H-1-NMR ASSIGNMENT AND CONFORMATIONAL-ANALYSIS OF GLUCITOL - COMPARISON WITH MALTITOL

The 1H NMR spectrum of glucitol in D2O solution contains eight strongly‐coupled multiplets covering a chemical shift range of about 0.21 ppm. The spectrum is severely complicated by second‐order effects and was assigned by a combination of 2D NMR experiments and extensive spin simulation of the 1D spectra. The magnitudes of the spin coupling constants were analysed in terms of preferred conformations about each bond in the backbone of glucitol, and the results were compared with those found for maltitol in solution, with crystal conformations of both glucitol and maltitol and with low‐energy conformations computed using the MM2CARB force field. The results show a surprising degree of agreement between computed low‐energy conformations and those observed in the crystal, but little correspondence with conformations observed by NMR in solution. Although maltitol is derived from glucitol sustituted in the 4‐position with the bulky α‐D‐glucopyranose moiety, NMR measurements indicate approximately free rotation about all bonds in the glucitol moiety of maltitol. This is in contrast to glucitol, where the proton‐proton trans conformer predominates (74%) for the C4–C5 bond which, in turn, stabilizes the gauche‐gauche conformer of the adjacent C5–C6 bond (53%) and destabilizes the trans conformer of the C3–C4 bond (3%). It is found from molecular modelling studies that steric factors alone cannot explain these results, and it is postulated that intramolecular hydrogen bonding stabilizes the local conformations in glucitol whereas disruption of the hydrogen‐bond network occurs as a result of substitution of the glucitol moiety in maltitol. Copyright © 1990 John Wiley & Sons, Ltd.