A Sustainable Approach for Synthesizing (R)-4-Aminopentanoic Acid From Levulinic Acid Catalyzed by Structure-Guided Tailored Glutamate Dehydrogenase

In this study, a novel enzymatic approach to transform levulinic acid (LA), which can be obtained from biomass, into value-added (R)-4-aminopentanoic acid using an engineered glutamate dehydrogenase from Escherichia coli (EcGDH) was developed. Through crystal structure comparison, two residues (K116 and N348), especially residue 116, were identified to affect the substrate specificity of EcGDH. After targeted saturation mutagenesis, the mutant EcGDHK116C, which was active toward LA, was identified. Screening of the two-site combinatorial saturation mutagenesis library with EcGDHK116C as positive control, the kcat/Km of the obtained EcGDHK116Q/N348M for LA and NADPH were 42.0- and 7.9-fold higher, respectively, than that of EcGDHK116C. A molecular docking investigation was conducted to explain the catalytic activity of the mutants and stereoconfiguration of the product. Coupled with formate dehydrogenase, EcGDHK116Q/N348M was found to be able to convert 0.4 M LA by more than 97% in 11 h, generating (R)-4-aminopentanoic acid with >99% enantiomeric excess (ee). This dual-enzyme system used sustainable raw materials to synthesize (R)-4-aminopentanoic acid with high atom utilization as it utilizes cheap ammonia as the amino donor, and the inorganic carbonate is the sole by-product.

Catalytic Asymmetric β-Oxygen Elimination

A catalytic enantioselective β-O-elimination reaction is reported in the form of a zirconium-catalyzed asymmetric opening of meso-ketene acetals. Furthermore, a regiodivergent β-O-elimination is demonstrated. The reaction proceeds under mild conditions, at low catalyst loadings, and produces chiral monoprotected 1,2-diol building blocks in good yield and enantiomeric excess. The combination with a Mitsunobu reaction then gives access to all 1,2-diol stereoisomers and trans-1,2-aminoalcohols in high enantiomeric purity. A stereochemical analysis supported by DFT calculations reveals that a high selectivity in the hydrozirconation step is also important for achieving high enantioselectivity, although it does not constitute the asymmetric step. This insight is crucial for the future development of related asymmetric β-elimination reactions.

Engineering of Reductive Aminases for Asymmetric Synthesis of Enantiopure Rasagiline

Reductive aminases (RedAms) for the stereoselective amination of ketones represent an environmentally benign and economically viable alternative to transition metal–catalyzed asymmetric chemical synthesis. Here, we report two RedAms from Aspergillus calidoustus (AcRedAm) and bacteria (BaRedAm) with NADPH-dependent features. The enzymes can synthesize a set of secondary amines using a broad range of ketone and amine substrates with up to 97% conversion. To synthesize the pharmaceutical ingredient (R)-rasagiline, we engineered AcRedAm through rational design to obtain highly stereoselective mutants. The best mutant Q237A from AcRedAm could synthesize (R)-rasagiline with >99% enantiomeric excess with moderate conversion. The features of AcRedAm and BaRedAm highlight their potential for further study and expand the biocatalytic toolbox for industrial applications.

Novel Bispidine-Monoterpene Conjugates—Synthesis and Application as Ligands for the Catalytic Ethylation of Chalcones

A number of new chiral bispidines containing monoterpenoid fragments have been obtained. The bispidines were studied as ligands for Ni-catalyzed addition of diethylzinc to chalcones. The conditions for chromatographic analysis by HPLC-UV were developed, in which the peaks of the enantiomers of all synthesized chiral products were separated, which made it possible to determine the enantiomeric excess of the resulting mixture. It was demonstrated that bispidine-monoterpenoid conjugates can be used as the ligands for diethylzinc addition to chalcone C=C double bond but not as inducers of chirality. Besides products of ethylation, formation of products of formal hydrogenation of the chalcone C=C double bond was observed in all cases. Note, that this formation of hydrogenation products in significant amounts in the presence of such catalytic systems was found for the first time. A tentative scheme explaining the formation of all products was proposed.

Steps toward Rationalization of the Enantiomeric Excess of the Sakurai–Hosomi–Denmark Allylation Catalyzed by Biisoquinoline N,N’-Dioxides Using Computations

Allylation reactions of aldehydes are chemical transformations of fundamental interest, as they give direct access to chiral homoallylic alcohols. In this work, we focus on the full computational characterization of the catalytic activity of substituted biisoquinoline-N,N’-dioxides for the allylation of 2-naphthaldehyde. We characterized the structure of all transition states as well as identified the π stacking interactions that are responsible for their relative energies. Motivated by disagreement with the experimental results, we also performed an assessment of 34 different density functional methods, with the goal of assessing DFT as a general tool for understanding this chemistry. We found that the DFT results are generally consistent as long as functionals that correctly account for dispersion interactions are used. However, agreement with the experimental results is not always guaranteed. We suggest the need for a careful synergy between computations and experiments to correctly interpret the data and use them as a design tool for new and improved asymmetric catalysts.

Biocatalytic Reductive Amination by Native Amine Dehydrogenases to Access Short Chiral Alkyl Amines and Amino Alcohols

Small optically active molecules, and more particularly short-chain chiral amines, are key compounds in the chemical industry and precursors of various pharmaceuticals. Their chemo-biocatalytic production on a commercial scale is already established, mainly through lipase-catalyzed resolutions leading to ChiPros™ products among others. Nevertheless, their biocatalytic synthesis remains challenging for very short-chain C4 to C5 amines due to low enantiomeric excess. To complement the possibilities recently offered by transaminases, this work describes alternative biocatalytic access using amine dehydrogenases (AmDHs). Without any protein engineering, some of the already described wild-type AmDHs (CfusAmDH, MsmeAmDH, MicroAmDH, and MATOUAmDH2) were shown to be efficient for the synthesis of hydroxylated or unfunctionalized small 2-aminoalkanes. Conversions up to 97.1% were reached at 50 mM, and moderate to high enantioselectivities were obtained, especially for (S)-1-methoxypropan-2-amine (98.1%), (S)-3-aminobutan-1-ol (99.5%), (3S)-3-aminobutan-2-ol (99.4%), and the small (S)-butan-2-amine (93.6%) with MsmeAmDH. Semi-preparative scale-up experiments were successfully performed at 150 mM substrate concentrations for the synthesis of (S)-butan-2-amine and (S)-1-methoxypropan-2-amine, the latter known as “(S)-MOIPA”. Modeling studies provided some preliminary results explaining the basis for the challenging discrimination between similarly sized substituents in the active sites of these enzymes.

Enantioselective synthesis of 6-methyloctanal and 8-methyldecanal, the characteristic aroma components in yuzu Citrus junos, and analysis of their enantiomeric compositions in yuzu essential oil

6-Methyloctanal and 8-methyldecanal are the characteristic aroma components of yuzu Citrus junos. However, their absolute configurations and enantiomeric compositions in yuzu essential oil have not been analyzed. A concise enantioselective synthesis of both aldehydes was successfully carried out to determine their absolute configurations and enantiomeric compositions. Both aldehydes in yuzu essential oil were found to be (S)-form with high enantiomeric excess.

Assignment-free chirality detection in unknown samples via microwave three-wave mixing

Straightforward identification of chiral molecules in multi-component mixtures of unknown composition is extremely challenging. Current spectrometric and chromatographic methods cannot unambiguously identify components while the state of the art spectroscopic methods are limited by the difficult and time-consuming task of spectral assignment. Here, we introduce a highly sensitive generalized version of microwave three-wave mixing that uses broad-spectrum fields to detect chiral molecules in enantiomeric excess without any prior chemical knowledge of the sample. This method does not require spectral assignment as a necessary step to extract information out of a spectrum. We demonstrate our method by recording three-wave mixing spectra of multi-component samples that provide direct evidence of enantiomeric excess. Our method opens up new capabilities in ultrasensitive phase-coherent spectroscopic detection that can be applied for chiral detection in real-life mixtures, raw products of chemical reactions and difficult to assign novel exotic species.

Enantioselective Synthesis, Tautomerism, Molecular Docking And Biological Evaluation of 4-Hydroxy-3-(3-Oxo-1-Phenylbutyl)-2H-Chromen-2-One Analogues

Keeping in view the aim of better alternatives of 4-Hydroxy-3-(3-oxo-1-phenylbutyl)-2H-chromen-2-one (warfarin), eleven analogs of warfarin have been synthesized with the goal to increase enantioselectivity of (S) enantiomer by using appropriate catalyst and minimize tautomerism by replacing methyl group of the side chain with aryl group. There are many reports of the serious complications with warfarin use, which are associated with the tautomeric forms of warfarin. The key step was highly enantioselective Michael addition of variously substituted chalcone and 4-hydroxycoumarin by using cinchona based 9-amino-9-deoxyepiquinine as chiral catalyst. Synthesized compounds were characterized by IR, 1HNMR,13CNMR, EIMS and CD studies. Enantiomeric excess (%ee) was determined by chiral HPLC which was upto 98%. Synthesized analogues were screened for anticoagulant, antibacterial and antifungal activities. In-vitro anticoagulant activity was evaluated by plasma recalcification time (PRT) method and out of eleven, ten synthesized compounds showed improved IC50 values as compaired to IC50 values of standard drug warfarin. Compound 4 showed 68.25% inhibation against staphylococcus aureus and compound 7 showed 68% inhibation against bacillus subtillis, gram positive strains of bacteria, compound 6 shows 70% inhibation against fungal strain candida albicans. Furthermore, molecular docking studies were carried out with Vitamin K1 epoxide reductase VKOR1 receptor 3kp9, a potential target of warfarin for anticoagulant activity.