Catalysis of Organic..

50Preparation of 4-Aminocarboxylic Acidin the ester group appeared at δ=4.1 ppm. Signals of the -CH 2 - protons in thecyclohexane ring originating from axial hydrogen atoms are at δ=1.4 ppm, whereasthose of equatorial atoms are at δ=1.6 ppm. The proton next to ester group gives asignal with a chemical shift of δ=2.43 ppm. The nitrogen atom of the amino groupshifted the signal of nearby proton in the cyclohexane ring to δ=2.84 ppm.Figure 2 shows the 1 H-NMR spectrum of the mixture of cis- and trans-4-aminocyclohexane carboxylic acid ethyl ester. The assignation of the signals ofprotons in the ester group and those of the -CH 2 - groups in the cyclohexane ring areas given above. However, the signals belonging to the protons next to the ester andamino groups in the trans isomer appeared at lower chemical shifts, δ=2.19 ppm andδ=2.64 ppm, respectively, than those of the cis isomer.Figure 1 NMR spectrum of cis-4-aminocyclohexane carboxylic acid ethyl ester.The 1 H-NMR spectrum of trans-4-aminocyclohexane carboxylic acid ethyl ester isenriched in a mixture containing ca. 10 % of the cis isomer is seen in Figure 3. Thesignals belonging to the protons next to the ester and amino groups in the transisomer appeared at chemical shifts of δ=2.20 ppm and δ=2.65 ppm, respectively. Theintensity of signals in the NMR spectra allows estimating the ratio of stereoisomers.