VIRTUAL H-1-H-1 SPIN-SPIN COUPLING IN A LINEAR 5-SPIN SYSTEM ON THE PYRANOSE RINGS OF SOME GLUCURONIDES

In the H-1 NMR spectra of methyl 2,3,4-tri-O-acetyl-beta D-glucopyrano syluronate-(1 --> 3)- 1,2:5,6-di-O-isopropylidene-alpha-D-glucofuranos e (6) and methyl 2,3,4-tri-O-acetyl-beta-D-glucopyranosyluronate-(l -- > -1,2:3,5-di-O-isopropylidene-alpha-D-glucofuranose (7), which were o btained by the reaction of 1,2;5,6-di-O-isopropylidene-alpha-D-glucofu ranose (1) with methyl 2,3,4-tri-O-acetyl-alpha-D-glucopyranosyluronat e bromide (5) in the presence of Hg(CN), in 1:1 benzene-nitromethane a t 45 degrees C, protons on both beta-D-glucopyranosyluronate rings wer e observed as very complex signals that could not be interpreted by fi rst-order analysis. Similar complex signals were also observed for the protons on the beta-D-glucopyranosyluronate rings that were sugar com ponents of some triterpenoidal glycosides (13-15). These complex signa ls were determined to be due to virtual long-range spin-spin coupling in the linear five-spin system on the glucopyranosyluronate rings of t he glucuronides by H-1, C-13, H-C COSY, 1D HOHAHA, and spin-simulation spectroscopies.