Phosphorus oxychloride → 1-Chloro-4-hydroxy-isoquinoline-3-carboxylic acid butyl ester

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A mixture of 17.3 g (66.2 mmol) of 1,4-dihydroxy-isoquinoline-3-carboxylic acid butyl ester and 100 ml of phosphorous oxychloride was stirred at ambient temperature for 1 h, and then heated slowly with stirring in the course of 2 h to reflux temperature. The mixture was refluxed gently with stirring for 30 min. After cooling to room temperature the excess phosphorous oxychloride was evaporated in vacuo, and the residue was dissolved in 100 ml of ethyl acetate. The solution was poured into 300 ml of a saturated aqueous sodium bicarbonate solution with stirring. The precipitate formed was removed by vacuum filtration. The organic phase was separated, and the aqueous phase was extracted with 3×100 ml of ethyl acetate. The combined aqueous phases were dried over sodium sulfate, filtered through a pad of silica gel and evaporated in vacuo to give a brown oil that solidified later. 11.37 g of the title compound were obtained; 1H NMR (CDCl3): δ=11.91 (s, 1H), 8.41 (m, 1H), 8.29 (m, 1H), 7.83 (m, 2H), 4.49 (t, 2H), 1.84 (m, 2H), 1.48 (m, 2H), 0.99 (t, 3H).

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