Thermally induced reactions of 10-diazoanthrone with suitably activated alkenes have been studied. The reactions with maleic anhydride, N-phenylmaleimide, acrylonitrile, and diethyl fumarate in benzene solution were accompanied by spontaneous loss of nitrogen and formation of spirocyclopropane-anthrone derivatives. Under the same conditions, however, the reaction with diethyl maleate proceeded without loss of nitrogen to yield the corresponding 2-pyrazoline. Formation of dissimilar products in the reactions with diethyl maleate and diethyl fumarate suggests an essential difference in the nature of the transition state leading to the first intermediate adduct, the 1-pyrazoline. It seems that molecular nitrogen is lost more readily from a more polar, diazonium-like transition state than from an azo-like activated complex.