N-Heterocyclic Carbene Catalyzed Formal [3+3] Cycloaddition

Significance

Here the authors report a highly stereoselective N-heterocyclic carbene catalyzed formal [3+3] cycloaddition of enals and azomethine imines. N-mesityl benzimidazolium salt 1 along with DBU was found to be effective for this reaction. The addition of the carbene catalyst to the enal generates a homoenolate intermediate that undergoes an addition-acylation sequence with the azomethine imine to afford the bicyclic pyridazinones. With 20 mol% of catalyst 1 moderate to high yields (51-93%) and high diastereoselectivities (>20:1 dr) are obtained for different enals with different azomethine imines.