The effect of an electron withdrawing group adjacent to cyclopropyl carbanion has been studied. The facilitv with which carbanion interconversion occurs can be attributed to the existence of"the anomeric effect. Cyclopropyl carbanions derived from optically active precursors are capable of retaining their configuration to a significant degree.I The calculated energy barrier for inversion of the cyclopropyl carbanion is 36.3 Kcal/mole.2 The initial hybridization of the carbon and the constraint in a three-membered ring (I-strain) are factors which undoubtedly affect the energy barrier for racemization of the derived cyclopropyl carbanion.3 Walborsky and coworkers have elegantly demonstrated that the configurational stability of the a-substituted cyclopropyl carbanion depends on the solvating property of the reaction medium and on the type of substituent present-whether it is a delocalizing or a nondelocalizing one. 4 The present work describes a case where substituents adjacent to the carbanionic center affect the formation and the geometry of its reaction product with electrophiles. Silylsulfonyl substituted 3H-pyrazole 2 was readily prepared by the 1,3-dipolar cycloaddition of 2-diazopropane with alkyne L. Photolysis of this material in benzene afforded cyclopropene 2 in excellent yield. The reaction proceeds via cyclization of a transient vinyl carbene intermediate formed by extrusion of nitrogen.5