[4 + + 2] Cycloadditions with an N ‐Methyl‐4 H ‐pyrazolium cation as electron‐deficient diazadiene system

3,6‐Diphenyl‐1,2,4,5‐tetrazine ( 6 ) reacts with dimethoxycarbene ( 7 ), generated from the 1,3,4‐oxadiazoline 8 , to yield the 4 H ‐pyrazole 10. This is alkylated to the N ‐methyl‐4 H ‐pyrazolium tetrafluoroborate 12 in high yield. The alkylated, cationic azine system of 12 serves as electron‐deficient diene in a [4 + + 2] cycloaddition with inverse electron demand and reacts diastereoselectively with several cyclic olefins as electron‐rich dienophiles. The initially formed cationic [4 + + 2] adduct, e.g. 15 , is unstable and rearranges probably via 16 to a racemic phthalazinecarboxylic ester of type 17. The crystal structures of the epoxyphthalazinecarboxylic ester 25 and of the trans ‐annulated cycloocta[ d ]pyridazine 27 were determined by X‐ray diffraction. In contrast to the cyclic olefins, cyclooctyne ( 28 ) reacts with the pyrazolium cation 12 to yield the cyclooctapyridiazinium tetrafluoroborate 30 via the cycloadduct 29 .