Solubility, metastable zone width, and racemic characterization of propranolol hydrochloride

Characterization of the racemic species, which can be a racemic compound, a racemic conglomerate, or a pseudoracemate (solid solution), is a prerequisite for the design of crystallization resolution processes. It is useful to determine the solid/liquid equilibrium solubility of the enantiomer mixtures for crystallization operation. For the beta-blocker drug propranolol hydrochloride, Gibbs free energy of formation of racemic compound and entropy of mixing of the (R)- and (S)- enantiomers in the liquid state for racemic conglomerate were calculated. The structural differences between (R, S)-propranolol hydrochloride and its (S)-enantiomer were further investigated by powder X-ray diffraction patterns, infrared spectra, and solid-state NMR spectra. The solubility and metastable zone width of (R, S)- propranolol hydrochloride in a mixed solvent of methanol and acetone were determined by cooling crystallization over the temperature range 3.5-42.5 degrees C. The ternary solubility diagram of (R)-, (S)-propranolol hydrochloride was constructed using the same mixed solvent. The diagram will be useful as a guide for choosing crystallization operation conditions to produce pure enantiomers.     

Comparison of different strategies for the lipase-catalyzed preparative resolution of racemic acids and alcohols: Asymmetric hydrolysis, esterification, and transesterification

Lipase from Candida cylindracea has been found to be a highly stereospecific catalyst suitable for preparative resolution of racemic acids and alcohols. Using (R, S)-2-(p-chlorophenoxy) propionic acid (whose R isomer is a herbicide) and (R, S)-sec-butanol (a versatile synthon) as model compounds, three alternative approaches to lipase-catalyzed resolutions-asymmetric hydrolysis, esterification, and transesterification-have been compared. Enzymatic esterification in biphasic systems has been ruled out for preparative resolutions because addition of the acids lowers the pH of the aqueous phase thereby greatly reducing the efficiency of the procedure. Both enzymatic hydrolysis and biphasic transesterification afforded resolution of the racemates on a gram scale. From the standpoint of productivity, ease of product separation, and the amount of steps required, lipasecatalyzed asymmetric hydrolysis has been judged to be superior for the practical resolution of racemic acids, and lipase-catalyzed asymmetric transesterification to be the method of choice for the practical resolution of racemic alcohols.     

Comparison of the biophysical properties of racemic and d-erythro-N-acyl sphingomyelins.

In this study stereochemically pure d-erythro-sphingomyelins (SMs) with either 16:0 or 18:1(cisDelta9) as the N-linked acyl-chain were synthesized. Our purpose was to examine the properties of these sphingomyelins and acyl-chain matched racemic (d-erythro/l-threo) sphingomyelins in model membranes. Liquid-expanded d-erythro-N-16:0-SM in monolayers was observed to pack more densely than the corresponding racemic sphingomyelin. Cholesterol desorption to beta-cyclodextrin was significantly slower from d-erythro-N-16:0-SM monolayers than from racemic N-16:0-SM monolayers. Significantly more condensed domains were seen in cholesterol/d-erythro-N-16:0-SM monolayers than in the corresponding racemic mixed monolayers, when [7-nitrobenz-2-oxa-1, 3-diazol-4-yl]phosphatidylcholine was used as a probe in monolayer fluorescence microscopy. With monolayers of N-18:1-SMs, both the lateral packing densities (sphingomyelin monolayers) and the rates of cholesterol desorption (mixed cholesterol/sphingomyelin monolayers) was found to be similar for d-erythro and racemic sphingomyelins. The phase transition temperature and enthalpy of d-erythro-N-16:0-SM in bilayer membranes were slightly higher compared with the corresponding racemic sphingomyelin (41.1 degrees C and 8.4 +/- 0.4 kJ/mol, and 39.9 degrees C and 7.2 +/- 0.2 kJ/mol, respectively). Finally, d-erythro-sphingomyelins in monolayers (both N-16:0 and N-18:1 species) were not as easily degraded at 37 degrees C by sphingomyelinase (Staphylococcus aureus) as the corresponding racemic sphingomyelins. We conclude that racemic sphingomyelins differ significantly in their biophysical properties from the physiologically relevant d-erythro sphingomyelins.     

Enzymatic kinetic resolution of racemic ibuprofen: past,present and future

This review is a journey concerning the investigations of the kinetic resolution of racemic ibuprofen for the last 20 years. The relevancy of the pharmacological uses of the S(?+?) enantiomer along with its higher cost compared with racemic profen are the driving forces of a variety of scientific research studies addressing the enzymatic resolution of ibuprofen through enantiomeric esterification using lipases as biocatalysts. Lipases of fungal sources such as Candida rugosa, Rhizomucor miehei and the lipase B of Candida antarctica have been extensively studied both in homogeneous and heterogeneous (immobilized on solid supports) processes. In this context, the various alcohols and organic co-solvents frequently used in the esterification of racemic ibuprofen are summarized and discussed in this review. Moreover, recent investigations using membranes as reactors coupled with the separation of the desired product and microfluidic devices are presented. Finally, some guidelines about future perspectives regarding the technology of the kinetic resolution of profens and research niches are given.     

A mixed stationary phase containing two versatile cyclodextrin-based selectors for the simultaneous gas chromatographic enantioseparation of racemic alkanes and racemic alpha-amino acid derivatives

In an effort to simultaneously enantioseparate racemic unfunctionalized alkanes and racemic alpha-amino acid derivatives by gas chromatography (GC) in forthcoming experiments related to the search for extraterrestrial homochirality, the two versatile modified cyclodextrin (CD) selectors octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin (Lipodex G) and heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin were dissolved in a polysiloxane and the mixed binary chiral selector system was coated onto a 50m x 0.25 mm i.d. fused silica capillary column. Whereas the former CD selector enantioseparates racemic unfunctionalized alkanes the latter CD selector preferentially resolves N-(O,S)-trifluoroacetyl-alpha-amino acid alkyl esters. With both CD selectors employed as mixed binary chiral selector system present in one chiral stationary phase (CSP), the simultaneous gas chromatographic enantioseparation of racemic alkanes and of racemic derivatized alpha-amino acids is achieved in a single temperature-programmed run. Also for other classes of racemic compounds, the scope of enantioseparation could be extended as compared to the conventional use of the single CD selectors in GC.     

Kinetics, stereospecificity, and expression of the malolactic enzyme.

Mass spectrometric measurement of carbon dioxide production was used to study malolactic fermentation (MLF) in Lactobacillus collinoides isolated from cider. The kinetics and stereospecificity of the malolactic enzyme (MLE) were studied, and the stoichiometry of the reaction sequence was investigated. The optimum pH for activity of the MLE was 4.9. MLF was more rapid (in both intact cells and cell extracts) when L-malic acid was used than when D-malic acid or the racemic mixture was added. The enzyme was found to be constitutively present in L. collinoides. Addition of L-malic acid (37 mM) to the growth medium resulted in increased MLE activity; addition of the D isomer alone or the racemic mixture resulted in lower activities. Addition of the main sugars in apple juice (fructose, sucrose, and glucose) to the growth medium in the presence of malic acid repressed production of MLE to similar extents in all three cases; in the absence of malic acid, instead of inhibiting MLF, addition of sugars to the growth medium somewhat increased the residual MLE activity.     

Solid-Phase Synthesis of Surfactin,a Powerful Biosurfactant Produced by <Emphasis Type="Italic">Bacillus subtilis</Emphasis>, and of Four Analogues

Using the Fmoc methodology, we report the chemical synthesis of surfactin and of four of its analogues, by stepwise solid-phase peptide synthesis (SPPS) on Sasrin resin. Formation of depsipeptide bond was performed with EDC (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. In developing our strategy, surfactin was used as a model and we synthesized both its racemic mixture and its R isoform. (R) 3-hydroxy fatty acid was obtained using Candida antarctica lipase from the racemic fatty acid, allowing a further identification of both R and S isoforms in the racemic mixture. Analogues were synthesized as racemic linear lipopeptides. Then, both enantiomers were separated and purified by adsorption chromatography on silicic acid, following cleavage from the resin. Linear R lipoheptapeptides were identified by TLC. They exhibit, in all cases, higher R_f values than those of the corresponding S isoforms. Cyclization was then performed independently for each enantiomer, using a HATU/DIEA coupling in solution. The yields were highly dependent on the position and on the nature of the modified amino acids.     

Propagation of Biochirality: Crossovers and Nonclassical Crystallization Kinetics of Aspartic Acid in Water

All experimental procedures discussed could be treated as a screening tool for probing the existence of molecular association among the chiral molecules and the solvent system. The molecular association phases of a racemic conglomerate solution (CS) and a racemic compound solution (RCS), and the templating effect of aspartic acid solid surface were observed to minimize the chance of redissolving racemic conglomerate and racemic compound aspartic acid in water and reforming an RCS in crossovers experiments. Only 1 %wt% of l‐aspartic acid was adequate enough to induce a transformation from a racemic compound aspartic acid to a racemic conglomerate aspartic acid. This would make the propagation of biochirality more feasible and sound. However, tetrapeptide, (l‐aspartic acid)₄, failed to induce enantioseparation as templates purely by crystallization. Nonclassical crystallization theory was needed to take into account the existence of a CS. Fundamental parameters of the crystallization kinetics such as the induction time, interfacial energy, Gibbs energetic barrier, nucleation rate, and critical size of stable nuclei of: (i) racemic compound aspartic acid, (ii) racemic compound aspartic acid seeded with 1 %wt% l‐aspartic acid, (iii) racemic conglomerate aspartic acid, and (iv) l‐aspartic acid were evaluated and compared with different initial supersaturation ratios. Morphological studies of crystals grown from the crystallization kinetics were also carried out.Chirality 25:768–779, 2013. © 2013 Wiley Periodicals, Inc.     

Determination of felodipine, its enantiomers, and a pyridine metabolite in human plasma by capillary gas chromatography with mass spectrometric detection

Sensitive methods based on capillary gas chromatography (GC) with mass spectrometric (MS) detection in a selected-ion monitoring mode (SIM) for the determination of racemic felodipine, its enantiomers, and a pyridine metabolite in human plasma are described. Following liquid-liquid extraction from plasma, enantiomers of felodipine were separated on a chiral HPLC column (Chiralcel OJ) and fractions containing each isomer were collected on a continuous basis using a fraction collector. These fractions were later analyzed by GC-MS-SIM. A similar method based on GC-MS-SIM detection was developed for the determination of racemic felodipine and its pyridine metabolite with a minor modification of sample preparation. The limits of quantitation in plasma were 0.1 ng/ml for both the R(+)- and S(−)-enantiomers of felodipine and 0.5 ng/ml for both racemic felodipine and its pyridine metabolite. The stereoselective assay was used to support a clinical study with racemic felodipine, and was capable of analyzing more than 30 plasma samples per day.     

Novel ketoconazole analogues based on the replacement of 2,4-dichlorophenyl group with 1,4-benzothiazine moiety: design, synthesis, and microbiological evaluation

As a part of a program to develop novel antifungal agents, new compounds which incorporate the 1,4-benzothiazine moiety into the structure of ketoconazole (KTZ) were prepared. These compounds were computationally investigated to assess whether the 1,4-benzothiazine moiety was a suitable bioisosteric replacement for the 2,4-dichlorophenyl group of KTZ in order to obtain a more potent inhibition of CYP51 enzyme of Candida albicans. Results of preliminary microbiological studies show that the racemic cis-7 analogue has a good in vivo activity, comparable to that of KTZ, but the best activity was observed in the racemic trans-7 analogue.     

Comparative activities of the S-enantiomer and racemic forms of equol on bone fragility in ovariectomized mice

We compared the effects of the S-enantiomer and racemic forms of equol on bone using ovariectomized (OVX) mice. Femoral bone mineral density and bone strength decreased in the OVX mice, but not in OVX mice administered 0.5 mg/d S-equol. This, however, did not hold for racemic equol. Serum and urine S-equol concentrations were higher in the mice administered S-equol than in those administered racemic equol. These results suggest that the inhibitory effects of S-equol on bone fragility in OVX mice are greater than those of racemic equol.     

Enantiomeric Crystallization from DL-Aspartic and DL-Glutamic Acids: Implications for Biomolecular Chirality in the Origin of Life

Amino acids in living systems consist almost exclusively of the L-enantiomer. How and when this homochiral characteristic of life came to be has been a matter of intense investigation for many years. Among the hypotheses proposed to explain theappearance of chiral homogeneity, the spontaneous resolution of conglomerates seems one of the most plausible. Racemic solids may crystallize from solution either as racemic compounds(both enantiomeric molecules in the same crystal), or lesscommonly as conglomerates (each enantiomer molecule separate indifferent enantiomeric crystals). Only conglomerates can developa spontaneous resolution (one of the enantiomeric molecule crystallizes preferentially, the other one remains in solution).Most of natural amino acids are racemic compounds at moderatetemperatures. How can we expect a hypothetical spontaneous resolution of these amino acids if they are not conglomerates?In this paper we show how DL-aspartic and DL-glutamic amino acids(racemic compounds), crystallize at ambient conditions as trueconglomerates. The experimental conditions here described,that allows this `anomalous' behaviour, could be also found innatural sedimentary environments. We suggest that these experimental procedures and its natural equivalents, have apotential interest for the investigation of the spontaneous resolution of racemic compounds comprising molecules associatedwith the origin of life.     

Pharmacokinetic data of propranolol enantiomers in a comparative human study with (S)- and (R,S)-propranolol

The pharmacokinetics of (S)-propranolol were compared after the oral administration of a 40 mg dose of the pure enantiomer and an 80 mg dose of a racemic mixture of (R,S)-propranolol. The results of this study indicate that the bioavailability of (S)-propranolol, as expressed by the mean area under the concentration-time curve (AUC) and maximum serum concentration, is lower after 40 mg of the optically pure drug than after the racemic drug.     

Structures and inclusion properties of racemic and enantiopure dimethyl and diphenyl 9,9'-bianthryl-2,2'-dicarboxylates

The authors prepared the dimethyl and diphenyl esters of 9,9'-bianthryl-2,2'-dicarboxylic acid in racemic and enantiopure (M) forms. The enantiopure dimethyl ester forms inclusion compounds with various organic compounds such as benzene, methanol, phenol, and aniline whereas the racemic form does this only with benzene. No guest molecules are included by the racemic and enantiopure diphenyl esters. These effects of substituents and homochirality on the inclusion properties are discussed on the basis of X-ray structures of some inclusion and guest-free compounds.     

Physicochemical properties, binary and ternary phase diagrams of ketoprofen

Compared to simulated moving bed (SMB) chromatography, fractional crystallization is a simple and economical method for enantioseparation. Therefore, the coupling of SMB chromatography and fractional crystallization is suggested for enantioseparation of racemic compounds. In this work, a nonsteroidal antiinflammatory drug, ketoprofen (Kp), was chosen to be studied. Kp was verified as a racemic compound by FTIR, PXRD, and thermodynamic calculations. To derive the condition where pure (S)-Kp could be crystallized from a solution, which was previously enantiomerically enriched, the binary melting phase diagram and the ternary solubility phase diagram in the mixed solvent of ethanol and water over a temperature range of 15-30 degrees C were obtained. From these phase diagrams the eutectic point was determined as 91.6% mole percent (S)-Kp from the binary phase diagram and 91% from the ternary phase diagram. The results may provide valuable experiment data for the possibility of coupling fractional crystallization with SMB for Kp separation.     

Meso and racemic organisms and the transition to asymmetry

In this paper we analyse extensive but previously unappreciated and scattered data on anomalous enzyme modulation by metabolite enantiomers for enzymes in the principle metabolic cycles and develop a coherent theory for their derivation from reciprocally coupled antipodal metabolisms in antecedent meso and racemic organisms. Structures for such chirally symmetric organisms are outlined, as is the gradual decoupling of their integrated antipodal metabolisms upon an evolutionary progression from meso organism to racemic organism to racemate of organisms. The driving forces and the dynamics for a transition to asymmetry are developed.     

Homochiral oligopeptides via surface recognition and enantiomeric cross impediment in the polymerization of racemic phenylalanine N-carboxyanhydride crystals suspended in water

As part of our program on the search of possible prebiotic routes for the formation of oligopeptides of homochiral sequence (isotactic) from racemic precursors in aqueous environment, we report the polymerization of racemic crystals of phenylalanine N-carboxyanhydrides, enantioselectively tagged with five deuterium atoms, suspended in water containing various amine initiators. Racemic mixtures of isotactic oligopeptides, comprising up to 25 repeat units of the same handedness, as the dominant component for each length, were observed in a MALDI-TOF mass spectrometry analysis. The racemic mixtures of the peptides could be desymmetrized by initiating the polymerization reaction with water-soluble methyl esters of either enantiopure alpha-amino acids or dipeptides. A three-step mechanism is proposed to account for these results: (i) Surface recognition of the chiral initiator by the chiral sites present at specific faces of the crystal; (ii) Oligopeptide elongation at the polymer/crystal interface; and (iii) Self-assembly of the short isotactic peptides into racemic antiparallel beta-sheets as templates followed by cross-enantiomeric impediment in the growth of enantiomeric chains at the peptide beta-sheet/crystal interface.     

Synthesis of 5'-C-methyl-1',3'-dioxolan-4'-yl nucleosides

Novel racemic 5'-C-methyl-1',3'-dioxolan-4'-yl nucleosides were synthesized from the key intermediate, 2-benzoyloxymethyl-4-oxo-5-C-methyl-1,3-dioxolane, which was prepared from racemic lactic acid.     

The application of preparative batch HPLC, supercritical fluid chromatography, steady-state recycling, and simulated moving bed for the resolution of a racemic pharmaceutical intermediate

This article discusses the chromatographic resolution of a racemic pharmaceutical intermediate. Preparative batch high performance liquid chromatography (HPLC), supercritical fluid chromatography (SFC), steady-state recycling (SSR), and simulated moving bed (SMB) were used to resolve a total of 12.2 kg of a racemic pharmaceutical intermediate. In this study, a first batch of 0.8 kg of racemate was separated on the preparative batch HPLC and SFC, and subsequently another 5.9 kg of racemate was separated on the SSR. Lastly, a third batch of 5.5 kg was separated on the SMB. The separation conditions and results of these techniques are discussed. The productivities and solvent costs of SFC versus HPLC are compared. The productivities and solvent costs of SMB, SSR, and HPLC are also compared. The analytical method development and process optimization of these processes are also discussed in this article.