A H-1-NMR spectroscopic investigation of the conformation of the acetamido group in some derivatives of N-acetyl-D-allosamine and -D-glucosamine

The population of the conformations obtained by rotation around the C(2)-N and the N-C(O) bonds of AllNAc, GlcNAc, and GlcNMeAc derivatives was investigated by H-1-NMR spectroscopy. The AllNAc-derived alpha-D- and beta-D-pyranosides 4-7, the AllNAc diazirine 16, and the GlcNAc-derived axial anomers alpha-D-8-10 prefer the (Z)-anti-conformation. A significant population of the (Z)-syn-conformer in the (Z)-syn/(Z)-anti-equilibrium for the equatorial anomers beta-D-8-10 and the GlcNAc diazirine 17 was evidenced by an upfield shift of H-C(2), downfield shifts of H-C(I) and H-C(3), and by NOE measurements. The population of the (Z)-syn-conformation depends on the substituent at C(1) and is highest for the hexafluoroisopropyl glycoside. The population of the (Z)-syn-conformation of beta-D-14 decreases with increasing polarity of the solvent, but a substantial population is still observed for solutions in D2O. Whereas the alpha-D-anomers of the hemiacetal 22 and the methyl glycoside 21 prefer the (Z)-anti-conformation in D2O solution, the corresponding beta-D-anomers are mixtures of the (Z)-anti-and (Z)-syn-conformers. The diazirine 17 self-associates in CD2Cl2 solution at concentrations above 0.005m at low temperatures. The axial anomers of the GlcNMeAc derivatives alpha-D-26- 28 are 2:1 to 3:1 mixtures of (Z)-anti- and (E)-anti-conformers, whereas the corresponding beta-D-glycosides are ca. 1:3:6 mixtures of(Z)-syn-, (Z)-anti-, and (E)-anti-conformers.