PURPOSE: To selectively obtain a desired optical isomer of the title compound by reacting a novel specific 10-methyl-2,4-dioxo-1,5-dioxaspiro[5,5]undecane derivative with diazomethane and then hydrolyzing.

CONSTITUTION: A novel substance shown by formula III [one of R1 and R2 is H and the other is aryl or furyl which may contain substituent group (lower alkyl, lower alkoxy, halogen or NO2); R3 is lower alkyl which may be replaced with phenyl] is reacted with diazomethane to give a novel substance shown by formula I, which is hydrolyzed with an alkali and optionally further esterified to selectively give a desired optical isomer of the aimed compound shown by formula II (R4 is H or lower alkyl) useful as an intermediate for drugs, agricultural chemicals, etc., from isomers shown by formula III or formula I. The raw material can be produced from a compound shown by formula V and compounds shown by formula VI-VII through a novel substance shown by formula IV and relatively readily isomerized in the stage of recrystallization.