PURPOSE: To permit high-selectivity, high-yield production of the title substance which is useful as an intermediate of organic synthesis, by reaction of an acetal with an organotin organothio compound in a solvent in the presence of a Lewis acid catalyst.

CONSTITUTION: The subject compound of formula III is obtained by allowing a compound of formula I (R1, R2 are H, hydrocarbon group; R3 is lower alkyl, lower alkoxyalkyl; R4 is hydrocarbon group, R1WR4 may bond to each other) to react with another compound of formula II (R5 is alkyl, aryl; R is alkyl, aryl) in the presence of a Lewis acid catalyst such as boron trifluoride etherate in a solvent such as toluene at low temperature (-20°C) for 1 to several hours.