A 1 H‐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 NC(O) bonds of AllNAc, GlcNAc, and GlcNMeAc derivatives was investigated by 1 H‐NMR spectroscopy. The AllNAc‐derived α‐ D ‐and β‐ D ‐pyranosides 4–7 , the AllNAc diazirine 16 , and the GlcNAc‐derived axial anomers α‐ 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 β‐ D ‐ 8–10 and the GlcNAc diazirine 17 was evidenced by an upfield shift of HC(2), downfield shifts of HC(1) and HC(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 β‐ D ‐ 14 decreases with increasing polarity of the solvent, but a substantial population is still observed for solutions in D 2 O. Whereas the α‐ D ‐anomers of the hemiacetal 22 and the methyl glycoside 21 prefer the ( Z )‐ anti ‐conformation in D 2 O solution, the corresponding β‐ D ‐anomers are mixtures of the ( Z )‐ anti ‐and ( Z )‐ syn ‐conformers. The diazirine 17 self‐associates in CD 2 Cl 2 solution at concentrations above 0.005 M at low temperatures. The axial anomers of the GlcNMeAc derivatives α‐ D ‐ 26–28 are 2:1 to 3:1 mixtures of ( Z )‐ anti ‐and ( E )‐ anti ‐conformers, whereas the corresponding β‐ D ‐glycosides are ca. 1:3:6 mixtures of ( Z )‐ syn ‐, ( Z )‐ anti ‐, and ( E )‐ anti ‐conformers.