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.