Boron tribromide → 2H-1-Benzopyran-2-one, 5-hydroxy-

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Reaction Steps

To a cold (-30° C.) solution of 2,6-dimethoxycinnamic acid methyl ester (5.2 g, 23.4 mmol, prepared as described above), in chloroform (250 ml), was added boron tribromide (80 ml of a 1M solution in methylene chloride). The mixture was allowed to stir under anhydrous conditions was poured into saturated aqueous sodium hydrogen carbonate solution (400 ml). [Caution Frothing] The organic phase was diluted with ethyl acetate (250 ml), the mixture was transferred to a separating funnel and extracted thoroughly. The organic phase was isolated, dried (MgSo4) and evaporated in vacuo to give a dark red oil which did not crystallize. The oil was purified further by chromatography on silica gel to give 5-hydroxycoumarin (3.4 g, 89%), as a pale-yellow crystalline powder. M.p. 226°-229 ° C. (Das Gupta et al. J. Chem. Soc. (C), 29, 1969 give m.p. 224°-225° C.) 1H NMR (DMSO-d6): 8.11 (dd, J=9,6,0.6Hz, 1H), 7.40 (t, J=8.4Hz, 1H), 6.78 (m, 2H), 6.34 (d, J=9.6Hz, 1H).

Provenance & Evidence