Pyrazolopyridazinones by 1,3-Dipolar Cycloaddition of Diazomethane to Pyridazin-3(2H)-ones

Abstract

N-methylpyridazin-3(2H)-onesi-; react slowly with diazomethane to give, a s main products, the En?-dimethylpyrazolo [3,4-d ] pyridazin-4(5g)-ones & and 1 & in a 1 : 2 ratio.The reaction of diazo- 1 6 methane with the N-methyl-4-bromopyridazin-3(2g)-one (4) leads to N ,Ndimethylpyrazolo 13.4-d] pyridazin-7(6E)-one (2).In the first case, the regioselectivity of the primary 1. 3-dipolar cycloaddition i s the reverse of that observed in enone systems.While 1,s-dipolar cycloaddition of diazoalkanes to substituted alkenes and a k y n e s has been extensively investigated 1 ' 2 ' 3 , very little i s known about the reaction with heterocyclic 4 dipolarophiles.However, examples a r e described in Which the cycloaddition to a pyridone and a pyridazinone a r e claimed a s the first step in a C-alkylation o r ring-expansion procedure, r e spectively.