Pseudoazulene als Dienophile in der [4 + 2]‐Cycloaddition mit inversem Elektronenbedarf

Pseudoazulenes as Dienophiles in the [4 + 2] Cycloaddition with Inverse Electron Demand [4 + 2] Cycloadditions of the pseudoazulenes 2, 6, 13, 18 , and 22 to the extremely electron‐deficient s‐cis ‐fixed diazadiene system of 3,6‐bis(trifluoromethyl)‐1,2,4,5‐tetrazine ( 1 ) are described. Probably according to a two‐step mechanism the pseudoazulenes 2, 6 , and 18 react with 1 to form the corresponding cycloadducts followed by elimination of nitrogen and oxidation to furnish the novel 14 systems 5, 12 , and 21 in rather high yields. In contrast, with the cyclopenta[ d ]‐pyridazines 13 and 22 the azines 17 and 26 , respectively, are formed exclusively.