urea in neat (no solvent) heating of 2,3-naphthalene dicarboxylic acid with urea (excess) in the presence of AlCl3 and a catalytic amt. of ammonium molybdate, 300 °C; repeated washing (0.5N HCl, 0.5N NaOH, 50 °C) or ultrasonication in alkaline medium (pH 14); 2-3 subls. at 300 °C (30 min., 0.5 mm Hg);

in quinoline refluxing for 2h; cooling; according to Mikhalenko, S. A.; Luk'yanets, E.A. J. Gen. Chem. USSR (Engl. Transl.) 1969, 39, 2495-2498; Zh. Obshch. Khim. 1969, 39, 2554-2558; filtration; washing (toluene, EtOH); air-drying; elem. anal.;