Chirality memory concept

The noncovalent helicity induction and chiral memory concept is versatile enough to produce and maintain either a right- or left-handed helix because the helix-sense is predetermined by the chirality of the enantiomeric amines used. Consequently, the opposite enantiomeric helicity induction and the memory requires the opposite enantiomeric amine, followed by replacement with achiral amines. However, both mirror-image enantiomeric helices can be produced with a high efficiency from a helical poly(phenylacetylene) induced by a single enantiomer (Fig. 26) [129]. This dual memory of enantiomeric... 

Ishi-i, T. Crego-Caiama, M. Timmerman, P. Reinhoudt, D.N. Shinkai, S. Bnantioselective formation of a dynamic hydrogen-bonded assembly based on chiral memory concept. J. Am. Chcm. Soc. 2002, 124 (49), 14631-14641, and references cited therein. 

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 concept of memory of macromolecular helicity has been introduced for 104 (Figure 14).186 As discussed above, a right- or left-handed helical conformation is induced for 104 with the interaction with chiral additives. For this system, it was found that the helical conformation is not lost even after the chiral additives are replaced with achiral additives. In the case shown in Figure 14, chiral 107 is replaced with achiral 2-aminoethanol. Hence, the helicity is memorized. The effectiveness of the memory depends sensitively on the structure of the achiral additive replacing the chiral additive. It should be noted that the memorized helical-sense excess increased on storage with achiral 2-aminoethanol complexed to 104. 

The concept of intramolecular alkylation of AT-substituted amino acid derivatives via 1,5-diradicals also turned out to be an excellent system for studying the different stereochemical course of spinisomers as discussed in Section 6.2.2. Thus, the a-ketoester 9, which contains an alanine moiety, was prepared. In contrast to aryl ketones, a-ketoesters are not completely converted into the triplet state after photochemical excitation. Upon addition of either a triplet quencher (naphthalene) or a triplet sensitizer (benzophenone), each of the two spin states may be forced (Scheme 3, Table 1). The chiral center at the d-position with respect to the keto carbonyl group raises the question whether a memory effect of chirality may be observed during the cyclization. The results summarized in Table 1 amply demonstrate the specific properties of spinisomeric biradicals. In the presence of naphthalene, which probably acts not only as a triplet quencher but also as a singlet sensitizer, the chiral information of the reactant 9 is almost entirely conserved in the helical diradical 10 because of its very short lifetime. In contrast, the addition of benzophenone results in almost complete racemization, and also the cis/trans selectivity is... 

The memory of chirality concept has been employed in a strategy for the synthesis of chiral a,/ -diamino- and a-amino-/ -hydroxy ester derivatives via asymmetric imino-aldol and aldol reactions, starting from protected aminoesters. The route can 0 be extended to the enantioselective synthesis of aziridines. 

There is no unique definition of the concept of memory of chirality(MOC). According to Fuji et al.," MOC can be described as a process in which central chirality at a carbon alpha to a carbonyl group is preserved as a transient... 

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