3-Azatricyclo [4.1.0.0^2.5] hept-3-ene derivatives prepared from pyridines via five steps were found to be useful synthons for fully unsaturated monocyclic seven-membered heterocyclic rings. Photolysis of the 7-oxa derivatives 15 resulted in valence isomerization with ring opening to give the 1, 4-oxazepines (19), which are the first examples of fully conjugated 1, 4-oxazepines. Similarly, the 7-aza derivatives 16, upon irradiation, afforded the novel 1H-1, 4-diazepines (20).
The 2, 7-dimethyl-1, 4-oxazepine (19c) and 2, 7-dimethyl-1, 4-diazepine (20c) underwent thermal ring conversion, giving rise to the 1, 3-oxazepine (22) and 1, 3-diazepine (24), respectively.