Cycloaddition reactions of cyclic ketene‐ N,S ‐acetals with 1,2,4,5‐tetrazines

Reaction of 3,6‐diphenyl‐, 3,6‐bis(2‐pyridyl)‐ and the unsubstituted 1,2,4,5‐tetrazine with 4,5‐dihydro‐1‐methyl‐2‐(methylthio)pyrrole ( 2 ) and 1‐raethyl‐2‐(methylthio)‐4.5,6,7‐tetrahydroazepine ( 3 ) gives 4,7‐di‐ R ‐2,3‐dihydro‐1‐methylpyrrolo[2,3‐d]pyridazine ( 4 , R = phenyl, 2‐pyridyl, hydrogen) and 6,9‐di‐ R ‐1‐methyl‐2,3,4,5‐tetrahydropyridazino[4,5‐6]azepine ( 5 ), R = phenyl, 2‐pyridyl, hydrogen), respectively, in reasonable to good yields. The compounds 4 (R = phenyl, hydrogen) are converted into their corresponding 1‐methylpyrrolo‐[2,3‐ d ]pyridazines 6 by reaction with potassium permanganate in butanone. Reaction of 3‐phenyl‐1,2,4,5‐te‐trazine with 2 and 3 leads to the exclusive formation of the 7‐phenyl isomer 4d and 9‐phenyl isomer 5d , respectively, indicating that the cycloaddition is regiospecific. The mechanism is discussed.