Enantioseparation of P-Stereogenic 1-Adamantyl Arylthiophosphonates and Their Stereospecific Transformation to 1-Adamantyl Aryl-H-phosphinates.

A focused library of 1-adamantyl arylthiophosphonates was prepared in racemic form. An enantioseparation method was developed for P-stereogenic thiophosphonates using (S)-1-phenylethylamine as the resolving agent. Under optimized conditions, three out of the five arylthiophosphonates were prepared in enantiopure form (ee > 99%). The subsequent desulfurization of optically active arylthiophosphonates gave the corresponding H-phosphinates without significant erosion of enantiomeric purity (ee = 95-98%). Hence, this reaction sequence can be considered an alternative method for the preparation of 1-adamantyl aryl-H-phopshinates. The absolute configuration of the (S)-1-adamantyl phenylphosphonothioic acid was assigned using single-crystal XRD and it allowed the confirmation that the removal of the P = S group proceeds with retention of configuration. The organocatalytic applicability of (S)-1-adamantyl phenylphosphonothioic acid was also evaluated as a P-stereogenic Brønsted acid.

Economic Separations of Organic Acidic or Basic Enantiomeric Mixtures-A Protocol Suggestion.

In this review, we aim to present new concepts for the revisited separation of enantiomers from racemic compounds and a protocol worth to be followed in designing the preparation of pure enantiomers. We have taken into account not only the influence of the properties (eutectic composition) and characteristics of the reactants (racemic compound, resolving agent), but also the behavior of the resulting diastereomers and the different conditions (e.g., crystallization time, solvents used, solvate-forming compounds, achiral additives, etc.). The examples discussed are resolutions developed by our research team, through which we will try to illustrate the impact of all these considerations, presenting the methodological investigations interpreting recent discoveries and observations. Some special solid-state analytical and structural investigations assisting us in the elucidation and invention design of the resolution processes of some active pharmaceutical ingredients, such as Tetramisole, tofisopam, and Amlodipine, are also shown.

Optical Resolution of Two Pharmaceutical Bases with Various Uses of Tartaric Acid Derivatives and Their Sodium Salts: Racemic Ephedrine and Chloramphenicol Base.

The optically active dibenzoyltartaric acid, tartaric acid, and its sodium salts were successfully applied to the optical resolution of (1R,2S)(1S,2R)-2-(methylamino)-1-phenylpropan-1-ol (EPH) and (1R,2R)(1S,2S)-2-amino-1-(4-nitrophenyl)propane-1,3-diol (AD) as resolving agents. It was observed that both compounds' resolution using a mixture of salts of quasi-racemic resolving agents showed a change in chiral recognition under the same conditions compared to the result of the use of the single enantiomeric resolving agent. The changes are followed by detailed analytical (XRD, FTIR, and DSC) studies. Meanwhile, the DASH indexing software package was also tested on powder XRD patterns of pure initial materials and intermediate salt samples of high diastereomeric excess.

Enantioseparation of P-Stereogenic Secondary Phosphine Oxides and Their Stereospecific Transformation to Various Tertiary Phosphine Oxides and a Thiophosphinate.

Secondary phosphine oxides incorporating various aryl and alkyl groups were synthesized in racemic form, and these products formed the library reported in this study. TADDOL derivatives were used to obtain the optical resolution of these P-stereogenic secondary phosphine oxides. The developed resolution method showed a good scope under the optimized reaction conditions, as 9 out of 14 derivatives could be prepared with an enantiomeric excess (ee) ≥ 79% and 5 of these derivatives were practically enantiopure >P(O)H compounds (ee ≥ 98%). The scalability of this resolution method was also demonstrated. Noncovalent interactions responsible for the formation of diasteromeric complexes were elucidated by single-crystal XRD measurements. (S)-(2-Methylphenyl)phenylphosphine oxide was transformed to a variety of P-stereogenic tertiary phosphine oxides and a thiophosphinate in stereospecific Michaelis-Becker, Hirao, or Pudovik reactions.

The resolution of acyclic P-stereogenic phosphine oxides via the formation of diastereomeric complexes: A case study on ethyl-(2-methylphenyl)-phenylphosphine oxide.

As an example of acyclic P-chiral phosphine oxides, the resolution of ethyl-(2-methylphenyl)-phenylphosphine oxide was elaborated with TADDOL derivatives, or with calcium salts of the tartaric acid derivatives. Besides the study on the resolving agents, several purification methods were developed in order to prepare enantiopure ethyl-(2-methylphenyl)-phenylphosphine oxide. It was found that the title phosphine oxide is a racemic crystal-forming compound, and the recrystallization of the enantiomeric mixtures could be used for the preparation of pure enantiomers. According to our best method, the (R)-ethyl-(2-methylphenyl)-phenylphosphine oxide could be obtained with an enantiomeric excess of 99% and in a yield of 47%. Complete racemization of the enantiomerically enriched phosphine oxide could be accomplished via the formation of a chlorophosphonium salt. Characterization of the crystal structures of the enantiopure phosphine oxide was complemented with that of the diastereomeric intermediate. X-ray analysis revealed the main nonbonding interactions responsible for enantiomeric recognition.

Effect of ultrasound-assisted crystallization in the diastereomeric salt resolution of tetramisole enantiomers in ternary system with O,O'-dibenzoyl-(2R,3R)-tartaric acid.

The diastereomeric salt resolution of racemic tetramisole was studied using ultrasound irradiation. We examined the effect of power and duration of ultrasonic irradiation on the properties of the crystalline phase formed by ultrasound-assisted crystallization and the result of the whole optical resolution. The results were compared with reference experiment without using ultrasound. The US time (5-30min) caused higher enantiomeric excess. Although yield was lower continuously high resolving efficiency could have been reached through ultrasound. We had the best results with 4.3W ultrasound power when resolvability was even higher than the best of reference. Furthermore, we accomplished a deep and thorough examination of the salts that possibly could form in this resolution. One of the four diastereomeric salts, which have been identified by powder X-ray diffraction, FTIR-spectroscopy, and differential scanning calorimetry (DSC) in the ternary system of the two tetramisole enantiomers and the resolving agent, namely the bis[(S)-tetramisole]-dibenzoyl-(R,R)-tartrate salt have been proven the key compound in the resolution process, and presented the highest melting point of 166°C (dec.) among the four salts. The originally expected diastereomeric bitartrate salts with 1:1M base:acid ratio [(S)-tetramisole-dibenzoyl-(R,R)-hydrogen-tartrate salt and (R)-tetramisole-dibenzoyl-(R,R)-hydrogen-tartrate salt] and their 'racemic' co-crystal [(RS)-tetramisole-dibenzoyl-(R,R)-hydrogen-tartrate salt] showed somewhat lower melting points (152, 145, and 150°C, respectively) and their crystallization was also prevented by application of ultrasound. Based on the melting points and enthalpies of fusion measured by DSC, all the binary and ternary phase diagrams have been newly established and calculated in the system with help of classical modelling equations of liquidus curves.

Selecting Resolving Agents with Respect to Their Eutectic Compositions.

In order to develop a resolution procedure for a given racemic compound, the first and the most important step is finding the most suitable resolving agent. We studied 18 individual resolutions that were carried out with resolving agents having high eutectic composition. We found that very high enantiomeric excess values were obtained in all cases. We assume that the eutectic composition of a given resolving agent is one of the most important properties that should always be considered during the search for the most efficient resolving agent.

Resolution of P-stereogenic P-heterocycles via the formation of diastereomeric molecular and coordination complexes (a review).

TADDOL derivatives and the Ca(2+)-salts of tartaric acid derivatives were found to be versatile and generally applicable resolving agents for the preparation of the enantiomers of P-stereogenic heterocyclic phosphine oxides and phosphinates via the formation of the corresponding diastereomeric molecular and coordination complexes. A few of the diastereomeric intermediates were characterized by single crystal X-ray crystallography to gain insights into the binding mode of the corresponding heterocyclic phosphine oxide ("guest") and the resolving agent ("host") and to study the underlying phenomenon of enantiomeric recognition.

Resolution of 1-n-butyl-3-methyl-3-phospholene 1-oxide with TADDOL derivatives and calcium salts of O,O'-Dibenzoyl-(2R,3R)- or O,O'-di-p-toluoyl-(2R,3R)-tartaric acid.

The resolution methods applying (-)-(4R,5R)-4,5-bis(diphenylhydroxymethyl)-2,2-dimethyldioxolane ("TADDOL"), (-)-(2R,3R)-α,α,α',α'-tetraphenyl-1,4-dioxaspiro[4.5]decan-2,3-dimethanol ("spiro-TADDOL"), as well as the acidic and neutral Ca(2+) salts of (-)-O,O'-dibenzoyl- and (-)-O,O'-di-p-toluoyl-(2R,3R)-tartaric acid were extended for the preparation of 1-n-butyl-3-methyl-3-phospholene 1-oxide in optically active form. In one case, the intermediate diastereomeric complex could be identified by single-crystal X-ray analysis. The absolute P-configuration of the enantiomers of the phospholene oxide was also determined by comparing the experimentally obtained and calculated CD spectra.

Separation of non-racemic mixtures of enantiomers: an essential part of optical resolution.

Non-racemic enantiomeric mixtures form homochiral and heterochiral aggregates in melt or suspension, during adsorption or recrystallization, and these diastereomeric associations determine the distribution of the enantiomers between the solid and other (liquid or vapour) phases. That distribution depends on the stability order of the homo- and heterochiral aggregates (conglomerate or racemate formation). Therefore, there is a correlation between the binary melting point phase diagrams and the experimental ee(I)vs. ee(0) curves (ee(I) refers to the crystallized enantiomeric mixtures, ee(0) is the composition of the starting ones). Accordingly, distribution of the enantiomeric mixtures between two phases is characteristic and usually significant enrichment can be achieved. There are two exceptions: no enrichment could be observed under thermodynamically controlled conditions when the starting enantiomer composition corresponded to the eutectic composition, or when the method used was unsuitable for separation. In several cases, when kinetic control governed the crystallization, the character of the ee(0)-ee(I) curve did not correlate with the melting point binary phase diagram.

A practical and efficient method for the resolution of 3-phospholene 1-oxides via coordination complex formation.

A simple, efficient, and economical method based on the combination of the exceptional behavior of o,o'-dibenzoyl- or o,o'-di-p-toluyl-(2R,3R)-tartaric acid in chiral recognition processes, and the coordination ability of calcium or magnesium ion was developed for the resolution of phospholene oxides 1. The calcium or magnesium salt of (-)-o,o'-dibenzoyl-(2R,3R)-tartaric acid 2,4-6 or calcium hydrogen (-)-o,o'-di-p-toluyl-(2R,3R)-tartrate 3 may form crystalline diastereomeric coordination complexes with the appropriate antipode of substituted 3-phospholene oxides 1 that makes possible efficient resolutions. Optically active phospholene oxides 1 were prepared directly by simply crystallization and digestion of the corresponding diastereomeric complexes so formed. Thermal behavior of the crystalline diastereomeric complexes was studied by simultaneous TG/DTA. The novel method may be of more general value in respect of the resolution of tertiary phosphine oxides.

Influence of benzylamine on the resolution of ibuprofen with (+)-(R)-phenylethylamine via supercritical fluid extraction.

The resolution of racemic ibuprofen was studied by partial diastereomer salt formation. The resolution was performed via two methods: resolution with (+)-(R)-phenylethylamine as chiral agent and resolution with a mixture of (+)-(R)-phenylethylamine and benzylamine. The diastereomers and unreacted enantiomers were separated by supercritical fluid extraction with carbon dioxide at 15 MPa and 33 degrees C. The influence of the achiral benzylamine on the resolution efficiency was studied by varying the concentrations of the structurally related amines in their mixtures, keeping the sum molar ratio of the amines to racemic ibuprofen constant at 0.55 +/- 0.02. The presence of benzylamine positively influenced the resolution efficiency at certain concentrations. The crystal structure of the salts of (+)-(R)-phenylethylamine with (-)-(R)-ibuprofen and (+)-(S)-ibuprofen, respectively, as well as the cocrystal of the benzylamine-ibuprofen salt with neutral ibuprofen molecules are presented. These structures were determined by single crystal X-ray diffraction, proving the significantly different stoichiometry of the related amines with the chiral acid, in accordance with mass balance calculations.

Optical resolution of 1-(1-naphthyl)ethylamine by its dicarboxylic acid derivatives: Structural features of the oxalic acid derivative diastereomeric salt pair.

Optical resolution methods were established for racemic 1-(1-naphthyl) ethylamine. The resolving agents were synthesized by N-derivatizing (R)-1-(1-naphthyl) ethylamine with dicarboxylic acids. Oxalic, malonic, and succinic acid derivatives were found to be suitable resolving agents. These resolutions are parallel to a series of optical resolutions of 1-phenylethylamine which had been previously performed by our research group using similar derivative resolving agents (Balint et al., Tetrahedron: Asymmetry 2001;12:1511-1518.) The comparison of the results of the enantiomer separations is performed. The diastereomeric salts formed with (R)-N-[1-(1-naphthyl)ethyl]oxalamic acid were investigated by single crystal X-ray diffraction. The crystal structures were compared with the previously published structures of the diastereomers of the phenyl-substituted analogue, namely (R)- and (S)-1-phenylethylammonium (R)-N-(1-phenylethyl)oxalamates (Balint et al., Tetrahedron: Asymmetry 2001;12:1511-1518).

Application of mixtures of tartaric acid derivatives in resolution via supercritical fluid extraction.

Racemic N-methylamphetamine (rac-MA) was resolved with 2R,3R-tartaric acid (TA) and its derivatives (O,O'-dibenzoyl-(2R,3R)-tartaric acid monohydrate (DBTA) and O,O'-di-p-toluoyl-(2R,3R)-tartaric acid (DPTTA)), individually and using them in different combinations. After partial diastereomeric salt formation, the free enantiomers were extracted by supercritical fluid extraction using carbon dioxide as solvent. DBTA and DPTTA are efficient resolving agents for rac-MA, the best chiral separation being obtained at a molar ratio of 0.25 resolving agent to racemic compound for both resolving agents (ee(E) = 82.5% and ee(E) = 57.9%, respectively). Compared with the two other acids, TA is practically unsuitable for enantiomer separation (ee(E) < 5%). Applying a mixture of one individually active and one ineffective acid in half the equivalent molar ratio, when the acids are in 1:1 ratio in the mixture, the resolution efficiency values obtained exceeded those obtained by using the components individually. Decreasing the molar ratio of resolving agent mixture to 0.25, at which the individual resolving agents give the best chiral separation, the obtained resolution efficiency values did not differ significantly from those expected. The outcome of the resolution process depended only on the amount of the individually active resolving agents in the mixture.

Enhanced efficiency due to the use of achiral additives in the optical resolution of 1-phenylethylamine by its glutaric acid derivative.

Racemic 1-phenylethylamine was optically resolved by its own derivative formed with glutaric acid namely (+)-(R)-N-(1-phenylethyl)glutaramic acid. The amide acid resolving agent was synthesized from (+)-(R)-1-phenylethylamine by N-derivatization. The glutaric acid derivative was the next in a homologous series of dicarboxilic acid derivatized resolving agents of racemic 1-phenylethylamine. Resolution results obtained with the oxalic, malonic, and succinic acid derivatives were previously discussed(1). Each of the above derivative resolving agents could be successfully applied as resolving agents of 1-phenylethylamine. The efficiency of the present optical resolution using (+)-(R)-N-(1-phenylethyl)glutaramic acid resolving agent was remarkably inferior to the results obtained by its shorter chained homologues(1). Use of achiral additives, like urea, thiourea, N-methylurea, and N,N'-dimethylurea caused large increase in the efficiency of the resolution by (+)-(R)-N-(1-phenylethyl)glutaramic acid resolving agent. Precipitated salts obtained in the resolutions performed in the presence of the additives were investigated by thermoanalysis, X-ray powder diffraction, and optical microscopy. Based on the analytical data, the improvement of the resolution results was attributed to the influence of the additives on the crystal nucleation processes of the diasteromeric salts.

Nonlinear chiral interactions in resolutions with benzylamine derivatives.

N-formylphenylalanine was subjected to optical resolution using a chiral agent such as phenylethylamine enantiomer, (R)-phenylglycine methyl ester, (S)-N-benzyl-2-aminobutanol, alone or in a mixture with each other or in a mixture with benzylamine.

Advantages of structural similaritiesof the reactants in optical resolution processes.

Advantages and limitations of the use of structurally similar compounds in racemate resolution viadiastereoisomeric salt formation are discussed in this review. An effective conception on "derivativeresolving agents" (use of the optically active derivatives of a racemate as resolving agents)is presented by examples and the method is extended to the homologous series of the derivative resolvingagents and/or achiral additives. A recently developed distillation version of the family approachto optical resolution and the novel, solvent-free resolution methods using a half equivalent amountof resolving agent are also discussed.

Optical resolution methods.

Despite the large number of elaborate enantioselective syntheses for the preparation of a single enantiomer to achieve industrial and scientific goals, the separation and purification of enantiomers (components of racemic compounds) is also necessary. Hence, we present the most often used thought-provoking modern methods based on momentous recognitions (e.g. spontaneous resolution, induced crystallization, resolution by formation of diastereomers, resolution by formation of non-covalent diastereomers, resolution by diastereomeric salt formation, resolution by diastereomeric complex formation, "half equivalent" methods of resolution, separation by crystallization, separation by distillation, separation by supercritical fluid extraction, resolution with mixtures of resolving agents, resolution with a derivative of the target compound, enantioselective chromatography, resolution by formation of covalent diastereomers, resolution by substrate selective reaction, kinetic resolution without enzymes, kinetic resolution by enzyme catalysis, hydrolytic and redox enzymes, kinetic and thermodynamic control, resolutions combined with 2nd order asymmetric transformations, enrichment of partially resolved mixtures, role of the solvent and methods of optimization in the separation of diastereoisomers, non-linear effects and selected examples of resolution on an industrial scale).

Enantioseparation of secondary alcohols by diastereoisomeric salt formation.

A general method was found for the resolution of the racemic 1-phenyl-1-propanol (1) and 1-phenyl-2-propanol (2) with various resolving agents. Monoesters of the alcohols were prepared, which were then reacted with different chiral bases. Successful optical resolutions were achieved only with the maleic acid monoesters (3 and 6). Alcohol 1 has been resolved to >99% enantiomeric excess by diastereoisomeric salt formation via its maleic acid monoester (3) using cinchonidine (9) as resolving agent. Alcohol 2 has been obtained in 98% enantiomeric excess by diastereoisomeric salt formation via its the maleic acid monoester (6) using (+)-dehydroabietylamine (11) as resolving agent.

Optical resolution by crystallization from melt using enantiomer derivatives as resolving agents: nonlinear behavior of resolving agent mixtures.

Substituted racemic 1-phenylethylamine derivatives were reacted in molten phase with chiral N-substituted 1-phenylethylamine dicarboxylic acid derivatives (4-9), and their mixtures (e.g., 4+6, 7+8, etc.) in different ratios, and the crystalline quasi-racemic diastereomers so formed were separated. Nonlinear behavior of the mixtures of resolving agents having related structure and considerable positive and negative chiral-chiral interactions were observed.