To a 250 mL three neck flask fitted with a condenser and Dean-Stark apparatus was added 4′-amino-biphenyl-2-sulfonic acid tert-butylamide (1.22 g, 4.0 mmol), p-toluenesulfonic acid monohydrate (152 mg, 0.8 mmol), 2,2,8-trimethyl-4H-[1,3]dioxino [4,5-c]pyridine-5-carbaldehyde (995 mg, 4.8 mmol) and toluene (120 ml). The reaction mixture was stirred at 120° C. under nitrogen atmosphere for 7 hours before concentrating to dryness. The resulting solid was then dissolved in acetic acid (20 mL), cooled to 0° C., followed by a slow addition of sodium borohydride (529 mg, 14 mmol). After the addition of sodium borohydride, dichloromethane (30 mL) was then added to the reaction mixture and stirring was continued at room temperature for an additional 3 hours. Sodium hydroxide (5 N) was added to neutralize the solution, and the reaction mixture was extracted with ethyl acetate, dried over MgSO4, filtered and evaporated. The crude mixture was purified by column chromatography on silica gel using a mixture of ethyl acetate and hexane (1:1) as eluant, to give 4′-[(2,2,8-trimethyl-4H-[1,3]dioxino[4,5-c]pyridin-5-ylmethyl)-amino]-biphenyl-2-sulfonic acid tert-butylamide (1) (0.457 g, 24% yield) as a colorless solid.
