The photo cycloaddition of alkenes to s-triazolo[4,3-b] and [2,3-b]pyridazines

s-Triazolo4,3-b]pyridazine (I) reacted with cyclohexene under the influence of ultraviolet light to yield 4a,5,7,8,8a,9-hexahydro-9-methylene-6H-s-triazolo1,5-a]indole (IV) and 9-cyanomethyl-4a,5,7,8,8a,9-hexahydro-6H-s-triazolo1,5-a]indole (V). These products were formed by the addition of the alkene to the 1,8 positions of I with a concurrent cleavage of the N₄? N₅ bond. Similar additions were observed with cyclopentene and 2,3-dimethyl-1,3-butadiene. The isomeric s-triazolo2,3-b]pyridazine (III) reacted with cyclohexene to form an isomer of IV, 4a,5,7,8,8a,9-hexahydro-9-methylene-6H-s-triazolo4,3-a]indole (XV) and two 2 + 2] cycloadducts (XVI and XVII).