NEW MATERIAL: cis-3,3-Dimethyl-2-[3-methyl-2-(2'-propynyl)-2-cyclopenten-1- on-4-yloxyhydroxyhydroxymethyl] cyclopropanecarboxylic acid lactone of formula I.

USE: A synthetic intermediate for a raw material of agricultural chemicals. Optically active cyclopentenolone which is useful as a raw material for insecticides can be readily synthesized.

PROCESS: An optically active lactone compound expressed by formula II (R is H or lower alkyl) is reacted with cyclopentenolone of formula III in the presence of an acidic catalyst while removing the compound of formula IV formed from the reaction to give the optically active compound of formula I. The resultant compound of formula I is crystallized in the presence or absence of a solvent, and purified by separating the crystalline part from the mother liquor.