... KGaA, Weinheim. Issue. European Journal of Organic Chemistry. ... In the 1 H NMR spectra of 7, 13, 21 and 25 the resonance for 3-H, which is typical for indole-2-carboxylic acids, is found at δ = 6.97–7.13 ppm as a doublet (J = 1.3–2.1 Hz). Conclusions. ...