~ ~ ~~Nucleophilic Singlet Carbenes in the [4 + 11 Cycloaddition with 1,2,4.5-Tetrazines: a New Synthesis of Isopyrazoles A series of 3,6-disubstituted 1,2,4,5-tetrazines 6, including by subsequent [4 + 21 cycloreversion. The acceptor-substi-C6H5, SCH3, S02CH3, N(CH&, CF3, C02CH3 groups, has been tuted isopyrazole 9e is characterized as an electron-deficient submitted to [4 + 1 1 cycloaddition with the nucleophilic sin-diene by some Diels-Alder reactions with inverse electron deglet carbenes 4, 15, and 20, which are generated from the pre-mand leading to the expected azo-bridged cycloadducts 23,