Asymmetric photochemical

That chiral molecules can be produced in a CPL field, either from achiral precursors by photo-activated synthesis or by preferential chiral photodestruction of a racemic mixture, is now well demonstrated and has been reviewed. [46] In all cases currently known, however, such processes have proved very inefficient. For example, asymmetric photochemical ring-closures of achiral helicene precursors induced by CPL have produced only about 0.2% e.e. in the products. Likewise, the CPL-induced photolysis of racemic camphor produced about 20 % e.e., but only after 99% photodestruction, and photolysis of D.L-glutamic acid produced only 0.22 % e.e. after 52 % photodecomposition. [71]... 

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

It will be interesting to follow the developments on the highly efficient and more stereoselective photocycloaddition and photoaddition to aromatic rings from the viewpoints for the synthesis of more complex compounds, including natural products. In addition, the chiral induction in the excited states should be more attractive projects in the near future. Although some excellent reviews about the asymmetric photochemical reactions have been reported in recent years [490-492], the highly enantioselective or diastereoselective photocycloaddition and photoaddition have been reported in only limited cases. 

Asymmetric Photochemical Reactions in Solution Simon R. L. Everitt and Yoshihisa Inoue... 

Crucially, not only temperature [148,149] but also other environmental factors, such as pressure [143], magnetic/electronic fields [150,151], and solvent [152] play vital roles in a variety of asymmetric photochemical reactions. In the enantiodifferentiating photoisomerization of (Z cyclo-octene 38Z, (7 )-(—)-( ,)-38E is produced predominantly at temperatures higher than the isoenantiodifferentiating temperature (r0), while the antipodal (S)-(+)-isomer prevails at temperatures lower than T0 even if... 

The description that we have developed also allows to describe the effect of MChA in other asymmetric photochemical processes. The case of MChA in photodecomposition can easily be related to Eqs. (12) and (13) by a proper choice of the parameters. 

Irradiation of tropolone alkyl ether 22 (Scheme 14) led to a 4ir-disrotatory ring closure to yield bicyclo[3.2.0]heptadienone 23 with two chiral centers, while prolonged irradiations led to the formation of a secondary product 24 [76-78]. As the same photocyclization was performed in chirally modified zeolites, it is interesting to compare the asymmetric photochemical behavior of 22 in the distinctly different chiral confined media of zeolites and cyclodextrins. Even in the... 

Lahav, Leiserowitz and their coworkers have used inclusion complexes of deoxycholic acid to carry out asymmetric photochemical reactions. The authors reported that acetophenone forms a 2 5 crystalline channel inclusion complex with deoxycholic acid 22 as the host and that irradiation of the complex in th4 solid state leads to abstraction of a hydrogen atom by the acetophenone 23a from C5 of the steroid followed by coupling of the resulting radical pair to produce1 photoproduct 24 (Scheme 9) [216,219]. The authors were able to follow the course of the reaction by x-ray crystallography. Only one diastereomer of the product was obtained in the reaction. Aoyama et al. have studied the reaction of AT,N-dialkylpyruvamides in deoxycholic acid inclusion crystals (Scheme 10) [246], Solid-state irradiation of the inclusion complex of 22 and 25 gave the... 

For example, crystallization of the achiral ester 117 (Scheme 6.50) from melt or from solution affords two chiral enantiomorphous crystal forms.715 Irradiation of a solid sample containing both forms then yields two dimers with the either (S,S,S,S)- or (R,R, R,R)-118 absolute configurations, which is an interesting example of asymmetric photochemical synthesis.588... 

Explain the following concepts and keywords photostationary state Rydberg state NEER principle bicycle-pedal mechanism phototransduction phototherapy photochemical deracemization enantiodifferentiating photosensitization photochromism di-Tt-methane rearrangement asymmetric photochemical synthesis single-crystal-to-single-crystal photochemistry photochemotherapy rule of five. 

The classification of asymmetric photochemical reactions is generalized in Table... 

It is clear that [2-1-2] photocycloaddition had dominated the recent literature of the asymmetric photochemical process in solution, and we have highlighted some of the more interesting examples that have appeared over the last 6 years. However, this chapter does not seek to describe this area in depth, and the reader s attention should be directed to the reviews mentioned to obtain a more specialized view of this area of photochemistry. 

With numerous researchers investigating the advantages associated with the thermal or biocatalytic control of asymmetric reactions, Ichimura and co-workers [89] considered the potential of photochemical asymmetric syntheses performed in continuous flow reactors. To investigate the hypothesis, the authors employed the asymmetric photochemical addition of MeOH to (R)-( + )-(Z)-limonene (159) as a model reaction, comparing three quartz micro reactors, with a standard laboratory cell as a means of highlighting the synthetic potential of this approach. 

The use of chiral sensitizers or of circularly polarized light can lead to asymmetric photochemical synthesis. For a review, see Inoue, Y. Chem. Rev. 1992, 92, 741. See also equation 12.99. 

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