Indigo revisited

Abstract

The synthesis of indigo and five derivatives containing electron-donating groups (methyl or methoxyl) by known methods, is described. The Baeyer-Drewsen method was followed in most cases. Another product containing ester groups was prepared via the corresponding 3-formylindole by a Dakin reaction. The blue dyes were characterized by their absorption (UV-Vis and IR) and NMR spectra, and elemental analysis or mass spectrometry. The objective of this study was to prepare and fully characterize indigo derivatives with electron-donating groups and to obtain and characterize a carboxylated indigo derivative.