1.2.4.5- Tetrazines 1.2.4- triazepines

Cycloaddition of azirines 5 to 1.2,4,5-tetrazines 6 is followed by loss of nitrogen and ring enlargement to yield 5//-1,2,4-triazepines 7, which tautomerize spontaneously by a [1,51-hydrogen shift to the 2/7-1,2,4-triazepines 8. The triazepinesare accompanied by variable amounts of pyrimidines and pyrazoles.335 - 338... 

Fully unsaturated 2H-1,2,4-triazepines (501) are formed by the cycloaddition of 1-azirines (499) to 1,2,4,5-tetrazines (500). The initial product rearranges by a 1,5-hydrogen shift to give (501) (74TL2303). 

Prior to 1981, there were many reports concerning the syntheses and reactions of the fully unsaturated monocyclic 1,2,4-triazepines, and all possible tautomers, H- (I), 2H- (2), AH- (3), and 6H- (4) forms, have been prepared by the cycloaddition of 1-azirines to 1,2,4,5-tetrazines <74CC45,... 

---