1.4 g (4.4 mmol) of the compound from Step 10.4 was dissolved in 20 mL of water and 1.5 g of hydrochloric acid (32%), the solution was cooled in an ice bath and to it was added over a period of 10 min 0.3 g (4.4 mmol) of sodium nitrite in 2 mL of water. The mixture was allowed to agitate for an additional 30 min after which a solution of 0.63 g (4.4 mmol) of 2-naphthol in 10 mL of isopropanol was added. The pH was adjusted to 8 to 9 by addition of saturated sodium carbonate solution which resulted in coupling with formation of a brown solution. After 1 hour, the solution was evaporated to dryness and, to isolate the pure dye, 80 mL of methanol was added to the residue. After an agitation period of 1 hour at room temperature, the inorganic salts were filtered off. The filtrate was concentrated to about 20% of its original volume and then 30 mL of isopropanol was added to induce crystallization. Cooling in an ice bath, filtration and drying at 40° C. under vacuum gave 1.6 g (67% of the original) of a red-brown product.
