Phosphorus oxychloride → 3-Methyl-thieno[3,2-b]pyridine-6-carboxylic acid ethyl ester

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A mixture of 3-methyl-7-oxo-4,7-dihydro-thieno[3,2-b]pyridine-6-carboxylic acid ethyl ester (WO 2003/059878 A2) (1.0 g) and phosphorus oxychloride (3 mL) was heated at 150° C. for 15 minutes in a microwave reactor. The reaction mixture was then cooled and poured into a mixture of ice-water and ethyl acetate. The resulting mixture was stirred for 10 minutes; the organic layer was separated, washed twice with water (50 mL) and with brine, dried over anhydrous sodium sulfate, filtered and evaporated under reduced pressure. To the solid residue dissolved in a mixture of ethyl acetate and isopropanol (10/1, 55 mL) sodium acetate trihydrate (2.0 g) and palladium hydroxide on carbon (20%, 0.3 g) were added and the resulting mixture was shaken in a Parr apparatus under hydrogen atmosphere (50 PSI) overnight. The reaction mixture was then filtered on a CELITE™ pad, the filtrate was evaporated under reduced pressure and the crude residue was purified by flash chromatography (EtOAc/hexane, 10/90) to afford 0.78 g of 3-methyl-thieno[3,2-b]pyridine-6-carboxylic acid ethyl ester.

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