The reaction of 2,2-dialkyl-4,6-diaryl-1,3-diazabicyclo[3.1.0]hex-3-enes with dichlorocarbene to give (1RS,5SR,6RS)-4-(alk-1-enyl)-1,5-diaryl-3,7,7- trichloro-2,4-diazabicyclo[4.1.0]hept-2-enes has been studied. Quantum-chemical calculations using the DFT B3LYP/6-31G(d) method have confirmed that the mechanism of the reaction includes the formation of azirino[c]imidazolium ylides and coordinated opening of the imidazole and aziridine rings with subsequent cyclization and dichloropropane formation.