General procedure for the synthesis of 6-aminoindolones from 2,4-dinitrophenylacetic acid: 2,4-dinitrophenylacetic acid (10 g, 44 mmol) was dissolved in ethanol (200 mL) and purged with nitrogen. 10% Pd/C catalyst (400 mg) was added and the reaction was hydrogenated at 45 psi hydrogen pressure for 2 hours. Upon completion of the reaction, the mixture was transferred to a round bottom flask, p-toluenesulfonic acid (150 mg) was added, and heated to reflux overnight. The reaction mixture was filtered through diatomaceous earth to remove the catalyst and the filtrate was subsequently concentrated. The product precipitated as brown crystals and 4.68 g (72% yield) was obtained by filtration. The product was characterized by 1H NMR (DMSO-d6, 400 MHz): δ 3.20 (s, 2H), 4.97 (br s, 2H), 6.07 (m, 2H), 6.76 (d, J = 8.7 Hz, 1H), 10.05 (s, 1H); HPLC retention time: 2.123 min; LRMS (M+) 148.7.
