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

In the 1H NMR spectra of methyl 2,3,4- tri-O-acetyl-beta-D-glucopyranosyluronate-(1-->3)- 1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranose (6) and methyl 2,3,4-tri-O-acetyl-beta-D-glucopyranosyluronate-(1-->6)- 1,2:3,5-di-O-isopropylidene-alpha-D-glucofuranose (7), which were obtained by the reaction of 1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranose (1) with methyl 2,3,4-tri-O-acetyl-alpha-D-glucopyranosyluronate bromide (5) in the presence of Hg(CN)2 in 1:1 benzene-nitromethane at 45 degrees C, protons on both beta-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 beta-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.