Synthesis and characterization of the two enantiomers of a chiral sigma-1 receptor radioligand: (S)-(+)- and (R)-(-)-[18F]FBFP

Racemic [¹⁸F]FBFP ([¹⁸F]1) proved to be a potent σ₁ receptor radiotracer with superior imaging properties. The pure enantiomers of unlabeled compounds (S)- and (R)-1 and the corresponding iodonium ylide precursors were synthesized and characterized. The two enantiomers (S)-1 and (R)-1 exhibited comparable high affinity for σ₁ receptors and selectivity over σ₂ receptors. The Ca²⁺ fluorescence assay indicated that (R)-1 behaved as an antagonist and (S)-1 as an agonist for σ₁ receptors. The ¹⁸F-labeled enantiomers (S)- and (R)-[¹⁸F]1 were obtained in >99% enantiomeric purity from the corresponding enantiopure iodonium ylide precursors with radiochemical yield of 24.4% ± 2.6% and molar activity of 86–214 GBq/µmol. In ICR mice both (S)- and (R)-[¹⁸F]1 displayed comparable high brain uptake, brain-to-blood ratio, in vivo stability and binding specificity in the brain and peripheral organs. In micro-positron emission tomography (PET) imaging studies in rats, (S)-[¹⁸F]1 exhibited faster clearance from the brain than (R)-[¹⁸F]1, indicating different brain kinetics of the two enantiomers. Both (S)- and (R)-[¹⁸F]1 warrant further evaluation in primates to translate a single enantiomer with more suitable kinetics for imaging the σ₁ receptors in humans.     

Univocal syntheses of 2- and 3-hydroxymethyl-2,3-dihydro[1,4]dioxino[2,3-b]pyridine enantiomers

The enantiomers of 2- and 3-hydroxymethyl substituted 2,3-dihydro[1,4]dioxino[2,3-b]pyridine 1 and 2, important chiral building blocks for the preparation of several biologically active compounds, were synthesized. (S)- and (R)-1 were obtained from either one or both the enantiomers of benzylglycerol, while (S)- and (R)-2 were obtained from (R)- and (S)-isopropylideneglycerol, respectively. The novel efficient synthetic strategies, which do not follow routes already reported for the corresponding racemates, ensure very high regioselectivity and maintenance of the enantiomeric purity of the starting materials. The enantiomeric composition of the title compounds was determined by chiral HPLC or NMR. The key intermediate in the synthesis of non-racemic 1, namely 1-benzyl-2-mesyl-3-tritylglycerol, is a new high melting chiral C₃ synthon, worth considering for its stability, versatility, easy isolation by simple crystallization and, potential of configuration inversion through a simple one-pot reaction sequence.     

Simple and straightforward synthesis of novel enantiopure ionic liquids via efficient enzymatic resolution of (±)-2-(1H-imidazol-1-yl)cyclohexanol

Both enantiomers of enantiopure imidazolium ionic liquids were synthesized by a simple and straightforward procedure from (R,R)- and (S,S)-2-(1H-imidazol-1-yl)cyclohexanol derivatives obtained via lipase-catalyzed resolution. Structural properties and thermal stability of these compounds have been studied to elucidate their potential applications in asymmetric catalysis.     

Synthesis,spectral characterization and biological evaluation of novel 1-(2-(4,5-dimethyl-2-phenyl-1H-imidazol-1-yl)ethyl)piperazine derivatives

A simple, highly useful synthesis of 1-(2-(4,5-dimethyl-2-phenyl-1H-imidazol-1-yl)ethyl)piperazine derivatives is achieved by a four component cyclo condensation of diacetyl, aromatic aldehyde, 2-(piperazin-1-yl)ethanamine and ammonium acetate using SO₄²⁻/Y₂O₃ as a catalyst in ethanol. The synthesized compounds were characterized by IR, ¹H and ¹³C NMR and mass spectral studies. All the synthesized compounds were screened for their in vitro antimicrobial studies. Among the newly synthesized compounds 5d, 5e, 5h and 5m showed excellent antibacterial and antifungal activities when compared to the standard drugs.     

Design,synthesis and evaluation of 2-(4-(substituted benzoyl)-1,4-diazepan-1-yl)-N-phenylacetamide derivatives as a new class of falcipain-2 inhibitors

The cysteine protease, falcipain-2 is an important drug target in human malaria parasite Plasmodium falciparum. A new series of 2-(4-(substituted benzoyl)-1,4-diazepan-1-yl)-N-phenylacetamide derivatives 5(a–t) were designed as per pharmacophoric requirements of falcipain-2 inhibitors using ligand-based approach. The target compounds were synthesized from the key intermediate, 2-(1,4-Diazepan-1-yl)-N-phenylacetamide, by coupling it with appropriate carboxylic acids using carbodiimide chemistry. Structural features of target compounds were characterized by spectral data (¹H NMR, and mass) and elemental analyses. The purity of the final compounds was confirmed by HPLC. The compounds were tested for their in vitro falcipain-2 inhibitor activity on recombinant falcipain-2 enzyme. Five compounds 5b, 5g, 5h, 5j, 5k showed good inhibitory activity (>60%), against falcipain-2 at 10 μM concentration, and fifteen compounds showed weak to moderate inhibitor activity. Compound 5g, the most potent compound from this series showed 72% inhibition at 10 μM concentrations.     

Practical synthesis of (E)- and (Z)-2-silyl-3-penten-1-ols with high enantiopurity

Practical methods for the synthesis of the optically active (E)- and (Z)-2-silyl-3-pentene-1-ols are described. The optically pure (E)-allylsilane was synthesized from commercially available (R)-3-butyn-2-ol in five steps involving hydrozirconation followed by alkylation of the resulting alkenylmetal with BnOCH₂Cl. On the other hand, both enantiomers of the corresponding (Z)-allylsilane were prepared from commercially available dimethylphenylvinylsilane through epoxidation, the regioselective epoxide-opening reaction with 1-propynylmagnesium bromide, and the subsequent optical resolution using a lipase.     

Synthesis and optical behaviors of 2-(9-phenanthrenyl)-, 2-(9-anthryl)-, and 2-(1-pyrenyl)-1-alkylimidazole homologues

Eight 2-(9-phenanthrenyl)-, 2-(9-anthryl)- and 2-(1-pyrenyl)-1-alkyl-benzimidazole compounds, three 2-(9-anthryl)-1-alkylphenanthroimidazole compounds and five 4,5-diphenyl-1-alkyl-2-(9-anthryl)imidazole compounds were synthesized by alkylation reactions of the corresponding benzimidazole, phenanthroimidazole or imidazole compounds. 2-(10-Bromo-9-anthryl)-1-alkyl-benzimidazole compounds were prepared by bromination reaction of 2-(9-anthryl)-1-alkylbenzimidazole compounds. All the synthesized compounds were characterized by elemental analysis, ¹H NMR, ¹³C NMR, MS or HRMS; their absorption coefficients (), maximum absorption λ_amax, fluorescence emission maximum λ_em, Stokes shifts and fluorescence quantum yields (Φ_F) in ethyl acetate were determined; their fluorescent lifetimes (T₁ and T₂) were measured in ethyl acetate and in solid state, respectively. The crystal structure of 2-(9-anthryl)-1-n-butyl-4,5-diphenylimidazole (12a) was determined to be triclinic, space group P-1 types, using single crystal X-ray crystallography technique. The results showed that these compounds exhibited moderate fluorescence-emission abilities and higher solubility in most organic solvents than their corresponding starting materials. The relationships between the optical behaviors and structures for these compounds were discussed.     

Synthesis of (E)-2-(1-ferrocenylmethylidene)malonic acid derivatives by a cobalt-catalyzed domino reaction of ethyl diazoacetate, carbon monoxide and ferrocenylimines

The domino reaction of ethyl diazoacetate, carbon monoxide and ferrocenylimines was investigated in the presence of Co₂(CO)₈ as catalyst. In most cases the main products are 2-(1-ferrocenylmethylidene) malonates formed by an N(1)-C(4) cleavage of the primarily derived β-lactams. The latter compounds could only be isolated when the reaction was carried out at relatively low CO pressure, using an excess of ethyl diazoacetate. trans-N-(tert-Butyl)-3-ethoxycarbonyl-4-ferrocenyl-β-lactam proved to be the most stable one among these compounds and could be isolated in 55% yield. N-alkyl β-lactams were shown to undergo acidic cleavage leading to the E isomers of 2-(1-ferrocenylmethylidene) malonates as the main products. The structures of the two new compounds, (E)-2-ethoxycarbonyl-3-ferrocenyl-N-((R)-1-phenylethyl)-2-propenamide and trans-N-(tert-butyl)-3-ethoxycarbonyl-4-ferrocenyl-β-lactam were confirmed by X-ray crystallography. The relative thermodynamical stability of the products as well as the energetics of the acid-mediated cleavage of the β-lactam ring was elucidated with DFT calculations.     

Crystal and molecular structure of methyl(±)-2-((1R,3R)-3-{ 2-[(3S)-1-ethyl-3-hydroxy-2-oxo-2,3-dihydro-1H-3-indolyl]acetyl}-2,2-dimethylcyclobutyl) acetate

The structure of methyl (±)-2-((1R,3R)-3-{ 2-[(3S)-1-ethyl-3-hydroxy-2-oxo-2,3-dihydro-1H-3-indolyl]acetyl}-2,2-dimethylcyclobutyl) acetate has been determined by single crystal X-ray diffraction. The crystal belongs to triclinic system; parameters of the unit cell are: a = 6.551(1) Å, b = 11.506(1) Å, c = 14.334(1) Å, α = 101.41(1)°, β = 97.57(1)°, γ = 104.72(1)°; space group P-1, Z = 2, composition C₂₁H₂₇NO₅. The structure of N-ethyloxindole fragment is usual for the present class of compounds. The configuration of the formed asymmetric carbon atom C(3) of the pyrrole ring along with the configuration of C(12) and C(14) atoms of 2,2-dimethylcyclobutane ring form the side chain of the molecule were determined. There is observed the generation of centrosymmetrical dimers in the crystal structure due to realized intermolecular hydrogen bond of O-H...O type, 2.808(2) Å.     

Stereocontrolled synthesis of the enantiomers of 1-[2-hydroxy-3-(4-phenyl-1-piperazinyl)-propyl]-pyrrolidin-2-one

The asymmetric synthesis of 1-[2-hydroxy-3-(4-phenyl-1-piperazinyl)-propyl]-pyrrolidin-2-one 1 is described. Enantiomers of compound 1 were obtained using the Sharpless asymmetric dihydroxylation (AD) or hydrolytic kinetic resolution (HKR) methods. The enantiomers of compound 1, which were obtained by HKR had higher enantiomeric excesses than those which were synthesized by AD and epoxidation. The enantiomeric purity of the synthesized compounds was determinated by capillary electrophoresis.     

Trichosporon cutaneum-promoted deracemization of (±)-2-hydroxindan-1-one: a mechanistic study

A mechanistic study of the deracemization of (±)-2-hydroxy-1-indanone mediated by the yeast Trichosporon cutaneum to afford pure (1S,2R)-1,2-indandiol is reported. The key aspect of the study was the use of pure (R)- and (S)-2-hydroxy-1-indanone enantiomers to ensure reliable conclusions. Experiments in the absence of yeast cells or using dead cells disclosed that the pure enantiomers were not racemized, which suggest that the whole dynamic kinetic resolution process is enzymatic in character. When living yeast cells were used the (R)-substrate was smoothly converted to (1S,2R)-1,2-indandiol, whilst the (S)-2-hydroxy-1-indanone was converted to the same diol through a more complex fashion, which requires a more lengthy oxidation–reduction pathway having the 1,2-indanedione as an achiral intermediate. An unexpected observation was that 1,2-indanone acts as a moderate inhibitor of the reductive enzymes acting in the conversion of (R)-2-hydroxy-1-indanone to (1S,2R)-1,2-indandiol.     

Efficient synthesis of optically active 1-(2-carboxymethyl-6-ethylphenyl)-1H-pyrrole-2-carboxylic acid: a novel atropisomeric 1-arylpyrrole derivative

An efficient synthesis and resolution of (±)-1-(2-carboxymethyl-6-ethylphenyl)-1H-pyrrole-2-carboxylic acid has been developed for the preparation of novel optically active atropisomers. The ee values were measured by a ¹H NMR spectroscopic method using quinidine as the chiral complexing agent. Absolute configurations of the separated enantiomers were determined using single crystal X-ray diffraction measurements of both the disodium salt and the (R)-1-phenylethylamine salt of the enantiomerically pure dicarboxylic acid, separately. The analysis of the CD spectra with the aid of TD-DFT quantum chemical calculations confirmed the assignment of configurations.     

Enantioseparation of Some New 1-(2-Naphthyl)-1-ethanol Ester Derivatives by HPLC on Chiralcel OD

The direct enantiomeric resolution of racemic 2-(1H-imidazole-1-yl)-1-naphthalene-2-yl)ethanol esters, 1-(naphthalene-2-yl)ethanol esters, and 1-(1-hydroxynaphthalene-2-yl)-2-(1H-imidazole-1-yl)ethanol on silica-based cellulose tris(3,5-dimethylphenylcarbamate) (Chiralcel OD) column is described. The separations were performed using mobile phases which consist of alcohol (methanol, ethanol or 2-propanol)/n-hexane in various proportions. The effect of structural features of the solutes along with the nature and concentration of alcohol in the mobile phase on the discrimination between the enantiomers was examined for different mobile phase compositions. The results suggest that not only the structure and concentration of alcohol in the mobile phase, but also the subtle structural differences in racemates can have a pronounced effect on enantiomeric separation and retention. Baseline separations were obtained for 2-(1H-imidazole-1-yl)-1-naphthalene-2-yl)ethanol esters carrying imidazole ring in addition to ester functional group in their structures. The α values of the resolved enantiomers of 2-(1H-imidazole-1-yl)-1-naphthalene-2-yl)ethanol esters were in the range of 1.49–1.62 while the R _s values varied from 4.20 to 6.75 when methanol/n-hexane (70:30 v/v) was used as mobile phase.

     

Enzyme-catalyzed synthesis and absolute configuration of (1S,2R,5S)- and (1R,2S,5R)-2-(1-hydroxyethyl)-1-(methoxymethyloxyethyl)cyclobutane-1-carbonitrile,key intermediates for the preparation of chiral cyclobutane-containing pheromones

Formal synthesis of chiral grandisol and the oleander scale pheromone and their antipodes can be achieved through a convenient lipase-catalyzed enantiodifferentiation process of the common cyclobutane intermediate (±)-2-(1-hydroxyethyl)-1-(methoxymethyloxyethyl)cyclobutane-1-carbonitrile 3. The resolution afforded both enantiomers in almost enantiomerically pure form and their absolute configurations were assigned on the basis of the Δδ values for their (R)- and (S)-MTPA esters.     

A synthesis of 1-substituted 5-[2-(acylamino)ethyl]-1H-pyrazole-4-carboxamides

A seven-step synthesis of 1-substituted 5-(2-acylaminoethyl)-1H-pyrazole-4-carboxamides 20 as the pyrazole analogues of histamine was developed. The synthesis starts with a three-step preparation of N(1)-substituted methyl 5-(2-tert-butoxycarbonylaminoethyl)-1H-pyrazole-4-carboxylates 7 from commercially available Boc-β-alanine (1). Subsequent four-step transformation of the key-intermediates 7 into the final products 20 was performed following two complementary reaction sequences comprising acidolytic removal of the Boc group, hydrolysis of the COOMe group, amidations of the COOH group, and acylations of the NH₂ group. The structures of pyrazole derivatives were determined by spectroscopic methods and by X-ray diffraction.     

Asymmetric synthesis and retroracemisation of amino acids via copper(II) complexes with schiff bases between chiral 1-(N,N-dimethylaminomethyl)-2-formylcymantrene and dipeptides

1-(N,N-Dimethylaminomethyl)-2-formylcymantrene (AFCMT) has been resolved into enantiomers through an intermediate formation of diastereomeric complexes with (S)-Ala-(S)-Ala, (S)-Ala-Gly and Gly-(S)-Ala. By the X-ray anomalous dispersion method the absolute configuration of its enantiomers has been determined: (-)₄₃₆ AFCMT-(S), (+)₄₃₆ AFCMT-(R).Alkylation of enantiomeric complexes (R)-and (S)-AFCMT-(GlyGly) Cu(II) with acetaldehyde gives, respectively, (R)-and (S)-Thr with an asymmetric yield of 92–98% and (R)- and (S)-allo-Thr with an asymmetric yield of 95–100%, only the N-terminal glycine being alkylated.The AFCMT enantiomers were also employed for retroracemisation of (R,S)-Ala-(R,S)-Nva; in this case an excess of (S)-Ala and (R)-Nva is obtained for (S)-AFCMT. (R)- and (S)-AFCMT are not liable to racemisation in the course of the threonine synthesis and retroracemisation of depeptides and can be repeatedly employed for these transformations.     

Chiral fluoroacetic acid: synthesis of (R)- and (S)-[2H1]-fluoroacetate in high enantiopurity

A two-step synthesis of (R)- and (S)-[²H₁]-fluoroacetate (sodium salts) in high enantioselectivity is reported. The synthesis is the development of a previous one in which the enantioselectivity has been increased from ～38% ee to >95% ee. The improvement in enantioselectivity applied Bio’s methodology, which involved a deoxyfluorination reaction with DAST on either enantiomer of [²H₁]-benzyl alcohol, adding TMS-morpholine to the reaction. The additive promotes an S_N2 inversion process, and suppresses a competing non-stereospecific S_N1 reaction course, and as a result significantly improves the stereointegrity of the C–F bond formation. The intermediate [²H₁]-benzyl alcohols, [²H₁]-benzyl fluorides and the product [²H₁]-fluoroacetates as their hexyl esters were separately assayed for their stereochemical integrity, using the Courtieu method. This method involved measuring their ²H NMR spectra in a chiral matrix of poly-γ-benzyl l-glutamate. The chiral assay demonstrated that there was no significant loss in stereointegrity during the deoxyfluorination reaction and showed that the enantiomers of [²H₁]-fluoroacetate were generated with high enantiomeric purity (95% ee).     

Enantiopure trans-1-amino-2-(arylsulfanyl)cyclohexanes: novel chiral motifs for ligands and organocatalysts

In order to obtain the title compounds (1R,2R)-cyclohexane-1,2-diol was stereoselectively converted into cis-(1R,2S)-2-(arylsulfanyl)cyclohexanols and these products were submitted to the nucleophilic substitution via the Mitsunobu reaction (HN₃, DEAD). Reduction of the isolated azides gave the desired trans-(1S,2S)-1-amino-2-(arylsulfanyl)cyclohexanes. The (1S,2S)-1-amino-2-(2-aminophenylsulfanyl)cyclohexanes thus prepared were reacted with 3,5-bis(trifluoromethyl)phenyl isothiocyanate to furnish the respective bis-thiourea compounds. An application of a derivative of this type as an organocatalyst (20 mol %) in the Baylis–Hillman reaction gave the respective product in up to 93% ee.     

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

Mexiletine [1-(2,6-dimethylphenoxy)-2-aminopropane], an orally effective antiarrhythmic agent, exhibits enantioselective pharmacokinetics and pharmacodynamics during mexiletine therapy. The purpose of this paper is to emphasize the advantage of tetrahydropyranyl-protected mandelic acid (THPMA) in the resolution of mexiletine enantiomers. Both enantiomers of mexiletine were obtained in 99% enantiomeric excess. Judging by the differential shielding effects in the ¹H and ¹³C NMR analyses, we have observed the opposite predominant conformation for the mexiletine mandelates in comparison with the O-methylmandelates.     

2-Alkyl-4-amino-5-(tert-butyl-NNO-azoxy)-2H-1,2,3-triazole 1-oxides: synthesis and reduction

A new approach to the synthesis of 4-amino-5-(tert-butyl-NNO-azoxy)-2-R-2H-1,2,3-triazole 1-oxides 1 was developed. Compounds 1 were obtained by reactions of 3-amino-4-(tert-butyl-NNO-azoxy)furoxan with aliphatic amines RNH₂ (R = Me, Et, Prⁱ, Bu, and Bu^t). 4-Amino-5-(tert-butyl-NNO-azoxy)-2-tert-butyl-2H-1,2,3-triazole 1-oxide was transformed under the action of acids into 4-amino-5-(tert-butyl-NNO-azoxy)-1-hydroxy-1H-1,2,3-triazole. Methylation of the latter with diazomethane mainly involves the O atom of the triazole oxide ring. Reduction of compounds 1 gave 4-amino-5-(tert-butyl-NNO-azoxy)-2-R-2H-1,2,3-triazoles and 4-amino-5-(tert-butyldiazenyl)-2-R-2H-1,2,3-triazoles (R = Me, Prⁱ, and Bu^t). The structures of the compounds obtained were confirmed by ¹H, ¹³C, and ¹⁴N NMR spectroscopy.