PURPOSE: To obtain a carboxylic acid derivative under extremely mild condition, by reacting a carbonyl compound with a thiazolium salt in the presence of a compound having isoalloxazine skeleton, and reacting the produced active acyl compound with a nucleophilic compound.

CONSTITUTION: The carboxylic acid derivative of formula IV is obtained by reacting the compound of formula I (R1 is aryl, alkyl, alkenyl, etc.; R2 is H or carboxy) with a thiazolium salt (e.g. the compound of formula III) in the presence of a compound having isoalloxazine skeleton (e.g. the compound of formula II), and reacting the produced active compound with the nucleophilic compound of formula H-Y (Y is alkoxy, amino, carboxylic acid residue, etc.). The amount of the compound having isoalloxazine skeleton is 0.01W1mol per 1mol of the substrate, and the reaction is carried out usually at 5W100°C under atmospheric pressure for 1hrW2 days.