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Molecules
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New Insights into the Preparation and Separation of Enantiomers
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New Insights into the Preparation and Separation of Enantiomers

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Chemistry".

Deadline for manuscript submissions: closed (31 December 2021) | Viewed by 9193

Special Issue Editor

Dr. Peter Bagi

E-Mail Website
Guest Editor
Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, Budapest, Hungary
Interests: P-stereogenic compounds; organophosphorus chemistry; optically active compounds; catalysis
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Chirality plays a major role in living processes, as individual enantiomers of a given chiral molecule may influence biological events in a different manner. Stereogenic element(s) appears in the structure of many pharmaceuticals, agrochemicals, flavors, and fragrances; thus, chiral chemistry has a strong connection with their research, development, and industrial production.

These facts underlie the importance of optically active molecules, whose preparation still represents a challenge. Consequently, many aspects of chiral chemistry attract considerable interest among industrial and academic research groups and chemists. Isolation and chemical transformation from natural products, enantioselective syntheses involving efficient (bio)catalysts, and separation racemates are the main methods for preparation of the corresponding enantiomerically pure compounds. The number and variety of these preparative methods are constantly increasing, and they are in the focus of many researchers. Stereogenic element(s) may include central chirality on either a carbon atom or heteroatom. Moreover, axial or planar chirality is becomming more and more common. Crystallographic, spectroscopic, and theoretical studies are also of particular importance for the determination of the structure and for absolute configuration. Finding a connection between the chiral structure of a molecule and its biological activity, self-disproportion of enantiomers, and the appearance of homochirality on Earth can also be considered to be hot topics.

This Special Issue is devoted to the latest advancements in the field of the preparation and separation of enantiomers. Researchers are encouraged to publish original papers, communications, and reviews covering a wide range of topics, namely:

  • Isolation and characterization of chiral molecules from natural sources
  • Enantioselective synthesis, development of enantioselective catalysts, and asymmetric biocatalysis
  • Separation of enantiomers from racemates by classical/kinetic resolution or chromatography, and racemization of undesired enantiomers
  • Self-amplification/disproportion of non-racemic mixtures
  • Biological activity of different enantiomers related to pharmaceutical and agricultural chemistry
  • Analytical chemistry of chiral molecules
  • Theoretical, chiroptical, or spectroscopic aspects of chiral chemistry
  • Industrial applications
  • Origin and evolution of homochirality in biological systems

Dr. Péter Bagi

Guest Editor

Keywords

  • chirality
  • enantioseparation
  • enantioselective synthesis
  • enantioselective catalysis
  • enantiomeric recognition
  • resolution of racemates
  • self-disproportion of enantiomers
  • chiroptical spectroscopy
  • industrial applications
  • origin of biological homochiraliry

Published Papers (4 papers)

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Research

14 pages, 2543 KiB  
Article
Experimental Determination of a Chiral Ternary Solubility Diagram and Its Interpretation in Gas Antisolvent Fractionation
by Márton Kőrösi, Csaba Varga, Péter Tóth, Noémi Buczkó, Erzsébet Varga and Edit Székely
Molecules 2023, 28(5), 2115; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules28052115 - 24 Feb 2023
Viewed by 1128
Abstract
Although crystallization has been widely applied for the enantiomeric enrichment of non-racemates both in research and in industrial applications, the physical–chemical background of chiral crystallizations is not as frequently discussed. A guide for the experimental determination of such phase equilibrium information is lacking. [...] Read more.
Although crystallization has been widely applied for the enantiomeric enrichment of non-racemates both in research and in industrial applications, the physical–chemical background of chiral crystallizations is not as frequently discussed. A guide for the experimental determination of such phase equilibrium information is lacking. In the current paper, the experimental investigation of chiral melting phase equilibria, chiral solubility phase diagrams and their application in atmospheric and supercritical carbon dioxide-assisted enantiomeric enrichment is described and compared. Benzylammonium mandelate is a racemic compound; it shows eutectic behavior when molten. A similar eutonic composition was observed in its methanol phase diagram at 1 °C. The influence of the ternary solubility plot could be unequivocally discovered in atmospheric recrystallization experiments, which proved that the crystalline solid phase and the liquid phase were in an equilibrium. The interpretation of the results obtained at 20 MPa and 40 °C, using the methanol–carbon dioxide mixture as a pseudo-component, was more challenging. Although the eutonic composition was found to be the limiting enantiomeric excess value in this purification process as well, the high-pressure gas antisolvent fractionation results were only clearly thermodynamically controlled in certain concentration ranges. Full article
(This article belongs to the Special Issue New Insights into the Preparation and Separation of Enantiomers)
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14 pages, 1511 KiB  
Article
Enantioseparation of P-Stereogenic 1-Adamantyl Arylthiophosphonates and Their Stereospecific Transformation to 1-Adamantyl Aryl-H-phosphinates
by Bence Varga, Levente Buna, Daniella Vincze, Tamás Holczbauer, Béla Mátravölgyi, Elemér Fogassy, György Keglevich and Péter Bagi
Molecules 2023, 28(4), 1584; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules28041584 - 07 Feb 2023
Viewed by 1113
Abstract
A focused library of 1-adamantyl arylthiophosphonates was prepared in racemic form. An enantioseparation method was developed for P-stereogenic thiophosphonates using (S)-1-phenylethylamine as the resolving agent. Under optimized conditions, three out of the five arylthiophosphonates were prepared in enantiopure form ( [...] Read more.
A focused library of 1-adamantyl arylthiophosphonates was prepared in racemic form. An enantioseparation method was developed for P-stereogenic thiophosphonates using (S)-1-phenylethylamine as the resolving agent. Under optimized conditions, three out of the five arylthiophosphonates were prepared in enantiopure form (ee > 99%). The subsequent desulfurization of optically active arylthiophosphonates gave the corresponding H-phosphinates without significant erosion of enantiomeric purity (ee = 95–98%). Hence, this reaction sequence can be considered an alternative method for the preparation of 1-adamantyl aryl-H-phopshinates. The absolute configuration of the (S)-1-adamantyl phenylphosphonothioic acid was assigned using single-crystal XRD and it allowed the confirmation that the removal of the P = S group proceeds with retention of configuration. The organocatalytic applicability of (S)-1-adamantyl phenylphosphonothioic acid was also evaluated as a P-stereogenic Brønsted acid. Full article
(This article belongs to the Special Issue New Insights into the Preparation and Separation of Enantiomers)
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14 pages, 1041 KiB  
Article
Synthesis of Novel Crown Ether-Squaramides and Their Application as Phase-Transfer Catalysts
by Zsuzsanna Fehér, Dóra Richter, Sándor Nagy, Péter Bagi, Zsolt Rapi, András Simon, László Drahos, Péter Huszthy, Péter Bakó and József Kupai
Molecules 2021, 26(21), 6542; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26216542 - 29 Oct 2021
Cited by 4 | Viewed by 3082
Abstract
This work presents the synthesis of six new phase-transfer organocatalysts in which the squaramide unit is directly linked to the nitrogen atom of an aza-crown ether. Four chiral skeletons, namely hydroquinine, quinine, cinchonine (cinchonas), and α-d-glucopyranoside were responsible for the asymmetric [...] Read more.
This work presents the synthesis of six new phase-transfer organocatalysts in which the squaramide unit is directly linked to the nitrogen atom of an aza-crown ether. Four chiral skeletons, namely hydroquinine, quinine, cinchonine (cinchonas), and α-d-glucopyranoside were responsible for the asymmetric construction of an all-carbon quaternary stereogenic center in α-alkylation and Michael addition reactions of malonic esters. We investigated the effects of these different chiral units and that of crown ethers with different sizes on catalytic activity and enantioselectivity. During extensive parameter investigations, both conventional and emerging green solvents were screened, providing valuable α,α-disubstituted malonic ester derivatives with excellent yields (up to 98%). Full article
(This article belongs to the Special Issue New Insights into the Preparation and Separation of Enantiomers)
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16 pages, 5075 KiB  
Article
Asymmetric Synthesis of Stereogenic Phosphorus P(V) Centers Using Chiral Nucleophilic Catalysis
by Ahmed Numan and Matthew Brichacek
Molecules 2021, 26(12), 3661; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules26123661 - 15 Jun 2021
Cited by 4 | Viewed by 3064
Abstract
Organophosphates have been widely used in agrochemistry, as reagents for organic synthesis, and in biochemistry. Phosphate mimics possessing four unique substituents, and thereby a chirality center, are useful in transition metal catalysis and as nucleotide therapeutics. The catalytic, stereocontrolled synthesis of phosphorus-stereogenic centers [...] Read more.
Organophosphates have been widely used in agrochemistry, as reagents for organic synthesis, and in biochemistry. Phosphate mimics possessing four unique substituents, and thereby a chirality center, are useful in transition metal catalysis and as nucleotide therapeutics. The catalytic, stereocontrolled synthesis of phosphorus-stereogenic centers is challenging and traditionally depends on a resolution or use of stochiometric auxiliaries. Herein, enantioenriched phosphorus centers have been synthesized using chiral nucleophilic catalysis. Racemic H-phosphinate species were coupled with nucleophilic alcohols under halogenating conditions. Chiral phosphonate products were synthesized in acceptable yields (33–95%) and modest enantioselectivity (up to 62% ee) was observed after identification of an appropriate chiral catalyst and optimization of the solvent, base, and temperature. Nucleophilic catalysis has a tremendous potential to produce enantioenriched phosphate mimics that could be used as prodrugs or chemical biology probes. Full article
(This article belongs to the Special Issue New Insights into the Preparation and Separation of Enantiomers)
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