Crystallization chiral resolution screening

Our approach for chiral resolution is quite systematic. Instead of randomly screening different chiral acids with racemic 7, optically pure N-pMB 19 was prepared from 2, provided to us from Medicinal Chemistry. With 19, several salts with both enantiomers of chiral acids were prepared for evaluation of their crystallinity and solubility in various solvent systems. This is a more systematic way to discover an efficient classical resolution. First, a (+)-camphorsulfonic acid salt of 19 crystallized from EtOAc. One month later, a diastereomeric (-)-camphorsulfonic acid salt of 19 also crystallized. After several investigations on the two diastereomeric crystalline salts, it was determined that racemic 7 could be resolved nicely with (+)-camphorsulfonic acid from n-BuOAc kinetically. In practice, by heating racemic 7 with 1.3equiv (+)-camphorsulfonic acid in n-BuOAc under reflux for 30 min then slowly cooling to room temperature, a cmde diastereomeric mixture of the salt (59% ee) was obtained as a first crop. The first crop was recrystallized from n-BuOAc providing 95% ee salt 20 in 43% isolated yield. (The optical purity was further improved to -100% ee by additional recrystallization from n-BuOAc and the overall crystallization yield was 41%). This chiral resolution method was more efficient and economical than the original bis-camphanyl amide method. 

Finally, libraries aimed to chiral resolution of racemates will be covered here in particular, the use of chiral stationary phases (CSPs) has recently been reported for the identification of materials to be used for chiral separation of racemates by HPLC. The group of Frechet reported the selection of two macroporous poly methacrylate-supported 4-aryl-1,4-dihydropyrimidines (DHPs) as CSPs for the separation of amino acid, anti-inflammatory drugs, and DHP racemates from an 140-member discrete DHP library (214,215) as well as a deconvolutive approach for the identification of the best selector phase from a 36-member pool library of macroporous polymethacrylate-grafted amino acid anilides (216,217). Welch and co-workers (218,219) reported the selection of the best CSP for the separation of a racemic amino acid amide from a 50-member discrete dipeptide iV-3,5-dinitrobenzoyl amide hbrary and the follow-up, focused 71-member library (220). Wang and Li (221) reported the synthesis and the Circular Dichroism- (CD) based screening of a 16-member library of CSPs for the HPLC resolution of a leucine ester. Welch et al. recentiy reviewed the field of combinatorial libraries for the discovery of novel CSPs (222). Dyer et al. (223) reported an automated synthetic and screening procedure based on Differential Scanning Calorimetry (DSC) for the selection of chiral diastereomeric salts to resolve racemic mixtures by crystallization. Clark Still rejxrrted another example which is discussed in detail in Section 9.5.4. 

Finally, the bacterial PTE mentioned above has also been exhaustively studied with regard to its enantioselectivity. Initial studies used the known crystal structure of PTE to identify the substrate-binding pocket. This was then rationally evolved for enhancement and relaxation of the stereospecificity.97 Most recently, a combinatorial library has been screened for the resolution of chiral phosphate, phosphonate, and phosphinate esters.124 This work identified two variants with markedly different preferences for 5p- and Rp-enantiomers of 4-acetylphenyl methyl phenyl phosphate. One variant preferentially catalyzed hydrolysis of the 5p-enantiomer by a factor of 3.7 x 105, while the other preferentially catalyzed hydrolysis of the A p-enantiomer by a factor of 9.7 x 102 - an enantioselective discrimination of 3.6 x 108. 

The desired (S)-isomer was obtaiued by resolution of rac-THpC performed by crystallization of its diastereomeric salts formed with (5)-(+)-camphorsulfonic acid in EtOH. This resolving agent was selected by screening a collection of chiral acids ((5)-(-)-malic acid, (H-)-o,o-p-toluyl-D-tartaric... 

One area of chirotechnology which is undergoing rapid development is chiral HPLC, whereby the use of chiral stationary phases (CSPs) permits the direct separation of racemic compounds into constituent enantiomers. Despite the capital outlay required, for example, for columns costing upwards of 3000, the use of preparative chiral HPLC in drug discovery has a number of benefits. After development of an appropriate method based on a previously defined analytical separation has been carried out, rapid and quantitative separation of racemates can be achieved, with evaporation of solvent from column fractions affording pure enantiomers directly. Although preparative chiral HPLC is less amenable to scale-up than other resolution techniques, it may be ideal for preliminary screening of both enantiomers in circumstances where manipulation of small quantities of material, for example, by crystallization, is impractical and prone to contamination problems. 

In another report by Aelterman et al., the development of a facile and large-scale preparation of the antitumor agent R116010 was demonstrated (Scheme 56.2). In their work, the key strategic improvement was the crystallization-induced diastereomeric dynamic resolution of the aminoketone rac-7, leading to the chiral ketone (S)-7 in 90% yield and 90% enantiomeric purity. After screening 22 chiral acids in numerous solvents, the resolution of aminoketone with ditoluoyltartaric acid in methanol was found to be the optimal process. This new process improves the overall yield from 0.26% to 18.8% without tedious chromatographic separations and hazardous reaction conditions. 

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