Conformational studies on pertrimethylsilyl derivatives of 2-acetamido-2-deoxyaldohexopyranoses by 220- and 300-MHz P.M.R. spectroscopy

Abstract

The complete interpretation of high-resolution p.m.r. spectra and the accurate chemical shifts and coupling constants, obtained after computer simulation of the spectra, of the trimethylsilyl (TMS) derivatives of six 2-acetamido-2-deoxy-D-aldo-hexopyranoses are given. All pyranoid rings occur in the 4C1(D) chair conformation. The preferred conformation of the C-5—CH2OTMS group depends on the configuration at C-4. Only the chemical shifts and the line widths of the signals of NH and H-1 of the beta-D-gluco- and the beta-D-galacto-pyranose derivatives were found to be strongly dependent on the temperature. This observation can be interpreted in terms of the preferred conformation of the acetamido group. At room temperature, the cis and trans conformations of this group, with respect to the ring, are present in a dynamic equilibrium. The intermediate conformations are populated to a very low extent. Because of the similarity in coupling constants JNH,2, it is proposed that this situation is also present in the alpha-D-gluco- and alpha-D-galacto-pyranose derivatives, whereas the trans conformation is populated in the D-mannopyranose derivatives.