Abstract
Full and unambiguous assignment of all 1H- and 13C-NMR resonances of the isomers due to restricted C-N amide bond rotation of N-formyl-o-toluidine and N,N’-bis-formyl-o-tolidine in DMSO-d6 is reported. The cis-isomer predominates in the equilibrium mixture of both compounds as 1D-NOE difference experiments show.
Highlights
The Madelung indole synthesis is a method for producing these heterocycles via a base-catalyzed thermal cyclization of N-acyl-o-toluidides [1,2]
The preferred conformation solution of N-formyl-o-toluidine (2) in DMSO-d6 is the cis one, based on the fact that the 1H-NMR signals for the N-H and the formyl proton of the major isomer present appear as singlets at δ = 9.84 and 8.57 respectively, i.e. with a coupling constant J≈ 0, whereas the signals corresponding to the minor isomer are present as doublets at δ = 10.01 and 8.66 respectively, showing a coupling constant J ≈ 10.63 Hz, i.e. in the latter isomer a coupling constant of greater magnitude is observed, which is in accordance with the observation made for formamide [20a, 21]
In the case of the N-methyl, N-ethyl- and N-tert-butylformamide signals for the formyl protons of the cis isomer (d, J = 0.3-2 Hz) as well as of trans isomer (d, J = 12 Hz) could be clearly observed in the 1H-NMR spectra (CDCl3) which coincides again with the observation made for formamide
Summary
The Madelung indole synthesis is a method for producing these heterocycles via a base-catalyzed thermal cyclization of N-acyl-o-toluidides [1,2]. In the corresponding 1H-NMR spectrum two methyl signals are found at δ = 2.79 and 2.94, together with a singlet at δ = 8.0 for the formyl proton.
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