Conformational analysis of some 11,12-dihydrodibenz[b,f][1,5]oxazocin-6-one derivatives by NMR spectroscopy

A variable temperature 200 MHz 1H NMR investigation of some N-acylsubstituted 11,12-dihydrodibenz[b, f][1, 5]oxazocin-6-ones showed the presence at room temperature of at least two conformationally restricted diastereomers (and their enantiomers) which molecular mechanics calculations strongly suggested to be a boat-like structure (major conformer) and a pseudo-chair and/or twistboat-like structure (minor conformer). The diastereomers interconvert through a bond rotation mechanism just above room temperature (51-54 °C, depending upon the nature of the N-acyl substituent), and the boat enantiomers interconvert at higher temperatures (75-95 °C, depending upon the nature of the N-acyl substituent and the substitution pattern of the aromatic rings) via a ring inversion process. The N-acyl groups exocyclic to the eight-membered ring are shown to wholly exist - probably as a result of severe dipole: dipole interactions - in the sterically disfavoured conformation where the alkyl substituent bonded to the exocyclic carbonyl carbon atom is trans to the benzylic methylene group. Corresponding N-unsubstituted derivatives exhibit rapid ring inversion on the NMR time scale at room temperature.