The general procedure for the synthesis of 2-formyl-1H-indole-6-carbonitrile from 2-hydroxymethylindole-6-carbonitrile (18; 2.10 g, 12.2 mmol) was as follows: 2-hydroxymethylindole-6-carbonitrile was dissolved in acetone (50 mL) and manganese dioxide (3.15 g, 36 mmol) was added. The reaction mixture was stirred at room temperature for 16 hours, followed by the addition of manganese dioxide (1.05 g, 12 mmol) and continued stirring for 24 hours. Upon completion of the reaction, the insoluble solids were removed by diatomaceous earth filtration and the solid residue was washed with acetone (20 mL). The filtrates were combined and concentrated under reduced pressure. The concentrated crude product was adsorbed onto silica gel (~100 mL, uncompressed) and eluted with ethyl acetate (250 mL) until product was no longer detected in the eluate. The eluate containing product was collected and concentrated under reduced pressure. The crude product was recrystallized by boiling a solvent mixture of dichloromethane/hexane to give 1.14 g (55% yield) of the product as an olive colored powder. Thin layer chromatography (TLC) analysis showed an Rf value of 0.45 (unfolding agent ratio 2:1 hexane/ethyl acetate). The 1H NMR (DMSO-d6) data of the product were as follows: δ 12.50 (s, 1H), 9.97 (s, 1H), 7.96-7.91 (m, 2H), 7.52-7.42 (m, 2H).
