Crystallization-induced enantiomer transformation

Another emerging resolution technique is applicable to atropisomers that form conglomerates. Chiral symmetry breaking of C-N axial chirality for N-aryl-2(lH)-pyrimidinone conglomerates was achieved by crystallization induced enantiomer transformation without any outside chiral... 

In summary, (R)-phenylglycine amide 1 is an excellent chiral auxiliary in the asymmetric Strecker reaction with pivaldehyde or 3,4-dimethoxyphenylacetone. Nearly diastereomerically pure amino nitriles can be obtained via a crystallization-induced asymmetric transformation in water or water/methanol. This practical one-pot asymmetric Strecker synthesis of (R,S)-3 in water leads to the straightforward synthesis of (S)-tert-leucine 7. Because (S)-phenylglycine amide is also available, this can be used if the other enantiomer of a target molecule is required. More examples are currently under investigation to extend the scope of this procedure. ... 

The asymmetric synthesis of a-amino acids is an important topic due to their extensive use in pharmaceuticals and agrochemicals and as chiral ligands. The Strecker reaction is historically one of the most versatile ways to produce a-amino acids, but this method has a maximum yield of only 50% for a single enantiomer. Higher yields can be achieved by using chiral auxiliaries, but auxiliaries have other drawbacks, such as high cost, low availability, the need for purification, and high loss rates. A possible solution to these problems would be to use a chiral auxiliary in a crystallization-induced asymmetric transformation. 

Either (S)-specific aminopeptidase catalyzed hydrolysis of racemic PGA11 or crystallization-induced asymmetric transformation of racemic PGA with (.S l-mandelic acid as resolving agent12 can be used to prepare (R)-PGA. As a result of its ready availability on large scale within DSM, we envisaged the application of (R)-PGA for the production of enantiomerically pure amine functionalized compounds using the chirality transfer concept. Obviously, (S)-phenylglycine amide is also available and can be used for the preparation of the opposite enantiomer of the amines described. 

Scheme 6.4 Crystallization induced asymmetric transformation of ( )-l-methyl-3-amino-5-pheny 1-1,4-benzodiazepine (6) into the (3S)-enantiomer...

A chiral compound, dissolved in a nematic liquid crystal phase, transforms this phase into a chiral phase that is very often a chiral nematic - cholesteric -phase. Under the same condition of concentration and temperature two enantiomers induce helical structures with the same pitch but of opposite sign. The helical pitch p is for low concentrations of the dopant a linear function of mole fraction x. The molecular measure for the chiral induction is the helical twisting power (ITTP) ... 

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