Memory effect of chirality

Scheme 5. Stereoselectivity caused by hydrogen bonds (a) and memory effect of chirality (b).

The third example clarifies the special feature of singlet photochemistry. In contrast with 34 and 36, the a-ketoester 39 reacts from the singlet state if irradiated in the presence of naphthalene as triplet quencher. Despite the low diastereoselectiv-ity, the chirality of the alanine derivative 39 is completely conserved during cycli-zation to the pyrrolidines 40a and 40b. This approach has been called the memory effect of chirality [16]. 

For the memory effect of chirality during the alkylation, the naphthalene ring in 23 is essential. The benzene analogue 29 gave a completely racemized product on its a-methylation under the identical reaction conditions. 

Matsumura and co-workers reported a memory effect of chirality in the electrochemical oxidation of 95 to give 96, although the enantioselectivity was modest (Scheme 3.25). The reaction is assumed to proceed via carbenium ion intermediate Q.46 The mechanism for asymmetric induction is not clear. A possible mechanism involves chiral acid (95)-mediated deracemization of racemic 96 produced by the electrochemical oxidation of 95. However, this suggestion may be eliminated based on the finding that treatment of racemic 96 with 95 in methanol containing 5% formic acid did not produce optically active 96. 

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... 

Scheme 3. Memory effect of chirality on photocyclization of a-ketoesters...

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...

Lord, B. S., Memory effect of diethyl-amine mobile phase additive on chiral separations on polysaccharide stationary phases, Chirality, 2004,16, 493-498. 

Diastereoselective reaction using memory effect of molecular chirality controiied by crystaiiization... 

T. (2007). Diastereoselective photocydoaddition using memory effect of molecular chirality controlled by crystallization. Chem.Commun., pp. 1632-1634... 

On removal of the low frequency field, in thick cells, the texture relaxes back to focal conic with residual light-scattering properties. This has been described as the storage mode or memory effect in chiral nematic systems [162]. If a high frequency ac field is now applied (o)> co ), the dielectric torque restores the nonscattering planar texture and, just as in smectic A materials, this effect can be used in electrooptic storage mode devices [165], since we have a low frequency write and high frequency erase mechanism. 

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... 

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). 

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...

The imbalance between the two diastereoisomers is prolonged when the chiral D-tartrate is removed by precipitation with ethylendiamine thus leading to an enantioenriched chiral double-rosette made of achiral components with 90% e.e. The memory effect in this case is even stronger then in the former example with activation energy towards racemization as high as 119 KJ/mol and a half-life time of one week at room temperature. 

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...

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