O=[C:2]1[C:11]2[C:6](=[N:7][CH:8]=[CH:9][CH:10]=2)[O:5][C:4]2[CH:12]=[CH:13][C:14]([CH:16]([CH3:20])[C:17]([OH:19])=[O:18])=[CH:15][C:3]1=2.[BH4-].[Na+].C1(C)C=CC=CC=1.Cl>[OH-].[Na+].O>[N:7]1[CH:8]=[CH:9][CH:10]=[C:11]2[CH2:2][C:3]3[CH:15]=[C:14]([CH:16]([CH3:20])[C:17]([OH:19])=[O:18])[CH:13]=[CH:12][C:4]=3[O:5][C:6]=124.75 g of 2-(5-oxo-5H-[1]benzopyrano[2,3-b]pyridin-7-yl)propionic acid is dissolved in a solution of 0.88 g of sodium hydroxide in 25 ml of water. To the solution are added 5 g of sodium borohydride and a small amount of toluene, and the mixture is heated under reflux for 3 hours. The reaction mixture is adjusted to pH 3 by adding concentrated hydrochloric acid under ice cooling. The resulting crystalline precipitate is filtered off, washed with water, and recrystallized from aqueous dioxane to give 2.5 g of 2-(5H-[1]benzopyrano[2,3-b]pyridin-7-yl)propionic acid as white needles melting at 182.5°-183.5°C.
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