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Isolation and structural characterization of a penta β-D-glucopyranosyl diterpene from Stevia rebaudiana
Abstract
From the commercial extract of the leaves of Stevia rebaudiana, a diterpene glycoside was isolated which was identified as 13-[(2-O-β-D-glucopyranosyl-3-O-β-D-glucopyranosyl-β-Dglucopyranosyl) oxy] ent-kaur-16-en-19-oic acid-(2-O-β-D-glucopyranosyl-β-D-glucopyranosyl) ester (1); also known as rebaudioside D. The complete 1H and 13C NMR assignment of rebaudioside D was achieved by the extensive NMR ( 1H and 13C, COSY, HSQC, HMBC) and mass spectral data. Further, hydrolysis studies were performed on rebaudioside D using various acid, base and enzymatic studies to identify partial degradation products, aglycone and sugar residues.
Systematic phytochemical studies of the extract of Luo Han Go (Siraitia grosvenorii) furnished six known cucurbitane glycosides, namely, mogrosides IVa, mogrosides V and VI, isomogroside V, 11-oxomogroside V, and siamenoside I. The structures of all the isolated compounds were characterized on the basis of extensive spectral and chemical studies. Though the partial NMR spectral data has been reported for some of the isolated compounds previously, the complete 1H and 13C NMR spectral assignments of all the isolated compounds are reported herewith for the first time based on COSY, HSQC, HMBC, and NOESY spectroscopic data.
Stability of a mixture of steviol glycosides was studied under a variety of conditions covering a typical pH range and various temperatures that simulated both relevant and extreme beverage storage conditions. Thus, steviol glycosides was evaluated in mock beverage solutions by simulating formulations used in commercial cola soft drinks (pH 2.8 and pH 3.2), lemon-lime soft drinks (pH 3.8), and root beer soft drinks (pH 4.2) but lacking the flavor components. The mock beverage samples, formulated at ~500 mg/l, were subjected to various temperature conditions (5, 20, 30, and 40°C) for 26-weeks at each pH. Samples were analyzed at scheduled intervals throughout the 26-week period for steviol glycosides, known impurities, known degradation products, and unidentified compounds greater than or equal to 0.100% of the starting concentration of steviol glycosides Experimental results indicated that steviol glycoside mixture did not undergo any major decomposition but yielded two minor compounds which were identified as 13-[(2-0-β-D-glucopyranosyl-3-0-β-D-gluopyranosyl-β-D-glucopyranosyl)oxy] ent-kaur-15-en-19-oic acid β-D-glucopyranosyl ester, and 13-[(2-0-β-D-glucopyranosyl-3-0-β-D-glucopyranosyl-β-D-glucopyranosyl)oxy]-16β-hydroxy-ent-kauran-19-oic acid β-D-glucopyranosyl ester, on the basis of 1H & 13C NMR, COSY, HSQC, HMBC, MS, MS/MS spectral data as well as hydrolysis studies.
From sweet leaves of Rubus suavissimus, 10 new kaurane-type diterpene glycosides named suaviosides were isolated, in addition to the known major sweet glycoside, rubusoside and several known minor glycosides. Of these minor glycosides, suaviosides B, G, H, I and J taste sweet, and suaviosides C1, D2 and F taste bitter, while suaviosides D1 and E are tasteless. Their structures were determined mainly on the basis of NMR spectral evidence.
Two new new diterpene glycosides, 13-[(2-O-(6-O-beta-D-glucopyranosyl)-beta-D-glucopyranosyl-beta-D-glucopyranosyl)oxy] kaur-16-en-18-oic acid beta-D-glucopyranosyl ester (1) and 13-[(2-O-beta-D-glucopyranosyl-3-O-beta-D-fructofuranosyl-beta-D-glucopyranosyl)oxy] kaur-16-en-18-oic acid beta-D-glucopyranosyl ester (2) were isolated from the leaves of Stevia rebaudiana, along with the known steviol glycosides stevioside, rebaudiosides A-F and dulcoside A. The structures of the two new compounds were established on the basis of extensive 2D NMR (COSY, HSQC, and HMBC), MS and chemical studies.
Rebaudioside A is a natural sweetener from Stevia rebaudiana in which four beta-D-glucopyranose units are attached to the aglycone steviol. Its (1)H and (13)C NMR spectra in pyridine-d(5) were assigned using 1D and 2D methods. Constrained molecular dynamics of solvated rebaudioside using NMR constraints derived from ROESY cross peaks yielded the orientation of the beta-D-glucopyranose units. Hydrogen bonding was examined using the temperature coefficients of the hydroxyl chemical shifts, ROESY and long-range COSY spectra, and proton-proton coupling constants.
A new dammarane-type triterpene glycoside, polysciasoside A (1), and three known compounds have been isolated from the leaves of Polyscias fulva. The structure of the new compound was established as 12-oxo-3beta,16beta,20(S)-trihydroxydammar-24-ene-3-O-alpha-L-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside.
Bioassay-guided fractionation of an ethanolic extract of the infructescences of Polyscias amplifolia resulted in the isolation of two new oleanolic acid saponins, polyfoliolides A (1) and B (2), in addition to the two known saponins 3-O-beta-D-galactopyranosyloleanolic acid (3) and 3-O-beta-D-galactopyranosyl-(1-->4)-beta-D-galactopyranosyloleanolic acid (4). The structures of the two new compounds were established as 3-O-beta- D-galactopyranosyl-(1-->4)-beta-D-xylopyranosyloleanolic acid (1) and 3-O-beta-D-galactopyranosyl-(1-->4)-alpha-L-arabinopyranosyloleanolic acid (2) on the basis of extensive 1D and 2D NMR spectroscopic data interpretation and chemical conversions. All the isolated compounds showed weak cytotoxicity against A2780 human ovarian cancer cell line, with IC50 values in the range 6.7 to 10.8 microg/mL.