New monocyclic monoterpenoid glycoside from Mentha haplocalyx Briq.
Gai-Mei Sheet al
Chemistry Central Journal 2012, 6:37
new monocyclic monoterpenoid glycosides, rel-(1R,2S,3R,4R) p-menthane-1,2,3-triol 3-O-β-D-glucopyranoside (1) were isolated from aqueous acetone extract of the aerial parts of Mentha haplocalyx Briq..
On the basis of spectroscopic methods, including 2D-NMR (HMQC, HMBC, 1H-1H COSY and NOESY), the structures of new were determined as rel- (1R,2S,3R,4R) p-menthane-1,2,3-triol 3-O-β-D-glucopyranoside (1)
Compound 1 was obtained as a pale amorphous powder. Its HR-ESI-MS displayed quasi-molecular-ion peak [M + Na]+ at m/z 373.1521 ([C16H30O8Na]+), and the EI-MS gave fragment-ion peaks at m/z 171 [M + 1–162(glucosyl)-H2O]+ and 153 [M + 1–162(glucosyl)-2H2O]+ corresponding to a molecular formula C16H30O8, with the presence of 16 carbon signals in the 13C-NMR spectrum.
The 1H- and 13C-NMR spectral data displayed the presence of two secondary methyl δ 0.81 (3H, dJ = 7.0 Hz, H-10), 0.92 (3H, dJ = 7.0 Hz, H-9)], a tertiary methyl δ 1.21 (3H, s, H-7)], two methylenes δ 1.37 (2H, dtJ = 11.7, 8.3 Hz, H-5), 1.40 (1H, m, H-6α) and 1.57 (1H, m, H-6β)], four methines (two of them was oxygenated) δ 3.82 (1H, dJ = 10.8, 9.2 Hz, H-3), 3.33 (1H, dJ = 10.8 Hz, H-2), 2.31 (1H, m, H-8), and 1.69 (1H, m, H-4)], and an oxygentated quaternary carbon, suggesting that compound 1 was a menthane-type monoterpene with three OH-groups
Moreover, 1H-1H COSY correlations were observed between H-C(9)/H-C(8)/H-C(10), H-C(8)/H-C(4), and H-C(6)/H-C(5)/H-C(4)/H-C(3)/H-C(2), that the deduced spin system implied that the three OH-groups were located at C(1), C(2) and C(3) in 1, respectively. In addition, one glucopyranosyl unit δ (H) 4.33 (1H, dJ = 8.2 Hz, H-(1′)), δ(C) 105.9 (C-1′)] was evident from 1H- and 13C-NMR of 1. The J value (8.2 Hz) of the anomeric proton concluded the β-configuration of the glucose moiety, suggesting that 1 was a p-menthane-1,2,3-triol glycoside.
This was further confirmed by the HMBC experiment, in which correlations of the glucosyl H-1′ (δ 4.33) with the C(3) (δ 81.9) were observed. Furthermore, other HMBC correlations confirmed the structure of compound 1. Thus, these 2D-NMR methods deduced compound 1 as p-menthane-1,2,3-triol 2-Oβ-D- glucopyranoside. The coupling constants of 10.8 Hz for H-C(3)/H-C(2), 9.2 Hz for H-C(3)/H-C(4) for 1 showed that H-C(2), H-C(3) and H-C(4) were axial protons.
The relative configuration at C(1) was determined from ROESY correlation of δ 1.21 (Me(7)) with H-2 (δ 3.33). It was in good agreement with those of rel-(1R,2S,3R,4R,6S) p-menthane-1,2,3,6-tetrol . Therefore 1 should possess rel-(1R,2S,3R,4R)-configuration.