PURPOSE: To obtain the subject compound as an intermediate for insecticides in one step by reacting a chloroketone such as 1,1,3,6-tetrachloro-4, 4-dimethyl-1- hexen5-one with an aqueous solution of an alkali metal hydroxide.

CONSTITUTION: A chloroketone of formula I (wherein Z is Cl or Br) (desirably, 1,1,3,6tetrachloro-4,4-dimethyl-1hexen-5-one of formula II) is reacted with an aqueous solution of an alkali metal hydroxide (especially desirably NaOH or KOH in an amount of 3-10 mol per mol of the chloroketone) to effect cyclization into trans-2, 2-dimethyl3-(2,2-dichlorovinyl) cyclopropanecarboxylic acid of formula III, namely trans-permethrinic acid. This is useful as an intermediate for the production of a pyrethroid insecticide.