Conformational behaviour of medium-sized rings. Part III. Heterocyclic analogues of 12,13-dihydro-11H-dibenzo[a, e]cyclononene, 6,11,12,13-tetrahydro-5H-dibenzo[a, e]cyclononene, and 5,6,7,12,13,14-hexahydrodibenzo[a, f]cyclodecene

The temperature dependence of the 1H n.m.r. spectra of a number of heterocyclic analogues (3a–c) of 12,13-dihydro-11H-dibenzo[a, e]cyclononene has been interpreted in terms of the interconversion of chair- and boat-like conformations. Conformational analysis on these molecules has been carried out with the aid of strain energy calculations on the thia-analogue (3c); in this case a useful correlation between calculated and experimental activation parameters was found. Variable temperature 1H n.m.r. spectroscopy and strain energy calculations have demonstrated that the heterocyclic analogues (4a–f) and (5a–d) of 6,11,12,13-tetrahydro-5H-dibenzo[a, f]-cyclononene and 5,6,7,12,13,14-hexahydrodibenzo[a, f]cyclodecene, respectively, all adopt flexible chair-like conformations with C² symmetry which undergo an inversion process involving torsion about single bonds.