[4+2] Cycloadditions of 1,2,4,5-Tetrazines and Cyclopropenes − Synthesis of 3,4-Diazanorcaradienes and Tetracyclic Aliphatic Azo Compounds

1,2,4,5-Tetrazines 1 readily react with cyclopropenes 2 to form 3,4-diazanorcaradienes 3, 4, 7 and 8 in a cycloaddition − cycloelimination sequence. Compounds 3 and 4 still act as 1,3-dienes with cyclopropenes 2, producing aliphatic azo compounds 5 and 6, versatile starting compounds in thermolysis and photolysis reactions.