Chirality memory

Lauceri, R., Raudino, A., Scolaro, L. M Mical, N., and Purrello, R. (2002). From achiral porphyrins to template-imprinted chiral aggregates and further self-replication of chiral memory from scratch. / Am. Chem. Soc., 124, 894-5. 

Purrello, R. (2003). Lasting chiral memory. Nature Mater., 2, 216-17. 

In 2008, Sakamoto and coworkers have reported the synthesis of optically active (3-lactams via photochemical intramolecular y-hydrogen abstraction reaction of thioimides [177]. This reaction provides the first example of a chiral-memory effect for the photochemical y-hydrogen abstraction reaction of thiocarbonyl or carbonyl compounds, and a useful synthetic methodology for preparing optically active (5-lactams (Scheme 78). 

Niwano H, Nakata M, Thisayukta J, Link DR, Takezoe H, Watanahe J (2004) Chiral memory on transition between the B2 and B4 phases in achiral banana-shaped molecular system. J Phys Chem B 108 14889-14896... 

The intramolecular alkylation of the enolate derived from phenylalanine derivatives 22a,b to form P-lactams 23a,b has also been achieved using Taddol as a chiral phase-transfer catalyst (Scheme 8.11) [23]. In this process, the stereocenter within enantiomerically pure starting material 22 is first destroyed and then regenerated, so that the Taddol acts as a chiral memory relay. Taddol was found to be superior to other phase-transfer catalysts (cinchona alkaloids, binol, etc.) in this reaction, and under optimal conditions (50 mol % Taddol in acetonitrile with BTPP as base), P-lactam 23b could be obtained with 82% et. The use of other amino acids was also studied, and the... 

Fig. 4 Schematic representations of the possible pathways for the construction of chiral supramolecular aggregates. A Chiral supramolecular aggregates from chiral components. B Chiral racemic supramolecular aggregates from chiral components. C Chiral enantioen-riched supramolecular aggregates from chiral components exploiting the chiral memory effect. D Chiral encapsulation achiral capsule binds an enantiopure primary guest forming a chiral space in the cavity, which is filled preferentially by one of the two enantiomers of a secondary guest...

Fig. 24 Enantioselective non covalent synthesis of double rosettes exploiting the chiral memory effect. Exchange of chiral barbiturate with the achiral cyanurate occurs faster then racemization by dissociation of the supramolecular structure and allows temporary preservation of the asymmetry instructed in the first diastereoselective step...

Fig. 26 A Chiral memory effect applied to the construction of a chiral softball homo-dimeric capsule made of achiral units. B Racemic softball binds a chiral guest leading preferentially to one diastereoisomer. Fast exchange, compared to capsule s racemization rate, with a better achiral guest allows preparation of enantioenriched capsule...

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... 

Fig. 26 Schematic representation the chiral memory effect observed in enantiomerically pure tetrahedron 6. Ligand 3 can replace up to three bridging ligands 5 in the (A A A A)-6 tetrahedron, while still maintaining its original AAAA-homochirality [149]...

Interestingly, a high chiral memory has been observed for this photochemical reaction, which probably involves triplet biradicals. For example, triplet-photosensitized electron transfer in the substrate of Scheme 9.4 leads to decarboxylation, followed by cyclization to a pyrrolobenzodiazepine with 86% enantiomeric excess... 

As presented in Scheme 1, induction of chirality is a necessary and preliminary step leading to its permanent transfer (i.e. memory). For this reason we present first a system which does not show chiral memory but represents a kind of milestone in the development of our research [26]. The rationale behind this study was to investigate systems more complex than those studied in literature at that time (1998). The reason was not simply to do something different or new but to understand if and how chirality transfers and propagates from a chiral template to a first shell of achiral guests and from here to a second shell of achiral guests. 

On the other hand, all the other couples of porphyrins bearing opposite charges H2T4-H2TPPS, H2T4-CuTPPS and CuT4-CuTPPS lead to the formation of supra-molecular species for which the chiral memory phenomenon occurs. All the three... 

The behaviour of the systems presented above is, however, even more complex and richer than what we have shown until now. In particular, a deeper study of the tetra-anionic porphyrin Ru-complexes system has revealed that, using proper experimental conditions, not only the porphyrin J-aggregates show the chiral memory phenomenon, but also that the chiral information can be cyclically erased and rewritten, switching on and off the porphyrin ICD signal by pH variations [51]. 

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