[CH2:1]([O:3][C:4]([C:6]1[N:7]=[CH:8][C:9]2[NH:10][C:11]3[C:16]([C:17]=2[C:18]=1[CH3:19])=[CH:15][CH:14]=[CH:13][CH:12]=3)=[O:5])[CH3:2].O.S(=O)(=O)(O)O.[I:26](O)(=O)=O>C(O)(=O)C.II>[CH2:1]([O:3][C:4]([C:6]1[N:7]=[CH:8][C:9]2[NH:10][C:11]3[C:16]([C:17]=2[C:18]=1[CH3:19])=[CH:15][C:14]([I:26])=[CH:13][CH:12]=3)=[O:5])[CH3:2]5.08 g of 4-methyl-β-carboline-3-carboxylic acid ethyl ester is dissolved in 40 ml of glacial acetic acid and combined with 0.96 ml of water, 0.24 ml of concentrated sulfuric acid, 688 mg of iodic acid, and 1.768 mg of iodine. The mixture is heated to 80° C. for 3 hours. After cooling, the mixture is suctioned off from undissolved material, and the filtrate is evaporated. The residue is taken up in ethanol/water. The thus-formed crystals are suctioned off, taken up in 500 ml of ethyl acetate, and extracted by stirring with 200 ml of 1N sodium hydroxide solution for 15 minutes. The organic phase is separated, concentrated by evaporation, the residue is extracted by stirring in ethyl acetate, and vacuum-filtered, thus obtaining 4.3 g of 6-iodo-4-methyl-β-carboline-3-carboxylic acid ethyl ester, mp 250°-255° C.
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