# Aromatic Amines in Organic Synthesis Part III; \(p\)-Aminocinnamic Acids and Their Methyl Esters
## 2 Results and Discussion  
The amine methyl esters of cinnamic acid derivatives were prepared according to the synthetic routes outlined in Figure 1.  
In the first method (Method A), the Knoevenagel condensation was used, and the corresponding cinnamic acids were obtained from amine derivatives of benzoic or naphthoic aldehydes. Six of these acids (2c, 2d, 2e, 2f, 2g, and 2k) have not been described in the literature yet. The reaction yield of cinnamic acid synthesis varies between 58% and 88%. A significantly lower yield of 29% was obtained for the 4-(dimethylamino)-2,6-dimethylcinamic acid, probably due to steric hindrance. The purity of the synthesized acids was very high, and they were used in the next stage without further purification. In the second stage, selected cinnamic acids were converted into methyl esters. Four of these compounds (3h, 3i, 3l, 3n) are new and have not been described in the literature so far. In search of the most effective esterification method, the process was carried out using three different reagents, i.e., thionyl chloride in methanol (Method B), sulfuric acid in methanol (Method C), and dimethyl sulfate in acetone (Method D). All these methods  
Figure 1: Synthesis of the derivatives of \(p\)-aminocinnamic acids and their esters. proved to be relatively effective, with reaction yields of 65-93% and high-purity products. The best, both in terms of the ease of carrying out the reaction (room temperature, no volatile by-products) and the highest yield, was the reaction with dimethyl sulfate (Method D). The yields of the reaction were greater than 75%, and the obtained esters were of high purity. The structure and purity of all the synthesized acids and esters were confirmed by spectroscopic methods, namely \({}^{1}\)H NMR, \({}^{13}\)C NMR, and IR (see spectra in Supplementary File), and by elemental analysis. The NMR spectra showed all signals from protons and carbons present in the product molecules, as described in the Experimental part. As can be seen, the proton signal of the carboxyl group occurs at about 12 ppm, whereas the characteristic signals from the protons of the methine group are in the range of 8.3 to 5.9 ppm. The coupling constant of these protons (\({}^{3}\)J\({}_{\rm H,H}\) = 15.8 Hz) indicates the _trans_ configuration. The signal from the carbon of the carboxyl group is in the range of 168-169 ppm and is shifted by about 1-2 ppm in relation to the analogous benzoic acids [39]. After the esterification, the proton signal of the carboxyl group disappears, while the methyl signal appears at approx. 3.7 ppm. The analysis of the IR spectra confirms the presence of the carboxyl group in the tested acids thanks to the characteristic, strong signal at frequencies from 1653 cm\({}^{-1}\) (2m) to 1704 cm\({}^{-1}\) (2j). In most cases, these values are 5-20 cm\({}^{-1}\) higher than for analogous benzoic acids [39]. After esterification, this signal shifts to the range of 1702-1716 cm\({}^{-1}\). Figure 2 shows the \({}^{1}\)H NMR spectra of 3-(1-methyl-1,2,3,4-tetrahydroroquinolin-6-yl)acrylic acid (2l) in DMSO-\(d_{6}\), its methyl ester, and the analogous benzoic acid to illustrate the shift in signal position associated with the elongation of the \(\pi\) electron system. Comparing these three spectra, the position of the proton signal of the carboxyl group changes little. For both the benzoic acid and the cinnamic acid derivative, it is at approx. 11.9 ppm. The protons of the methine group give signals at 7.41 and 6.15 ppm, with a coupling constant of approximately 16 Hz, indicating a trans configuration. Moreover, there is a visible shift in the signals coming from the protons of the aromatic ring, located in the _ortho_ position in relation to the methine group, by about 0.2-0.4 ppm towards lower \(\delta\) values (they are shielded more strongly than in benzoic acid).