Amplifications of chirality

Maeda K, Yashima E (2006) Dynamic Helical Structures Detection and Amplification of Chirality. 265 47-88... 

Weissbuch I, Leiserowitz L, Lahav M (2005) Stochastic Mirror Symmetry Breaking via Self-Assembly, Reactivity and Amplification of Chirality Relevance to Abiotic Conditions. 259 123-165... 

Chiral Discotic Molecules Expression and Amplification of Chirality... 

Nuckolls and Katz have synthesized discotic liquid crystalline molecules in which the core is a helix in its own right.37 Nonracemic helicene 33 was found to assemble into a columnar mesophase in which the helicenes stack on top of each other. CD spectroscopy showed a strong increase of the Cotton effect upon going from the molecularly dissolved state to the aggregated state, exhibiting an amplification of chirality. These helical columns give rise to a strong expression of chirality because the intrinsic shape of the helicenes... 

A second example of homochiral columns formed by discotics are the complexes of tetrazoles (59 and 60) with l,3,5-tris(4,5-dihydroimidazol-2-yl)benzene (61).74 Four molecules self-assemble to give a supramolecular disc and these discs subsequently form columns in nonpolar solvents. Chiral discs were obtained from the self-assembly of the chiral tetrazole (60) with 61. The chirality of the side chains was found to induce a bias in the helic-ity of the supramolecular assembly. Sergeants-and-soldiers measurements75 were performed for which chiral (60) and achiral (59) molecules were mixed. The experiments showed no amplification of chirality, thus revealing that in these systems chirality transfer from the side chains into the column is... 

Figure 6.11 (Top) Plot showing chiral induction via sergeants and soldiers experiment, expressed in terms of Cotton effect (mdeg), as function of fraction of sergeants added. At higher concentrations maximum Cotton effect has been reached after addition of only 10% sergeant 22 to soldiers 20. (Bottom) Graphical representation of columns formed by 22 ( ) and 20 in hexane with 80-fold amplification of chirality and molecular dissolved nature in chloroform.

Brunsveld, L., Chiral Discotic Molecules Expression and Amplification of Chirality, 24, 373. Buckingham, D. A., Conformational Analysis and Configurational Effects for Chelate Complexes, 6, 219. 

These possibilities rectify the proposed subsequent appearance and amplification of chiral autocatalytic molecules and hypercydes. [190] Any autocatalytic systems would propagate [191] throughout an extensive adjoining hydrated porous network already rich in layered amphiphiles, lipids, polymeric materials, amino acids, thiols, and so forth. In addition, amphiphiles are known to be organized into lipid membranes by interaction with the inner surfaces of porous minerals. [136] It is a small organizational jump from these membranes to frilly formed lipid vesides. 

ASYMMETRIC AUTOCATALYSIS WITH AMPLIFICATION OF CHIRALITY AND ORIGIN OF CHIRAL HOMOGENEITY OF BIOMOLECULES... 

We describe highly enantioselective asymmetric autocatalysis with amplification of chirality and asymmetric autocatalysis initiated by chiral triggers. Asymmetric autocatalysis correlates between the origin of chirality and the homochirality of organic compounds. We also describe spontaneous absolute asymmetric synthesis in combination with asymmetric autocatalysis. 

Asymmetric autocatalysis using (5)-pyrimidyl alkanol 2a with only 2% ee afforded (5)-2a with an increased ee of 10%, [Eq. (9.4)]. The (5)-2a obtained with 10% ee was then used as an asymmetric autocatalyst for the following asymmetric autocatalysis. (5)-Pyrimidyl alkanol 2a with an increased ee of 57% was obtained. The subsequent consecutive asymmetric autocatalysis and the use of that product as an asymmetric autocatalyst for the following round gave (5)-pyrimidyl alkanol 2a with 81 % and 88% ee, respectively. Thus, the overall process was the asymmetric autocatalysis of (5)-2a starting from a low ee of 2% with significant amplification of chirality to 88% ee, with the increase in the amount without need for other chiral auxiliary. ° This stands as the first example of an asymmetric autocatalysis with amplification of ee. In addition, one-pot asymmetric autocatalysis of pyrimidyl alkanol 2b also significantly increased the chirality from 0.28 to 87% ee. ... 

As described, pyrimidyl alkanols 2 act as highly efficient asymmetric autocatalysts with significant amplification of chirality. 3-Quinolyl alkanoland 5-carbamoyl-3-pyridyl alkanol also serve as efficient asymmetric autocatalysts with amplification of chirality. 

ASYMMETRIC AUTOCATALYSIS AND ITS ROLE IN THE ORIGIN AND AMPLIFICATION OF CHIRALITY... 

Thus, a slight enantiomeric imbalance in compounds induced by CPL was correlated for the first time to an organic compound with very high ee by asymmetric autocatalysis with amplification of chirality. Moreover, various chiral organic compounds such as 1,1-binaphthyl,[2.2]paracyclophanes, and primary alka-nols due to deuterium substitution have been found to serve as chiral triggers in asymmetric autocatalysis. 

We thought that when i-Pr2Zn was treated with pyrimidine-5-carbaldehyde without adding any chiral substance, extremely slight enantioenrichment would be induced statistically in the initially formed zinc alkoxide of the pyrimidyl aUca-nol, and that the subsequent amplification of chirality by asymmetric autocatalysis would afford the pyrimidyl alkanol with detectable enantioenrichment [Eq. (9.11)]. Indeed, we found that pyrimidyl alkanol with an ee that is above the detection level was formed.Pyrimidine-5-carbaldehyde was reacted with /-Pr2Zn, and the resulting pyrimidyl alkanol was used as an asymmetric autocatalyst for the subsequent asymmetric autocatalysis. The consecutive asymmetric autocatalysis afforded pyrimidyl alkanol of either 5 or 7 configuration with enantiomeric enrichment above the detection level. °... 

As described, asymmetric autocatalysis with amplification of chirality is a powerful tool to correlate the origin of chirality with highly enantioenriched organic compounds. 

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