are mixed in a solvent such as water, methanol and ethanol, and reacted at, 50 to 300??? C. for 0.5 to 10 hours with stirring, or 1 mole part of an acid anhydride shown by the above-described general formula [27] and 2 to 20 mole parts of a formamide are reacted at 50 to 150??? C. for 0.5 to 10 hours with stirring without solvent or in a hydrocarbon solvent such as toluene and xylene, followed by