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A mixture of indoline (145 g, 1.2 mol) and formic acid (90%, 92 g, 1.8 mol) was brought to reflux in toluene with removal of water via a Dean-Stark apparatus. After 6 hours the reaction mixture was cooled to room temperature, washed with water and concentrated in vacuo. N-formylindoline (158 g, 1.1 mol, 88% yield) was isolated as an off-white solid and used in the next step without further purification. Mixture of rotamers: 1H-NMR (CDCl3) δ 8.92 (s, 0.85H), 8.51 (s, 0.15H), 8.06 (d, J=8.3 Hz, 0.15H), 7.26-7.16 (m, 3.85H), 7.16-7.02 (m, 1H), 4.13-4.02 (m, 2H), 3.21-3.12 (dt, 2H).

1. washwashed with water
2. concentrationconcentrated in vacuo