PURPOSE: To industrially and advantageously obtain the titled compound useful as a synthetic intermediate for vitamin Ds, brassinolides which are plant growth regulators and ecdysone derivatives which are insect molting hormones, by applying ene reaction to side chain synthesis of a steroid compound.

CONSTITUTION: A compound expressed by formula I [formula II is formula IIIWV R2 and R3 are lower alkyl, benzyl, etc.)] and a compound expressed by formula VI {Z is -R1 (R1 is 1W4C alkyl, etc.), -CH2OR' [R' is lower alkyl, (substituted)benzyl, etc.] or -CO2R7 (R7 is lower alkyl)} are subjected to ene reaction to afford the aimed compound expressed by formula VII.