(R)- and (S)-5-hydroxy-2-(dipropylamino)tetralin (5-OH DPAT): assessment of optical purities and dopaminergic activities

Racemic 5-hydroxy-2-(dipropylamino)tetralin (5-OH DPAT), a potent and selective dopamine (DA) D2-receptor agonist, was resolved into the enantiomers by a new method. The enantiomers of 5-OH DPAT were determined by chiral ion-pair chromatography using N-benzyloxycarbonylglycyl-L-proline as the counter ion. The enantiomeric purity of (R)-5-OH DPAT was found to be greater than 99.7%. The ability of the enantiomers to change the rat brain DOPA levels was evaluated in vivo. The results indicate that (R)-5-OH DPAT is a weakly potent DA D2-receptor antagonist.     

HPLC resolution of diacylglycerol moieties of natural triacylglycerols on a chiral phase consisting of bonded (R)-(+)-1-(1-naphthyl)ethylamine

Chiral phase high performance liquid chromatographic resolution of sn-1,2(2,3)- and X-1,3-diacylglycerols generated by partial Grignard degradation from natural triacylglycerols was carried out using a chiral column (25 cm x 4.6 mm i.d.) containing (R)-(+)-1-(1-napthyl)ethylamine polymer chemically bonded to 300A wide pore spherical silica (5 microns particles). The diacylglycerols were chromatographed as 3,5-dinitrophenyl-urethanes and detected at 226 or 254 nm UV. By an isocratic elution with n-hexane- 1,2-dichloroethane-ethanol 40:10:1 (v/v/v) as the mobile phase, the sn-1,2(2,3)-diacylglycerols from corn, linseed, and menhaden oils were resolved into two clearly distinguishable enantiomer groups, although some peak overlappings between the enantiomers were observed in the linseed and menhaden oil diacylglycerols. In addition to the excellent enantiomer resolution, each enantiomer and the X-1,3-isomers were partially resolved into several peaks, which could be tentatively identified on the basis of equivalent carbon number. It is concluded that chiral phase high performance liquid chromatography can be utilized for effective resolution, identification, and quantitation of enantiomeric diacylglycerols from complex natural mixtures.     

Asymmetric synthesis and stereochemical structure-activity relationship of (R)- and (S)-8-[1-(2,4-dichlorophenyl)-2-imidazol-1-yl-ethoxy] octanoic acid heptyl ester, a potent inhibitor of allene oxide synthase

The preparation of both enantiomers of 8-[1-(2,4-dichlorophenyl)-2-imidazol-1-yl-ethoxy] octanoic acid heptyl ester (JM-8686), a potent inhibitor of allene oxide synthase, has been achieved using 2,4-dichlorophenacyl bromide as a starting material. The key step was the asymmetric reduction of 1-(2,4-dichlorophenyl)-2-imidazol-1-yl-ethanone with chiral BINAL-H. The products were purified by chiral high-performance liquid chromatography (HPLC) to afford pure (R)-JM-8686 and (S)-JM-8686. The inhibitory activities and binding affinities of these enantiomers toward allene oxide synthase were determined. We found that the inhibition potency of (R)-JM-8686 is approximately 200 times greater than that of (S)-JM-8686, with IC(50) values of approximately 5+/-0.2 nM and 950+/-18 nM, respectively. The dissociation constants of (R)-JM-8686 and (S)-JM-8686 with respect to the recombinant allene oxide synthase were approximately 1.4+/-0.3 microM and 4.8+/-0.6 microM, respectively.     

Separation and aquatic toxicity of enantiomers of 1-(substituted phenoxyacetoxy)alkylphosphonate herbicides

A series of organophosphorous compounds (OPs), 1-(substituted phenoxyacetoxy)alkylphosphonates containing a chiral carbon atom, show notable herbicidal activities. In this study, the enantioselective separation and biological toxicity of all these compounds were investigated. The enantioselective separation on the columns of Chiralpak AD, Chiralpak AS, Chiralcel OD, and Chiralcel OJ were compared under various chromatographic conditions. All the analytes investigated obtained baseline resolution (R(s) > 1.5) on Chiralpak AD column, which showed best chiral separation capacity. Further investigation was carried out on Chiralpak AD to evaluate the influence of the mobile phase composition and column temperature. The effect of the structural features on discrimination was also examined. The resolved enantiomers were distinguished by their signs of circular dichroism. The acute aquatic toxicity of enantiomers and racemate to Daphnia magna (D. magna) were assessed. The in vivo assays showed that compound 3 was about 2-148.5 times more toxic than the other four analogues to D. magna. The racemates of compounds 3 and 5 showed intermediate toxicity compare to their enantiomers, while those of compounds 1, 2, and 4 showed synergistic or antagonistic effect. These results suggest that the biological toxicity of chiral OPs to nontarget organisms is enantioselective and therefore should be evaluated with their pure enantiomers.     

Synthesis, resolution, stereochemistry, and molecular modeling of (R)- and (S)-2-acetyl-1-(4'-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline AMPAR antagonists

Recently we identified (R,S)-2-acetyl-1-(4'-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (6) as a potent non-competitive AMPA receptor antagonist able to prevent epileptic seizures. We report here the optimized synthesis of compound 6, its resolution by chiral preparative HPLC, and the absolute configuration of (R)-enantiomer established by X-ray diffractometry. The biological tests of the single enantiomers revealed that higher anticonvulsant and antagonistic effects reside in (R)-enantiomer as also suggested by molecular modeling studies.     

Synthesis and antipicornavirus activity of (R)- and (S)-1-[5-(4'-chlorobiphenyl-4-yloxy)-3-methylpentyl]-3-pyridin-4-yl-imidazolidin-2-one

The new pyridyl imidazolidinone derivative, 1-[5-(4'-chlorobiphenyl-4-yloxy)-3-methylpentyl]-3-pyridin-4-yl-imidazolidin-2-one (+/-)-1a, was synthesized and found to have an excellent antiviral activity against EV71 (IC50 = 0.009 microM). Therefore, both the enantiomers, (S)-(+)-1a and (R)-(-)-1a, have been prepared starting from readily available monomethyl (R)-3-methylglutarate (7) as a useful chiral building block and their antiviral activity was evaluated in a plaque reduction assay. Interestingly, we observed that the enantiomer (S)-(+)-1a was 10-fold more active against enterovirus71 (EV71) (IC50 = 0.003 microM) than the corresponding enantiomer (R)-(-)-1a (IC50 = 0.033 microM). Similar results were found against all five strains (1743, 2086, 2231, 4643, and BrCr) of EV71 tested. This demonstrated that the absolute configuration of the chiral carbon atom at the 3-position of the alkyl linker considerably influenced the anti-EV71 activity of these pyridyl imidazolidinones.     

Enantioseparation, absolute configuration determination, and anticonvulsant activity of (+/-)-1-(4-aminophenyl)-7,8-methylenedioxy-1,2,3,5-tetrahydro-4H-2,3-benzodiazepin-4-one

The resolution of 1-(4-aminophenyl)-7,8-methylenedioxy-1,2,3,5-tetrahydro-4H-benzodiazepin-4-one (+/-)-(R,S)-2 was accomplished by chiral HPLC. The absolute configuration of (+)-2, determined by X-ray crystallographic analysis, was R. The in vivo anticonvulsant activity of the enantiomers (+)-(R)-2 and (-)-(S)-2 is reported. It has been also demonstrated that compound (+/-)-(R,S)-2 in vivo undergoes oxidative metabolism to derivative 1.     

Separation and quantitation of (R)- and (S)-atenolol in human plasma and urine using an alpha 1-AGP column

A method for the determination of (R)- and (S)-atenolol in human plasma and urine is described. The enantiomers of atenolol are extracted into dichloromethane containing 3% heptafluorobutanol followed by acetylation with acetic anhydride at 60 degrees C for 2 h. The acetylated enantiomers were separated on a chiral alpha 1-AGP column. Quantitation was performed using fluorescence detection. A phosphate buffer pH 7.1 (0.01 M phosphate) containing 0.25% (v/v) acetonitrile was used as mobile phase. The described procedure allows the detection of less than 6 ng of each enantiomer in 1 ml plasma. The relative standard deviation is 4.4% at 30 ng/ml of each enantiomer in plasma. The plasma concentration of (R)- and (S)-atenolol did not differ significantly in two subjects who received a single tablet of racemic atenolol. The R/S ratio of atenolol in urine was approximately 1.     

Formation of glycine conjugate and (-)-(R)-enantiomer from (+)-(S)-2-phenylpropionic acid suggesting the formation of the CoA thioester intermediate of (+)-(S)-enantiomer in dogs.

It has been proposed that the chiral inversion of the 2-arylpropionic acids is due to the stereospecific formation of the (-)-R-profenyl-CoA thioesters which are putative intermediates in the inversion. Accordingly, amino acid conjugation, for which the CoA thioesters are obligate intermediates, should be restricted to those optical forms which give rise to the (-)-R-profenyl-CoA, i.e., the racemates and the (-)-(R)-isomers. We have examined this problem in dogs with respect to 2-phenylpropionic acid(2-PPA). Regardless of the optical configuration of 2-phenylpropionic acid administered, the glycine conjugate was the major urinary metabolite and this was shown to be exclusively the (+)-(S)-enantiomer by chiral HPLC. Both (-)-(R)- and (+)-(S)-2-phenylpropionic acid were present in plasma after the administration of either antipode, and further evidence of the chiral inversion of both enantiomers was provided by the presence of some 25% of the opposite enantiomer in the free 2-phenylpropionic acid and its glucuronide excreted in urine after administration of (-)-(R)- and (+)-(S)-2-phenylpropionic acid. The (+)-(S)-enantiomer underwent chiral inversion to the (-)-(R)-antipode when incubated with dog hepatocytes. These data suggests that both enantiomers of 2-phenylpropionic acid are substrates for canine hepatic acyl CoA ligase(s) and thus undergo chiral inversion, but that the CoA thioester of only (+)-(S)-2-phenylpropionic acid is a substrate for the glycine N-acyl transferase. These studies are presently being extended to the structure and species specificity of the reverse inversion and amino acid conjugation of profen NSAIDs.     

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.     

High-performance liquid chromatographic enantioseparation of 1-(aminoalkyl)-2-naphthol analogs on polysaccharide-based chiral stationary phases

The enantiomers of various 1-(alpha-aminobenzyl)-2-naphthol and 1-(aminoalkyl)-2-naphthol analogs were separated on cellulose-tris-3,5-dimethylphenyl carbamate-based chiral stationary phases (Chiralcel OD-H and Chiralcel OD-RH), using n-hexane/2-propanol/diethylamine or phosphate buffer/organic modifier mobile phases. The 3,5-dimethylphenyl carbamoylated cellulose columns were effective in both normal and rev ersed-phase modes. The effects of the mobile phase composition, the pH, the buffer concentration, and the structures of the substituents on the 2-naphthol on the enantioseparations were studied. The absolute configuration and elution sequence were determined for 1-(1-amino-2-methylpropyl)-2-naphthol: the elution sequence was S < R.     

Partial resolution of racemic trans-4-[5-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxyl-2-yl]benzoic acids by the diastereomer method with (R)- or (S)-1-phenylethylamine

The partial resolution is described of a series of racemic trans-4-[5-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxyl-2-yl]benzoic acids (1), which are the key intermediates for the synthesis of chiral organic radical liquid crystalline compounds and are crystallized to give racemic compounds. Racemic acid 1 [(+/-)-1] with a long alkyl chain (C7 to C13) could be resolved by the conventional diastereomeric salt formation using (R)- or (S)-1-phenylethylamine (2) as the resolving agent, whereas resolution of (+/-)-1 with a short alkyl chain (C4 to C6) was unsuccessful. Use of six equiv of (R)- or (S)-2 for the initial diastereomeric salt formation of (+/-)-1 with a C7-C13 alkyl chain, followed by recrystallization of the resulting salts once or twice, gave 2S,5S- or 2R,5R-enriched 1, respectively, in an ee range of 75-92% and with an overall recovery of 11-27%, based on the original quantity of (+/-)-1.     

Chiral discrimination of multiple profens as diastereomeric (R)-(+)-1-phenylethylamides by achiral dual-column gas chromatography

Profens were converted into diastereomeric (R)-(+)-1-phenylethylamides using ethyl chloroformate and triethylamine in dichloromethane. Gas chromatographic analysis on dual-columns with different polarities provided complete enantioresolution of eight profens, facilitating chiral discrimination based on matching with retention index sets characteristic of each enantiomer. The present method was linear (r >/= 0.9992) with good precision (0.8-6.0%) and accuracy (-9.3 to 0.003%), allowing detection of trace (R)-profens in optical purity test on four (S)-profen mixture in a single run. And the method allowed simultaneous enantiomeric screening for ibuprofen enantiomers and their chiral metabolites excreted in urine following administration of racemic ibuprofen.     

Diphenylethanediamine (DPEDA) derivatives as chiral selectors: IV. A comparison of 3,5-dinitrobenzoylated (S,S)- and (S,R)-DPEDA-derived chiral stationary phases with Pirkle's standard (R)-phenylglycine-derived phase in normal phase HPLC

Undecanoyl bound 3,5-dinitrobenzoyl-(S,R)-1,2-diphenylethane-1,2-diamine [(1S,2R)-DNB-DPEDA] as chiral selector (SO) has been synthesized and used as a chiral stationary phase (CSP II) for normal-phase enantioselective HPLC. It is compared with the already published diastereomeric (1S,2S)-DNB-DPEDA-derived CSP I and with the “standard” Pirkle DNB-(R)-phenylglycine-derived CSP III. Chromatographic data for about 100 racemic analytes reveal that CSP II is able to separate especially well enantiomers of derivatized aromatic carboxylic acids and analytes having a benzyl substituent bound at the chiral center. However, CSP I was found to be superior to CSP II and III in its general applicability and its ability to resolve enantiomers of heterocyclic drugs. © 1994 Wiley-Liss, Inc.     

Binding and photochemistry of enantiomeric 2-(3-benzoylphenyl)propionic acid (ketoprofen) in the human serum albumin environment.

Global analysis of circular dichroism multiwavelength data and time resolved fluorescence was applied to investigate the interaction of R(-)- and S(+)-ketoprofen (KP) with human serum albumin (HSA) in buffer solution at neutral pH. The most stable drug:protein adducts of 1 : 1 and 2 : 1 stoichiometry were characterized as regards the stability constants and the absolute circular dichroism spectra. The spectra of the diastereomeric 1 : 1 conjugates are negative with minima at ca. 350 nm for R(-)-KP and 330 nm for S(+)-KP, those of the 2 : 1 complexes are both negative with minimum at 340 nm and quite similar in shape to each other, thereby showing that the protein loses chiral recognition capability upon multiple binding. HSA intrinsic time resolved fluorescence data obtained exciting at 295 nm point to Trp 214 being located in the secondary binding site for both KP enantiomers. The photodegradation of the S(+)- and R(-)-KP:HSA complexes was studied by steady state photolysis using lambda(irr) > 320 nm. No decrease of the photodegradation quantum yields was observed in 1 : 1 complexes. An induction time for the photodegradation course in 2 : 1 complexes was observed. Transient absorption spectroscopy at lambda(exc) = 355 nm showed that triplet KP species were formed with stereo-differentiated lifetimes and high quantum yields (0.7-0.9). Secondary transients were consistent with the occurrence of photodecarboxylation and/or photoreduction within the protein matrix.     

Asymmetric synthesis and teratogenic activity of (R)- and (S)-2-ethylhexanoic acid, a metabolite of the plasticizer di-(2-ethylhexyl)phthalate

The stereoselectivity of the teratogenic activity of 2-ethylhexanoic acid (EHXA), a metabolite of the widely-used plasticizer di-(2-ethylhexyl)phthalate, was investigated. The enantiomers of EHXA were prepared via asymmetric synthesis with the aid of the chiral auxiliaries (R)- and (S)-1-amino-2-(methoxymethyl)pyrrolidine (RAMP, SAMP). The aqueous solutions of the sodium salts of (R)- and (S)-EHXA and the racemic EHXA [+/- )-EHXA) were injected each morning and evening of day 7 and 8 of gestation in the NMRI mouse (500 mg/kg, i.p.), a period highly sensitive in regard to the production of neural tube defects (exencephaly) by branched-chain carboxylic acids. (S)-EHXA did not yield any teratogenic or embryotoxic response in this model, while (R)-EHXA was highly teratogenic (59% of living fetuses exhibited exencephaly) and embryotoxic (as indicated by embryolethality and fetal weight retardation); the exencephaly rate induced by (+/- )-EHXA was between those of the two enantiomeres (32%). It is therefore likely that stereoselective interactions of the enantiomers of EHXA with chiral molecules in the embryo are decisive in regard to the teratogenic response. This first example of the stereoselectivity of the teratological activity of an environmental pollutant suggests that the safety of man-made chemicals can be improved by the use of pure enantiomers instead of racemates.     

Efficient asymmetric syntheses, determination of absolute configurations and biological activities of 1-[4,4-bis(4-fluorophenyl)butyl]-4-[2-hydroxy-3-(phenylamino)propyl]piperazine as a novel potent dopamine uptake inhibitor in the central nervous system

An efficient asymmetric synthesis of the chiral N-(3-chloro-2-hydroxypropyl)anilines (2a and 2b) was achieved through the regioselective ring-opening reaction of chiral epichlorohydrin with aniline. This was applied to an asymmetric synthesis of the enantiomers of 1-[4,4-bis(4-fluorophenyl)butyl]-4-[2-hydroxy-3-(phenylamino)propyl]piperazine 1 as a novel potent dopamine uptake inhibitor. Both enantiomers as trihydrochlorides, 4a.3HCl and 4b.3HCl, could be synthesized in good total yields and optical purities of 100% ee in three steps synthesis, respectively. The absolute configurations of 4a.3HCl and 4b.3HCl were determined using the modified Mosher's method with the related compounds, the intermediates (2a and 2b) and the free bases (4a and 4b). The analytical results indicated that 4a.3HCl and 4b.3HCl have the (S)- and (R)-configuration, respectively, and a series of reactions to provide them proceeded without the apparent influence on the stereochemistry at the chiral centers. In in vitro pharmacological evaluations, 4a.3HCl and 4b.3HCl showed potent dopamine transporter binding affinities, high dopamine, moderate serotonin, and weak norepinephrine uptake inhibitory activities, and 4a.3HCl exhibited a more potent and selective dopamine uptake inhibition over the serotonin or norepinephrine uptake inhibition as compared with 4b.3HCl. An ex vivo evaluation revealed that the oral administrations of both enantiomers at a dose of 30 mg/kg in rats displayed apparent dopamine uptake inhibitory activities and 4a.3HCl had a stronger tendency to inhibit dopamine uptake compared with 4b.3HCl.     

Determination of the absolute configuration of quercivorol, (1S,4R)-p-menth-2-en-1-ol, an aggregation pheromone of the ambrosia beetle Platypus quercivorus (Coleoptera: Platypodidae)

A pair of enantiomers of trans-p-menth-2-en-1-ol, an aggregation pheromone of Platypus quercivorus, was synthesized from (S)- and (R)-limonene. The retention time of the aggregation pheromone from the insect coincided with that of (1S,4R)-p-menth-2-en-1-ol synthesized from (S)-limonene from GC analyses with a chiral column, enabling the absolute configuration of the aggregation pheromone to be determined as (1S,4R).     

Lipase-catalyzed irreversible transesterification of 1-(2-furyl)ethanol using isopropenyl acetate

Asymmetric acetylation of racemic 1-(2-furyl)ethanol with the innocuous acyl donor isopropenyl acetate catalyzed by lipases in organic media afforded the chiral alcohol and acetate in high enantiomeric excess (up to 99%). The effect of molecular sieves as well as organic solvents on the kinetic resolution were studied. An effective separation of the enantiomers of both substrate and product was performed using gas chromatography on the chiral stationary phase heptakis-(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin.