CAL-B-catalyzed resolution of some pharmacologically interesting β–substituted isopropylamines

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).

Transaminase-mediated synthesis of enantiopure drug-like 1-(3',4'-disubstituted phenyl)propan-2-amines

Transaminases (TAs) offer an environmentally and economically attractive method for the direct synthesis of pharmaceutically relevant disubstituted 1-phenylpropan-2-amine derivatives starting from prochiral ketones. In this work, we report the application of immobilised whole-cell biocatalysts with (R)-transaminase activity for the synthesis of novel disubstituted 1-phenylpropan-2-amines. After optimisation of the asymmetric synthesis, the (R)-enantiomers could be produced with 88-89% conversion and >99% ee, while the (S)-enantiomers could be selectively obtained as the unreacted fraction of the corresponding racemic amines in kinetic resolution with >48% conversion and >95% ee.

Amphetamine-like Neurochemical and Cardiovascular Effects of α-Ethylphenethylamine Analogs Found in Dietary Supplements

Dietary supplements often contain additives not listed on the label, including α-ethyl homologs of amphetamine such as N,α-diethylphenethylamine (DEPEA). Here, we examined the neurochemical and cardiovascular effects of α-ethylphenethylamine (AEPEA), N-methyl-α-ethylphenethylamine (MEPEA), and DEPEA as compared with the effects of amphetamine. All drugs were tested in vitro using uptake inhibition and release assays for monoamine transporters. As expected, amphetamine acted as a potent and efficacious releasing agent at dopamine transporters (DAT) and norepinephrine transporters (NET) in vitro. AEPEA and MEPEA were also releasers at catecholamine transporters, with greater potency at NET than DAT. DEPEA displayed fully efficacious release at NET but weak partial release at DAT (i.e., 40% of maximal effect). In freely moving, conscious male rats fitted with biotelemetry transmitters for physiologic monitoring, amphetamine (0.1–3.0 mg/kg, s.c.) produced robust dose-related increases in blood pressure (BP), heart rate (HR), and motor activity. AEPEA (1–10 mg/kg, s.c.) produced significant increases in BP but not HR or activity, whereas DEPEA and MEPEA (1–10 mg/kg, s.c.) increased BP, HR, and activity. In general, the phenethylamine analogs were approximately 10-fold less potent than amphetamine. Our results show that α-ethylphenethylamine analogs are biologically active. Although less potent than amphetamine, they produce cardiovascular effects that could pose risks to humans. Given that MEPEA and DEPEA increased locomotor activity, these substances may also have significant abuse potential.

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.

Reductive Amination in the Synthesis of Pharmaceuticals

Reductive amination plays a paramount role in pharmaceutical and medicinal chemistry owing to its synthetic merits and the ubiquitous presence of amines among biologically active compounds. It is one of the key approaches to C-N bond construction due to its operational easiness and a wide toolbox of protocols. Recent studies show that at least a quarter of C-N bond-forming reactions in the pharmaceutical industry are performed via reductive amination. This Review concisely compiles information on 71 medical substances that are synthesized by reductive amination. Compounds are grouped according to the principle of action, which includes drugs affecting the central nervous system, drugs affecting the cardiovascular system, anticancer drugs, antibiotics, antiviral and antifungal medicines, drugs affecting the urinary system, drugs affecting the respiratory system, antidiabetic medications, drugs affecting the gastrointestinal tract, and drugs regulating metabolic processes. A general synthetic scheme is provided for each compound, and the description is focused on reductive amination steps. The green chemistry metric of reaction mass efficiency was calculated for all reactions.

Upgraded Bioelectrocatalytic N2 Fixation: From N2 to Chiral Amine Intermediates

Enantiomerically pure chiral amines are of increasing value in the preparation of bioactive compounds, pharmaceuticals, and agrochemicals. ω-Transaminase (ω-TA) is an ideal catalyst for asymmetric amination because of its excellent enantioselectivity and wide substrate scope. To shift the equilibrium of reactions catalyzed by ω-TA to the side of the amine product, an upgraded N₂ fixation system based on bioelectrocatalysis was developed to realize the conversion from N₂ to chiral amine intermediates. The produced NH₃ was in situ reacted with l-alanine dehydrogenase to generate alanine with NADH as a coenzyme. ω-TA transferred the amino group from alanine to ketone substrates and finally produced the desired chiral amine intermediates. The cathode of the upgraded N₂ fixation system supplied enough reducing power to synchronously realize the regeneration of reduced methyl viologen (MV^•+) and NADH for the nitrogenase and l-alanine dehydrogenase. The coproduct, pyruvate, was consumed by l-alanine dehydrogenase to regenerate alanine and push the equilibrium to the side of amine. After 10 h of reaction, the concentration of 1-methyl-3-phenylpropylamine achieved 0.54 mM with the 27.6% highest faradaic efficiency and >99% enantiomeric excess (ee_p). Because of the wide substrate scope and excellent enantioselectivity of ω-TA, the upgraded N₂ fixation system has great potential to produce a variety of chiral amine intermediates for pharmaceuticals and other applications.

A Stable Heroin Analogue That Can Serve as a Vaccine Hapten to Induce Antibodies That Block the Effects of Heroin and Its Metabolites in Rodents and That Cross-React Immunologically with Related Drugs of Abuse

An improved synthesis of a haptenic heroin surrogate 1 (6-AmHap) is reported. The intermediate needed for the preparation of 1 was described in the route in the synthesis of 2 (DiAmHap). A scalable procedure was developed to install the C-3 amido group. Using the Boc protectng group in 18 allowed preparation of 1 in an overall yield of 53% from 4 and eliminated the necessity of preparing the diamide 13. Hapten 1 was conjugated to tetanus toxoid and mixed with liposomes containing monophosphoryl lipid A as an adjuvant. The 1 vaccine induced high anti-1 IgG levels that reduced heroin-induced antinociception and locomotive behavioral changes following repeated subcutaneous and intravenous heroin challenges in mice and rats. Vaccinated mice had reduced heroin-induced hyperlocomotion following a 50 mg/kg heroin challenge. The 1 vaccine-induced antibodies bound to heroin and other abused opioids, including hydrocodone, oxycodone, hydromorphone, oxymorphone, and codeine.

Controlling stereoselectivity by enzymatic and chemical means to access enantiomerically pure (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinoline derivatives

A chemoenzymatic strategy for the synthesis of enantiomerically pure novel alkaloids (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinolines is presented. The key steps are the biocatalytic stereoselective reductive amination of substituted 1-phenylpropan-2-one derivatives to yield chiral amines employing microbial ω-transaminases, and the diastereoselective reduction of a Bischler–Napieralski imine intermediate by catalytic hydrogenation in the presence of palladium on charcoal, leading exclusively to the desired cis-isomer.

Deracemization of mexiletine biocatalyzed by omega-transaminases

(S)- as well as (R)-mexiletine [1-(2,6-dimethylphenoxy)-2-propanamine], a chiral orally effective antiarrhythmic agent, was prepared by deracemization starting from the commercially available racemic amine using omega-transaminases in up to >99% ee and conversion with 97% isolated yield by a one-pot two-step procedure. The absolute configuration could be easily switched to the other enantiomer, just by switching the order of the applied transaminases. The cosubstrate pyruvate needed in the first oxidative step was recycled by using an amino acid oxidase.

Highly Selective Enzymatic Kinetic Resolution of Primary Amines at 80 °C: A Comparative Study of Carboxylic Acids and Their Ethyl Esters as Acyl Donors

Optimization of the kinetic resolution of 2-amino-4-phenyl-butane was achieved at 80 degrees C using CAL-B-catalyzed aminolysis of carboxylic acids and their ethyl esters. The reactions carried out with long chain esters and the corresponding acids as acyl donors proceeded with remarkably high enantioselectivity. The use of carboxylic acids as acylating agents led to a marked acceleration of the reaction rate compared to their ester counterparts. Lauric acid led to enantiomeric excesses superior to 99.5% for both the remaining amine and the corresponding lauramide at 50% conversion (reached in 3 h). These optimized conditions were applied to the resolution of a series of aliphatic and benzylic amines.

Thiyl Radical Mediated Racemization of Nonactivated Aliphatic Amines

The racemization of nonactivated aliphatic amines has been mediated with alkanethiols and with methyl thioglycolate in the presence of AIBN. The process involves reversible H-abstraction at the chiral center, in a position alpha relative to nitrogen, by thiyl radical. The knowledge of the reaction enthalpy is critical to select the appropriate thiol. In the absence of experimental values, theoretical calculations of the alpha-C-H BDEs and the S-H BDE provide a useful guide.