¹H and ¹³C NMR studies on the conformation of E,Z‐diastereomers of N‐desmethyl‐N,O‐diacetyl‐α‐metazocine (sp²‐hybridized nitrogen derivatives of a benzomorphan narcotic analgesic)

The ¹H (300 MHz) and ¹³C (75.5 MHz) NMR spectra (CDCl₃) of N‐desmethyl‐N,O‐diacetyl‐α‐metazocine showed two (E,Z)‐amide diastereomers (ratio ≈ 1 : 2, respectively). All four vicinal proton‐proton coupling constants in each NCH₂CH₂C moiety were readily measured owing to effective chemical shift dispersion caused by the acetamido function. All vicinal coupling constants were also measured in the E. COSY 2D NMR spectrum. Comparison of the vicinal coupling constants involving sp²‐N piperidine ring protons in the (E,Z)‐amides and the corresponding values involving sp³‐N piperidine ring protons in morphine free base and hydrochloride N‐methyl diastereomers shows them to be similar. The R‐factor method was used to estimate a 54(1)° value for the NCH₂CH₂C dihedral angle in the sp²‐N piperidine ring for both solution‐state E,Z‐diasteromers. The corresponding dihedral angle within the sp³‐N piperidine ring in solid‐state (±)‐6‐desmethyl‐8‐deoxy‐α‐metazocine · HCl was found to be 53.8° by x‐ray crystallography. Both sp² and sp³ N‐hybridrization seem to afford basically the same slightly flattened chair‐type conformations for the piperidine‐ring.