Asymmetric Reduction of Cyclic Imines by Imine Reductase Enzymes in Non-Conventional Solvents

The first enantioselective reduction of 2-substituted cyclic imines to the corresponding amines (pyrrolidines, piperidines, and azepines) by imine reductases (IREDs) in non-conventional solvents is reported. The best results were obtained in a glycerol/phosphate buffer 1 : 1 mixture, in which heterocyclic amines were produced with full conversions (>99 %), moderate to good yields (22-84 %) and excellent S-enantioselectivities (up to >99 % ee). Remarkably, the process can be performed at a 100 mM substrate loading, which, for the model compound, means a concentration of 14.5 g L-1 . A fed-batch protocol was also developed for a convenient scale-up transformation, and one millimole of substrate 1 a was readily converted into 120 mg of enantiopure amine (S)-2 a with a remarkable 80 % overall yield. This aspect strongly contributes to making the process potentially attractive for large-scale applications in terms of economic and environmental sustainability for a good number of substrates used to produce enantiopure cyclic amines of high pharmaceutical interest.

A facile, one-pot reductive alkylation of aromatic and heteroaromatic amines in aqueous micellar media: a chemoenzymatic approach

A facile, green, selective and practical method for the catalytic N-alkylation of amines using molecular hydrogen as the reductant was developed. This procedure involves a lipase-mediated one-pot chemoenzymatic cascade wherein an amine undergoes a reductive amination with an aldehyde generated in situ. The imine formed thereby is reduced to give the corresponding amine. This process represents a convenient, environmentally benign and scalable one-pot process for the synthesis of N-alkyl amines. We report for the first time chemoenzymatic reductive alkylation in aqueous micellar media with an E-factor of 0.68.

3D printed ceramics as solid supports for enzyme immobilization: an automated DoE approach for applications in continuous flow

In recent years, 3D printing has emerged in the field of chemical engineering as a powerful manufacturing technique to rapidly design and produce tailor-made reaction equipment. In fact, reactors with complex internal geometries can be easily fabricated, optimized and interchanged in order to respond to precise process needs, such as improved mixing and increased surface area. These advantages make them interesting especially for catalytic applications, since customized structured bed reactors can be easily produced. 3D printing applications are not limited to reactor design, it is also possible to realize functional low cost alternatives to analytical equipment that can be used to increase the level of process understanding while keeping the investment costs low. In this work, in-house designed ceramic structured inserts printed via vat photopolymerization (VPP) are presented and characterized. The flow behavior inside these inserts was determined with residence time distribution (RTD) experiments enabled by in-house designed and 3D printed inline photometric flow cells. As a proof of concept, these structured inserts were fitted in an HPLC column to serve as solid inorganic supports for the immobilization of the enzyme Phenolic acid Decarboxylase (bsPAD), which catalyzes the decarboxylation of cinnamic acids. The conversion of coumaric acid to vinylphenol was chosen as a model system to prove the implementation of these engineered inserts in a continuous biocatalytic application with high product yield and process stability. The setup was further automated in order to quickly identify the optimum operating conditions via a Design of Experiments (DoE) approach. The use of a systematic optimization, together with the adaptability of 3D printed equipment to the process requirements, render the presented approach highly promising for a more feasible implementation of biocatalysts in continuous industrial processes. Graphical abstract.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s41981-021-00163-4.

Advances in deep eutectic solvents and water: applications in metal- and biocatalyzed processes, in the synthesis of APIs, and other biologically active compounds

This review highlights recent advances in metal- and biocatalyzed transformations, in the synthesis of APIs and other biologically active compounds, when employing deep eutectic solvents and water as environmentally responsible solvents.

Eco-Friendly 1,3-Dipolar Cycloaddition Reactions on Graphene Quantum Dots in Natural Deep Eutectic Solvent

Due to their outstanding physicochemical properties, the next generation of the graphene family-graphene quantum dots (GQDs)-are at the cutting edge of nanotechnology development. GQDs generally possess many hydrophilic functionalities which allow their dispersibility in water but, on the other hand, could interfere with reactions that are mainly performed in organic solvents, as for cycloaddition reactions. We investigated the 1,3-dipolar cycloaddition (1,3-DCA) reactions of the C-ethoxycarbonyl N-methyl nitrone 1a and the newly synthesized C-diethoxyphosphorylpropilidene N-benzyl nitrone 1b with the surface of GQDs, affording the isoxazolidine cycloadducts isox-GQDs 2a and isox-GQDs 2b. Reactions were performed in mild and eco-friendly conditions, through the use of a natural deep eutectic solvent (NADES), free of chloride or any metal ions in its composition, and formed by the zwitterionic trimethylglycine as the -bond acceptor, and glycolic acid as the hydrogen-bond donor. The results reported in this study have for the first time proved the possibility of performing cycloaddition reactions directly to the p-cloud of the GQDs surface. The use of DES for the cycloaddition reactions on GQDs, other than to improve the solubility of reactants, has been shown to bring additional advantages because of the great affinity of these green solvents with aromatic systems.

Deep Eutectic Solvents as Effective Reaction Media for the Synthesis of 2-Hydroxyphenylbenzimidazole-based Scaffolds en Route to Donepezil-Like Compounds

An unsubstituted 2-hydroxyphenylbenzimidazole has recently been included as a scaffold in a series of hybrids (including the hit compound PZ1) based on the framework of the acetylcholinesterase (AChE) inhibitor Donepezil, which is a new promising multi-target ligand in Alzheimer’s disease (AD) treatment. Building upon these findings, we have now designed and completed the whole synthesis of PZ1 in the so-called deep eutectic solvents (DESs), which have emerged as an unconventional class of bio-renewable reaction media in green synthesis. Under optimized reaction conditions, the preparation of a series of 2-hydroxyphenylbenzimidazole-based nuclei has also been perfected in DESs, and comparison with other routes which employ toxic and volatile organic solvents (VOCs) provided. The functionalization of the aromatic ring can have implications on some important biological properties of the described derivatives and will be the subject of future studies of structure-activity relationships (SARs).

Organocatalytic Asymmetric Conjugate Addition of Aldehydes to Maleimides and Nitroalkenes in Deep Eutectic Solvents

A chiral primary amine-salicylamide is used as an organocatalyst for the enantioselective conjugate addition of α,α-disubstituted aldehydes to maleimides and nitroalkenes. The reactions are performed in deep eutectic solvents as reaction media at room temperature, leading to the corresponding adducts with enantioselectivities up to 88% (for maleimides) and 80% (for nitroalkenes). Catalyst and solvent can be recovered and reused.

Deep Eutectic Solvents as Efficient Solvents in Biocatalysis

'Ideal' solvents in biocatalysis have to fulfill a large number of requirements, such as high substrate solubility, high enzyme activity and stability, and positive effects on reaction equilibrium. In the past decades, many enzymatic synthesis routes in water-based and nonaqueous (organic solvents, ionic or supercritical fluids) reaction media have been developed. However, no solvent meets every demand for different reaction types at the same time, and there is still a need for novel solvents suited for different reaction types and applications. Deep eutectic solvents (DESs) have recently been evaluated as solvents in different biocatalytic reactions. They can improve substrate supply, conversion, and stability. The best results were obtained when the DES is formed by the substrates of an enzymatic reaction.

The Future of Polar Organometallic Chemistry Written in Bio-Based Solvents and Water

There is a strong imperative to reduce the release of volatile organic compounds (VOCs) into the environment, and many efforts are currently being made to replace conventional hazardous VOCs in favour of safe, green and bio-renewable reaction media that are not based on crude petroleum. Recent ground-breaking studies from a few laboratories worldwide have shown that both Grignard and (functionalised) organolithium reagents, traditionally handled under strict exclusion of air and humidity and in anhydrous VOCs, can smoothly promote both nucleophilic additions to unsaturated substrates and nucleophilic substitutions in water and other bio-based solvents (glycerol, deep eutectic solvents), competitively with protonolysis, at room temperature and under air. The chemistry of polar organometallics in the above protic media is a complex phenomenon influenced by several factors, and understanding its foundational character is stimulating in the perspective of the development of a sustainable organometallic chemistry.

Deep Eutectic Solvents as Novel and Effective Extraction Media for Quantitative Determination of Ochratoxin A in Wheat and Derived Products

An unprecedented, environmentally friendly, and faster method for the determination of Ochratoxin A (OTA) (a mycotoxin produced by several species of Aspergillus and Penicillium and largely widespread in nature, in wheat and derived products) has, for the first time, been set up and validated using choline chloride (ChCl)-based deep eutectic solvents (DESs) (e.g., ChCl/glycerol (1:2) and ChCl/ urea (1:2) up to 40% (w/w) water) as privileged, green, and biodegradable extraction solvents. This also reduces worker exposure to toxic chemicals. Results are comparable to those obtained using conventional, hazardous and volatile organic solvents (VOCs) typical of the standard and official methods. OTA recovery from spiked durum wheat samples, in particular, was to up to 89% versus 93% using the traditional acetonitrile-water mixture with a repeatability of the results (RSD_r) of 7%. Compatibility of the DES mixture with the antibodies of the immunoaffinity column was excellent as it was able to retain up to 96% of the OTA. Recovery and repeatability for durum wheat, bread crumbs, and biscuits proved to be within the specifications required by the current European Commission (EC) regulation. Good results in terms of accuracy and precision were achieved with mean recoveries between 70% (durum wheat) and 88% (bread crumbs) and an RSD_r between 2% (biscuits) and 7% (bread).

Towards the development of continuous, organocatalytic, and stereoselective reactions in deep eutectic solvents

Different deep eutectic solvent (DES) mixtures were studied as reaction media for the continuous synthesis of enantiomerically enriched products by testing different experimental set-ups. L-Proline-catalysed cross-aldol reactions were efficiently performed in continuo, with high yield (99%), anti-stereoselectivity, and enantioselectivity (up to 97% ee). Moreover, using two different DES mixtures, the diastereoselectivity of the process could be tuned, thereby leading to the formation, under different experimental conditions, to both the syn- and the anti-isomer with very high enantioselectivity. The excess of cyclohexanone was recovered and reused, and the reaction could be run and the product isolated without the use of any organic solvent by a proper choice of DES components. The dramatic influence of the reaction media on the reaction rate and stereoselectivity of the process suggests that the intimate architecture of DESs deeply influences the reactivity of different species involved in the catalytic cycle.

An Expeditious and Greener Synthesis of 2-Aminoimidazoles in Deep Eutectic Solvents

A high-yield one-pot two-step synthesis of 2-aminoimidazoles (2-AI), exploiting an under-air heterocyclodehydration process between α-chloroketones and guanidine derivatives, and using deep eutectic solvents (DESs) as nonconventional, “green” and “innocent” reaction media, has been accomplished successfully. The combination of either glycerol or urea with choline chloride (ChCl) proved to be effective for decreasing the reaction time to about 4–6 h in contrast to the 10–12 h usually required for the same reaction run in toxic and volatile organic solvents and under an argon atmosphere. In addition, the use of the ChCl–urea as a DES also enables the direct isolation of triaryl-substituted 2-AI derivatives by means of a simple work-up procedure consisting in filtration and crystallization, and allows the recycle of the DES mixture. A plausible mechanism highlighting the potential role played by hydrogen bonding catalysis has also been illustrated.

Carbon–carbon bond formation in acid deep eutectic solvent: chalcones synthesis via Claisen–Schmidt reaction

Claisen–Schmidt reaction in acid deep eutectic solvent without the use of acid or basic catalyst.

Acetaldehyde: A Small Organic Molecule with Big Impact on Organocatalytic Reactions

Stereocontrolled formation of carbon-carbon and carbon-heteroatom bonds through asymmetric organocatalysis is a formidable challenge for modern synthetic chemistry. Among the most significant contributions to this field are the transformations involving the use of acetaldehyde or α-heteroatom-substituted acetaldehydes for constructing valuable synthons (e.g., amino acid derivatives and hydroxycarbonyl). In this Minireview, versatile (enantioselective) organocatalytic transformations are discussed.

Recent Advances in Biocatalytic Promiscuity: Hydrolase-Catalyzed Reactions for Nonconventional Transformations

Enzymes have emerged in recent decades as ideal catalysts for synthetic transformations under mild reaction conditions. Their capacity to accelerate a myriad of biotransformations with high levels of selectivity and broad substrate specificity including excellent atom economy has led to a current full recognition. The six classes of enzymes (oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases) possess outstanding abilities to perform specific modifications in target molecules. Nevertheless, in the last fifteen years, novel examples have appeared related to nonconventional processes catalyzed by various classes of biocatalysts. Amongst these, hydrolases have received special attention since they display remarkable activities in initially unexpected reactions such as carbon-carbon and carbon-heteroatom bond-formation reactions, oxidative processes and novel hydrolytic transformations. In this review, the main findings in this area will be disclosed, highlighting the catalytic properties of hydrolases not only to catalyze single processes but also multicomponent and tandem nonconventional reactions.

Acetaldehyde in asymmetric organocatalytic transformations

This review summarize both the recent developments in the organocatalysed use of acetaldehyde as a substrate in various organic transformations and its application in the synthesis of bioactive molecules.

Recent advances in the application of deep eutectic solvents as sustainable media as well as catalysts in organic reactions

This review highlights the recent advances using deep eutectic solvents (DESs), deep eutectic ionic liquids (DEILs), low-melting mixtures (LMMs) or low transition temperature mixtures (LTTMs) as green media as well as catalysts in organic reactions.