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Study of the mechanism of optical resolutionsvia diastereoisomeric salt formation

V. Thermoanalytical investigation of the optical resolution of the N-methylamphetamine by tartaric acid

Journal of Thermal Analysis and Calorimetry
Authors:
D. Kozma
,
C. Novák
,
G. Pokol
, and
E. Fogassy

The products obtained in the optical resolution of the N-methylamphetamine by the Pope-Peachy method, using half equivalent R, R-tartaric acid and half equivalent hydrochloric acid in absolute ethanol were investigated by thermoanalytical methods. The DSC measurements of the precipitated salts provide sufficient information for following the progress of the resolution. In this way, the results of the resolution can be estimated with the precision of about 5% optical purity.

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Abstract  

The enantioselectivity of the diastereomeric supramolecular compound (SMC) formation between O,O'-dibenzoyl-(2R,3R)-tartaric acid (DBTA) and chiral secondary alcohols was investigated. On the basis of TG measurements the DBTA:chiral alcohol molar ratio in the SMC-s is nearly 1:1. Among the investigated complexes the most stable SMC is trans-2-iodo-cyclohexanol-DBTA. The SMC forming capability and the enantioselectivity depends on the space filling of the alcohol side chain or ring. In the case of trans-2-halogen-cyclohexanols a relationship can be observed between the thermal stability of the SMC-s and the enantioselectivity of SMC forming.

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Abstract  

The optical resolution of racemic mandelic acid (I) by S-2-benzylamino-butanol (II) was performed in water, ethyl acetate, and water saturated ethyl acetate. It was found that the efficiency of the resolution is three times higher in water saturated ethyl acetate than in either water or ethyl acetate. The salt mixtures produced during the resolutions and the pure diastereoisomeric salts were analyzed by TG, DSC and X-ray powder diffraction measurements. The R-(-)-IS-(+)-II salt has the higher melting point and heat of fusion value which indicates that this is the more stable salt. Though the general assumption is that diastereoisomeric salt pairs of successful optical resolutions form eutectic systems, the R-mandelic acid-S-2-benzylamino-butanol and the S-mandelic acid-S-2-benzylamino-butanol system was found to behave in a different way. Melting did not start at or near the estimated eutectic temperature. The difference can be explained either by miscibility in the solid phase (solid solution) or by a blocked interaction between the crystals of the two solid salts. This unusual behaviour of the salt pair should be responsible for the unusual difference in the efficiency of the resolutions performed in different solvents

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Abstract  

Chiral drugs are generally resolved through formation of diastereoisomeric salts of different solubility. When the diastereoisomer salts from a eutectic in the solid phase, a good estimate can be made for the efficiency of the resolution (S), through a calculation based on the eutectic composition (x eu). The eutectic composition can be computed from thermal data of a differential scanning calorimetric (DSC) measurement of the diastereoisomeric salts [1]. In this work we investigated the influence of uncertainties of thermal data on the estimation of the efficiency of resolution (S), spreading throughout the calculations involved. Performance of our error-estimating method is demonstrated on 6 diastereoisomer salt pairs. The obtained deviations are close to the deviations of resolution experiments. Thus, it can be concluded that the DSC measurements are useful means in predicting the efficiency of resolutions.

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Abstract  

The diastereoisomeric salt pair formed between α-phenyl-ethyl-amine and R-1-phenylethylsuccinamic acid were investigated by physico-chemical methods. Melting and solubility phase diagrams were determined, the coincidence of the eutectic points of the two phase diagrams were demonstrated. The large difference in physico-chemical properties of the salt pair explains the efficient enantiomer separation.

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Study of the mechanism of optical resolutions via diastereoisomeric salt formation

2. Thermoanalytical and X-ray study of solid solution formation during the resolution of racemic malic acid by R-α-phenylethylamine

Journal of Thermal Analysis and Calorimetry
Authors:
D. Kozma
,
Klára Tomor
,
C. Novák
,
G. Pokol
, and
E. Fogassy

Racemic malic acid (I) was resolved by R-α-phenylethylamine(II). The S-(−)-I.R-(+)-II diastereoisomer was in excess in the precipitated salt. DSC curves and X-ray powder diffractograms proved that the diastereoisomeric salt mixture precipitated during the resolution was isomorphous with the optically pure S-(−)-I.R-(+)-II salt. The diastereoisomeric salt mixture containing the R-(−)-I.R-(+)-II salt in abundance bound crystal solvate (water or methanol) when produced by the total evaporation of the mother liquor, while the optically pure R-(−)-I.R-(+)-II salt crystallized without solvate. It is generally assumed that solid solution formation takes place when the two diastereoisomers are alike and the high similarity results in less efficient enantiomer separation.

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Abstract  

A non-isothermal experimental study using thermogravimetry (TG) and differential scanning calorimetry (DSC) was conducted for investigation the oxidation reactivity of natural phosphate and its demineralised products. The analyses were carried out in oxygen atmosphere and at different heating rate (5, 10, 20, 30, 50, 60C min-1) up to 1000C. The results indicated that the material washed with HCl from the original phosphate, mainly apatite and carbonates of calcium and magnesium, as well as with HCl/HF, silicates minreals, had an inhibition effect during oxidation reactions of organic material. The increase of the heating rate shifted the reactions to higher temperatures. In addition, kinetic parameters were determined by assuming a single first-order kinetic model, using the Coats-Redfern method. The influences of demineralization process of natural phosphate and the heating rate were examined and discussed.

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Abstract  

The resolution of three chiral alcohols with O,O'-dibenzoyl-(2R,3R)-tartaric acid (DBTA) via diastereoisomeric supramolecular compound formation was investigated with thermoanalytical methods. On the basis of TG measurements the DBTA:chiral alcohol molar ratio in the compounds is 1:1 which agrees with the results of single-crystal X-ray diffraction analysis. The DBTA – chiral alcohol supramolecular compounds have different supramolecular structure than the DBTA – achiral alcohol supramolecular compounds. The supramolecular compounds containing cyclohexanols have higher thermal stability than the compounds containing acyclic aliphatic alcohols. The amount of unreacted DBTA monohydrate in the solid phase can be determined both with DSC and with TG measurements.

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