*4.5. (*±*)-(3R,4S)-2-Ethyl-1-oxo-4-(4-phenyl-1H-1,2,3-triazol-1-yl)-3-(p-tolyl)-1,2,3,4 tetrahydroisoquinoline-4-carboxylic Acid (***18***)*

The diacid **15** (50 mg, 0.15 mmol) was dissolved in DMF (0.5 mL, dry) in a screw-cap, and DCC (1.1 equiv.) was added with stirring. After 3 h, *N*-(4-methylbenzylidene)ethanamine (1.1 equiv.) was added, and the reaction mixture was kept for a day at room temperature. The solution was then filtered through celite, EtOAc (15 mL) and 10 mL of brine were added to the filtrate, the precipitate formed was filtered off, and then organic layer of the filtrate was washed with brine (10 mL × 3), dried over sodium sulfate and evaporated. The residue was treated with diethyl ether (1 mL), after which pentane (3 mL) was added and the solid was thoroughly ground. After cooling to −20 ◦C for 20 min, the liquid was decanted. The resulting solid was dried in vacuo to give pure title compound. The substance undergoes decarboxylation easily and is therefore unstable in solutions even at room temperature.

Yield 35 mg, 50%. Colorless amorphous solid. 1H NMR (400 MHz, DMSO-*d*6) *δ* 14.25 (s, 1H), 8.83–8.72 (m, 1H), 8.17–8.07 (m, 1H), 7.88–7.79 (m, 3H), 7.75–7.62 (m, 2H), 7.50–7.40 (m, 2H), 7.39–7.31 (m, 1H), 7.16–6.90 (m, 4H), 6.05 (s, 1H), 3.69 (dq, *J* = 14.0, 7.0 Hz, 1H), 3.18 (dq, *J* = 14.0, 7.0 Hz, 1H), 2.24 (s, 3H), 0.90 (t, *J* = 7.0 Hz, 3H). 13C NMR (101 MHz, DMSO-*d*6) *δ* 167.7, 161.6, 146.2, 138.4, 133.3, 132.4, 131.5, 130.9, 130.4, 130.0, 129.4, 129.4, 128.8, 128.5, 127.9, 125.6, 122.1, 72.2, 65.6, 42.1, 21.1, 13.3. HRMS (ESI/Q-TOF) *m*/*z*: [M + Na+] <sup>+</sup> Calcd for C27H24N4O3Na+ 475.1741; Found 475.1730.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27238462/s1. Copies of NMR and HRMS spectra. *X*-ray data [23–25].

**Author Contributions:** Conceptualization, O.B. and D.D.; methodology, O.B. and D.D.; investigation, N.M. and A.K.; data curation, O.B.; writing—original draft preparation, M.K.; writing—review and editing, O.B. and D.D.; supervision, M.K.; funding acquisition, O.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** We gratefully acknowledge the financial support from the Russian Foundation for Basic Research (grant# 20-03-00922) and Megagrant of the Government of Russian Federation (# 075-15-2021-637).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data available from the corresponding authors upon reasonable request.

**Acknowledgments:** We thank the Research Centre for Magnetic Resonance and the Center for Chemical Analysis and Materials Research and the Centre for *X*-ray Diffraction Methods of Saint Petersburg State University Research Park for obtaining the analytical data.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

**Sample Availability:** Samples of the compounds **9** are available from the authors upon reasonable request.
