General procedure for the synthesis of ethyl 5-hydroxyindole-2-carboxylate from ethyl 5-benzyloxyindole-2-carboxylate: ethyl 5-benzyloxyindole-2-carboxylate (10 g, 34 mmol) was dissolved in ethanol (250 mL), 10% Pd/C catalyst was added, and the hydrogenation reaction was carried out for 20 h at atmospheric pressure (using a double-walled balloon). Upon completion of the reaction, the reaction mixture was diluted with ethyl acetate and filtered through diatomaceous earth to remove the catalyst. The filtrate was concentrated in vacuum to give ethyl 5-hydroxyindole-2-carboxylate (7 g, 99% yield) in brown solid form. The product was characterized by 1H NMR (400 MHz, CDCl3): δ 8.86 (broad peak, 1H), 7.29 (d, J = 8.80 Hz, 1H), 7.11 (s, 1H), 7.06 (s, 1H), 6.94 (d, J = 8.70 Hz, 1H), 4.83 (broad peak, 1H), 4.42 (q, J = 7.09 Hz, 2H ), 1.41 (t, J = 7.08Hz, 3H).
