Green synthesis of enantiopure (S)-1-(benzofuran-2-yl)ethanol by whole-cell biocatalyst

Enzymatic Synthesis of Indole-Based Imidazopyridine using α-Amylase

The imidazo[1,2-a]pyridine scaffold has gained significant attention due to its presence as a lead structure in several commercially available pharmaceuticals like zolimidine, zolpidem, olprinone, soraprazan, etc. Further, indole-based imidazo[1,2-a]pyridine derivatives have been found interesting due to their anticancer and antibacterial activities. However, limited methods have been reported for the synthesis of indole-based imidazo[1,2-a]pyridines. In this study, we have successfully developed a biocatalytic process for synthesizing indole-based imidazo[1,2-a]pyridine derivatives using the α-amylase enzyme catalyzed Groebke-Blackburn-Bienayme (GBB) multicomponent reaction of 2-aminopyridine, indole-3-carboxaldehyde, and isocyanide. The generality and robustness of this protocol were shown by synthesizing differently substituted indole-based imidazo[1,2-a]pyridines in good isolated yields. Furthermore, to make α-amylase a reusable catalyst for GBB multicomponent reaction, it was immobilized onto magnetic metal-organic framework (MOF) materials [Fe3 O4 @MIL-100(Fe)] and found reusable up to four consecutive catalytic cycles without the significant loss in catalytic activity.

Microbial alcohol dehydrogenases: recent developments and applications in asymmetric synthesis

Alcohol dehydrogenases are a well-known group of enzymes in the class of oxidoreductases that use electron transfer cofactors such as NAD(P)+/NAD(P)H for oxidation or reduction reactions of alcohols or carbonyl compounds respectively. These enzymes are utilized mainly as purified enzymes and offer some advantages in terms of green chemistry. They are environmentally friendly and a sustainable alternative to traditional chemical synthesis of bulk and fine chemicals. Industry has implemented several whole-cell biocatalytic processes to synthesize pharmaceutically active ingredients by exploring the high selectivity of enzymes. Unlike the whole cell system where cofactor regeneration is well conserved within the cellular environment, purified enzymes require additional cofactors or a cofactor recycling system in the reaction, even though cleaner reactions can be carried out with fewer downstream work-up problems. The challenge of producing purified enzymes in large quantities has been solved in large part by the use of recombinant enzymes. Most importantly, recombinant enzymes find applications in many cascade biotransformations to produce several important chiral precursors. Inevitably, several dehydrogenases were engineered as mere recombinant enzymes could not meet the industrial requirements for substrate and stereoselectivity. In recent years, a significant number of engineered alcohol dehydrogenases have been employed in asymmetric synthesis in industry. In a parallel development, several enzymatic and non-enzymatic methods have been established for regenerating expensive cofactors (NAD+/NADP+) to make the overall enzymatic process more efficient and economically viable. In this review article, recent developments and applications of microbial alcohol dehydrogenases are summarized by emphasizing notable examples.

Optimization of asymmetric bioreduction conditions of 1-(thiophen-2-yl)ethanone by Weissella cibaria N9 using a desirability function-embedded face-centered optimization model

Prochiral ketones can be effectively bio-reduced to chiral secondary alcohols by whole-cell biocatalysts, which are possible useful precursors to synthesize physiologically active chemicals and natural products. When whole-cell biocatalysts strains are used, bioreduction process can be influenced by various cultural factors, and it is vital to optimize these factors that affect selectivity, conversion rate, and yield. In this study, Weissella cibaria N9 was used as whole-cell biocatalyst for bioreduction of 1-(thiophen-2-yl)ethanone, and cultural design factors were optimized using a desirability function-embedded face-centered optimization model. For this, effects of pH (4.5-5.5-6.5, x1), (2) temperature (25-30-35 °C, x2), (3) incubation period (24-48-72 h, x3), and (4) agitation speed (100-150-200 rpm, x4) on two response variables; (1) ee (%) and (2) cr (%) were tested. Next, desirability function-embedded face-centered optimization model revealed that a pH of 6.43, a temperature of 26.04 °C, an incubation period of 52.41 h, and an agitation speed of 150 rpm were the optimum levels and the estimated ee and cr responses were 99.31% and 98.16%, respectively. Importantly, the actual experimental ee and cr responses were similar to the estimated values indicating the capability of the offered desirability function-embedded face-centered optimization model when using the optimum cultural conditions.

Gram-scale synthesis of (S)-1-(thiophen-2-yl)ethanol in high enantiomeric purity under Enterococcus faecium BY48 biocatalysts

Sulfur-containing chiral heterocyclic secondary alcohols are relevant intermediates in the preparation of enantiomerically pure compounds endowed with biological activity. In this report, we evaluated the reductive potential of different lactic acid bacteria as whole-cell biocatalysts of the enantioselective reduction of 1-(thiophen-2-yl)ethanone (1). Enterococcus faecium BY48, isolated from boza, a cereal-based fermented beverage, was found to be the best biocatalyst in our initial investigations. Using whole-cell preparations of E. faecium BY48, we then systematically analyzed the reaction parameters (pH, incubation period, agitation speed, and temperature) to optimize the yield, the enantiomeric excess (e. e.), and the conversion leading to (S)-1-(thiophen-2-yl)ethanol [(S)-2]. The target derivative, which is a precursor in the synthesis of biologically active chiral compounds, was obtained in enantiomerically pure form, gram-scale amounts, and high yield. This is also the first report that the manufacture of (S)-2 in excellent conversion, yield, enantiopure form, and gram scale used whole-cell biocatalyst. This whole-cell E. faecium BY48 biocatalyzed reaction is a clean, environmentally friendly, and cost-effective process, representing a valuable alternative to chemical catalysis or previously reported biocatalytic procedures in the preparation of (S)-2.

Regioselective asymmetric bioreduction of trans-4-phenylbut-3-en-2-one by whole-cell of Weissella cibaria N9 biocatalyst

There is a considerable interest in the asymmetric production of chiral allylic alcohols, the main building blocks of many functional molecules. The asymmetric reduction of α,β-unsaturated ketones is difficult with traditional chemical protocols in a regioselective and stereoselective manner. In this study, the reductive capacity of whole cell of Leuconostoc mesenteroides N6, Weissella paramesenteroides N7, Weissella cibaria N9, and Leuconostoc pseudomesenteroides N13 was investigated as whole-cell biocatalysts in the enantioselective reduction of (E)-4-phenylbut-3-en-2-one (1). The biocatalytic reduction of 1 to (S,E)-4-phenylbut-3-en-2-ol ((S,E)-2) using the whole cell of W. cibaria N9 isolated from Turkish sourdough was developed in a regioselective fashion, occurring with excellent conversion and recovering the product in good yield. In biocatalytic reduction reactions, the conversion of the substrate and the enantiomeric excess (ee) of the product are significantly affected by optimization parameters such as temperature, agitation rate, pH, and incubation time. Effects of these parameters on ee and conversion were investigated comprehensively. In addition, to our knowledge, this is the first report on production of (S,E)-2 using whole-cell biocatalyst in excellent yield, conversion with enantiopure form and at gram scale. These findings pave the way for the use of whole cell of W. cibaria N9 for challenging higher substrate concentrations of different α,β-unsaturated ketones for regioselective reduction at industrial scale.

Taguchi analysis and asymmetric keto-reduction of acetophenone and its derivatives by soil filamentous fungal isolate: Penicillium rubens VIT SS1

Microbial asymmetric reduction of ketone is an efficient tool for the synthesis of chiral alcohols. This research focuses on exploring the soil fungal isolates for their ability toward the keto reduction of acetophenone and its derivatives to their corresponding chiral alcohols using growing cells. Bioreduction of acetophenone, 4-fluoro acetophenone, 4-methyl acetophenone, and 3-hydroxy acetophenone was carried out using different fungal cultures isolated from soil. Among the fungal isolates, Penicillium sp. and Aspergillus sp. showed significant bioconversion with varying enantio-selectivity. However, the Penicillium sp. has shown the maximum ability of bioreduction. The potential isolate was characterized using the internal transcribed spacer (ITS) region and found to be Penicillium rubens VIT SS1 (Genbank accession number: MK063869.1), which showed higher conversion and selectivity > 90%. The biocatalyst production and the reaction conditions were optimized using Taguchi analysis. The process conditions such as pH, temperature, media components, cosolvent, and substrate dosing were evaluated for the bioreduction of 3-hydroxy acetophenone, which is a key chiral intermediate of Phenylephrine and Rivastigmine using P. rubens VIT SS1. This study concludes about the potential of fungal cultures for sustainable synthesis of key chiral intermediates of Phenylephrine and Rivastigmine, similarly many aromatic chiral alcohols in simpler, novel, and cost-effective manner.

D-Lactic Acid as a Metabolite: Toxicology, Diagnosis, and Detection

Two enantiomers of lactic acid exist. While L-lactic acid is a common compound of human metabolism, D-lactic acid is produced by some strains of microorganism or by some less relevant metabolic pathways. While L-lactic acid is an endogenous compound, D-lactic acid is a harmful enantiomer. Exposure to D-lactic acid can happen by various ways including contaminated food and beverages and by microbiota during some pathological states like short bowel syndrome. The exposure to D-lactic acid cannot be diagnosed because the common analytical methods are not suitable for distinguishing between the two enantiomers. In this review, pathways for D-lactic acid, pathological processes, and diagnostical and analytical methods are introduced followed by figures and tables. The current literature is summarized and discussed.

Green and scalable synthesis of chiral aromatic alcohols through an efficient biocatalytic system

Chiral aromatic alcohols have received much attention due to their widespread use in pharmaceutical industries. In the asymmetric synthesis processes, the excellent performance of alcohol dehydrogenase makes it a good choice for biocatalysts. In this study, a novel and robust medium‐chain alcohol dehydrogenase RhADH from Rhodococcus R6 was discovered and used to catalyse the asymmetric reduction of aromatic ketones to chiral aromatic alcohols. The reduction of 2‐hydroxyacetophenone (2‐HAP) to (R)‐(‐)‐1‐phenyl‐1,2‐ethanediol ((R)‐PED) was chosen as a template to evaluate its catalytic activity. A specific activity of 110 U mg⁻¹ and a 99% purity of e.e. was achieved in the presence of NADH. An efficient bienzyme‐coupled catalytic system (RhADH and formate dehydrogenase, CpFDH) was established using a two‐phase strategy (dibutyl phthalate and buffer), which highly raised the tolerated substrate concentration (60 g l⁻¹). Besides, a broad range of aromatic ketones were enantioselectively reduced to the corresponding chiral alcohols by this enzyme system with highly enantioselectivity. This system is of the potential to be applied at a commercial scale.