Configurational effects on 13C chemical shifts of 1,6-anhydro-hexopyranoses and related compounds. Utility of 13C–1H coupling patterns for signal assignments

Abstract

An analysis of configurational effects on the 13C chemical shifts of bicyclic compounds of the 1,6-anhydrohexopyranose and 2,7-anhydroheptulopyranose type is presented. As with other saturated ring Systems, strong upfield shifts are found in association with an axial orientation of the substituents, although in this respect 13C nuclei of the anhydro ring appear to exhibit atypical behaviour. Deshielding changes characterize a syn-diaxial orientation of hydroxyl groups, and also of a hydroxyl group with respect to the bridgehead bonds. No appreciable effects of antiperiplanar hydroxyl groups on 13C chemical shifts have been detected. Possible interrelationships between 13C or 1H chemical shifts and steric interactions are discussed with reference to the relative stability of isomeric anhydrosugars and C—H bond polarization. Some characteristics of one-, two-, and three-bond 13C–1H coupling are described, and are shown to be of value in several instances for the assignment of 13C signals. The 1H-coupled signal for methylenic C-6 of 1,6-anhydrides is commonly non-first order, in contrast to the simpler pattern exhibited by methylenic C-1 of 2,7-anhydrides.