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Triterpenoids and coumarins from the leaves of Sterculia foetida Linn.

Introduction: Sterculia foetida Linn. is widely distributed in tropical countries. As the continuous study on the hexane and ethyl acetate extracts of Sterculia foetida leaves, the isolation and struc-tural determination of four triterpenoids and two coumarins were addressed. Method: The crude extract was prepared from dried power of Sterculiafoetida leaves by maceration method in ethanol. This extract was then separated by liquid-liquid partition with n-hexane, chloroform, and ethyl acetate, respectively, to obtain the corresponding extracts. The hexane and ethyl acetate extracts were applied to multiple silica gel column chromatography to yield six compounds. Their chemical structures were determined by the NMR data analysis as well as the comparison their spectroscopic data and physical properties with those of reported literature. Results: Four triterpenoid compounds, including betulinic acid (1) , conyzasaponin G (2) , taraxerol (3) , and taraxer-14-ene-1 a ,3 b -diol (4) , and two coumarins fraxetin (5) , and aesculin (6) were identified. Conclusion: To the best of our knowledge, they have not been reported in the leaves of Sterculia foetida before, and compound 2 was known to present in Sterculia genus for the first time.


INTRODUCTION
Sterculia foetida Linn. belonging to Sterculiaceae is grown in tropical areas 1 . Leaves of Sterculia foetida Linn. are used in traditional medicine as an aperient, diuretic, and insect repellent 2,3 . There were some previous studies on chemical constituents of different parts of this species, which reported the presence of steroids, flavonoids, phenolic, coumarins, phenylpropanoids, and cerebrosides [2][3][4][5] . Previously, we reported the isolation of the organic compounds from leaves of this species collected in Binh Thuan province, including some triterpenoids, quercetin derivatives, and phenolic compound 6,7 . Herein, the continuous chemical study on the hexane and ethyl acetate extracts of Sterculia foetida leaves was discussed.

General experimental procedures
The NMR spectra were obtained on a BRUKER AC 500 spectrometer (500 MHz for 1 H-NMR and 125 MHz for 13 C-NMR). The SCIEX X500 QTOF and X500 QTOF machines were used to record the high resolution-mass spectra and the ESI/APCI mass spectra, respectively. Column chromatography was ap-plied to the silica gel normal-phase (Kieselgel 60, 230-400 mesh, Merck). The thin-layer chromatography technique was done by using silica gel plates (Kieselgel 60 F 254 , 0.25 mm, Merck).

Plant material
The S. foetida leaves were collected in Binh Thuan Province in October 2017. The scientific name of this plant was determined by botanist Dr. Dang Van Son, Institute of Tropical Biology. A voucher specimen, coded No.SFC/TUYEN-1017A, was deposited at the laboratory of Faculty of Environmental Science, Saigon University.

RESULTS
The chemical investigation on the S. foetida leaves collected in Binh Thuan Province led to the isolation of six compounds by the use of efficient separation techniques. From the hexane extract, three compounds 1, 3, and 4 were isolated, while 2, 5, and 6 were isolated from ethyl acetate extract. Their 13 C-NMR data were performed in Table 1, and the following data were 1 H-NMR data.
• . Its 13 C-NMR spectrum showed 30 carbon signals of a triterpene skeleton whose most of the carbon signals resonated in the high magnetic zone from 14 to 56 ppm. An oxygenated methine signal at δ C 79.0 was assigned . Therefore, the presence of isopropenyl group in lupane skeleton was proved. Five rest singlet methyl proton signals from 0.76 to 0.98 ppm were assigned to H-23 to H-27. The axial-position of the oxygenated methine proton was confirmed by the signal at δ H 3.19 (1H, dd, 11.5, 5.0), since the hydroxy group at C-3 was determined as equatorial-or β -position. Based on the above analysis, 1 was elucidated as betulinic acid 8 . The NMR analysis of 2 and 1 showed that 2 had one more hydroxyl group at C-2, one more hydroxymethylene instead of the methyl group, and two more sugar units. Two oxymethylene protons presented as two doublet signals at δ . Based on the above analysis, the structure of 6 was determined as aesculin 13 . CONCLUSION In the continuous study on the leaves of Sterculia foetida Linn. collected in Binh Thuan province, four triterpenoids and two coumarin derivatives were isolated, including betulinic acid (1), taraxerol (3), and taraxer-14-ene-1α,3β -diol (4) isolated from the hexane extract and conyzasaponin G (2), fraxetin (5), and aesculin (6) isolated from the ethyl acetate extract. Their chemical structures were determined by the NMR, MS data analysis, and their spectroscopic data and physical properties with those reported in the literature.