Fluorescent Brightener 367

Preparation of 1, 4-bis(benzoxazol-2'-yl)naphthalene in the Closed System; 0.71 g (3.3 mmol) of naphthalene-1, 4-dicarboxylic acid and 0.81 g (7.4 mmol) of o-aminophenol were suspended in 2.4 g of N-methylpyrrolidone in a pressure-resistant glass cuvette with stirring. The ammonium salt-containing suspension thus prepared was, after adding 169 mul of titanium tetrabutoxide, exposed to microwave irradiation of 300 W in the cuvette sealed pressure-tight with stirring and external cooling for 15 min. A temperature of approx. 225 C. measured by means of IR sensor was attained, and the pressure rose to nearly 20 bar. Subsequently, the reaction mixture was cooled to room temperature within 10 minutes, in the course of which the product crystallized out in the form of yellowish needles. The conversion based on naphthalene-1, 4-dicarboxylic acid was 83%. After filtration, washing of the crystals with ethanol, extraction of residual acid by stirring with alcoholic sodium hydroxide solution and drying, 1, 4-bis(benzoxazol-2'-yl)naphthalene was obtained with more than 99.5% purity. A further 15-minute microwave irradiation of the mother liquor which has been dried to remove water of reaction and consists essentially of solvent and unconverted reactants afforded a further 14% conversion (based on the amount of naphthalene-1, 4-dicarboxylic acid originally used).