Synthesis and applications of carbohydrate based chiral ionic liquids as chiral recognition agents and organocatalysts

Preparation of amino acid chiral ionic liquid and visual chiral recognition of glutamine and phenylalanine enantiomers

In this paper, the amino acid chiral ionic liquid (AACIL) was prepared with L-phenylalanine and imidazole. It was characterized by CD, FT-IR, 1H NMR, and 13C NMR spectrum. The chiral recognition sensor was constructed with AACIL and Cu(II), which exhibited different chiral visual responses (solubility or color difference) to the enantiomers of glutamine (Gln) and phenylalanine (Phe). The effects of solvent, pH, time, temperature, metal ions, and other amino acids on visual chiral recognition were optimized. The minimum concentrations of Gln and Phe for visual chiral recognition were 0.20 mg/ml and 0.28 mg/ml, respectively. The mechanism of chiral recognition was investigated by FT-IR, TEM, SEM, TG, XPS, and CD. The location of the host-guest inclusion or molecular placement has been conformationally searched based on Gaussian 09 software.

Expeditious Discovery of Small-Molecule Thermoresponsive Ionic Liquid Materials: A Review

Ionic liquids (ILs) are a class of low-melting molten salts (<100 °C) constituted entirely of ions, and their research has gained tremendous attention in line with their remarkably growing applications (>124,000 publications dated 30 August 2023 from the Web of ScienceTM). In this review, we first briefly discussed the recent developments and unique characteristics of ILs and zwitterionic liquids (ZILs). Compared to molecular solvents and other conventional organic compounds, (zwitter) ionic liquids carry negligible volatility and are potentially recyclable and reusable. For structures, both ILs and ZILs can be systematically tailor-designed and engineered and are synthetically fine-tunable. As such, ionic liquids, including chiral, supported, task-specific ILs, have been widely used as powerful ionic solvents as well as valuable additives and catalysts for many chemical reactions. Moreover, ILs have demonstrated their value for use as polymerase chain reaction (PCR) enhancers for DNA amplification, chemoselective artificial olfaction for targeted VOC analysis, and recognition-based affinity extraction. As the major focus of this review, we extensively discussed that small-molecule thermoresponsive ILs (TILs) and ZILs (zwitterionic TILs) are new types of smart materials and can be expeditiously discovered through the structure and phase separation (SPS) relationship study by the combinatorial approach. Using this SPS platform developed in our laboratory, we first depicted the rapid discovery of N,N-dialkylcycloammonium and 1,3,4-trialkyl-1,2,3-triazolium TILs that concomitantly exhibited LCST (lower critical solution temperature) phase transition in water and displayed biochemically attractive Tc values. Both smart IL materials were suited for applications to proteins and other biomolecules. Zwitterionic TILs are ZILs whose cations and anions are tethered together covalently and are thermoresponsive to temperature changes. These zwitterionic TIL materials can serve as excellent extraction solvents, through temperature change, for biomolecules such as proteins since they differ from the common TIL problems often associated with unwanted ion exchanges during extractions. These unique structural characteristics of zwitterionic TIL materials greatly reduce and may avoid the denaturation of proteins under physiological conditions. Lastly, we argued that both rational structural design and combinatorial library synthesis of small-molecule TIL materials should take into consideration the important issues of their cytotoxicity and biosafety to the ecosystem, potentially causing harm to the environment and directly endangering human health. Finally, we would concur that before precise prediction and quantitative simulation of TIL structures can be realized, combinatorial chemistry may be the most convenient and effective technology platform to discover TIL expeditiously. Through our rational TIL design and combinatorial library synthesis and screening, we have demonstrated its power to discover novel chemical structures of both TILs and zwitterionic TILs. Undoubtedly, we will continue developing new small-molecule TIL structures and studying their applications related to other thermoresponsive materials.

Synthesis of the Three Most Expensive l-Hexose Thioglycosides from d-Glucose

The biologically important l-hexoses, which are less widespread than d-hexoses, cannot be obtained from natural sources or can only be extracted very costly. Due to the complexity of their synthesis, their commercially available derivatives (which are sold mostly in free form) are also very expensive, which is further exacerbated by the current rapid rise in prices. In the present work, starting from the cheapest d-hexose, d-glucose, using inexpensive and readily available chemicals, a reaction pathway was developed in which the three most expensive l-hexoses (l-idose, l-altrose, and l-talose) were successfully prepared in orthogonally protected thioglycoside form, ready for glycosylation. The l-ido and l-talo derivatives were synthesized by C-5 epimerization of the corresponding 5,6-unsaturated thioglycosides. From the l-ido derivatives, the orthogonally protected thioglycosides of l-altrose were then prepared by C-4 epimerization. Different approaches to the preparation of the key intermediates, 5,6-unsaturated thioglycoside derivatives, were systematically investigated in the presence of various protecting groups (ether and ester) and using commercially available reagents.

Chiral Porous Carbon Surfaces for Enantiospecific Synthesis

Chiral surfaces, developed in the last decade, serve as media for enantioselective chemical reactions. Until today, they have been based mostly on developments in silica templating, and are made mainly from imprints of silicate materials developed a long time ago. Here, a chiral porous activated carbon surface was developed based on a chiral ionic liquid, and the surface chemistry and pore structure were studied to lay a new course of action in the field. The enantioselectivities of surfaces are examined by using variety of methods such as circular dichroism, linear sweep voltammetry and catalysis. These techniques revealed a 28.1% preference for the D enantiomer of the amino acid proline, and linear sweep voltammetry confirmed chirality recognition by another probe. An aldol surface chiral catalytic reaction was devised and allowed to determine the root of the enantiomeric excess. These results affirm the path toward a new type of chiral surface.

Synthesis of Four Orthogonally Protected Rare l-Hexose Thioglycosides from d-Mannose by C-5 and C-4 Epimerization

l-Hexoses are important components of biologically relevant compounds and precursors of some therapeuticals. However, they typically cannot be obtained from natural sources and due to the complexity of their synthesis, their commercially available derivatives are also very expensive. Starting from one of the cheapest d-hexoses, d-mannose, using inexpensive and readily available chemicals, we developed a reaction pathway to obtain two orthogonally protected l-hexose thioglycoside derivatives, l-gulose and l-galactose, through the corresponding 5,6-unsaturated thioglycosides by C-5 epimerization. From these derivatives, the orthogonally protected thioglycosides of further two l-hexoses (l-allose and l-glucose) were synthesized by C-4 epimerization. The preparation of the key intermediates, the 5,6-unsaturated derivatives, was systematically studied using various protecting groups. By the method developed, we are able to produce highly functionalized l-gulose derivatives in 9 steps (total yields: 21-23%) and l-galactose derivatives in 12 steps (total yields: 6-8%) starting from d-mannose.

Recent Advances in Organocatalyzed Asymmetric Reduction of Prochiral Ketones: An Update

Chiral alcohols are important synthetic intermediates and building blocks for the synthesis of drugs, agrochemicals, and natural products. Asymmetric reduction of prochiral ketones has been the most investigated method for accessing chiral alcohols. In this regard, organocatalyzed asymmetric reduction, as a complementary method to transition-metal- and enzyme-catalyzed reactions, has attracted tremendous interest in the past decades due to the reactions with such catalysts being metal-free and easy to operate, and principally, the ease of recovery and the ability to reuse the catalysts. Following up on a comprehensive overview on organocatalyzed asymmetric reductions of prochiral ketones in early 2018, this short review is intended to summarize the recent progress in this area from the beginning of 2018 until the end of August 2021.

1 Introduction

2 Boron-Based Chiral Organocatalysts

2.1 Boron-Containing Chiral Schiff Base Catalysts

2.2 Chiral Alpine-Borane Catalysts

2.3 Boron-Containing Chiral Frustrated Lewis Pair Catalysts

2.4 Chiral Borate Ester–Amine Complex Catalysts

3 Phosphorus-Based Chiral Organocatalysts

3.1 Chiral Phosphoric Acid Organocatalysts

3.2 Chiral Phosphinamide and Phosphoramide Organocatalysts

4 Chiral Ionic Liquid Organocatalysts

5 Chiral-Oxazoline-Based Organocatalysts

6 Conclusion and Outlook

Screening of simple carbohydrates as a renewable organocatalyst for the efficient construction of 1,3-benzoxazine scaffold

A convenient protocol for the two component preparation of 1,3-benzoxazines by using several protected and unprotected carbohydrate molecules as organocatalysts have been developed which is broadly applicable to condensation reaction between variety of Mannich bases and paraformaldehyde. This study revealed that fructose have much higher catalytic activity than the other carbohydrates and can be an alternative to metal-containing catalysts as a green renewable organocatalyst for efficient and rapid construction of 1,3-benzoxazine skeleton. In this context, 21 benzoxazine compounds were successfully synthesized and spectral characterizations of these compounds were carried out by spectroscopic methods and elemental analysis. Furthermore, density functional theory (DFT) calculations have been performed to study the detailed mechanism of organocatalyst assisted synthesis of the benzoxazine monomers. The results obtained from these calculations showed that the more realistic reaction pathway involves formation of a phenolate based intermediate which loses a water molecule to form benzenaminium ion. Subsequently, this ion provides the formation of the corresponding benzoxazines with good yields through the intramolecular ring closure step.

Recent advances of the ionic chiral selectors for chiral resolution by chromatography, spectroscopy and electrochemistry

Ionic chiral selectors have been received much attention in the field of asymmetric catalysis, chiral recognition, and preparative separation. It has been shown that the addition of ionic chiral selectors can enhance the recognition efficiency dramatically due to the presence of multiple intermolecular interactions, including hydrogen bond, π-π interaction, van der Waals force, electrostatic ion-pairing interaction, and ionic-hydrogen bond. In the initial research stage of the ionic chiral selectors, most of work center on the application in chromatographic separation (capillary electrophoresis, high-performance liquid chromatography, and gas chromatography). Differently, more and more attention has been paid on the spectroscopy (nuclear magnetic resonance, fluorescence, ultraviolet and visible absorption spectrum, and circular dichroism spectrum) and electrochemistry in recent years. In this tutorial review as regards the ionic chiral selectors, we discuss in detail the structural features, properties, and their application in chromatography, spectroscopy, and electrochemistry.

Sugar-Based Ionic Liquids: Multifaceted Challenges and Intriguing Potential

Carbohydrates represent a promising option in transitioning from oil-based chemical resources to renewable ones, with the goal of developing chemistries for a sustainable future. Cellulose, hemicellulose, and largely available monosaccharides already provide useful chemical building blocks, so-called platform chemicals, such as levulinic acid and hydroxymethyl furfural, as well as solvents like cyrene or gamma-valerolactone. Therefore, there is great anticipation for novel applications involving materials and chemicals derived from sugars. In the field of ionic liquids (ILs), sugar-based ILs have been overlooked for a long time, mainly on account of their multistep demanding preparation. However, exploring new strategies for accessing sugar-based ILs, their study, and their exploitation, are attracting increasing interest. This is due to the growing concerns about the negative (eco)toxicity profile of most ILs in conjunction with their non-sustainable nature. In the present review, a literature survey concerning the development of sugar-based ILs since 2011 is presented. Their preparation strategies and thermal behavior analyses, sorted by sugar type, make up the first two sections with the intention to provide the reader with a useful guide. A final overview of the potential applications of sugar-based ILs and their future perspectives complement the present analysis.

K, Rayadurgam J, Rajasekhara Reddy S. Naturally derived sugar-based ionic liquids: an emerging tool for sustainable organic synthesis and chiral recognition

In the past decade, the synthesis of sugar-based ionic liquids (SILs) from natural sugars has been described as a promising strategy.

Synthesis of glucoside-based imidazolium salts for Pd-catalyzed cross-coupling reaction in water

Sugar-based imidazolium salts (IMSs) represent an outstanding type of material making them eye-catching for a wide variety of applications. Herein, a series of glucoside-based IMSs (Glu-IMSs) combining glucoside and imidazolium head groups with different substituents were synthesized. The catalytic activities of these Glu-IMSs were evaluated by Pd-catalyzed Heck-Mizoroki and Suzuki-Miyaura reactions in water. Among them, the Glu-IMSs contain both -OH and NHCs coordination sites was found to be the most efficient ancillary ligand in comparison with other Glu-IMSs with just single NHCs coordination site. The HR-TEM analysis showed that the palladium nanoparticles stabilized by the Glu-IMSs with an average size of ~4.0 nm was formed in the reaction system, which may be act as an efficient real catalytic species. Under the optimized reaction conditions, a series of novel fluorine-cored organic small molecule functional materials were synthesized with favorable yields.

Insights into the Properties and Potential Applications of Renewable Carbohydrate-Based Ionic Liquids: A Review

Carbohydrate-derived ionic liquids have been explored as bio-alternatives to conventional ionic liquids for over a decade. Since their discovery, significant progress has been made regarding synthetic methods, understanding their environmental effect, and developing perspectives on their potential applications. This review discusses the relationships between the structural properties of carbohydrate ionic liquids and their thermal, toxicological, and biodegradability characteristics in terms of guiding future designs of sugar-rich systems for targeted applications. The synthetic strategies related to carbohydrate-based ionic liquids, the most recent relevant advances, and several perspectives for possible applications spanning catalysis, biomedicine, ecology, biomass, and energy conversion are presented herein.

Chiral Ionic Liquids: Structural Diversity, Properties and Applications in Selected Separation Techniques

Ionic liquids (ILs) are chemical compounds composed of ions with melting points below 100 °C exhibiting a design feature. ILs are commonly used as the so-called green solvents, reagents or highly efficient catalysts in varied chemical processes. The huge application potential of ionic liquids (IL) justifies the growing interest in these compounds. In the last decade, increasing attention has been devoted to the development of new methods in the synthesis of stable chiral ionic liquids (CILs) and their application in various separation techniques. The beginnings of the successful use of CILs to separate enantiomers date back to the 1990 s. Most chiral ILs are based on chiral cations or chiral anions. There is also a limited number of CILs possessing both a chiral cation and a chiral anion. Due to the high molecular diversity of both ions, of which at least one has a chiral center, we have the possibility to design a large variety of optically active structures, thus expanding the range of CIL applications. Research utilizing chiral ionic liquids only recently has become more popular. However, it is the area that still has great potential for future development. This review aimed to describe the diversity of structures, properties and examples of applications of chiral ionic liquids as new chiral solid materials and chiral components of the anisotropic environment, providing chiral recognition of enantiomeric analytes, which is useful in liquid chromatography, countercurrent chromatography and other various CIL-based extraction techniques including aqueous biphasic (ABS) extraction systems, solid-liquid two-phase systems, liquid-liquid extraction systems with hydrophilic CILs, liquid-liquid extraction systems with hydrophobic CILs, solid-phase extraction and induced-precipitation techniques developed in the recent years. The growing demand for pure enantiomers in the pharmaceutical and food industries sparks further development in the field of extraction and separation systems modified with CILs highlighting them as affordable and environmentally friendly both chiral selectors and solvents.