PM3 calculations of possible pathways for azetidine-ring formation reaction of imine with allene was performed, indicating that the reaction proceeds via a nonconcerted two-step mechanism in which the allene attacks toward the lone pair of nitrogen atom of imine moiety. Isolation of two stereoisomeric 2:1 cycloadducts from the reaction of 3,3-dimethyl-3H-indole with phenylsulfonyl propadiene strongly supports the presence of a zwittcrionic reaction intermediate.