standardization of the butanol extract from this herb in the next step.

Results and Discussion
The 60% aq. EtOH extract of Averrhoa carambola L. root was successively separated by cyclohexane, EtOAc and n-BuOH. From the n-BuOH fraction, fifteen known compounds (1-15, listed in the Abstract, structures shown on Figures 1 and 2) were isolated and their structures confirmed by detailed FTIR, NMR ( 1 H, 13 C) data comparison with those in the literature [11][12][13][14][15][16][17][18][19][20][21][22]. 13 C-NMR data of the compounds is summarized in Tables 1 and 2. 1 H-NMR data is given in the Experimental. These nine phenolic and six lignan glycosides still not been previously isolated and identified from this plant. Just as mentioned in the Introduction, Our preliminary investigations have shown that the ethanolic extract of Averrhoa carambola L. root could relieve the lipid peroxide reactions and has a significant hypoglycemic effect in the streptozotocin-induced diabetic mice, suggesting it may be a potential hypoglycemic agent for the treatment of diabetes and its complications [3]. Compounds 6 (418.9 mg) and 7 (534.7 mg), as the main chemical ingredients of the n-BuOH extract, may play a key role in the observed anti-diabetic effect. To explore the active substances in this plant, these two compounds' hypoglycemic activities will be investigated in the near future. The structures of the compounds are shown on Figures 1 and 2.

Plant Materials
The roots of Averrhoa carambola L. were collected from Linshan County, Guangxi Province, China, in June 2010 and were identified by Prof. Maoxiang Lai. The voucher specimen (No. 20100605) was deposited in the herbarium of the Guangxi Institute of Chinese Medicine & Pharmaceutical Science (Guangxi, China).

Conclusions
The 1 H and 13 C-NMR data of 15 compounds isolated from the roots of Averrhoa carambola L. for the first time were identical with the literature values.