Optical activity memory

Optical lithography, in compound semiconductor processing, 22 193 Optically active citronellol, 24 506 Optically transparent porous gel-silica, 23 75, 76 Optical materials nonlinear, 17 442-460 second-order nonlinear, 17 444—453 third-order nonlinear, 17 453-457 Optical memory, photochromic material application, 6 602 Optical microscopy, 16 467-487 history of, 16 467-469 in kinetic studies, 14 622 liquid immersion, 15 186 Optical mode density, 14 849, 850-852 Optical multichannel analyzers (OMAs), 23 143... 

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). 

The cooperativity of amplification, switching, and memory in synthetic helical polymers might thus be shared with ideas of a scenario for the biomolec-ular homochirality, autocatalytic mechanism in chiral chemical synthesis, and bifurcation equilibrium mechanisms in crystallization of chiral crystals. Indeed, amplification phenomena in several optical activity and helicity of synthetic polymers in isotropic solution appears to be common and are now established as sergeants and soldiers experiment and majority rules in polymer stereochemistry [17,18]. Any minute chiral forces caused by intramolecular and intermolecular systems can be detectable, when a proper model polymer system is chosen to elucidate the cooperativity of amplification, switching, and memory. 

The one-handed helicity of 28 induced by a small amount of chiral (R)-39 and subsequently amplified by an achiral amine 40 (Fig. 16) can also be memorized in the same way by the replacement of (R)-39 and 40 with achiral amines [87]. The chiral amplification combined with the macromolecular helicity memory will offer a highly sensitive, chirality sensing method for chiral molecules even when their optical activities are too small to detect by conventional spectroscopic means. 

However, despite these and other early observations, it was not until considerably later that Cushny demonstrated clearly that one enantiomer of a molecule could be much more active pharmacologically than the other enantiomer [23], Thus ( — )-hyoscyamine was 12-20 times as active as the (+ )-enantiomer with respect to their action on motor nerve endings. In his Charles E. Dohme Memorial Lecture (1925), Cushny [23] stated with reference to optical activity The chemists have long been mildly interested in this form of vital activity, the pharmacologists have shown its importance in their sphere, but biologists in general do not seem to have appreciated it as a very definite and measurable feature of living matter . 

When the bulk of the crystals of 68 (Table 13) in a test tube was irradiated at 15°C under an argon atmosphere, intramolecular [2 + 2] cyclization proceeded effectively without melting down, and two diastereomeric oxetanes, 69 and 70, were obtained in 95 and 5% yield, respectively. The enantiomeric purity of the main product 69 was determined as > 99%. When 68 was irradiated after dissolving in THF at various temperatures, optically active oxetanes were isolated below — 20°C, whereas the racemic oxetanes were naturally obtained from the photolysis above 0°C in THF. The photolysis in THF at — 60°C gave 87% ee of 69 and 62% ee of 70, in 76 and 24% chemical yields, respectively. The memory of... 

In the early twentieth century Leuchs reported a surprising example of the a-chlorination of chiral ketone 73, which gave optically active 74 in the absence of additional chiral sources.36 From a mechanistic point of view, however, there remains some ambiguity. Possible mechanisms for the formation of optically active 74 include (1) asymmetric chlorination via an enol intermediate (i.e., memory of chirality), (2) direct electrophilic chlorination of the C-H bond at the stereogenic carbon center, (3) complex formation of an achiral enol intermediate with optically active 73, (4) resolution of dl-74 by co-crystallization with optically active 73, and (5) simultaneous resolution of dl-74. 

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. 

Yashima and co-workers reported the memory of macromolecular helicity of poly((4-carboxyphenyl)acetylene) (poly-98). Poly-98 itself possesses a large number of short helical units with many helix-reversal points, and is therefore achiral. However, in the presence of optically active amine 99, which can interact with the polymer s carboxyl groups, one-handed macromolecular helicity is induced in the polymer. When achiral amino alcohol 100 is added to the helical complex, chiral amine 99 bound to poly-98 is replaced by stronger base 100. Nevertheless, the newly formed complex still shows a one-handed helical conformation. Even after the removal of 99 by gel permeation chromatography, the poly-98-100 complex retains a one-handed helical conformation without a loss of helical intensity. Thus the helicity of poly-98 induced by complexation with a chiral amine was memorized after replacement by an achiral one. The half-life of the chiral memory is as long as four years at room temperature.48... 

More detailed kinetic information about the motions and reactions of the alkyl/aryloxy radical pairs can be obtained if the alkyl carbon atom making an ether bond is chiral and the alkyl aryl ether is optically active. Because the radical centers of the alkyl fragments derived from irradiation of such ethers are either planar or invert very rapidly, they exist initially with their aryloxy partners as prochiral pairs. Their ability to retain the memory of the chirality of their parent ethers and impart it to the rearrangement products depends on the ratio between the in-cage rates of radical tumbling and radical pair combination. 

The corresponding esters are much less informative because the centers of chirality in their acyl radicals are structurally protected from racemization like that experienced by translational or rotational motions of prochiral alkyl radicals. In addition, the decarbonylated radicals derived from them are formed long after their acyl precursors have moved to orientations with respect to their aryloxy partners that result in a loss of the memory of their host stereochemistry within a cage see above. Thus, of the Claisen-like photoproducts from irradiation of (7 )-lb, only the BzON (i.e., 3b) retains a measurable amount of optical activity even in the solid phases of long -aIkane. However, in polyethylene hlms, all of the Claisen products from irradiation of (7 )-lb—2-BN, 4-BN, and 3b—exhibit signihcant ee values. In the same media, the photo-Fries products from lb retain virtually all of the enantiomeric purity of the... 

The imprinting effects of MIPs prepared with optically active compounds as the print molecules are readily demonstrated by chromatographic evaluations. For example, when the L-enantiomer of an amino acid derivative is used as the print species, a column packed with the resulting polymer will retain the L-enantiomer longer than the o-enantiomer and vice versa when the o-enantiomer is used as the print molecule. Reference polymers prepared with the racemate or without print molecule will not be able to resolve the racemate. A steroselective memory is hence induced in the polymers by the print molecules and is in many cases very precise. 

Optical Activity and Energy Discrimination (H.G. Smith Memorial Lecture 1972). 

L. Kuhnert. A new optical photochemical memory device in a light-sensitive chemical active medium. Nature (London), 337 393-394, 1986. 

Conformational memory in the reductive decyanation of optically active cyanohydrins [31] is shown in Scheme 21. In order to obtain optically active products by this process, reduction of the initially-formed radical intermediate must be faster than any other racemizing process such as ring inversion. Typically in these reductions, the products possess only modest enantiomeric excesses (entries 1 and 2), but if high concentrations of lithium in ammonia are used, then the reduced products can be obtained in up to 90% ee (entry 3). 

Conformational memory is also observed in Barton radical decarboxylations of optically active tetrahydropyrans [31]. Photolysis of thiohydroxamate esters derived from optically pure tetrahydropyrans in the presence of various hydrogen atom... 

Conformational memory has also been demonstrated in intramolecular cyclizations to form spiro compounds [32]. In the event, treatment of an optically active cyanohydrin with LiDBB in THF at -78 °C for 10 min alfords a high yield of a single diastereomeric c -spiroether in 42% ee (Scheme 23). The cyclization proceeds most likely via an equatorial a-alkoxylithium, which arises from ring inversion of the kinetic axial alkyllithium intermediate [34]. 

The kinetics of the solvolysis of linalyl p-nitrobenzoate were published in note form 18 years ago, but a full discussion of the reaction, including the mechanism for retention of optical activity in the cyclized products, has now been published as one of the Winstein memorial papers. Another paper on the reaction of linalool with phosphorus pentachloride reports 88 % yield of a ca. 3 1 mixture of geranyl and linalyl chlorides (after 4 h at —10 °C). Acetylation of linalool is notoriously fickle on account of ready rearrangements now a method using t-butyl acetate and sodium methoxide is said to give a 90% yield of the unrearranged acetate. The rearrangements involved in the acid decomposition of the... 

Figure 6 Schematic illustration of a preferred-handed helicity induction in achiral st-PMMA in the presence of Ceo with (S)- or (R)-PEA, memory of the induced helicity after removal of PEA, and subsequent optically active stereocomplex formation after the addition of it-PMMA, resulting from replacement of the encapsulated Ceo molecules by it-PMMA strands. Reproduced with permission from Yashima, E. Maeda, K. lida, H. etal. Chem. Rev. 2009, 109, 6102. Copyright 2009 American Chemical Society.

Optically active polymers n. Examples of these materials are organic polymers, metallorganic optical materials, and bio-molecular optical materials. These materials may be used for various non-linear optical and electroptical applications, principally in communications, information processing, memory/storage, and optical limiting. 

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