**4. Conclusions**

In conclusion, we have found efficient and extraordinarily easy (room temperature) access to ethynylpyrroles via decarbonylation of available acylethynylpyrroles. The reaction proceeds in the MeCN-THF/*t*-BuOK system via the addition of CH2CN− anion to the carbonyl group of acylethynylpyrroles followed by *retro*-Favorsky reaction of the intermediated propargylic alcohols. Thermodynamic aspects of the intermediate alcohol decomposition have been considered in the framework of B2PLYP/6-311G\*\*//B3LYP/6- 311G\*\*+C-PCM/acetonitrile methodology. The substrate scope of the reaction includes benzoyl-, furoyl-, thenoylethynylpyrroles with alkyl, vinyl, aryl and hetaryl substituents at 1(4,5)-positions of the pyrrole ring, and methyl, benzyl, and vinyl moieties at the nitrogen atom, as well as acylethynyl derivatives of 1-methylindole and menthofuran.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules28031389/s1. Synthesis of ethynylpyrroles **4a**–**k**, ethynylindole **6**, ethynylfuran **8** (S3–S7); Synthesis of propargyl alcohols **3a**,**c**,**d**,**f** (S7–S8); Quantum chemical calculations details (S9–S12); 1H and 13C NMR spectra of synthesized compounds **3a**,**c**,**d**,**f**, **4a**–**k**, **6**, **8** (S13–S44). References [36,64–74] are cited in the Supplementary Materials.

**Author Contributions:** Conceptualization, B.A.T.; investigation, D.N.T.; writing—review and editing, D.N.T., L.N.S. and B.A.T.; writing—original draft preparation, D.N.T. and L.N.S.; formal analysis, I.A.U.; calculation part, A.M.B. and A.B.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Education and Science of Russian Federation (State Registration No. 121021000199-6).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available in the Supplementary Material.

**Acknowledgments:** This work was supported by the research project plans in the State Register of the IPC RAS no. 121021000199-6. The authors acknowledge Baikal Analytical Center for the collective use of SB RAS for the equipment. A.B.T. gratefully acknowledges Grant No. FZZE-2020- 0025 from the Ministry of Science and Higher Education of the Russian Federation. A.M.B. thanks the Irkutsk Supercomputer Center of SB RAS for providing computational resources of the HPC-cluster "Akademik V. M. Matrosov".

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of compounds **1a**–**k**, **5**, and **7** are available from the authors.

## **References**


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**Luka Ciber 1, Franc Požgan 1, Helena Brodnik 1, Bogdan Štefane 1, Jurij Svete 1, Mario Waser 2,\* and Uroš Grošelj 1,\***


**Abstract:** Ten novel bifunctional quaternary ammonium salt phase-transfer organocatalysts were synthesized in four steps from (+)-camphor-derived 1,3-diamines. These quaternary ammonium salts contained either (thio)urea or squaramide hydrogen bond donor groups in combination with either trifluoroacetate or iodide as the counteranion. Their organocatalytic activity was evaluated in electrophilic heterofunctionalizations of β-keto esters and in the Michael addition of a glycine Schiff base with methyl acrylate. α-Fluorination and chlorination of β-keto esters proceeded with full conversion and low enantioselectivities (up to 29% ee). Similarly, the Michael addition of a glycine Schiff base with methyl acrylate proceeded with full conversion and up to 11% ee. The new catalysts have been fully characterized; the stereochemistry at the C-2 chiral center was unambiguously determined.

**Keywords:** asymmetric organocatalysis; quaternary ammonium salts; phase-transfer catalysts (PTCs); camphor; camphor-derived diamines; β-keto esters; enantioselective α-fluorination; electrophilic α-chlorination; asymmetric Michael addition
