A method of preparing 1-phenyl-1-diethylaminocarbonyl-2-phthalimidomethyl-cyclopropane-Z and key intermediates therefor are disclosed.This method is characterized by the following successive steps:opening of the 1-phenyl-2-oxo-3-oxa-bicyclo(3:1:0)-hexane by diethylamine or a tertiary amine with the aid of a Lewis acid/amine complex:the 1-phenyl-1-diethylaminocarbonyl-2-hydroxymethyl-cyclopropane-Z thus obtained is converted into a 2-chlorinated derivative by the action of a chlorination reagent such as thionyl chloride; andthe 1-phenyl-1-diethylaminocarbonyl-2-chloromethylcyclopropane-Z is reacted with a phthalimide salt in an organic solvent to produce the 1-phenyl-1-diethylaminocarbonyl-2-phthaliumidomethyl-cyclopropane-Z.