Reaction of 2-diazoacenaphthenone with anils in the presence of a catalytic amount of borontrifluoride etherate is reported. The diazoketone reacts with benzylidenanilines to afford a mixture of stereoisomers of spiro[acenaphthenone-2′,4-imidazolidine], whose structures correspond to compounds derived from 1:2 adducts of the diazoketone and anils with the elimination of nitrogen. A pathway involving