Crystallization, preferential

Interestingly, when R1 and R2 are hydrogens, the -configurated amino nitriles 1 arc obtained, whereas one or two methoxy substituents on the aromatic ring leads to (S)-diastereomers. This surprising effect is caused by the preferential crystallization of the (R)- or the (.S )-diastereomers, respectively. If the pure diastereomers of 1 are dissolved in methanol, equilibration occurs. On concentration, the optically pure diastereomer again crystallizes from the solution45. 

The method is very useful for the synthesis of physiologically interesting a-mcthylamino acids, e.g., methyl dopa from the 3,4-dimethoxybenzyl derivative. The excellent stereoselection achieved in the process, however, is caused by the preferential crystallization of one pure diastereomerfrom the equilibrium mixture formed in the reversible Strecker reaction. Thus, the pure diastcrcomers with benzyl substituents, dissolved in chloroform or acetonitrile, give equilibrium mixtures of both diastereomers in a ratio of about 7 347. This effect has also been found for other s-methylamino nitriles of quite different structure49. If the amino nitrile (R1 = Bn) is synthesized in acetonitrile solution, the diastereomers do not crystallize while immediate hydrolysis indicates a ratio of the diastereomeric amino nitriles (S)I(R) of 86 1447. 

It is known the case of i-PP, for which the copolymerization with small amounts of ethylene tends to stabilize the y form [84] for instance, by melt crystallization of a copolymer with 6% by mol of ethylene more than 80% of the crystalline phase is in the y form [85], It is also known that the obtainment of the y form by melt crystallization, is also favored for samples of low molecular mass [86, 87] and for stereoblock fractions [88]. This seems to suggest that, whenever the preferential crystallization of the y-form is observed, there is the concomitant occurrence of a reduction in the polymer of the length of the chain stretches with polypropylene head to tail constitution and isotactic configuration. 

Enantiomers, preferential crystallization of 59 Endo selectivity 798 Ene reactions 808, 809 Enones, synthesis of 732 Enthalpies of formation 102, 103 Enynes, synthesis of 956 Enzymatic kinetic resolution 829 Epimerization 399 Episulphides, oxidation of 237 Episulphones 650, 775 Episulphoxides, photolysis of 742 a,/J-Epoxysulphones reactions of 811, 812 rearrangement of 685 synthesis of 612 / ,y-Epoxysulphones 781 y,<5-Epoxysulphones 627, 628 Epoxysulphoxides reactions of 613 rearrangement of 744 synthesis of 327, 612 Erythronolides 831... 

COlfen H (2007) Bio-inspired Mineralization Using Hydrophilic Polymers. 271 1-77 Collin J-P, Heitz V, Sauvage J-P (2005) Transition-Metal-Complexed Catenanes and Rotax-anes in Motion Towards Molecular Machines. 262 29-62 Collins BE, Wright AT, Anslyn EV (2007) Combining Molecular Recognition, Optical Detection, and Chemometric Analysis. 277 181-218 Collyer SD, see Davis F (2005) 255 97-124 Commeyras A, see Pascal R (2005) 259 69-122 Coquerel G (2007) Preferential Crystallization. 269 1-51 Correia JDG, see Santos I (2005) 252 45-84 Costanzo G, see Saladino R (2005) 259 29-68 Cotarca L, see Zonta C (2007) 275 131-161 Credi A, see Balzani V (2005) 262 1-27 Crestini C, see Saladino R (2005) 259 29-68... 

Compound (S)Sn(S)c-(75) was obtained 27) by preferential crystallization of the less abundant and less soluble compound of the 40/60 equilibrium mixture of epimerizing (S)s (S)c and (R)s (S)c compounds. Since the two diastereomers have different... 

Crystallization Crystallization is used to separate the API from its solvent and impurities, or to separate racemic mixtures in solution. Crystallization occurs from a supersaturated solution. Important conditions are the temperature, concentration, stirring rate, and heating and cooling rate. Seeding with the desired API can assist in providing nucleation sites for the preferential crystallization of the API. 

In the optical resolution of DL-threonine mixtures by batch preferential crystallization, changes of solution concentration and crystal purity were measured. The mechanism of nucleation of the un-seeded enantiomer was discussed to explain the purity decrease of the resolved crystals. From the observation of crystallization behavior of the seed crystals of L-threonine, it was concluded that the existence of the D-enantiomer on the surface of the seed caused the sudden nucleation when they grew to attain sufficient amounts. 

From the consecutive measurements of solution concentrations and crystal purities during the optical resolution by preferential crystallization, the crystallization of D-threonlne other than the seeded component (L-threonine) was observed In the later stage of the resolution. Washing of the seed crystals was found to be effective to delay the purity decrease. D-threonine was believed to be Introduced... 

Batch crystallizers, applications, 102 Batch preferential crystallization, purity decrease of L-threonine crystals in optical resolution, 251-259 Bayer proce description, 329 Benzene, hi -pressure crystallization from benzene-cydohexane mixture, 281-289... 

Figure 11.2 shows a cyclohexane oxidation reactor. The further oxidation of the ketone and alcohol to adipic acid is very complex but occurs in good yield, 94%, despite some succinic and glutaric acid by-products being formed because the adipic acid can be preferentially crystallized and centrifuged. 

However, cases known as second-order asymmetric transformation, which involve preferential crystallization of one diastereomer from an equilibrating mixture, can be excluded. 

Table 1. Some Examples of Compounds Resolvable by Preferential Crystallization of Derivatives3...

The diamine (99) was prepared from (S)-proline90b) or (S)-glutamic acid I15) maintaining the asymmetric center. Racemic 2-(anilinomethyl)pyrrolidine, prepared from (RS)-5-oxopyrrolidine-2-carboxylic acid, was effectively resolved into a pair of enantiomers by fractional crystallization of its mandelic acid salt U6). Moreover, the preferential crystallization of its 4-hydrobenzoic acid salt was found to produce both enantiomers in high optical purities by alternate seeding116). 

Such equilibria are driven by thermodynamics and therefore a selective synthetic route towards one of these species and isolation of such heteroleptic zincates in pure form is often very difficult or impossible. Only if one of the species has a sufficiently enhanced thermodynamic stability compared to the others in the equilibrium is its isolation as a pure compound possible. This is often the case when the various groups bound to zinc have a sufficiently different electronegativity, for example when one of the groups is bound to zinc via a heteroatom, or when the steric requirements of the groups bound to zinc are rather different. Sometimes it is possible to isolate one of the species present in the Schlenk equilibrium as a solid material, for example when one of the species preferentially crystallizes from solution. 

Reaction of two equivalents of 2-[(dimethylamino)methyl]ferrocenyUithium with ZnCl2 affords the corresponding diorganozinc compound (68) in which two 2-[(dimethylamino) methyl]ferrocenyl ligands are C,Af-chelate bonded to zinc. Due to the chirality of the bidentate ligands 68 exists in two diastereoisomeric forms. Indeed, in solution a meso one and an S,S)/ R,R) enantiomeric parr in a 1 4 ratio were observed by H and C NMR spectroscopy. However, it is only the S,S)/ R,R) enantiomeric pair that preferentially crystallizes from solution. An X-ray crystallographic study of such crystals afforded the structure of (R,R)/(S,S)-6S in the solid state (Figure 35). 

Careful stepwise crystallization of cobalt acetylacetonate from solutions of the partially resolved chelate produced surprising results (14). A typical experiment is summarized in Table VI. The molecular rotation of the filtrates steadily increased as each crystal crop was removed until no solute remained in solution— at this time all optical activity had, of course, been lost. All crystal crops were racemic It seems that the racemate is being preferentially crystallized from solution and at the same time a surface racemization is taking place to make up the deficient enantiomorph as the d, l crystals are formed. 

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