1,3‐Dipolar cycle‐addition of 2‐diazoalkanes to pyridazin‐3(2H)‐one derivatives. The formation of 3H‐pyrazolo[3,4‐d]pyridazin‐4(5H)‐one and 3H‐pyrazolo[3,4‐d]pyridazin‐7(6H)‐one derivatives

Abstract 1,3‐Dipolar cycloadditions of diazoalkanes to pyridazin‐3(2H)‐ones 1–7 and pyridazin‐3(2H)‐thiones 8 and 9 are regioselective producing 3H‐pyrazolo[3,4‐d]pyridazin‐4(5H)‐ones 15–19, 27–29 and 34–38 as the major products. In some instances, the isomeric 3H‐pyrazolo[3,4‐d]pyridazin‐7(6H)‐ones, such as 20 and 23 were isolated as the minor products. From 3 and 6 the primary 3a,7a‐dihydro cycloadducts 25 and 26, and rearranged 1,2‐dihydro intermediate 31 were isolated. From 10 and 1‐diazoindane the isomeric exo‐ and endospiro products 39 and 40 were formed.