Racemization and stereoselective alcoholysis of temazepam

Enantiomers of temazepam (TMZ), an anxiolytic drug in clinical use, were resolved and stabilized by the use of aprotic solvents in chiral stationary phase high‐performance liquid chromatography (CSP‐HPLC). The enantiomers were used to study racemization and heteronucleophilic substitution (alcoholysis) reactions in anhydrous acidic methanol and ethanol. Kinetics of spontaneous racemization and stereoselective conversions to 3‐O‐methyltemazepam (MeTMZ) and 3‐O‐ethyltemazepam (EtTMZ) were determined by circular dichroism (CD) spectropolarimetry and CSP‐HPLC. The rates of conversion of rac‐TMZ to rac‐MeTMZ (or rac‐EtTMZ) were determined by ultraviolet‐visible spectrophotometry. The results indicated that, for example, both N4‐protonated and unprotonated forms of (S)‐TMZ underwent spontaneous racemization and its 3S‐hydroxyl group was highly stereoselectively substituted at C3 position by the ethoxy group of ethanol to form EtTMZ enriched in (S)‐EtTMZ. Similar stereoselective reactions occurred for TMZ enantiomers in acidic methanol. Chirality 11:179–186, 1999. Published 1999 Wiley‐Liss, Inc.     

Highly stereoselective acid-catalyzed homonucleophilic substitution reactions

Enantiomeric 3-O-methyltemazepam and 3-Oethyltemazepam were highly stereoselectively substituted by the 3-methoxy group of methanol in acidic anhydrous methanol and by the 3-ethoxy group of ethanol in acidic anhydrous ethanol, respectively. The stereoselectivity of the homonucleophilic substitution reactions was determined by circular dichroism spectropolarimetry and gas chromatography-mass spectrometry. In anhydrous solutions containing 0.5 M D₂SO₄ at 50°C, for example, the stereoselectivity was ∼63:1 for enantiomeric 3-O-methyltemazepam in CD₃OD and ∼94:1 for enantiomeric 3-O-ethyltemazepam in C₂D₅OD. The high stereoselectivity at C3 position was primarily due to the presence of a methyl group at N1 position. © 1996 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Acid-catalyzed stereoselective heteronucleophilic substitution and racemization of 3-O-methyloxazepam and 3-O-ethyloxazepam

Enantiomers of 3-O-methyloxazepam (MeOX) and 3-O-ethyloxazepam (EtOX) were resolved by chiral stationary phase high-performance liquid chromatography (CSP-HPLC). Reaction kinetics and deuterium isotope effects of acid-catalyzed racemization of enantiomeric MeOX in ethanol and enantiomeric EtOX in methanol were studied by spectropolarimetry. The acid-catalyzed heteronucleophilic substitution reactions of racemic MeOX in ethanol and racemic EtOX in methanol were studied by reversed-phase HPLC. Thermodynamic parameters involved in the reactions were obtained by temperature-dependent reaction rates. The effects of solvent's dielectric constant on the heteronucleophilic substitution reactions were also determined. A nucleophilically solvated and transient C3 carbocation intermediate resulting from an N4-protonated enantiomer, derived from a 1,4-benzodiazcpine either in M (minus) or P (plus) conformation, is proposed to be an intermediate and responsible for the acid-catalyzed stereoselective nucleophilic substitution and the resulting racemization. © 1994 Wiley-Liss, Inc.     

Substitution and racemization of 3-hydroxy- and 3-alkoxy-1,4-benzodiazepines in acidic aqueous solutions

Oxazepam (OX), 3-O-methyloxazepam, 3-O-ethyloxazepam, temazepam (TMZ), 3-O-methyltemazepam, and 3-O-ethyltemazepam underwent acid-catalyzed nucleophilic substitution reaction (hydrolysis) in an acetonitrile–oxygen-18 water mixture to form either OX or TMZ in which the 3-hydroxyl group was either partially or fully labeled with an oxygen-18 atom. The dependence of the hydrolysis rates on solvent composition, temperature, ionic strength, and in deuterated solvent was studied by reversed-phase high-performance liquid chromatography (HPLC). The rates of racemization of enantiomeric compounds in acidic aqueous solutions were studied by both spectropolarimetry and chiral stationary phase HPLC. In acetonitrile: 2.5 M H₂SO₄ (4:1, v/v) at 50°C, enantiomers of OX and TMZ underwent racemization at rates ≥40-fold faster than the rates of hydrolysis. Enantiomeric 3-O-alkyl derivatives of OX and TMZ in acidic aqueous solutions did not themselves undergo racemization and it was their hydrolysis products (either OX or TMZ) that underwent racemization. © 1995 Wiley-Liss, Inc.     

Acid-Catalyzed nucleophilic substitution and racemization of 3-methoxy-N-desmethyldiazepam enantiomers in methanol

pK_a1 values of 3-methoxy-N-desmethyldiazepam in acetonitrile and methanol containing various acid concentrations were determined by spectrophotometry to be 3.5 and 1.3, respectively. Temperature-dependent racemization of enantiomeric 3-methoxy-N-desmethyldiazepam in methanol containing 0.5 M H₂SO₄ was studied by circular dichroism spectropolorimetry and the racemization reactions were found to follow apparent first-order kinetics. Thermodynamic parameters of the racemization reaction were found to be: E_act = 18.8 kcal/mol, and at 25°C: ΔH^? = 18.3 kcal/mol, ΔS^? = ?14.8 entropy unit, and ΔG^? = 22.7 kcal/mol, respectively. The racemization had an isotope effect (k_H/k_D) of 1.6 at 42°C. Based on the results of this report and those of earlier reports by other investigators, a nucleophilically solvated C3 carbocation intermediate resulting from either a P (plus) or an M (minus) conformation is proposed to be an intermediate and responsible for the stereoselective nucleophilic substitution and the subsequent racemization of 3-methoxy-N-desmethyldiazepam enantiomers. © 1993 Wiley-Liss, Inc.     

Racemization of the gastrointestinal antisecretory chiral drug esomeprazole magnesium via the pyramidal inversion mechanism: A theoretical study

The pyramidal inversion mechanisms of the 6‐methoxy and the 5‐methoxy tautomers of (S)‐omeprazole were studied, employing ab initio and DFT methods. The conformational space of the model molecule (S)‐2‐[(3‐methyl‐2‐pyridinyl)methyl]sulfinyl‐1H‐benzimidazole was calculated, with respect to rotations around single bonds, at the B3LYP/6‐311G(d,p) level. All of the resulting conformations were used as starting points for full optimizations of (S)‐omeprazole, at B3LYP/6‐31G(d), B3LYP/6‐311G(d,p), B3LYP/6‐311++G(d,p), B3LYP/6‐311G(2df,2pd), MP2/6‐31G(d), and MP2/6‐311G(d,p) levels. Four distinct pathways were found for enantiomerization via the pyramidal inversion mechanism for each of the tautomers of (S)‐omeprazole. Each transition state, in which the sulfur, the oxygen and the two carbon atoms connected directly to the sulfur are in one plane, connects two diastereomeric minima. The enantiomerization is completed by free rotation around the sulfur–methylene bond, and around the methylene–pyridine ring bond. The effective Gibbs' free energy barrier for racemization ΔG of the two tautomers of (S)‐omeprazole are 39.8 kcal/mol (5‐methoxy tautomer) and 40.0 kcal/mol (6‐methoxy tautomer), indicating that the enantiomers of omeprazole are stable at room temperature (in the gas phase). The 5‐methoxy tautomer of (S)‐omeprazole was found to be slightly more stable than the 6‐methoxy tautomer, in the gas phase. The energy barrier (ΔG^?) for the(S,M) (S,P) diastereomerization of (S)‐omeprazole due to the rotation around the pyridine chiral axis was very low, 5.8 kcal/mole at B3LYP/6‐311G(d,p). Chirality 2010. © 2010 Wiley‐Liss, Inc.     

Easy Access to Enantiomerically Pure Nonproteinogenic Amino Acids

The protease from Bacillus licheniformis (alcalase) shows a remarkable broad substrate tolerance and high enantioselectivity against nonproteinogenic racemic amino acid derivatives. N‐acetyl protected amino acid esters of mono‐, di‐ or tri‐substituted phenyl alanines and even tert.‐leucine were hydrolyzed with high enantioselectivity. The obtained mixtures of (S)‐N‐acetyl amino acid and (R)‐N‐acetyl amino acid ester can easily be separated. The R‐ or S‐amino acids were obtained by acidic cleavage of the optically pure derivatives or the (R)‐ester was racemized by treatment with potassium t‐butylate.     

Base-catalyzed racemization of 3-O-acyloxazepam

Enantiomers of 3-O-acyloxazepam (oxazepam 3-acetate; OXA) underwent base-catalyzed hydrolysis and racemization. Kinetics of reaction products formed from an OXA enantiomer in buffered and unbuffered alkaline solutions were analyzed by chiral stationary phase high-performance liquid chromatography. Racemization occurred with varying rates in aqueous solutions with pH ranging from 7.5 to 14. Racemization mechanism was studied by the dependence of rates of hydrolysis and racemization on temperature and pH. Mass spectral analysis of racemization products derived from an OXA enantiomer in a deuterated solvent indicated that racemization was accompanied by a proton exchange with the solvent. The results indicated that a base-catalyzed keto-enol tautomerism between the C2-carbonyl group and the C3 carbon was responsible for the observed racemization. © 1994 Wiley-Liss, Inc.

1 This article is a US Goverment work and, as such, is in the public domain in the United States of America.

     

Production of highly optically active (R)-3-chloro-1,2-propanediol using a bacterium assimilating the (S)-isomer

A bacterium that assimilates (S)-3-chloro-1,2-propanediol [monochlorohydrin (MCH)] was isolated from soil by enrichment culture. The bacterium was identified as Pseudomonas sp. by taxonomic studies. The strain grew in a medium containing racemic MCH as a source of carbon and degraded (S)-MCH stereoselectively, liberating chloride ions. The residual isomer was the (R)-form [99.5% enantiomeric excess (ee)], which was obtained from the racemate in a final yield of 36% by using this strain. Subsequently, highly optically active (R)-glycidol (GLD) (99.3% ee) was prepared from the (R)-MCH obtained by reaction in alkaline solution. The cell-free extracts of the cells had both dehalogenating and epoxide-opening activities, which converted various halohydrins to the corresponding epoxides and epoxides to the corresponding diols, respectively. Correspondence to: T. Suzuki     

OX40-OX40L Interaction Promotes Proliferation and Activation of Lymphocytes via NFATc1 in ApoE-Deficient Mice

Background

Our previous studies have shown that OX40-OX40L interaction regulates the expression of nuclear factor of activated T cells c1(NFATc1) in ApoE^−/− mice during atherogenesis. The aim of this study was to investigate whether OX40-OX40L interaction promotes Th cell activation via NFATc1 in ApoE^−/− mice.

Methods and Results

The lymphocytes isolated from spleen of ApoE^−/− mice were cultured with anti-CD3 mAb in the presence or absence of anti-OX40 or anti-OX40L antibodies. The expression of NFATc1 mRNA and protein in isolated lymphocytes were measured by real time PCR (RT-PCR) and flow cytometry (FCM), respectively. The proliferation of lymphocytes was analyzed by MTT method,and the expression of IL-2, IL-4 and IFN-γ in the cultured cells and supernatant were measured by RT-PCR and enzyme-linked immunosorbent assary (ELISA), respectively. After stimulating OX40-OX40L signal pathway, the expression of NFATc1 and the proliferation of leukocytes were significantly increased. Anti-OX40L suppressed the expression of NFATc1 in lymphocytes of ApoE−/− mice. Anti-OX40L or the NFATc1 inhibitor (CsA) markedly suppressed the cell proliferation induced by anti-OX40. Moreover, the expression of IL-2 and IFN-γ was increased in lymphocytes induced by OX40-OX40L interaction. Blocking OX40-OX40L interaction or NFATc1 down-regulated the expression of IL-2 and IFN-γ, but didn’t alter the expression of IL-4 in supernatants.

Conclusion

These results suggest that OX40-OX40L interaction promotes the proliferation and activation of lymphocytes through NFATc1.     

Effects of solvent on inclusion complexation of a chiral dipeptide toward racemic BINOL

The effects of reaction solvent on inclusion complexation of a chiral dipeptide (3S,6S)‐ 1 derived from (S)‐proline toward racemic BINOL was investigated, discovering that the reaction solvent played a crucial role in determining the inclusion complexation behavior of dipeptide (3S,6S)‐ 1 toward rac‐BINOL. (3S,6S)‐ 1 did not show any chiroselective or achiroselective complexation toward rac‐BINOL in polar protic solvents such as methanol and ethanol, polar aprotic solvents including trichloromethane and THF, while in polar aprotic solvent ethyl acetate and apolar aprotic solvents benzene, (3S,6S)‐ 1 displayed achiroselective complexation toward rac‐BINOL. However, the resulting heterocomplex HC‐ 2 from benzene and HC‐ 3 from ethyl acetate have a different composition. Single crystal X‐ray diffraction analysis demonstrates that the two heterocomplexes are formed via different H‐bond interaction patterns, in which the reaction solvent has a dramatic effect. Furthermore, this work provides a relatively green method for quantitative enantiomeric enrichment of nonracemic BINOL, in which unacceptable and toxic benzene was replaced by ethyl acetate.     

Axially chiral N,N′‐dioxides ethers for catalysis in enantioselective allylation of aldehydes

A series of axially chiral ethers synthesized from biscarboline N,N′‐dioxides, (S)‐ 1a to (S)‐ 1n , was investigated in enantioselectivity addition reactions of allyltrichlorosilane with a series of substituted aldehydes, including bulky substituted aldehydes. High enantioselectivities (up to 96%ee) were achieved using the catalyst (S)‐ 1k at 1 mol % loading.     

Benzopyran Derivatives and an Aliphatic Compound from a Mangrove Endophytic Fungus Penicillium citrinum QJF‐22

Two new benzopyran derivatives, (2R,4S)‐5‐methoxy‐2‐methyl‐3,4‐dihydro‐2H‐1‐benzopyran‐4‐ol and (2S,4R,2′S,4′R)‐4,4′‐oxybis(5‐methoxy‐2‐methyl‐3,4‐dihydro‐2H‐1‐benzopyran), and a new aliphatic compound, (3E,5Z,8S,10E)‐8‐hydroxytrideca‐3,5,10,12‐tetraen‐2‐one, together with three known benzopyran derivatives, were obtained from a mangrove endophytic fungus Penicillium citrinum QJF‐22 collected in Hainan island. Their structures were determined by analysis of spectroscopic data and the relative configuration of (2R,4S)‐5‐methoxy‐2‐methyl‐3,4‐dihydro‐2H‐1‐benzopyran‐4‐ol was also confirmed by single‐crystal X‐ray diffraction. The absolute configurations of four compounds were established by comparison of ECD spectra to calculations. The configuration of (3E,5Z,8S,10E)‐8‐hydroxytrideca‐3,5,10,12‐tetraen‐2‐one was confirmed by comparison of optical value to the similar compound. The configurations of the compounds (2S,4S)‐5‐methoxy‐2‐methyl‐3,4‐dihydro‐2H‐1‐benzopyran‐4‐ol and (2R,4R)‐5‐methoxy‐2‐methyl‐3,4‐dihydro‐2H‐1‐benzopyran‐4‐ol were first determined. (3R,4S)‐3,4,8‐Trihydroxy‐3,4‐dihydronaphthalen‐1(2H)‐one exhibited moderate inhibitory effects on LPS‐induced NO production in RAW264.7 cells with IC₅₀ of 44.7 μM, and without cytotoxicity to RAW264.7 cells within 50 μM.     

Stereoselectivity in acid-catalyzed heteronucleophilic substitution of enantiomeric 3-O-methyloxazepam and 3-O-ethyloxazepam

Kinetics of acid-catalyzed heteronucleophilic substitution and racemization of enantiomeric MeOX in ethanol and enantiomeric EtOX in methanol were studied by quenching reaction products at various times by neutralization. Enantiomeric contents of remaining substrate and reaction product were determined by chiral stationary phase high-performance liquid chromatography. The experimental procedure allowed the determination of the stereoselectivity (i.e., the enantiomeric ratio of a substitution product formed from an enantiomerically pure substrate) involved in the heteronucleophilic substitution reactions. The stereoselectivity was found to vary between 58:42 and 87:13, depending on the acid concentration, substrate, solvent, and temperature. The enantiomeric purity of remaining substrates was identical to that of the starting substrate, indicating that the enantiomeric substrates did not undergo a ring-opening reaction. The results provided additional evidence supporting the mechanism proposed earlier in acid-catalyzed stereoselective heteronucleophilic and homonucleophilic substitutions and the resulting racemization of enantiomeric 3-alkoxy-1,4-benzodiazepines in alcoholic solvents. © 1995 Wiley-Liss, Inc.     

Enantioselective hydrolysis of racemic 2-phenylpropionitrile and other (R,S)-2-arylpropionitriles by a new bacterial isolate,Agrobacterium tumefaciens strain d3

Bacteria were enriched from soil samples with succinate as carbon source and racemic 2-phenylpropionitrile as sole source of nitrogen. One of the isolates, strain d3, converted (R,S)-2-phenylpropionitrile with high enantioselectivity to (S)-2-phenylpropionic acid. Strain d3 was identified as Agrobacterium tumefaciens. Resting cells hydrolysed 2-phenylpropionitrile via 2-phenylpropionamide to 2-phenylpropionic acid. Racemic 2-phenylpropionitrile as well as 2-phenylpropionamide were converted to (S)-2-phenylpropionic acid with an enantiometric excess above 96%. The nitrile hydratase and the amidase were both shown to convert preferentially the S enantiomer of their respective substrate. These two enzymes were induced in the presence of (R,S)-2-phenylpropionitrile but only in the absence of ammonia. In addition to 2-phenylpropionitrile strain d3 could utilize various aliphatic and aromatic nitriles as nitrogen sources. Resting cells of strain d3 also converted (R,S)-2-phenylbutyronitrile, ibuprofen nitrile, ketoprofen nitrile and -aminophenylacetonitrile with high enantioselectivity. The nitrile- and amide-converting enzyme activities were also found in cell-free extracts.     

Synthesis and changes in affinity for NOP and opioid receptors of novel hexapeptides containing β2-tryptophan analogues

We report the synthesis and the biological activity of new analogues of Ac-RFMWMK-NH₂ and Ac-RYYRWK-NH₂, modified in position 4 and 5, respectively, with incorporation of newly synthesized β²-tryptophan analogues. Trp was substituted by the (S)-2-(1-methyl-1H-indol-3-yl)propionic residue or by (S)-2-(5-methoxy-1H-indol-3-yl)propionic residue. The biological activity (pEC₅₀ and E_max) of these compounds was tested on electrically stimulated preparations of rat vas deferens. The 5-methoxy β-tryptophan group reverses the affinity of the compounds.     

Bicyclo[3,2,1]octanoid,neolignans from Aniba simulans

Six bicyclo[3,2,1]octanoid neolignans, isolated from the benzene extract of Aniba simulans Allen (Lauraceae) trunk wood, are shown to derive from two basic structures: 1-allyl-8-hydroxy-6-(3′-methoxy-4′,5′-methylenedioxyphenyl)-7-methyl-3-oxobicyclo[3,2,1]octane, substituted by 4-hydroxy, 4-hydroxy-5-methoxy, 4-methoxy or 4,5-dimethoxy groups; and 1-allyl-8-hydroxy-6-(3′-methoxy-4′,5′-methylenedioxyphenyl)-7-methyl-4-oxobicyclo[3,2,1]oct-2-ene, substituted by 3-hydroxy or 3-hydroxy-5-methoxy groups. The structural proposals are based on spectral data, interconversions synthesis of a derivative from the known (2R,3S,3aS)-3a-allyl-5-methoxy-2-(3′-methoxy,4′,5′-methylenedioxyphenyl)-3-methyl-2,3,3a,6-tetrahydro-6-oxobenzofuran.     

Enantioselective preparation of (R) and (S)-3-hydroxycyclopentanone by kinetic resolution

A straightforward approach to enantiomerically enriched (R) and (S)-3-hydroxycyclopentanone is described. The key step involves a kinetic resolution of racemic 3-hydroxycyclopentanone using commercial Pseudomonas cepacia lipase immobilized on diatomite (Amano lipase PS-DI). The absolute stereochemistry of the product was determined by derivatization into (R)-3-(benzyloxy)cyclopentanone.     

Classroom Enters the Courtroom: Stereochemistry of SN1 and SN2 Reactions in Enantiomer Patent Litigations of the Antidepressant Escitalopram

The role of elementary stereochemistry is illustrated in the patent litigations of the blockbuster antidepressant drug escitalopram oxalate. An undergraduate student of organic chemistry would recognize the stereochemical courses of the intramolecular S_N2 and S_N1 reactions of the single‐enantiomer (S)‐diol intermediate in the synthesis of the blockbuster antidepressant drug escitalopram oxalate: retention of configuration of the chiral carbon atom under basic conditions and racemization under acidic conditions, respectively. He/she, in searching for a stereoselective ring‐closure reaction of the enantiomeric diol, will think of an S_N2 reaction in a basic medium. From these points of view, the process claim in the enantiomer patents of escitalopram is obvious/lacks an inventive step. An organic chemistry examination problem based on this scenario is offered. Chirality 28:39–43, 2016. © 2015 Wiley Periodicals, Inc.     

Single Enantiomer Versus Racemate: Chiral Distinction in the Proton Pump Inhibitors Omeprazole and Esomeprazole

Chiral distinction in the proton pump inhibitor drugs omeprazole and in its chiral‐switch esomeprazole magnesium was studied employing the Density Functional Theory (DFT) method. At B3LYP/6‐311G(d,p), the 6‐methoxy???6‐methoxy and 5‐methoxy???5‐methoxy homochiral and heterochiral dimers were calculated. The chiral distinction free energies (ΔΔG_{298,(RS‐SS)}) between the cyclic C₂‐(S,S)‐ and C_i‐(R,S)‐dimers with two intermolecular hydrogen bonds are 3.8, 1.9 (with BSSE counterpoise correction), and –6.9 (with D3 dispersion and BSSE counterpoise corrections) kJ/mol. Adding water as an implicit solvent (polarized continuum model [PCM] model) resulted in a chiral distinction energy of –3.3 kJ/mol, indicating a reversal of the order of the relative stabilities of C₂‐(S,S)‐ and C_i‐(R,S)‐dimers. The chiral distinction free energies between the corresponding (less stable) C₁‐dimers with one intermolecular hydrogen bond are –9.3, –5.8 (with BSSE CC), 17.6 (D3 + BSSE CC), and –3.2 (H₂O) kJ/mol. The results highlight the contention that omeprazole is not just a superposition of its enantiomer constituents. They are consistent with the pharmacological evidence of enantiomer–enantiomer interactions in omeprazole versus esomeprazole and the differences between the drugs omeprazole and esomeprazole magnesium and support the lodged application for regulatory supplementary protection certificate (SPC) exclusivity for the esomeprazole‐related combination drug Vimovo. Chirality 26:214–227, 2014. © 2014 Wiley Periodicals, Inc.