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

Abstract

A study of the conformations of 15 methyl aldohexo- and pentofuranosides in aqueous solution was carried out, based primarily on measurements of coupling by 13C-1 and 13C-2 with protons of the 5-membered ring. From these stereochemically-dependent data, and their correspondence with vicinal 1H-1H couplings, the conformation favored by each glycoside was deduced. Although in general the coupling patterns exhibited by homologous pairs of hexoses and pentoses are closely similar, the .alpha.-gluco and .alpha.-xylo pair provide an exception: the latter apparently favors the 2E conformation, whereas the glucoside is characterized by interconverting E4 and 3E conformers. A quasi-equatorial exocyclic side chain and a quasi-axial C-1,O-1 bond (anomeric effect) are apparently equally important stabilizing factors in furanosides. In a number of instances, the current findings differ from those obtained in earlier studies for configurationally-related furnose derivatives in organic media.