Conformations of the Pyranoid Sugars. IV. Infrared Absorption Spectra of Some Fully Acetylated Pyranoses

The infrared absorption spectra of twenty pyranose acetates in the range of 5000 to 250 cm^-1 are reported. The conformation adopted by each of fourteen of the corresponding methyl glycopyranosides (or their acetates) had previously been assigned by us from a study of their infrared spectra. Analysis of the spectra revealed, for the pyranose acetates (as for the methyl glycopyranosides and their acetates), groups of absorption bands which showed a concerted shift on change of anomeric disposition. Assignment of conformation by the methods earlier developed indicated that, for most of the compounds examined, the conformation is the same for the pyranose acetate, methyl glycopyranoside, and acetylated methyl glycopyranoside of one anomer of a monosaccharide. The following new assignments of conformation were made: the CA conformation for penta-O-acetyl-α-l-xylo-hexulopyranose and hexa-O-acetyl-α-d-gluco-heptulopyranose; and, possibly, either a non-chair conformation or a mixture of the CA and CE conformations for hexa-O-acetyl-l-glycero-β-d-gluco-heptopyranose and hexa-O-acetyl-d-glycero-β-d-galacto-heptopyranose.