The formal [2+3] cycloaddition of a twofold excess of diphenylcyclopropenone to azomethine imines generated from 6-aryl-1,5-diazabicyclo[3.1.0]hexanes 1 under thermolysis conditions presumably occurs via the initially formed unstable dipolar bicyclic intermediates 4. Isomerization of these intermediates and subsequent addition of the second molecule of diphenylcyclopropenone with the extrusion of one molecule of carbon monoxide results in the formation of tricyclic 4a,7b-diazacyclopenta[cd]inden-7-ones 8 in good yields.
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Molchanov et al. (2002) studied this question.
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