PURPOSE: To efficiently obtain a diastereoisomerically pure compd. as the raw material for medicines or the like by reacting the specified ketoprophene of a raceme with acid chloride in the presence of tertiary amine, reacting the produced ketene with alcohols and thereafter hydrolyzing the same.

CONSTITUTION: The ketoprophene of a raceme expressed by formula I (Me is methyl) is reacted with an acid chloride in the presence of tertiary amine to produce ketene expressed by formula II, after that, this ketene is moved into other reactor, where it is brought to react with alcohol expressed by the formula R1-OH [R1 is a group of formula III (R2 is a 1-10C alkyl, 3-5C cycloalkyl, benzyl or phenyl; Y is a 1-10C alkoxy, 3-10C cycloalkoxy, benzyloxy, phenoxy or the like) or a cyclic amide group in accordance with formula IV (R3 is a 2-4C alkylene or the like; X is O, amine or the like)] at a temp. of -100 to +25°C in the (non)presence of an inert organic solvent to obtain the diastereoisomerically pure compd. expressed by formula V and formula VI.