Pappalardo, Giovanni published the artcileIndoles. II. Ultraviolet spectra of ar-mononitro-2,3-dimethylindoles, Category: indole-building-block, the publication is Gazzetta Chimica Italiana (1958), 564-73, database is CAplus.
cf. C.A. 52, 6312f. Ultraviolet spectra of indole (I), skatole (II), and 2,3-dimethylindole (III), and of 4-, 5-, 6-, 7-nitro-2,3-dimethylindoles (IV, V, VI, VII) were measured in C6H12, alc., 0.05N HCl and 0.05N NaOH and were tabulated. PhNHN:CMeEt cyclized with polyphosphoric acid according to Kissmann, et al. (C.A. 48, 5850e), the product steam-distilled, and the distillate converted to the nitroso derivative, m. 63.5-4.5° (dilute alc.), and reconverted gave pure III, m. 105.5-6.5° (petr. ether). 3-O2NC6H4NHN:CMeEt (12.0 g.) cyclized with concentrated HCl, the product [m. 99.0-9.5° (dilute alc.)] chromatographed in 200 ml. C6H6 on Al2O3 (Brockmann), eluted successively with C6H6 and 1:10 C5H5NC6H6, and the fractions rechromatographed from C6H6 and crystallized gave 2.5 g. IV, m. 176.5-7.5° (C6H6 and dilute alc.), and 3.5 g. VI, m. 141-2° (alc.). Cyclization of 4-O2NC6H4NHN:CMeEt [85%, m. 128-9° (dilute alc.)] with concentrated HCl and chromatography in C6H6 on Al2O3 gave V, m. 193-4°. Similarly 2-O2NC6H4NHNH2 condensed with MeCOEt gave 63% 2-O2NC6H4NHN:CMeEt, m. 75.5-6.5° (alc.), cyclized with concentrated HCl to VII, m. 162.5-3.5°. The spectrum of III in C6H12 was similar to that of I and II and the variations were in accord with those predicted on the basis of the effect of Me on the indole system. The notable modifications produced by the introduction of NO2 groups into the aromatic ring varied according to the position and IV and VI showed certain similarities, though all, with the exception of V, showed bathochromic shifts. The shift was particularly pronounced with VII and the results in general confirmed the hypothesis of significant effects of the NO2 group on the conjugation of the 2 nuclei, with participation of the hetero atom.
Gazzetta Chimica Italiana published new progress about 20538-12-9. 20538-12-9 belongs to indole-building-block, auxiliary class Indole,Ester,Ether, name is Ethyl 7-methoxy-1H-indole-2-carboxylate, and the molecular formula is C12H13NO3, Category: indole-building-block.
Referemce:
https://www.nature.com/articles/s41429-020-0333-2,
Preparation of Indole Containing Building Blocks for the Regiospecific Construction of Indole Appended Pyrazoles and Pyrroles