[O:1]1[C:12]2[C:11]3[CH:10]=[CH:9][CH:8]=[CH:7][C:6]=3[NH:5][C:4]=2[CH:3]=[C:2]1[C:13]([O:15]CC)=[O:14].Cl>[OH-].[Na+]>[O:1]1[C:12]2[C:11]3[CH:10]=[CH:9][CH:8]=[CH:7][C:6]=3[NH:5][C:4]=2[CH:3]=[C:2]1[C:13]([OH:15])=[O:14]To ethyl furo[3,2-b]indole-2-carboxylate (10 g), 10% aqueous sodium hydroxide solution (100 ml) was added, then the mixture was stirred for an hour at 60° C. The resulting solution was acidified with hydrochloric acid, and the precipitate formed was collected by filtration to give furo[3,2-b]indole-2-carboxylic acid (8.5 g). To furo[3,2-b]-indole-2-carboxylic acid (8.5 g), thionyl chloride (40 ml) and benzene (50 ml) were added, the resulting solution was refluxed for 5 hours, and the excess amounts of thionyl chloride and the benzene were evaporated to give furo[3,2-b]indole-2-carbonyl chloride (7.1 g).
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