Conformations of the Pyranoid Sugars. III. Infrared Absorption Spectra of Some Acetylated Aldopyranosides

The infrared absorption spectra of twenty-four acetylated aldopyranosides in the range of 5,000 to 250 cm^-1 are reported. The conformation adopted by each of seventeen of the corresponding unacetylated glycosides had previously been assigned by us from a study of their infrared spectra. Analysis of the spectra revealed, for the acetylated glycosides (as for the parent glycosides), groups of absorption bands which showed a concerted shift on change of anomeric disposition. Assignment of conformation by the methods developed earlier led to the conclusion that each acetylated glycoside has the same conformation as its parent glycoside. Intercomparison of the spectra of four of the remaining acetates with those of related acetates, especially in regard to the characteristic groups of absorption bands, afforded evidence that the anomeric methoxyl group is axial in methyl hepta-O-acetyl-4-O-β-d- glucopyranosyl-α-d-mannopyranoside, and equatorial in methyl tetra-O-acetyl-β-d-manno pyranoside, methyl penta-O-acetyl-d-glycero-β-l-manno-heptopyranoside, and methyl hepta- O-acetyl-4-O-β-d-glucopyranosyl-β-d-mannopyranoside. For lack of spectra of related acetylated aldopyranosides, assignments cannot yet be made for methyl hepta-O-acetyl- 6-O-β-d-glucopyranosyl-β-d-glucopyranoside, methyl penta-O-acety-d-glycero-β-d-ido-heptopyranoside, and methyl hepta-O-acetyl-4-O-β-d-galactopyranosyl-β-d-altropyranoside.