Proton magnetic resonance spectra of three known diastereoisomers 1,4:3,6-dianhydro-D-glucitol, 1,4:3,6-dianhydro-D-mannitol, 1,4:3,6-dianhydro-L-iditol, and their diacetyl and dimesyl derivatives have been analyzed. The coupling constants obtained have been utilized to determine the conformation of the five-membered rings. Constancy of the couplings between ring protons throughout the series of compounds indicates the existence of one conformation common to all, and this appears to be a composite of the C, and Cz forms. It is significant that intramolecular hydrogen bonding, which occurs in two of the three diastereoisomers, has little or no effect on ring conformation.Canadian Journal of Chemistry, 47, 2395 (1969) It is well known that the hexitols L-iditol, D-glucitol, and D-mannitol readily form 1,4:3,6-dianhydrides under acidic conditions (1). These 1,4:3,6-dianhydrides each have two free hydroxyl groups located at the C-2 and C-5 positions.From inspection of molecular models the 1,4:3,6-dianhydrohexitols have two nearly planar cis-fused five-membered rings in the form of a "V" with the hydroxyl groups at the C-2 and C-5 positions either inside or outside the "V" shaped wedge. As in other ring systems the configuration of a group inside the wedge is designated as endo whereas one outside the wedge is exo. In the L-iditol derivative 1 both hydroxyl groups are exo, in the D-mannitol derivative 3 both are endo, and in the D-glucitol derivative 2 the hydroxyl at C-2 is exo and that at C-5 is endo.The stereochemistry of these compounds has been discussed by Mills (2) and the order of stability of the three compounds as deduced from their ease of formation (3, 4), epimerization (5), and general chemical properties (6, 7) indicates that the presence of an exo substituent increases the stability of the ring to which it is attached, by virtue of the diminished opportunities for interaction between non-bonded substituents. Furthermore the faces of the rings inside the "V" are less accessible than the faces outside, so exo and endo isomers exhibit differences in reactivity in the direction determined by the steric requirements of the reaction in question (8, 9). The extent of intramolecular hydrogen bonding, as determined by infrared spectroscopy in this series of compounds (10) illustrates the influence of structure on such bonding. The C-2 and C-3, and C-4 and C-5 oxygen bonds in the iditol derivative are trans related as in L-threitan so that intramolecular bonding is precluded. A cis relation of the C-4 and C-5 oxygen bonds exists in the D-glucitol analogue permitting a single intramolecular hydrogen bond. In D-mannitol the C-2 and C-3, and C-4 and C-5 oxygen bonds are cis related; two intramolecular hydrogen bonds may occur and the absence of detectable absorption for free hydroxyl groups indicates the completeness of the hydrogen bonding.Correlation of nuclear magnetic resonance Can. J. Chem. Downloaded from www.nrcresearchpress.com by 52.36.4.81 on 05/11/18For personal use only.