Conformational behaviour of medium-sized rings. Part 4. Heterocyclic analogues of 7,8,13,14-tetrahydrobenzo[6,7]cyclonona[1,2,3-de]- naphthalene and 7,8,15,16-tetrahydrocyclodeca[l,2,3-de: 6,7,8-d'e']dinaphthalene

The temperature dependences of the ¹H n.m.r. spectra of 8N.15H-dinaphtho[1,8-bc: 1',8'-gh] [1,5]dioxecin (4) and of heterocyclic analogues (2a, b, e, and f) of 7.8.1 3.1 4-tetrahydrobenzo[6,7]cyclonona[l,2,3-de]naphthalene have been interpreted in terms of ring inversion between enantiomeric twist-boat conformations. The temperature dependences of the ¹H n.m.r. spectra of the dithionins (2c and d) have been interpreted in terms of interconversions between chair and twist-boat conformations. A comparison of activation parameters shows that when periannelated naphthalene rings replace ortho-annelated benzene rings in '6.8.6' systems (1). chair-like conformations are destablised relative to boat-like conformations and the energy barriers to ring inversions involving pseudorotational processes are considerably higher.