6-Amino-2,6-dideoxy- or -2,3,6-trideoxyhexono-1,6-lactams: Synthesis and Conformation

6-Amino-2,6-dideoxy-D-ribo-hexono-1,6-lactam (1a), 6-amino-2,6-dideoxy-D-arabino-hexono-1,6-lactam (2a), 6-amino-2,3,6-trideoxy-L-threo-hexono-1,6-lactam (3a) and per-O-acetyl derivatives 1b-3b were synthesized and their seven-membered lactam ring conformation was studied. 1H and 13C NMR spectra of the named lactams, measured at low temperature, always disclosed the presence of both 1,NC4 and 4C1,N conformations in ratios which were affected mainly by the stereochemistry of cyclohexane. There were no CD extremes over 200 nm found in water solutions of the lactams 1a and 2a, probably owing to the symmetry of the C2-C6 parts of their seven-membered rings. These results contrast with those previously found for the lactams having OH or OAc at C2, and support a concept of the directive role of the C2 substituent in conformation equilibrium.