Chiral NMR discrimination of amines: analysis of secondary, tertiary, and prochiral amines using (18-crown-6)-2,3,11,12-tetracarboxylic acid

Enantiomeric discrimination is observed in the 1H and 13C NMR spectra of secondary and tertiary amines in the presence of (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (1). Nonequivalence of the resonances of prochiral nuclei in primary and secondary amines is also observed when they associate with 1. The amines are added in their neutral form and are protonated by the carboxylic acid groups of 1 to produce the corresponding ammonium and carboxylate ions. Secondary amines associate with 1 through two hydrogen bonds and an ion pair interaction. Tertiary amines can only form one hydrogen bond to accompany the ion pairing. Chiral discrimination in the 1H and 13C NMR spectra of a series of aryl-containing secondary amines is of sufficient magnitude to determine enantiomeric purities. The discrimination in the spectra of tertiary amines with 1 is smaller, but 13C NMR spectra provided enough distinction for the determination of enantiomeric purity.     

Enantiomeric impurities in chiral catalysts,auxiliaries, and synthons used in enantioselective syntheses. Part 5

The enantiomeric excess of chiral starting materials is one of the important factors determining the enantiopurity of products in asymmetric synthesis. Fifty‐one commercially available chiral reagents used as building blocks, catalysts, and auxiliaries in various enantioselective syntheses were assayed for their enantiomeric purity. The test results were classified within five impurities level (ie, <0.01%, 0.01%‐0.1%, 0.1%‐1%, 1%‐10%, >10%). Previously from 1998 to 2013, several reports have been published on the enantiomeric composition of more than 300 chiral reagents. This series of papers is necessitated by the fact that new reagents are forthcoming and that the enantiomeric purity of the same reagent can vary from batch to batch and/or from supplier to supplier. This report presents chiral liquid chromatography (LC) and gas chromatography (GC) methods to separate enantiomers of chiral compounds and evaluate their enantiomeric purities. The accurate and efficient LC analysis was done using newly introduced superficially porous particle (SPP 2.7 μm) based chiral stationary phases (TeicoShell, VancoShell, LarihcShell‐P, and NicoShell).     

Synthesis of new chiral cis-3-hydroxyazetidines and their application in diethylzinc addition to aldehydes

A new series of chiral cis-3-hydroxyazetidines have been prepared from (R)-1-phenylethylamine. They have excellent catalytic activities and enantiomeric selectivities in asymmetric addition of diethylzinc to aromatic aldehydes.     

Enantiomeric resolution by HPLC of axial chiral amides using amylose tris[(S)-1-phenylethylcarbamate]

The enantiomeric resolution of a series of N-arylamides was examined on amylose tris[(S)-1-phenylethylcarbamate] coated onto aminopropylated 7 μm silica with 500 Å diameter pores and on naked silica 5 μm particle size with 500 Å diameter pores. The enantiomeric resolution obtained for this series was excellent on both columns. The enantioselectivity of cellulose and amylose tris (3,5-dimethylphenylcarbamate) coated onto APS-Hypersil (120 Å pore size, 5 μm particle size) was also investigated for this series of compounds. Chirality 9:109–112, 1997. © 1997 Wiley-Liss, Inc.     

Catalyzed asymmetric dialkylzinc addition to benzaldehyde in the presence of new chiral ligands--delta-(1-phenethyl)aminoalcohols

A series of delta-aminoalcohols prepared from (S)- and (R)-N,N-dimethyl-1-phenethylamines was found to catalyze the enantioselective addition of diethylzinc to benzaldehyde to yield optically active 1-phenylpropanol with enantiomeric excess 60-85%.     

Stereochemistry and enantiomeric purity of a novel anxiolytic agent, deramciclane fumarate.

The synthesis, stereostructure, and enantiomeric separation by chromatography of a new, chiral anxiolytic agent, deramciclane fumarate (2, (-)-[1R,2S,4R]-2-(2-dimethylaminoethoxy)-2-phenyl-1,7, 7-trimethylbicyclo[2.2.1]heptane fumarate, EGIS-3886), is described. The optical antipode and the racemate of compound 2 were also prepared. The structure was determined by single crystal X-ray diffraction analysis. The enantiomeric separation was accomplished by HPLC on Chiralcel OD (250 x 4.6 mm; 10 microm) and hexane-ethanol (99.5:0.5) as mobile phase at room temperature. The enantiomeric purity of the synthesized drug substance proved to be very high (>99. 9%). Some statements published earlier on the stereostructure of deramciclane fumarate are critically discussed.     

Research on L-nucleosides. Synthesis and biological evaluation of a series of L- and D-2',3'-dideoxy-3'-[tris(methylthio)methyl]-beta-pentofuranosyl nucleosides

Novel nucleoside analogues of both D and L enantiomeric series were prepared by coupling reaction between a 2',3'-dideoxy-3'-modified furanose moiety and four different nucleobases. Though in all cases anomeric mixtures of nucleosides were obtained, the presence of the sterically bulky 3'-tris(methylthio)methyl group allowed a good stereoselectivity level. All the compounds of both enantiomeric series showed high IC(50) values as HSV-1 TK inhibitors and scarce ability to be phosphorylated by HSV-1 TK. In order to overcome possible problems related to the first phosphorylation step and to facilitate the penetration of the molecule through the cellular membrane, a monophosphate prodrug containing a long lipophilic chain was synthesized. No appreciable antiviral activity was exhibited by this molecule.     

Characterization of Arabidopsis thaliana pinoresinol reductase, a new type of enzyme involved in lignan biosynthesis

A lignan, lariciresinol, was isolated from Arabidopsis thaliana, the most widely used model plant in plant bioscience sectors, for the first time. In the A. thaliana genome database, there are two genes (At1g32100 and At4g13660) that are annotated as pinoresinol/lariciresinol reductase (PLR). The recombinant AtPLRs showed strict substrate preference toward pinoresinol but only weak or no activity toward lariciresinol, which is in sharp contrast to conventional PLRs of other plants that can reduce both pinoresinol and lariciresinol efficiently to lariciresinol and secoisolariciresinol, respectively. Therefore, we renamed AtPLRs as A. thaliana pinoresinol reductases (AtPrRs). The recombinant AtPrR2 encoded by At4g13660 reduced only (-)-pinoresinol to (-)-lariciresinol and not (+)-pinoresinol in the presence of NADPH. This enantiomeric selectivity accords with that of other PLRs of other plants so far reported, which can reduce one of the enantiomers selectively, whatever the preferential enantiomer. In sharp contrast, AtPrR1 encoded by At1g32100 reduced both (+)- and (-)-pinoresinols to (+)- and (-)-lariciresinols efficiently with comparative k(cat)/K(m) values. Analysis of lignans and spatiotemporal expression of AtPrR1 and AtPrR2 in their functionally deficient A. thaliana mutants and wild type indicated that both genes are involved in lariciresinol biosynthesis. In addition, the analysis of the enantiomeric compositions of lariciresinol isolated from the mutants and wild type showed that PrRs together with a dirigent protein(s) are involved in the enantiomeric control in lignan biosynthesis. Furthermore, it was demonstrated conclusively for the first time that differential expression of PrR isoforms that have distinct selectivities of substrate enantiomers can determine enantiomeric compositions of the product, lariciresinol.     

Bioreduction of aldehydes and ketones using Manihot species

Biocatalysis constitutes an important tool in organic synthesis, especially for the preparation of chiral molecules of biological interest. A series of aliphatic and aromatic aldehydes and two ketones were reduced using plant cell preparations from Manihot esculenta and Manihot dulcis roots. The reduced products were typically obtained in excellent yields (80-96%), and with excellent enantiomeric excess (94-98%), except for vanillin. Esters, a nitrile, and an amide were also examined, but were not reduced. Preliminary conversion rate studies are reported. This is the first attempt to perform the biotransformation of carbonyl compounds using Manihot species.     

Chiral investigation of midodrine, a long-acting alpha-adrenergic stimulating agent

Midodrine hydrochloride is a peripheral alpha(1)-adrenoreceptor agonist that induces venous and arterial vasoconstriction. Midodrine, after oral or intravenous administration, undergoes enzymatic hydrolysis and releases deglymidodrine, a pharmacologically active metabolite. Midodrine and deglymidodrine have a chiral carbon in the 2-position. To investigate the bioactivity of racemates and enantiomers of the drug and metabolite, three chromatographic chiral stationary phases, Chiralcel OD-H, Chiralcel OD-R, and alpha(1)-AGP, were evaluated for enantiomeric resolution. Good enantioseparation of midodrine racemate was obtained using the Chiralcel OD-H column. This stationary phase was then used to collect separately the midodrine enantiomers. By alkaline hydrolysis of rac-midodrine and each separated enantiomer, rac-deglymidodrine and its enantiomers were prepared. The control of the enantiomeric purity was carried out by alpha(1)-AGP stationary phase, while the hydrolysis of rac-midodrine and its enantiomers was controlled by capillary electrophoresis using trimethyl-beta-cyclodextrin as chiral selector. The pharmacological activity of the two racemates and the two enantiomeric pairs was tested in vitro on a strip of rabbit descending thoracic aorta. The tests continued that the activity of the drug and metabolite is due only to the (-)-enantiomer because neither of the (+)-enantiomers is active.     

Determination of enantiomeric composition of (-)-(R)-2-tert-butyltetrahydroimidazolidin-4-one by polarimetry, 1H NMR, and chiral SFC

Partial resolution of rac-2-tert-butyltetrahydroimidazolidin-4-one was carried out by recrystallization of diastereomeric salts. The enantiomeric composition of enriched samples was estimated by polarimetry, (1)H NMR, and chiral SFC. Enantiomeric composition estimated by polarimetry or by (1)H NMR was directly proportional to that estimated by chiral SFC. The occurrence of solute self-association in chloroform was detected through measurements of optical and specific rotation at variable concentration of (-)-(R)-2-tert-butyltetrahydroimidazolidin-4-one. Our data suggest that solute self-association in chloroform might be independent of enantiomeric composition.     

An efficient system for the asymmetric acylation of (R,S)-3-n-butylphthalide catalyzed by novozyme 435

Novozyme 435 could be a highly efficient catalyst in the asymmetric acylation of (R,S)-3-n-butylphthalide in tetrahydrofuran-hexane solvents. The effect of various reaction parameters such as agitation velocity, water content, mixed media, temperature, concentration of Novozyme 435, molar ratio of acetic anhydride to (R,S)-3-n-butylphthalide, reaction time, enantiomeric excess of substrate (ee(S)), enantiomeric excess of product (ee(P)), and enantioselective ratio (E) were studied. Tetrahydrofuran markedly improved (R,S)-3-n-butylphthalide conversion, enantiomeric excess of remaining 3-n-butylphthalide, and enantiomeric ratio. The optimum media were 50% (v/v) tetrahydrofuran and 50% (v/v) hexane. Other ideal reaction conditions were an agitation velocity of 150 rpm, 0.4% (v/v) water content, temperature of 30 °C, 8 mg/mL dosage of Novozyme 435, 8:1 (0.4 mmol: 0.05 mmol) molar ratio of acetic anhydride to (R,S)-3-n-butylphthalide, and a reaction time of 48 hr. Under the optimum conditions, 96.4% ee(S) and 49.3% conversion of (R,S)-3-n-butylphthalide were achieved. In addition, enantiomeric excess of the product was above 98.0%.     

Stereoselectivity of the carbonyl reduction of dolasetron in rats,dogs, and humans

The initial step in the metabolism of dolasetron or MDL 73, 147EF [(2α,6α,8α,9aβ)-octahydro-3-oxo-2,6-methano-2H-quinolizin-8-yl 1H-indol-3-carboxylate, monomethanesulfonate] is the reduction of the prochiral carbonyl group to give a chiral secondary alcohol “reduced dolasetron.” An HPLC method, using a chiral column to separate reduced dolasetron enantiomers, has been developed and used to measure enantiomers in urine of rats, dogs, and humans after dolasetron administration. In all cases, the reduction was enantioselective for the (+)-(R)-enantiomer, although the dog showed lower stereoselectivity, especially after iv administration. An approximate enantiomeric ratio (+/?) of 90:10 was found in rat and human urine. The contribution of further metabolism to this enantiomeric ratio was considered small as preliminary studies showed that oxidation of the enantiomeric alcohols by human liver microsomes demonstrated only minor stereoselectivity. Further evidence for the role of stereoselective reduction in man was obtained from in vitro studies, where dolasetron was incubated with human whole blood. The enantiomeric composition of reduced dolasetron formed in human whole blood was the same as that found in human urine after administration of dolasetron. Enantioselectivity was not due to differences in the absorption, distribution, metabolism, or excretion of enantiomers, as iv or oral administration of rac-reduced dolasetron to rats and dogs lead to the recovery, in urine, of essentially the same enantiomeric composition as the dose administered. It is fortuitous that the (+)-(R)-enantiomer is predominantly formed by carbonyl reductase, as it is the more active compound. © 1995 Wiley-Liss, Inc.     

Measurements of concentration dependence and enantiomeric purity of terpene solutions as a test of a new commercial VCD spectrometer

Vibrational circular dichroism (VCD) spectra of (+)-alpha-pinene solutions in carbon tetrachloride have been measured in the range of volume fractions 5-100% (v/v) in the mid-infrared region. The concentration dependence measured was statistically analyzed with the aim of obtaining a reliable correlation between the VCD band areas and the concentrations of individual enantiomers. The quality of the spectra was estimated by means of noise spectra which were defined as half the difference of the two following blocks of scans. In addition to this, the enantiomeric purity was studied. This study was carried out for both (+)- and (-)-alpha-pinene enantiomers in the range of the percent enantiomeric excess in the interval 10-100%. The relationship between VCD intensity and enantiomeric purity was determined by least-square regression and statistically evaluated. All measurements performed in this study were intended as a basic tool for testing of a new commercial VCD setup from Bruker. Copyright 2000 Wiley-Liss, Inc.     

Methylenecyclopropane analogues of nucleosides: synthesis, absolute configuration, and enantioselectivity of antiviral effect of (R)-(-)- and (S)-(+)-synadenol.

Synthesis, absolute configuration and antiviral activity of enantiomeric antiviral agents (R)-(-)- and (S)-(+)-synadenol (2 and 3a) are described.     

A practical synthesis of the serotonin 5-HT2A receptor antagonist MDL 100907, its enantiomer and their 3-phenolic derivatives as precursors for [11C]labeled PET ligands

A practical synthesis of the 3-phenolic precursor of MDL 100907, a selective 5-HT2A receptor antagonist, is described. The route was also applied to the enantiomeric series, thus affording the direct precursors of both 3-[11C]MDL 100907 and its enantiomer as ligands for positron emission tomography. Similar methodology was developed for the direct synthesis of MDL 100907 and its enantiomer, MDL 100009. The routes utilized classical optical resolution of the N-nor intermediates in at least 98% enantiomeric excess and easily afforded multigram amounts of the chiral precursors of a variety of N- and 3-O-substituted enantiomers.     

Species-dependent enantioselective pharmacokinetics of PNU-103017, a Pyrone HIV protease inhibitor

PNU-103017, 4-Cyano-N-(3-(cyclopropyl(5,6,7,8,9,10-hexahydro-4-hydroxy-2-oxo-2H-cycloocta(b) pyran-3-yl)methyl)phenyl)-benzenesulfonamide, is a selective HIV aspartyl protease inhibitor under evaluation as a potential oral treatment of Acquired Immunodeficiency Diseases. PNU-103017 is a racemic mixture of two enantiomers, designated PNU-103264 (R-) and PNU-103265 (S-). Stereoselective pharmacokinetics of the two enantiomers of PNU-103017 were observed in the dog, rat, and human after single and multiple dose administration of the racemate and were apparently species-dependent. Mean enantiomeric ratios of plasma concentrations (R-/S-) at each time point were greater than 1 in the dog, ranging from 1.22 to 3.06, but less than 1 in the rat and in the human, ranging from 0.44 to 0.80 and 0.23 to 0.73, respectively. A trend towards increased or decreased (farther from 1:1, R-/S-) enantiomeric ratio of plasma concentrations with time after each administration was also observed. The enantiomeric ratio remained unchanged after multiple dose administration in the rat, dog, and human although enzyme induction and increased plasma clearance were observed for both enantiomers. Chirality 10:210–216, 1998. © 1998 Wiley-Liss, Inc.     

Stereoselective formation of benz[a]anthracene (+)-(5S,6R)-oxide and (+)-(8R,9S)-oxide by a highly purified and reconstituted system containing cytochrome P-450c

The principal oxidative metabolites formed from benz[a]anthracene (BA) by the rat liver microsomal monooxygenase system are the 5,6- and 8,9-arene oxides. In order to determine the enantiomeric composition and absolute configuration of these metabolically formed arene oxides, an HPLC procedure has been developed to separate the six isomeric glutathione conjugates obtained synthetically from the individual enantiomeric arene oxides. Both (+)- and (?)-BA 5,6-oxide gave the two possible positional isomers, but only one positional isomer was formed in each case from (+)- and (?)-BA 8,9-oxide. When [¹⁴C]-BA was incubated with a highly purified and reconstituted monooxygenase system containing cytochrome P-450c, and glutathione was allowed to react with the arene oxides formed, radio-active adducts were formed predominantly (>97%) from the (+)-(5S,6R) and (+)-(8R,9S) enantiomers. The present results are in accord with theoretical predictions of the steric requirements of the catalytic binding site of cytochrome P-450c.     

Study of the synthesis, antiproliferative properties, and interaction with DNA and polynucleotides of cisplatin-like Pt(II) complexes containing carcinogenic polyaromatic amines

The chemical and biological features of two newly synthesized [PtCl₂(L)(2-aminonaphthalene)] complexes (L is NH₃ or 2-aminonaphthalene) were compared with those of two already reported enantiomeric complexes of formula [PtCl₂(DABN)] [DABN is (R)-1,1′-binaphthyl-2,2′-diamine or (S)-1,1′-binaphthyl-2,2′-diamine]. Solution behavior, lipophilicity, cytotoxicity with regard to one colorectal (HCT116) and two ovarian (A2780 and A2780Cp8) human carcinoma cell lines, and in vitro DNA- and G-quadruplex-binding properties were evaluated. In particular, the cytotoxicity of [PtCl₂(NH₃)(2-aminonaphthalene)] was better than that of cisplatin for all cell lines, and rather resembled that of oxaliplatin. The solution behavior of the whole series of complexes and the absence of an evident relationship between lipophilicity and cytotoxicity seem to suggest that all these experimental parameters are probably smoothed out during the 3-day cytotoxicity experiments and do not strongly affect the half-maximal inhibitory concentrations. The results of electrophoretic studies indicate that different kinds of interaction with DNA can be involved in the mode of action of these complexes, with intercalation in double-stranded DNA and stacking on G-quadruplex DNA being strongly implicated in particular for [PtCl₂(NH₃)(2-aminonaphthalene)].     

Absolute configuration of labdanes and ent-clerodanes from Chromolaena pulchella by vibrational circular dichroism

The aerial parts of Chromolaena pulchella biosynthesize two groups of diterpenes belonging to opposite enantiomeric series, specifically, the furanoid ent-clerodanes (5R,8R,9S,10R)-(-)-hardwikiic acid (1), methyl (5R,8R,9S,10R)-(-)-hardwikiate (2), (5S,8R,9S,10R)-(-)-hautriwaic acid lactone (3), methyl (5R,8R,9S,10R)-(-)-nidoresedate (4) and methyl (8R,9R)-(-)-strictate (5), as well as the labdanes (5S,8R,9R,10S)-(+)-(13E)-labd-13-ene-8,15-diol (6) and (5S,8R,9R,10S)-(+)-isoabienol (7). The absolute configuration of the two groups of diterpenes was unambiguously assigned by comparison of the vibrational circular dichroism spectra of 3 for ent-clerodanes, and of 7 for labdanes with their theoretical spectra obtained by density functional theory calculations. The results support a biogenetic proposal to diterpenes found in the studied botanical species.