Preparation of highly enantiopure stereoisomers of 1-(2,6-dimethylphenoxy)-2-aminopropane (mexiletine)

1,3-Dianionic annulation of ketones with ketene dithioacetal: a modified route to 3-aryl/cyclopropyl-5-thiomethyl-phenols and 1-(methylthio)-9,10-dihydrophenanthren-3-ols

A simple and efficient base-mediated [3 + 3] cyclization of 1,3-dianionic ketones with 3,3-bis(methylthio)-1-arylprop-2-en-1-ones was developed to afford 3-hydroxy-biaryls, hydroxy-xylenes, and hydroxy-teraryls. Various tri- and tetra-substituted phenols were prepared from different symmetric and asymmetric ketones. The reaction of 2-(bis(methylthio)methylene)-3,4-dihydronaphthalen-1(2H)-ones with different ketones provided 1-(methylthio)-9,10-dihydrophenanthren-3-ols in very good yield. The scope of the reaction was further extended by the synthesis of cyclopropyl-functionalized phenols. One of the compounds was crystallized, and its structure was confirmed using the single-crystal X-ray approach.

In Vitro and In Vivo Metabolic Activation and Hepatotoxicity of Environmental Pollutant 2,6-Dimethylphenol

2,6-Dimethylphenol (2,6-DMP) is an environmental pollutant found in industrial wastewater. Exposure to 2,6-DMP is of increasing concern as it endangered reportedly some aquatic animals. In this study, we investigated the metabolic activation and hepatotoxicity of 2,6-DMP. 2,6-DMP was metabolized to an o-quinone methide intermediate in vitro and in vivo. The electrophilic metabolite was reactive to the sulfhydryl groups of glutathione, N-acetyl cysteine, and cysteine. NADPH was required for the formation of the reactive metabolite. The quinone methide intermediate reacted with cysteine residues to form hepatic protein adduction. A single dose of 2,6-DMP induced marked elevation of serum ALT and AST in mice. Both the protein adduction and hepatotoxicity of 2,6-DMP showed dose dependency.

Enantioselective Analysis of Mexiletine Using Chromatographic Techniques: A Review

Background:

Mexiletine belongs to the β-amino-aryl-ether group of pharmaceutical and applied in the diagnosis of antiarrhythmics, allodynia, and myotonic disorders. In its chemical structure, it possesses a chiral center and practiced in the form of the racemic mixture. The production and accessibility of mexiletine have accompanied with a meaningful development in awareness of its pharmacologic actions. But in clinical arrhythmias and binding experiments on cardiac sodium channels, the (R)-enantiomer of mexiletine is more potent than the (S)-enantiomer. Also, (S)-enantiomer is further active in the diagnosis of allodynia than the (R)-enantiomer.

Methods:

During the last two decades, chromatographic techniques such as HPLC, and GC coupled with mass spectrometry or field ionization detector was used for the stereoselective analysis of MEX enantiomers.

Results:

The direct enantioresolution deal with the use of chiral stationary phases (CSPs) with or no pre-derivatization which depend on a chromophoric entity in the racemates whereas indirect HPLC process involved the use of chiral derivatization reagents (CDR) for the synthesis of diastereomeric derivatives of racemates. Different techniques have their strengths and weaknesses.

Conclusion:

Regulation of enantiomeric purity and estimation of particular enantiomers of drug molecules stays an essential topic for therapeutic, diagnostic, and regulatory uses and to promote a precise assessment of the hazards to human health by false enantiomers. This review aims to offer a systematic survey of the analytical methods (chromatography based) used in the enantioselective analysis of MEX developed in the last two decades (the year 2000 onwards).

Characterization of the 2,6-Dimethylphenol Monooxygenase MpdAB and Evaluation of Its Potential in Vitamin E Precursor Synthesis

2,6-Dimethylphenol (2,6-DMP) is a widely used chemical intermediate whose residue has been frequently detected in the environment, posing a threat to some aquatic organisms. Microbial degradation is an effective method to eliminate 2,6-DMP in nature. However, the genetic and biochemical mechanisms of 2,6-DMP metabolism remain unknown. Mycobacterium neoaurum B5-4 is a 2,6-DMP-degrading bacterium isolated in our previous study. Here, a 2,6-DMP degradation-deficient mutant of strain B5-4 was screened. Comparative genomic, transcriptomic, gene disruption, and genetic complementation data indicated that mpdA and mpdB are responsible for the initial step of 2,6-DMP degradation in M. neoaurum B5-4. MpdAB was predicted to be a two-component flavin-dependent monooxygenase system, which shows 32% and 36% identities with HsaAB from Mycobacterium tuberculosis CDC1551. The transcription of mpdA and mpdB was substantially increased upon exposure to 2,6-DMP. Nuclear magnetic resonance analysis showed that purified 6×His-MpdA and 6×His-MpdB hydroxylated 2,6-DMP and 2,3,6-trimethylphenol (2,3,6-TMP) at the para-position using NADH and flavin adenine dinucleotide (FAD) as cofactors. The apparent K_m values of MpdAB for 2,6-DMP and 2,3,6-TMP were 0.12 ± 0.01 and 0.17 ± 0.01 mM, respectively, and the corresponding k_cat/K_m values were 4.02 and 2.84 s^-1 mM^-1, respectively. Since para-hydroxylated 2,3,6-TMP is a major precursor for vitamin E synthesis, the potential of MpdAB in vitamin E synthesis was preliminarily evaluated using whole-cell catalysis. Low expression levels of MpdA and 2,3,6-TMP cytotoxicity limited the efficiency of whole-cell catalysis. Together, this study reveals the genetic and biochemical basis for the initial step of 2,6-DMP biodegradation and provides candidate enzymes for vitamin E synthesis. IMPORTANCE Although the microbial degradation of the six isomers of dimethylphenol has been extensively studied, the genetic and biochemical mechanisms of 2,6-DMP degradation remain unclear. This study identified the genes responsible for the initial step in the 2,6-DMP catabolic pathway in M. neoaurum B5-4. Moreover, MpdAB also catalyzed the transformation of 2,3,6-TMP to 2,3,5-trimethylhydroquinone (2,3,5-TMHQ), a crucial step in vitamin E synthesis. Overall, this study provides candidate enzymes for both the bioremediation of 2,6-DMP contamination and the development of a green method to synthesize vitamin E.

Enantioselective liquid-liquid extraction of tryptophan enantiomers by a recyclable aqueous biphasic system based on stimuli-responsive polymers

The hydrophobic organic solvents/water biphasic system had been always used in the traditional enantioselective liquid-liquid extraction (ELLE). In recent years, aqueous biphasic systems (ABSs) are considered as a promising method used in the ELLE. In the present work, a recyclable ABS composed of a temperature-responsive polymer poly(MAH-β-CD-co-NIPAAm) (P_N-CD) and a pH-responsive polymer poly(AA-DMAEMA-BMA) (P_ADB) was employed in the enantioseparation of tryptophan enantiomers. The polymer P_N-CD acted as not only the phase-forming component but also the chiral selector, which can be recycled by changing the temperature. The polymer P_ADB can be used as the phase-forming component, which can also be recycled by adjusting the pH. The phase behaviors of this P_N-CD/P_ADB ABS had been studied. The influencing parameters were studied for this chiral separation process, including the polymer concentration, initial tryptophan concentration, extraction temperature, and system pH. The maximum separation factor (α) of 1.42 was obtained by one-step extraction under the optimal conditions. Meanwhile, the distribution coefficients of L-tryptophan (L-Trp) and D-tryptophan (D-Trp) were 2.79 and 1.96, respectively. This study develops a green and sustainable strategy for enantioseparation by using the ELLE.

Molecular-level insights into the self-assembly driven enantioselective recognition process

The importance of the orientation of functional groups in a chiral environment on enantioselective recognition has been demonstrated. Orientation controlled interactions of functional groups in (R)/(S)-MA lead to a visually differentiable morphology with an arginine-based gelator. The crucial role of various molecular-level interactions discriminating the enantioselective self-assembly has been established using different analytical techniques, crystal structure analysis, and DFT calculations.

Biodegradation of plastic monomer 2,6-dimethylphenol by Mycobacterium neoaurum B5-4

2,6-Dimethylphenol (2,6-DMP), an important chemical intermediate and the monomer of plastic polyphenylene oxide, is widely used in chemical and plastics industry. However, the pollution problem of 2,6-DMP residues is becoming increasingly serious, which is harmful to some aquatic animals. Microbial degradation provided an effective approach to eliminate DMPs in nature, which is considered as a prospective way to remediate DMPs-contaminated environments. But the 2,6-DMP-degrading bacteria is not available and the molecular mechanism of 2,6-DMP degradation is unclear as well. Here, a 2,6-DMP-degrading bacterium named B5-4 was isolated and identified as Mycobacterium neoaurum. M. neoaurum B5-4 could utilize 2,6-DMP as the sole carbon source for growth. Furthermore, M. neoaurum B5-4 could degrade 2,6-DMP with concentrations ranging from 1 to 500 mg L^-1. Six intermediate metabolites of 2,6-DMP were identified and a metabolic pathway of 2,6-DMP in M. neoaurum B5-4 was proposed, in which 2,6-DMP was initially converted to 2,6-dimethyl-hydroquinone and 2,6-dimethyl-3-hydroxy-hydroquinone by two consecutive hydroxylations at C-4 and γ position; 2,6-dimethyl-3-hydroxy-hydroquinone was then subjected to aromatic ring ortho-cleavage to produce 2,4-dimethyl-3-hydroxymuconic acid, which was further transformed to citraconate, and subsequently into TCA cycle. In addition, toxicity bioassay of 2,6-DMP in water using zebrafish indicates that 2,6-DMP is toxic to zebrafish and M. neoaurum B5-4 could effectively eliminate 2,6-DMP in water to protect zebrafish from 2,6-DMP-induced death. This work provides a potential strain for bioremediation of 2,6-DMP-contaminated environments and lays a foundation for elucidating the molecular mechanism and genetic determinants of 2,6-DMP degradation.

Enantiodivergent syntheses of (+)- and (-)-1-(2,6-dimethylphenoxy)propan-2-ol: A way to access (+)- and (-)-mexiletine from D-(+)-mannitol

Chiron approach was used to acquire optically pure (R)- and (S)-1-(2,6-dimethylphenoxy)propan-2-ol, immediate precursors of (S)- and (R)-mexiletines, respectively. Two different routes were followed from a D-mannitol-derived optically pure common precursor to get the enantiomeric alcohols separately. Comparison of their specific rotation values with the corresponding literature values as well as exact mirror-image relationship between their CD curves proved their high enantiopurity. These alcohols were then transformed to the corresponding amine-drugs in an efficient one-step process instead of two steps described in the literature.

A High-Throughput Amenable Colorimetric Assay for Enantioselective Screening of Nitrilase-Producing Microorganisms Using pH Sensitive Indicators

Based on the color change of an indicator due to the release of hydrogen ion from a nitrilase-catalyzed reaction, a rapid colorimetric method was established for the enantioselective screening of nitrilase-producing microorganisms. The formation of acids due to the nitrilase-mediated hydrolysis of nitriles causes a drop in the pH, which in turn results in a change of color of the solution (containing indicator) that can be observed visually. The buffer (0.01 M phosphate, pH 7.2) and indicator (Bromothymol blue, 0.01%) were selected in such a way that both have the same affinity for the released protons. The enantioselectivity of nitrilases was estimated by comparing the hydrolysis of ( R)-mandelonitrile with that of racemate under the same conditions. The method was used to screen a library of nitrilase-producing microorganisms, isolated in the authors' laboratory for their ability to enantioselectively hydrolyze mandelonitrile to mandelic acid, an important chiral building block. ( Journal of Biomolecular Screening 2003:559-565).

Synthesis of six mexiletine derivatives with isoindolines attached as potential antioxidants and their evaluation as cardioprotective agents

Some mexiletine derivatives with isoindoline based antioxidants attached have been shown to have significant cardioprotective properties.

Selective phenol methylation to 2,6-dimethylphenol in a fluidized bed of iron-chromium mixed oxide catalyst with o-cresol circulation

BACKGROUND

2,6-dimethylphenol (2,6-DMP) is a product of phenol methylation, especially important for the plastics industry. The process of phenol methylation in the gas phase is strongly exothermic. In order to ensure good temperature equalization in the catalyst bed, the process was carried out using a catalyst in the form of a fluidized bed - in particular, the commercial iron-chromium catalyst TZC-3/1.

RESULTS

Synthesis of 2,6-dimethylphenol from phenol and methanol in fluidized bed of iron-chromium catalyst was carried out and the fluidization of the catalyst was examined. Stable state of fluidized bed of iron-chromium catalyst was achieved. The measured velocities allowed to determine the minimum flow of reactants, ensuring introduction of the catalyst bed in the reactor into the state of fluidization. Due to a high content of o-cresol in products of 2,6-dimethylphenol synthesis, circulation in the technological node was proposed. A series of syntheses with variable amount of o-cresol in the feedstock allowed to determine the parameters of stationary states.

CONCLUSION

A stable work of technological node with o-cresol circulation is possible in the temperature range of350-380°C, and o-cresolin/phenolin molar ratio of more than 0.48. Synthesis of 2,6-DMP over the iron-chromium catalyst is characterized by more than 90% degree of phenol conversion. Moreover, the O-alkylation did not occur (which was confirmed by GC-MS analysis). By applying o-cresol circulation in the 2,6-DMP process, selectivity of more than 85% degree of 2,6-DMP was achieved. The participation levels of by-products: 2,4-DMP and 2,4,6-TMP were low. In the optimal conditions based on the highest yield of 2,6-DMP achieved in the technological node applying o-cresol circulation, there are 2%mol. of 2,4-DMP and 6%mol. of 2,4,6-TMP in the final mixture, whereas 2,4,6-TMP can be useful as a chain stopper and polymer's molar mass regulator during the polymerization of 2,6-DMP.

Enantioselective nitrilase from Pseudomonas putida: cloning, heterologous expression, and bioreactor studies

Nitrilases have attracted tremendous attention for the preparation of optically pure carboxylic acids. This article aims to address the production and utilization of a highly enantioselective nitrilase from Pseudomonas putida MTCC 5110 for the hydrolysis of racemic mandelonitrile to (R)-mandelic acid. The nitrilase gene from P. putida was cloned in pET 21b(+) and over-expressed as histidine-tagged protein in Escherichia coli. The histidine-tagged enzyme was purified from crude cell extracts of IPTG-induced cells of E. coli BL21 (DE3). Inducer replacement studies led to the identification of lactose as a suitable and cheap alternative to the costly IPTG. Effects of medium components, various physico-chemical, and process parameters (pH, temperature, aeration, and agitation) for the production of nitrilase by engineered E. coli were optimized and scaled up to a laboratory scale bioreactor (6.6 l). Finally, the recombinant E. coli whole-cells were utilized for the production of (R)-(-)-mandelic acid.

Studies on the production of enantioselective nitrilase in a stirred tank bioreactor by Pseudomonas putida MTCC 5110

Nitrilases constitute an important class of hydrolases, having numerous industrial applications. The present work aims to address the production of nitrile hydrolyzing enzymes from Pseudomonas putida MTCC 5110 in a 6l bioreactor. Effect of various physico-chemical conditions and process parameters like pH, temperature, aeration and agitation rates and inducer concentration was studied. Further, the enzyme activity was enhanced by adopting the inducer feeding strategy. Various biochemical engineering parameters pertaining to the cultivation of P. putida in different physico-chemical conditions were reported. Finally, segregation of growth phase from the enzyme production phase allowed significant reduction in total fermentation time.

Facile entry to (-)-(R)- and (+)-(S)-mexiletine

The title compounds, 1a and 1b, have been synthesized in a three-step sequence starting from (-)-(S) and (+)-(R)-propylene oxide, respectively, in acceptable overall yields. The enantiomeric excess values for 1a and 1b were 96% and 93% respectively, as assessed by HPLC analysis on a chiral stationary phase of the corresponding N-acetyl derivatives. The synthetic route herein presented may represent a facile entry to highly enriched mexiletine enantiomers, alternative to those previously reported in the literature.