Diastereomeric 2-aminomethyl-1,4-benzodioxane mandelates: phase diagrams and resolution

The diastereomeric salts of (R)- and (S)-2-aminomethyl-1,4-benzodioxane with unichiral mandelic acid form a simple eutectic, whose binary phase melting point diagram shows the unique eutectic at 0.35 M ratio of the less soluble diastereomer. Such an eutectic composition, near to 0.5, is consistent with the modest efficiency previously reported for their separation via crystallization from ethanol/ethyl acetate. However, the ternary solubility phase diagram, obtained from solubility measurements in methanol, shifts the eutectic to a lower molar ratio (0.10) of the less soluble diastereomer, thus indicating an optimal resolvability of the diastereomeric mandelates. This was confirmed by the highly efficient resolution of racemic 2-aminomethyl-1,4-benzodioxane with (R)-mandelic acid via a single crystallization from methanol. The ready availability of both the racemic substrate and the resolving acid makes this simple and efficient resolution procedure very attractive to obtain the enantiomers of 2-aminomethyl-1,4-benzodioxane, which are important synthetic intermediates.     

First optically active P*-chiral phosphapalladacycle

The first cyclopalladated complex containing an asymmetric P^*-donor atom in the phosphapalladacycle has been prepared in an enantiomerically pure state via resolution of diastereomeric (S)-prolinate derivatives of the racemic dimer. The absolute configuration of the P^*-stereocenter (S_P) and the geometric configuration of the palladium environment (Z) were determined by an X-ray diffraction investigation of the less soluble diastereomer of the (S)-prolinate adduct.     

“Dutch”拆分研究进展

本文综述了第一代Dutch拆分、第二代Dutch拆分及反义Dutch拆分的拆分机理和研究进展,并展望了Dutch拆分的应用前景。Dutch拆分能以高收率和近于100％ee值与所有的实验消旋体迅速形成非对映体的结晶。第一代Dutch拆分中Family拆分试剂在形成非对映体晶体过程是典型的固溶体行为。在第二代Dutch拆分中,成核抑制剂改变了非对映异构体的亚稳区宽度,溶解度较大的非对映体盐比溶解度较小的非对映体盐的结晶温度下降得更多,使溶解度较小的非对映体盐更容易析出。     

The Dutch Resolution variant of the classical resolution of racemates by formation of diastereomeric salts: family behaviour in nucleation inhibition

The resolution of racemates through their diastereomeric salts can be positively affected by the addition of small amounts of suitable nucleation inhibitors. This discovery is a logical extension of "Dutch Resolution", in which equimolar amounts of resolving agents that are members of the same family (i.e., structurally related) are used. We conducted a systematic search for nucleation inhibitors of the resolving agent 1-phenylethylamine. A wide range of amines that bear possible family resemblances to 1-phenylethylamine was investigated. It was found that (R)-1-phenylbutylamine is a good inhibitor of (R)-1-phenylethylamine. Results of turbidity measurements showed that, for the model case of mandelic acid resolution, the chief effect of this inhibitor was to widen the metastable zone for the more soluble diastereomer. This observation is in accordance with previous experience. Further scouting for possible family members revealed a wide variation in the effectiveness of inhibitors, dependent on their structure. By far the most effective inhibitors are bifunctional 1-phenylethylamine and/or 1-phenylbutylamine analogues. The effect of racemic inhibitors was found to approach that of enantiomerically pure inhibitors of the same absolute configuration of the 1-phenylethylamine used for resolution. The most effective inhibitors were tested for the resolution of a structural variety of racemates, and were shown to be broadly applicable.     

Optical resolution of racemic stilbenediamine using N*-chiralortho-palladated matrix

Optical resolution of racemic stilbenediamine (Stien) was performed by recrystallization of its diastereomeric adducts withortho-palladated(S)-N-isopropyl-α-methylbenzylamine. The less soluble (S _C R _N′,SS) diastereomer was stuided by X-ray diffraction analysis. It was established that the crystal of this diastereomer consists of dimers formedvia association of two molecules of the mononuclear cationic complex with an additional molecule of free diamine of the same absolute configuration. The association occurs through a system of hydrogen bonds and weak agostic interactions. The association occurs through a system of hydrogen bonds and weak agostic interactions. Based on the X-ray diffraction data, the procedure was improved for the resolution of stilbenediamine due to the more profitable use of theortho-palladated reagent. The Stien/Pd ratio in the diastereomer isolated was inceased up to 3∶2. The conformational features of the complex are discussed on the basis of¹H NMR spectroscopy data. The preliminary results were reported previously, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 184–195, January, 1999.     

Highly efficient resolutions of 1,4-benzodioxane-2-carboxylic acid with para substituted 1-phenylethylamines

The salts of (S)- and (R)-1,4-benzodioxane-2-carboxylic acid with eight (S)-1-arylethylamines were prepared. The determination of their melting points and of their solubilities in alcohol solvents revealed large differences between the diastereomeric benzodioxanecarboxylates of (S)-1-(p-nitrophenyl)ethylamine and of (S)-1-(p-methylphenyl)ethylamine. Therefore, these latter amines were selected to resolve (±)-1,4-benzodioxane-2-carboxylic acid by diastereoselective crystallization finding that both of them display a very high resolution ability for such a substrate, which contrasts with the null efficiency of unsubstituted 1-phenylethylamine. These results are consistent with DSC evidences, which indicated that the two successfully resolved diastereomeric systems are binary mixtures exhibiting a eutectic with a high content of the more soluble diastereomeric salt. The new procedures can advantageously replace the two resolutions we had previously reported, that of the same acid with dehydroabietylamine and that of glycerol acetonide, a precursor of 1,4-benzodioxane-2-carboxylic acid, with 1-phenylethylamine.     

Optical resolution of N-formylphenylalanine succeeds by crystal growth rate differences of diastereomeric salts

Optical resolution of racemic-phenylalanine through its N-formyl derivative with a 1-phenylethylamine resolving agent is an effective procedure. Differential scanning calorimetry, single crystal X-ray diffraction and optical microscopy were used in the investigation of the resolution process. It was found that the thermodynamic properties of the given system would not allow the efficient enantiomer separation. Kinetic effects during the crystal formation have been discovered by the comparison of the crystal morphologies of the two diastereomers. The crystal structure of the less soluble diastereomer (S)-(−)-1-phenylethylammonium (S)-(+)-N-formylphenylalaninate salt has been determined and discussed.     

高效液相色谱法测定非天然氨基酸的光学纯度

 以（1- 氟-2,4-二硝基苯基）-5-L-缬氨 酰胺为手性试剂、反相高效液相色谱法测定非天然氨基酸的光学 纯度。梯度洗脱,流动相A：含体积分数为0.1％的三氟乙酸的乙腈,流动相B：体积分 数为0.1％的三氟乙酸水溶液。L-和D-对映体得到良好分离。准确测定了25种非天然 氨基酸L-和D-对映体的光学纯度。     

A novel separation technique of diastereomeric esters of pyridylethanols by extraction: formal total synthesis of PNU-142721, HIV-1 reverse transcriptase inhibitor

The separation of diastereomeric esters derived from (±)-pyridylethanols and 3β-acetoxyetienic acid were achieved by an extraction technique using diethyl ether and aqueous hydrochloric acid. A formal total synthesis of PNU-142721 was effectively carried out to prepare the chiral, non-racemic synthon 1-furo[2,3-c]pyridin-5-yl-ethanol (1) by means of this technique. The structure optimized using MOPAC calculations on each diastereomer suggested the presence of intramolecular CH/π interaction in only the (S)-isomer of the diastereomers.     

Asymmetric syntheses with r-α -hydroxypropionaldehyde : Temperature dependence of the diastereomeric product ratio

In an effort to test the applicability of the semi-empirical model for asymmetric induction⁵ to reactions of α-hydroxycarbonyl compounds, we monitored the diastereomeric ratio of diols obtained from the reactions of R-α-hydroxypropionaldehyde with LAH, MeLi and PhLi as a function of temperature and reagent concentration. In all these reactions the predominant diastereomer had the RR-configuration. However, whereas the diastereomeric product ratio of the first two reactions was enthalpy controlled, i.e. enthalpy favored the “correct” or predominant diastereomer, that of the reaction involving PhLi favored the “incorrect”, or minor diastereomer. In this reaction the diastereomeric ratio was entropy controlled. The usefulness and limitations of this model are discussed.     

Resolution of 1-cyclohexylethylamine via diastereomeric salt formation with enantiopure 2-phenylacetic acids

The resolution of 1-cyclohexylethylamine 1 with enantiopure 2-phenylacetic acids via diastereomeric salt formation was investigated. (R)-2-Methoxy-2-phenylacetic acid 3 and the (S)-2-phenylpropionic acid 5 were found to be efficient resolving agents for obtaining the single enantiomer (S)-1 as the correspondingly less-soluble diastereomeric salt (resolution efficiency = 48% and 52%, respectively).     

Resolution of enantiomers of [α-hydroxy-(o-chlorophenyl)methyl]phosphinic acid via diastereomeric salt formation with enantiopure 1-phenylethylamines

The resolution of racemic α-hydroxy-H-phosphinic acid with enantiopure 1-phenylethylamines via diastereomeric salt formation was investigated. X-Ray crystallographic analysis of the salt revealed that (R)-1-phenylethylamine to be efficient resolving agent for obtaining a single enantiomer of [α-hydroxy-(o-chlorophenyl)methyl]phosphinic acid. Resolving racemic α-hydroxy-H-phosphinic acid with (S)-2-phenylethylamine also gave access to (S)-α-hydroxyalkylphosphinic acid in good yield.     

A scalable and expedient method of preparing diastereomerically and enantiomerically enriched pseudonorephedrine from norephedrine

Norephedrine has been efficiently converted into the corresponding diastereomer pseudonorephedrine using a three step, one-pot reaction. The three step process involves treatment of norephedrine with di-tert-butyl dicarbonate (Boc₂O); cyclization by way of mesylate formation at the alcohol; and lithium hydroxide mediated hydrolysis of the oxazolidinone. The diastereomeric purity was determined by HPLC and the enantiomeric purity was determined by optical activity measurements and chiral stationary phase HPLC analysis of the pseudonorephedrine oxazolidinone derivatives.     

Resolution of racemic 1-benzyl-5-oxo-3-pyrrolidinecarboxylic acid with enantiopure (S)-phenylalanine N-benzylamide via diastereomeric salt formation

The resolution of racemic 1-benzyl-5-oxo-3-pyrrolidinecarboxylic acid 1, a potent chiral synthon with high pharmacological activity, was investigated with a variety of basic chiral resolving agents via diastereomeric salt formation. The findings in the optimization of resolution conditions aiming at an industrial-scale production revealed that (S)-phenylalanine benzylamide (S)-10 and 2-propanol containing ca. 4 mol % of water to (RS)-1 were the best conditions for obtaining enantiopure less-soluble diastereomeric salt, (S)-1/(S)-10/0.5H₂O (81%, 98% de, E 79%). X-ray crystallographic analysis of the salt clearly revealed that water molecules play a key role in crystallizing the enantiopure salt crystals, while stabilizing the crystal structure via three types of hydrogen-bond network associated with water molecules in addition to usual acid–base hydrogen bond.     

The effect of SDE on the separation of diastereomeric salts: a case study for the resolution of mandelic acid derivatives with Pregabalin

A resolution method has been elaborated for mandelic acid and 2-chloromandelic acid applying the (R)-(−)-3-(aminomethyl)-5-methylhexanoic acid (Pregabalin) as the resolving agent. The formation of the corresponding diastereomers was kinetically controlled. This observation was rationalized by the behavior of enantiomeric mixtures of mandelic acid, 2-chloromandelic acid, and 3-(aminomethyl)-5-methylhexanoic acid. It was found that the eutectic composition of Pregabalin influenced the diastereomeric excess of the diastereomers formed under kinetic control.     

Stereospecific co-crystallization of reactant and palladium complex in the resolution of stilbendiamine using N*-chiral ortho-palladated matrix

Resolution of racemic 1,2-diphenyl-1,2-ethanediamine was performed through recrystallization of its diastereomeric adducts 2a,b with N*-chiral ortho-palladated complex (S_CR_N)-1. The structure and absolute configuration of the less soluble (S_CR_N,SS)-2a diastereomer was determined by X-ray diffraction. The complex has an unusual dimeric structure with a palladacycle:diamine ratio of 2:3; the association of 2a with an additional diamine molecule of the same (SS) absolute configuration is achieved due to hydrogen bonds between the coordinated and solvate diamine.     

Optically active (2-aminomethylferrocenyl) phosphines with the phosphorus chiral center

The reaction of an enantiomeric planar-chiral palladium derivative of dimethyl-aminomethylferrocene with PhMePLi in THF at room temperature afforded a 1.5∶1.0 mixture of diastereomeric aminophosphines containing the phosphorus asymmetrical center along with a chiral plane. The absolute configuration of the phosphorus atom was determined based on the X-ray diffraction data for the complex of the minor diastereomer with PdI₂. The presence of the (S)-chiral plane in the initial palladium compound favors the predominant formation of the product with the (S)-configuration of the phosphorus center.     

Toward the computational design of diastereomeric resolving agents: an experimental and computational study of 1-phenylethylammonium-2-phenylacetate derivatives

The crystal structures, including two new polymorphs, of three diastereomerically related salt pairs formed by (R)-1-phenylethylammonium (1) with (S&R)-2-phenylpropanoate (2), (S&R)-2-phenylbutyrate (3), and (S&R)-mandelate (4) ions were characterized by low-temperature single crystal or powder X-ray diffraction. Thermal, solubility, and solution calorimetry measurements were used to determine the relative stabilities of the salt pairs and polymorphs. These were qualitatively predicted by lattice energy calculations combining realistic models for the dominant intermolecular electrostatic interactions and ab initio calculations for the ions' conformational energies due to the distortion of their geometries by the crystal packing forces. Crystal structure prediction studies were also performed for the highly polymorphic diastereomeric salt pair (R)-1-phenylethylammonium-(S&R)-2-phenylbutyrate (1-3) in an attempt to predict the separation efficiency without relying on experimental information. This joint experimental and computational investigation provides a stringent test for the reliability of lattice modeling approaches to explain the origins of chiral resolution via diastereomer formation (Pasteurian resolution). The further developments required for the computational screening of single-enantiomer resolving agents to achieve optimal chiral separation are discussed.     

A highly efficient method for the resolution of 8,8′-dihydroxy-1,1′-binaphthyl

The diastereomeric mixture of monoesters of racemic 8,8′-dihydroxy-1,1′-binaphthyl and N-CBZ-l-proline was easily separated. Each diastereomer gave enantiomerically pure (R)- or (S)-8,8′-dihydroxy-1,1′-binaphthyl in good yields after hydrolysis.     

Simple and highly diastereoselective synthesis of trifluoromethyl-containing myosmines via reaction between 2-(aminomethyl)pyridine and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione

The reaction between 1,1,1,5,5,5-hexafluoro-2,4-pentanedione and 2-(aminomethyl)pyridine, or its salts with carboxylic acids, was found to produce a mixture of diastereomeric 2-(2′-pyridyl)-3-hydroxy-3,5-bis-trifluoromethyl-1-pyrrolines with high (up to 85% de) of kinetic (3R*,5S*)-diastereoselectivity. The thermodynamic (3R*,5R*) diastereomer was prepared as a major product (90% de) by epimerization of the kinetic (3R*,5S*) diastereomer with triethylamine.