1//-1,2-Diazepines, electrocyclic ring closure

Ring contraction and intramolecular cyclization constitute a convenient route to ring-fused systems that would be difficult to synthesize in other ways. H- 1,2-Diazepines (538) undergo electrocyclic ring closure to the fused pyrazole system (539) (71CC1022). Azepines also undergo similar valence bond isomerizations. 

Electrocyclic ring-closure of the diazo compounds (35) and (37) has given the thieno[3,2-cf][l,2]diazepine (36) and the thieno[2,3-i7][l,2]diazepine (38) respectively in very good yields (80JCS(P1)1718>. However, the 3,4-disubstituted thiophene (39) did not give any cyclized product only products ascribable to a carbene intermediate could be isolated. 

An unusual 1,7-electrocyclic ring closure has been observed in the diazoalkane (48) to give the diazepine (49). In contrast, irradiation of the isomeric... 

---