Conformation of Eight-Membered Benzoannulated Lactams by Combined NMR and DFT Studies

The title compounds were synthesized, and their structure
and conformational
behavior in solution (NMR and DFT), in the gas phase (DFT), and, for
some of them, in the solid state (X-ray) were investigated. The variable-temperature
NMR spectra were employed to determine the conformational equilibria
and the activation energy of the conformational changes of the eight-membered
ring. The coalescence effects are assigned to racemization of the
chiral ground-state conformation with a ring inversion barrier in
the range of 38–100 kJ mol–1 depending on
the relative setting of the two strong conformational constraints:
benzoannulation and the amide function. The second conformational
process, interconversion between two different conformers, in the
molecules of benzo­[c]­azocin-3-one, benzo­[d]­azocin-2-one, and benzo­[d]­azocin-4-one
was observed. The natures of the conformers observed in solution were
elucidated by analysis of experimental and calculated NMR data. The
present results are discussed in conjunction with previous experimental
and theoretical data on (Z,Z)-cyclooctadienes
and their benzo analogues.