A process for advantageously producing compounds of general formula (III), wherein R1 represents hydrogen, alkyl having 1 to 6 carbon atoms, alkoxyl having 1 to 4 carbon atoms, halogen, hydroxyl, trifluoromethyl, nitro or amino; R2 represents substituted or unsubstituted phenyl or pyridyl; R?3 and R4¿ each represents hydrogen, alkyl having 1 to 6 carbon atoms, hydroxyalkyl having 1 to 4 carbon atoms, cycloalkyl having 3 to 8 carbon atoms, or substituted or unsubstituted phenyl; and Y represents alkylene having 1 to 12 carbon atoms, and salts thereof.