Styrene Oxide Isomerase Catalyzed Meinwald Rearrangement Reaction: Discovery and Application in Single-Step and One-Pot Cascade Reactions

Molecular structure characteristics and pyrolytic volatiles of hydrosoluble colored fractions from three melanoidins

The melanoidins produced by the Maillard reaction give foods a golden, brown or black color and release aromatic compounds. Investigating the molecular structure and pyrolysis characteristics of melanoidins provides insight into improving food quality. Seven hydrosoluble colored fractions were isolated from three melanoidins (glycine/glucose (Gly-Glc), proline/glucose (Pro-Glc), and phenylalanine/glucose (Phe-Glc)) using size exclusion and C18 chromatography. Fractions with lower molecular weights (Mw: 5958- 20,106 Da) exhibited deeper colors (browning index: 0.783-3.044). For melanoidins derived from polar amino acids (glycine), low-polarity fractions contributed more to color, while for those from non-polar amino acids (proline), high-polarity fractions were more significant. Mass spectrometry revealed consistent polymerization patterns involving glyoxylic acid and glyoxal across fractions (mass-to-charge differences of 74 m/z and 58 m/z). NMR analysis indicated that Pro-Glc fractions contained more glucose skeletons, whereas Phe-Glc fractions exhibited benzene-ring features. After pyrolysis, furans, pyrroles, and cyclopentenones were major volatiles in Gly-Glc fractions, with pyrroles and DDMP dominating Pro-Glc fractions, and phenyl-containing compounds prevalent in Phe-Glc fractions. These findings suggest amino acids influence both melanoidin structure and glucose-derived skeletons, providing a foundation for controlling color and aroma formation during processing such as baked foods, roasted meats, coffee, dairy products and fermented foods.

Amino Acids as Catalysts for the Regioselective Rearrangement of Epoxides into Ketones

The combination of a natural amino acid (AA) (l-Glu, l-Tyr, and l-Lys) and tetrabutylammonium iodide was efficiently employed as a catalytic framework for the Meinwald rearrangement of various epoxides into their respective ketones under microwave irradiation and solvent-free conditions. The designed catalytic system has shown a good catalytic performance in the conversion of epoxides with different functional groups. The reaction is regioselective toward ketone formation, and these have been obtained in good to excellent yields. l-Glu was found to be the most active AA for this catalytic transformation. A mechanistic proposal, supported by computational simulations, is also provided to reinforce our experimental findings. The suggested reaction mechanism was obtained by computing the minimum energy path at the density functional theory level through the climbing-image nudged elastic band method. A localized molecular orbital analysis based on intrinsic bond orbitals was also performed to better understand the catalytic role of the AA -TBAI combination along the computed reaction path. A key role of both the carboxylic moiety and the iodide anion was identified in different reaction steps of our proposed mechanism, facilitating hydride transposition to form the final product.

Review on Catalytic Meinwald Rearrangement of Epoxides

The past few decades have witnessed tremendous development within epoxides. Among the many known reactions involving epoxide, Meinwald rearrangements represent one of the most important and attractive approaches, which can transform epoxides into versatile carbonyl compounds. Given the high efficiency of this protocol, substantial efforts have been made by researchers by utilizing multiple catalyst systems. This review provides an overview of recent advances in the Meinwald rearrangement (from 2014 onward), along with detailed discussions on mechanistic insights. This review aims to highlight the importance and value of these methodologies, thereby promoting further investigation and application.

Green and Enantioselective Synthesis via Cascade Biotransformations: From Simple Racemic Substrates to High-Value Chiral Chemicals

Asymmetric synthesis of chiral chemicals in high enantiomeric excess (ee) is pivotal to the pharmaceutical industry, but classic chemistry usually requires multi-step reactions, harsh conditions, and expensive chiral ligands, and sometimes suffers from unsatisfactory enantioselectivity. Enzymatic catalysis is a much greener and more enantioselective alternative, and cascade biotransformations with multi-step reactions can be performed in one pot to avoid costly intermediate isolation and minimise waste generation. One of the most attractive applications of enzymatic cascade transformations is to convert easily available simple racemic substrates into valuable functionalised chiral chemicals in high yields and ee. Here, we review the three general strategies to build up such cascade biotransformations, including enantioconvergent reaction, dynamic kinetic resolution, and destruction-and-reinstallation of chirality. Examples of cascade transformations using racemic substrates such as racemic epoxides, alcohols, hydroxy acids, etc. to produce the chiral amino alcohols, hydroxy acids, amines, and amino acids are given. The product concentration, ee, and yield, scalability, and substrate scope of these enzymatic cascades are critically reviewed. To further improve the efficiency and practical applicability of the cascades, enzyme engineering to enhance catalytic activities of the key enzymes using the latest microfluidics-based ultrahigh-throughput screening and artificial intelligence-guided directed evolution could be a useful approach.

Structural basis of the Meinwald rearrangement catalysed by styrene oxide isomerase

Membrane-bound styrene oxide isomerase (SOI) catalyses the Meinwald rearrangement-a Lewis-acid-catalysed isomerization of an epoxide to a carbonyl compound-and has been used in single and cascade reactions. However, the structural information that explains its reaction mechanism has remained elusive. Here we determine cryo-electron microscopy (cryo-EM) structures of SOI bound to a single-domain antibody with and without the competitive inhibitor benzylamine, and elucidate the catalytic mechanism using electron paramagnetic resonance spectroscopy, functional assays, biophysical methods and docking experiments. We find ferric haem b bound at the subunit interface of the trimeric enzyme through H58, where Fe(III) acts as the Lewis acid by binding to the epoxide oxygen. Y103 and N64 and a hydrophobic pocket binding the oxygen of the epoxide and the aryl group, respectively, position substrates in a manner that explains the high regio-selectivity and stereo-specificity of SOI. Our findings can support extending the range of epoxide substrates and be used to potentially repurpose SOI for the catalysis of new-to-nature Fe-based chemical reactions.

One-pot Tandem Synthesis and Spontaneous Product Separation of N-heterocycles based on Bifunctional Small-molecule Photocatalyst

Homogeneous and heterogeneous reactions wherein the resulting products remain dissolved in solvents generally require complicated separation and purification process, despite the advantage of heterogeneous systems allowing retrieval of catalysts. Herein, we have developed an efficient approach for the one-pot tandem synthesis of quinazolines, quinazolinones and benzothiadiazine 1,1-dioxides from alcohols and amines utilizing a bifunctional bipyridinium photocatalyst with redox and Lewis acid sites using air as an oxidant. Through solvent-modulation strategy, the photocatalytic system exhibits high performance and enables most products to separate spontaneously. Consequently, the homogeneous catalyst can be reused by direct centrifugation isolation of the products. Notably, the method is also applicable to the less active substrates, such as heterocyclic alcohols and aliphatic alcohols, and thus provides an efficient and environmentally friendly photocatalytic route with spontaneous separation of N-heterocycles to reduce production costs and meet the needs of atomic economy and green chemistry.

Enzymatic reactions towards aldehydes: An overview

Many aldehydes are volatile compounds with distinct and characteristic olfactory properties. The aldehydic functional group is reactive and, as such, an invaluable chemical multi-tool to make all sorts of products. Owing to the reactivity, the selective synthesis of aldehydic is a challenging task. Nature has evolved a number of enzymatic reactions to produce aldehydes, and this review provides an overview of aldehyde-forming reactions in biological systems and beyond. Whereas some of these biotransformations are still in their infancy in terms of synthetic applicability, others are developed to an extent that allows their implementation as industrial biocatalysts.

Understanding the in silico aspects of bacterial catabolic cascade for styrene degradation

Styrene is a nonpolar organic compound used in very high volume for the industrial scale production of commercially important polymers such as polystyrene resins as well as copolymers like acrylonitrile butadiene styrene, latex, and rubber. These resins are widely used in the manufacturing of various products including single-use plastics such as disposable cups and containers, protective packaging, heat insulation, and so forth. The large-scale utilization leads to the over-accumulation of styrene waste in the environment causing deleterious health risks including cancer, neurological impairment, dysbiosis of central nervous system, and respiratory problems. To eliminate the accumulating waste. Microbial enzyme-based system represents the most environmental friendly and sustainable approach for elimination of styrene waste. However, comprehensive understanding of the enzyme-substrate interaction and associated pathways would be crucial for developing large-scale disposal systems. This study aims to understand the molecular interaction between the protein-ligand complexes of the styrene catabolic reactions by bacterial enzymes of sty operon. Molecular docking analysis for catalytic enzymes namely, styrene monooxygenase (SMO), styrene oxide isomerase (SOI), and phenylacetaldehyde dehydrogenase (PAD) of the bacterial sty operon was carried out with their individual substrates, that is, styrene, styrene oxide, and phenylacetic acid, respectively. The binding energy, amino acids forming binding cavity, and binding interactions between the protein-ligand binding sites were calculated for each case. The obtained binding energies showed a stable association of these complexes indicating the future scope of their utilization for large-scale bioremediation of styrene, and its commercially used polymers and copolymers.

Styrene Oxide Isomerase-Catalyzed Meinwald Rearrangement in Cascade Biotransformations: Synthesis of Chiral and/or Natural Chemicals

Styrene oxide isomerase (SOI) catalyzes the Meinwald rearrangement of aryl epoxides to carbonyl compounds via a 1,2-trans-shift in a stereospecific manner. A number of cascade biotransformations with SOI-catalyzed epoxide isomerization as a key step have been developed to convert readily available substrates into valuable chiral chemicals. Cascade conversion of terminal or internal alkenes into chiral acids, alcohols or amines was achieved, which involved SOI-catalyzed enantio-retentive isomerization of terminal epoxides via 1,2-H shift, or internal epoxides via 1,2-methyl shift. SOI-involved cascades were also developed to convert racemic epoxides into chiral acids or amines via dynamic kinetic resolution. Additionally, combining SOI-catalyzed isomerization with enantioselective C-C bond forming enzymes enabled the synthesis of chiral amino acids or amino alcohols from racemic epoxides. Finally, integration of SOI-involved cascades with biosynthesis pathways allowed for the direct utilization of renewable substrates for the sustainable synthesis of high-value natural chemicals such as alcohols, acids, and esters.

Widely targeted volatileomics analysis reveals the typical aroma formation of Xinyang black tea during fermentation

Xinyang black tea (XYBT) is characterized by the honey sugar-like aroma which is produced during the fermentation process. However, the formation of this typical aroma is still unclear. We here performed widely targeted volatileomics analysis combined with GC-MS and detected 116 aroma active compounds (AACs) with OAV > 1. These AACs were mainly divided into terpenoids, pyrazine, volatile sulfur compounds, esters, and aldehydes. Among them, 25 significant differences AACs (SDAACs) with significant differences in fermentation processes were identified, comprising phenylacetaldehyde, dihydroactinidiolide, α-damascenone, β-ionone, methyl salicylate, and so forth. In addition, sensory descriptions and partial least squares discriminant analysis demonstrated that phenylacetaldehyde was identified as the key volatile for the honey sugar-like aroma. We further speculated that phenylacetaldehyde responsible for the aroma of XYBT was probably produced from the degradation of L-phenylalanine and styrene. In conclusion, this study helps us better understand the components and formation mechanism of the honey sugar-like aroma of XYBT, providing new insight into improving the processing techniques for black tea quality.

Efficient chemoenzymatic synthesis of α-aryl aldehydes as intermediates in C-C bond forming biocatalytic cascades

Multi-enzyme biocatalytic cascades are emerging as practical routes for the synthesis of complex bioactive molecules. However, the relative sparsity of water-stable carbon electrophiles limits the synthetic complexity of molecules made from such cascades. Here, we develop a chemoenzymatic platform that leverages styrene oxide isomerase (SOI) to covert readily accessible aryl epoxides into α-aryl aldehydes through a Meinwald rearrangement. These unstable aldehyde intermediates are then intercepted with a C-C bond forming enzyme, ObiH, that catalyzes a transaldolase reaction with l-threonine to yield synthetically challenging β-hydroxy-α-amino acids. Co-expression of both enzymes in E. coli yields a whole cell biocatalyst capable of synthesizing a variety of stereopure non-standard amino acids (nsAA) and can be produced on gram-scale. We used isotopically labelled substrates to probe the mechanism of SOI, which we show catalyzes a concerted isomerization featuring a stereospecific 1,2-hydride shift. The viability of in situ generated α-aryl aldehydes was further established by intercepting them with a recently engineered decarboxylative aldolase to yield γ-hydroxy nsAAs. Together, these data establish a versatile method of producing α-aryl aldehydes in simple, whole cell conditions and show that these intermediates are useful synthons in C‒C bond forming cascades.

Enzyme-Catalyzed Meinwald Rearrangement with an Unusual Regioselective and Stereospecific 1,2-Methyl Shift

The Meinwald rearrangement is a synthetically useful reaction but often lacks regioselectivity and stereocontrol. A significant challenge in the Meinwald rearrangement of internal epoxides is the non-regioselective migration of different substituents to give a mixture of products. Herein, an enzyme-catalyzed regioselective and stereospecific 1,2-methyl shift in the Meinwald rearrangement of internal epoxides is reported. Styrene oxide isomerase (SOI) catalyzed the unique isomerization of internal epoxides through 1,2-methyl shift without 1,2-hydride shift to give the corresponding aldehydes and a cyclic ketone as the sole product. SOI-catalyzed isomerization showed high stereospecificity, fully retaining the stereoconfiguration. The synthetic utility of this enzymatic Meinwald rearrangement was demonstrated by its incorporation into three new types of enantioselective cascades, to convert trans-β-methyl styrenes into the corresponding R-configured alcohols, acids, or amines in high ee and yield.