The conformations of furanosides. A 13 C nuclear magnetic resonance study

Abstract

A study of the conformations of 15 methyl aldohexo- and pentofuranosides in aqueous solution has been carried out, based primarily on measurements of coupling by 13 C-1 and 13 C-2 with protons of the five-membered ring. From these stereochemically-dependent data, and their correspondence with vicinal 1 H– 1 H couplings, the conformation favored by each glycoside has been deduced. Although, in general, the coupling patterns exhibited by homologous pairs of hexoses and pentoses are closely similar, the α-gluco and α-xylo pair provide an exception: their data indicate that the latter favors the 2 E conformation, whereas the glucoside is characterized by interconverting E 4 and 3 E conformers. It appears that a quasi-equatorial exocyclic side chain and a quasi-axial C-1,O-1 bond (anomeric effect) are equally important stabilizing factors in furanosides. In a number of instances, the current findings differ from those obtained in earlier studies for configurationally-related furanose derivatives in organic media.