1.50 g of 2-{4-[N-(5,6-diphenylpyrazin-2-yl)-N-isopropylamino]butyloxy}acetic acid was dissolved in 20 ml of anhydrous tetrahydrofuran and 0.500 ml of triethylamine was added and then 0.376 ml of ethyl chlorocarbonate was added while stirring under ice cooling. After stirring under ice cooling for 45 minutes, a solution of saturated ammonia in 20 ml of tetrahydrofuran was added, followed by stirring for one hour. After removing an ice bath and stirring at room temperature for 18 hours, almost all of the solvent was evaporated. The residue was combined with water and then extracted with ethyl acetate. The extract was washed in turn with an aqueous saturated sodium hydrogen carbonate solution, 1N sodium hydroxide solution and water and dried over anhydrous magnesium sulfate, and then the solvent was evaporated under reduced pressure. The residue was washed with diethyl ether and then dried under reduced pressure to obtain 1.19 g of the desired product.
