The Chemistry of Indoles. XIII. Syntheses of Substituted Indoles carrying an Amino, Nitro, Methoxycarbonyl, or Benzyloxy Group at the 4-Position and Their 1-Hydroxy Derivatives

Various 1-hydroxyindoles carrying a nitro, methoxycarbonyl, or benzyloxy group at the 4 position were prepared by the controlled reduction of 6-substituted trans-β-dimethylamino-2-nitrostyrenes with either aqueous titanium (III) chloride or zinc in aqueous ammonium chloride. The stability of 4-substituted 1-hydroxyindoles decreased in the following order : 4-nitro-»4-methoxycarbonyl->4-benzyloxy-1-hydroxyindole. This result clearly indicates that an electron-withdrawing group at the 4-position can stabilize the 1-hydroxyindole structure. It was also found that 4-hydroxy-and 4-benzyloxy-indole were readily accessible by the reduction of 6-benzyloxy-2-nitrophenyl acetaldehyde. A unique route to 4-or 7-aminoindole from cinnoline is also described.