5-thio-D-ribopyranose : Part III. Conformational equilibria in the methyl D-ribopyranosides,their 1-thio, 5-thio,and 1,5-dithio analogues,and the related triacetates

Conformational equilibria have been estimated by n.m.r. spectroscopy for the methyl 2,3,4-tri-O-acetyl-α- and -β-D-ribopyranosides (1′a and 1′b), their 1-thio (2′a, 2′b), their 5-thio (3′a, 3′b), and their 1,5-dithio (4′a, 4′b) analogues. Only 1′b shows a preference for the 1C conformation; the others favour the C1 form to various extents. These results are discussed in terms of polar and steric effects. Similar estimations have been made on the unacetylated D-ribopyranosides (1–4) and, where a definite conformational assignment is possible, these follow the same trend as the triacetates (1′–4′). These results are compared, where possible, with the results of X-ray crystallographic studies.