Chair-boat interconversions in cyclic seven-membered amides and related structures

The transition state to ring reversal has been examined in a series of 7-membered rings related to caprolactam. The structural changes associated with introduction of Me groups, with alteration of the nature of the CN double bond, with ring fusion, or with introduction of a second, nonconjugated N atom perturb the transition state to ring reversal. From ¹H and ¹³C NMR studies, we find that the lactam bond itself favors a different transition state from that present in cycloheptene. With some exceptions, the observed barriers are consistent with TS II, in which ring reversal begins with movement of the C₅ end of the ring, or with TS III, in which ring reversal begins with C₃-C₄ bond torsion. The conclusions are consistent with the structural changes that result from the functional and steric alterations.