Influence of intramolecular hydrogen-bonding on the conformation of 3-deoxy-3-thioureido sugars

3-Deoxy-3-thioureido derivatives of 1,2:5,6-di-O-isopropylidene-α-d-allofuranose and 1,2:5,6-di-O-isopropylidene-α-d-glucofuranose were prepared, and their conformational properties in chloroform-d solutions were studied by temperature variable 1H NMR spectroscopy. The whole results, which include temperature coefficient data for the exchangeable pseudoamide protons, support that the presence of the sugar NHC( S) E-rotamer for d-gluco N,N′-disubstituted derivatives can be essentially ascribed to the existence of an intramolecular hydrogen bond analogous to that characteristic of peptide γ-turns.