PURPOSE: To selectively obtain one of steric isomers of a lactam by radiating light to a clathrate compound consisting of a specific compound and an optically active host compound.

CONSTITUTION: A clathrate compound consisting of a compound of formula I (R1 and R2 are alkyl; aryl, H or halogen; X is ≤10C bivalent organic group; (n) and (m) are 0-3; the ring may have substituent) and an optically active host compound (e.g. tartaric acid derivative) is irradiated with light to obtain a compound of formula II. The irradiation is carried out by pulverizing the crystal of the obtained clathrate compound and preferably stirring the powder to uniformly apply the light to the whole powder. There is no particular restriction on the wavelength and the kind of the light source. The process enables the repeated use of the host compound and is a selective photochemical reaction advantageous for the synthesis of a compound useful in the field of fine chemistry such as pharmaceuticals and agrichemicals in high yield.