Chirality stirring-induced

Our hypothesis is that J-aggregates are inherently chiral and exist in aqueous solution as racemate. We anticipate that their enantiomorphic distribution can be altered by vortex action the enantiomer favoured by stirring is deposited on the cuvette wall, the other remains in solution. This is possible because stirring induces a thermodynamic unbalancing in the racemate solution. The situation is complicated because, being a weak thermodynamic force, vortex action competes with other forces, such as, for example, the (stronger) thermodynamic effect exerted by the presence of high concentrations of chiral templates. 

When pyrimidine-5-carbaldehyde 11 was treated with z-Pr2Zn in the presence of powdered [CD(+)260]-crystal, (S)-pyrimidyl alkanol with 73% ee was obtained in 88% yield (Scheme 16). On the other hand, in the presence of [CD(-)260]-crystal, the opposite enantiomer (R)-12 with 89% ee was isolated in 89% yield. When the crystals, grown from the stirred methanol solution of hippuric acid using each enantiomorph of hippuric acid as the seed crystal, were used in asymmetric autocatalysis, the same correlation between the chirality of crystal and the product 12 was observed with excellent reproducibility. It should be noted that nearly enantiopure (S)- and (K)-pyrimidyl alkanols 12 with > 99.5% ee were obtained by consecutive asymmetric autocatalysis [64], In this system, after the enantiomorphs of the crystal induced the chirality of an external organic compound, the subsequent asymmetric autocatalysis gave a greater amount of enantiomerically amplified product. 

Arteaga O, Canillas A, Purrello R et al (2009) Evidence of induced chirality in stirred solutions of supramolecular nanofibers. Opt Lett 34 2177-2179... 

In the second case, a nonnatural amino acid of commercial value was synthesized in a Strecker reaction using a chiral amine to help induce chirality in the product. In this case, the authors were interested in the synthesis of 3-ethyl-L-norvaline. So, as shown in Scheme 12.33, a mixture of (5)-(-)-a-methylbenzylamine, potassium cyanide (KCN), and 2-ethylbutyraldehyde in dilute aqueous methanolic HCI was allowed to stir for several days at room temperature. During this time, the hydrochloride salt of the Strecker adduct precipitated and, under these conditions, the precipitate was found to be enriched in the levorotatory isomer (>30 1 by chromatographic separation of the diastereomers). Hydrolysis of the appropriate amino nitrile with cold concentrated sulfuric acid produced 3-ethyl-N -[(lS)-phenylethyl]-L-norvalinamide and debenzylation was accomplished by hydrogenation at lOOpsi of hydrogen (H2) over a 5% Pd/C catalyst. Finally, the amide was hydrolyzed in concentrated HCI to produce 3-ethyl-L-norvaline hydrochloride. 

With these experimental results at hand, it became of interest to evaluate the inducing ability of appropriate chiral auxiliary by introducing them into the acyloxyborane intermediate. Reaction of the monacylated tartaric acid with one equiv of borane should result in the formation of chiral acyloxyborane intermediate. Acrylic acid was then added to this catalyst and the mixture was treated with cyclopentadiene at -78 C After being stirred for 24 h, the product was isolated in high yield. The product showed a specific rotation of -86 which corresponded to 78% optical yield. 

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