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

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 13C-1 and 13C-2 with protons of the five-membered ring. From these stereochemically-dependent data, and their correspondence with vicinal 1H–1H 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 2E conformation, whereas the glucoside is characterized by interconverting E4 and 3E 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.