AbstractΔ1‐pyrazolines, in which the double bond is situated between the two nitrogen atoms can be prepared by the addition of diphenyldiazomethane to the diphenyl ester of citraconic acid and to the trimethyl ester of aconitic acid. These compounds split off nitrogen at higher temperatures forming cyclopropane derivatives.Diphenyl diazomethane combines directly with citraconic acid anhydride, splitting off nitrog