Virtual 1H- 1H spin-spin coupling in a linear five-spin system on the pyranose rings of some glucuronides

Abstract

In the 1H NMR spectra of methyl 2,3,4-tri- O-acetyl-β- d-glucopyranosyluronate-(1 → 3)-1,2:5,6-di- O-isopropylidene-α- d-glucofuranose ( 6) and methyl 2,3,4-tri- O-acetyl-β- d-glucopyranosyluronate-(1 → 6)-1,2:3,5-di- O-isopropylidene-α- d-glucofuranose ( 7), which were obtained by the reaction of 1,2;5,6-di- O-isopropylidene-α- d-glucofuranose ( 1) with methyl 2,3,4-tri- O-acetyl-α- d-glucopyranosyluronate bromide ( 5) in the presence of Hg(CN) 2 in 1:1 benzene-nitromethane at 45°C, protons on both β- d-glucopyranosyluronate rings were observed as very complex signals that could not be interpreted by first-order analysis. Similar complex signals were also observed for the protons on the β- d-glucopyranosyluronate rings that were sugar components of some triterpenoidal glycosides ( 13–15). These complex signals were determined to be due to virtual long-range spin-spin coupling in the linear five-spin system on the glucopyranosyluronate rings of the glucuronides by 1H, 13C, H-C COSY, 1D HOHAHA, and spin-simulation spectroscopies.