Memory of chirality

Karabatsos, G. J., Rotational Isomerism about sp2-sp3Carbon-Carbon Single Bonds, 5, 167. Kawabata, T., Memory of Chirality Asymmetric Induction Based on the Dynamic Chirality of Enolates, 23, 175. 

This method can be regarded as an example of memory of chirality,71 a phenomenon in which the chirality of the starting material is preserved in a reactive intermediate for a limited time. The example in Scheme 2-35 can also be explained by the temporary transfer of chirality from the a-carbon to the t-BuCH moiety so that the newly formed chiral center t-BuCH acts as a memory of the previous chiral center. The original chirality can then be restored upon completion of the reaction. 

Rychnovsky et al. considered the formation of achiral conformers from chiral molecules and trapping the prochiral radical with a hydrogen atom donor based on memory of chirality (Scheme 12) [41], The photo-decarboxylation of optically active tetrahydropyran 40 leads to an intermediate 43, which now does not contain a stereocenter. If the intermediate 43 can be trapped by some hydrogen atom source before ring inversion takes place, then an optically active product 41 will be formed. This is an example of conformational memory effect in a radical reaction. It was reported that the radical inversion barrier is low (< 0.5 kcal/mol) while the energy for chair flip 43 44 is higher (5 to... 

Scheme 12 Memory of chirality reduction of Barton esters...

The concept of memory of chirality —in which the chirality of the starting material is preserved in a reactive intermediate for a limited time—is discussed with particular reference to the C-alkylation of enolates of chiral ketones. ... 

The /V-alkyl-/V-chloroacetyl amino acid derivatives [180] have been reported to undergo the base-promoted cyclization to 3-lactams [194]. The stereoselectivity, due to memory of chirality, was highly dependent on the substituents of the starting amino acids. The amino acid side-chain (R3) appeared to be the principal stereodirecting element, offering additional support for the explanation that the memory of chirality was caused by a hindered rotation around the C-N bond (Scheme 87). 

Sakamoto, M., Iwamoto, T., Nono, N., Ando, M., Arai, W., Mino, T., and Fujita, T. (2003) Memory of Chirality Generated by Spontaneous Crystallization and Asymmetric Synthesis Using the Frozen Chirality, J. Org. Chem., 68, 942-946. 

Kawabata, T. Kawakami, S. Majumdar, S. Asymmetric cyclization via memory of chirality a concise access to cyclic amino acids with a quaternary stereocenter. J. Am. Chem. Soc. 

Nitrogen heterocycles with contiguous quaternary and tertiary stereocentres (118) have been prepared in high enantiomeric purity (<95% ee) by intramolecular conju- gate addition of enolates generated from a-amino acid derivatives (117) via memory of chirality.160... 

Giese, B., Wettstein, P., Stahelin, C., Barbosa, F., Neuburger, M., Zehnder, M., andWessig, P. (1999) Memory of chirality in photochemistry. Angewandte Chemie, International Edition, 38, 2586-2587. 

The phenomenon of memory of chirality [60] was recently extended to PET-cyclization reactions. An interesting example was the highly... 

However, a careful critical analysis of the kinetic properties of these species makes clear that it is possible to store-erase-rewrite the memory of chirality. As previously shown, the storing process is readily accessible if we have the right molecular components the game is easy. 

Memory of chirality is demonstrated by a simple experiment addition of an excess of A-[Ru(phen)3]2+ (15 pM) to J aggregates preformed in the presence of A-[Ru(phen)3]2+ (10 pM) does not cause inversion of the CD bands in the visible region (where the absorption features of porphyrins dominate) but only in the UV region (the spectroscopic region where absorption of the inorganic complex occurs). This straightforward experiment (Fig. 26) shows that this type of J aggregates remembers the chirality imprinted at the very onset of their formation [51]. 

These results can be explained under the light of the experiment shown in the previous section the inertness and catalytic properties [47] of the initially formed aggregates (seeds), which allow the J species to retain the memory of chirality transferred from A- or A-[Ru(phen)3]2+ [51]. The increase of the pH value to around... 

Mammana A, De Napoli M, Lauceri R et al (2005) Induction and memory of chirality in porphyrin hetero-aggregates the role of the central metal ion. Bioorg Med Chem 13 ... 

SCHEME 102. Memory of chirality enantioselective methylation of a phenylalanine derivative lithium enolate in the presence of no external source of chirality501... 

To explain the high degree of memory of chirality, it was proposed that there is a high activation barrier for the rotation around the central C-N single bond of the 1,7-biradical intermediate. Then the axial chirality can be preserved during the whole process. 

Memory of Chirality Asymmetric Induction Based on the Dynamic Chirality of Enolates... 

VII. Mechanism of Memory of Chirality in Asymmetric Alkylation of a-Amino Acid... 

C. Memory of Chirality in a Cyclization Reaction via an Enolate Intermediate... 

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