Asymmetric Photochemical Synthesis

The Use of Chirally Modified Zeoiites in Photochemical Asymmetric Synthesis... 

Scheme 5 Examples of the zeolite chiral inductor and chiral auxiliary methods for photochemical asymmetric synthesis...

Asymmetric synthesis, either enantioselective or diastereoselective, has seldom been performed by photochemical reactions. One of the first examples that may be classified as a photochemical asymmetric synthesis is the photoalkylation of the most simple amino acid, glycine. Elad and Sperling 220) demonstrated that, if glycine is part of a polypeptide chain, there is good control (up to 40 % e.e.) in the creation of the new chiral center. A radical mechanism operates after the first step of photoinitiation of the process. 

With numerous researchers investigating the advantages associated with thermally or biocatalytically controlled asymmetric syntheses, some of which have been performed in continuous flow reactors, few have considered the prospects of photochemical asymmetric synthesis, an idea... 

There are a couple of comprehensive reviews on general asymmetric photochemistry in solution [133,134] and also on asymmetric photosensitization [135,136]. An account on multidimensional control of asymmetric photoreaction by environmental factors has also appeared recently [137]. This reflects a keen interest in chiral photochemistry and photochemical asymmetric synthesis [29]. In this section, we will concentrate mostly on the asymmetric photosensitization of cycloalkenes. 

In seeking ways to capitalize on this particular advantage that photochemistry enjoys over ground-state chemistry, it would be desirable to be able to carry out the A — B transformation enantioselectively. Aside from the intellectual challenge posed by such a problem, the preparation of theoretically interesting, highly strained compounds in optically pure form could be of considerable interest in subsequent mechanistic studies of the chemical behavior of such species as well as in their use as synthons in total syntheses. The present volume, as well as a number of recent review articles and symposia [1], attest to the growing interest in the field of photochemical asymmetric synthesis. 

On the assumption that asymmetric reduction of an intermediate by a chiral metal hydride complex could occur during the course of photocycli-zation of the enamide 133, as exemplified by reductive photocyclization, Ninomiya et al. (16) have undertaken and completed the photochemical asymmetric synthesis of (—)-xylopinine. 

For an early example of photochemical asymmetric synthesis in irradiation of L-erythritol 1,4-dicinnamate derivatives see B. S. Green, A. T. Hagler, Y. Rabinsohn and M. RejtO, Isr. J. Chem., 916ITI, 15, 124. 

Ando has published a monograph dealing with the synthetic uses of ketone and alkene photochemistry. A review by Overend has focused attention on the synthesis of novel carbohydrates, including applications of photochemical methods. Other reviews have examined the developing area of photochemical asymmetric synthesis. Rau has reviewed asymmetric photochemistry in solution. 

Photochemical asymmetric synthesis with native and modified CDx s is certainly one of the most potentially successful examples of supramolecular photochirogen-esis, which is applicable in principle to most photoreactive substrates of appropriate size and shape. Despite this wide applicability, the enantiodifferentiating ability of CDx and hence the product ee obtained are not sufficiently high in many cases. Furthermore, the elucidation of the chiral discrimination mechanism and the rationalization of the product chirality and ee are generally more difficult in photochemical asymmetric induction. For more simple and well-defined host-guest interactions (in the ground as well as excited states), several approaches to photochemical asymmetric induction with newly designed synthetic chiral hosts have been reported. 

Table 4 Photochemical Asymmetric Synthesis Using the Solid -State Ionic Chiral Auxiliary Method... 

Ring and open-chain derivatives of L-erythritol 1,4-dicinnamate have been irradiated yielding products of photochemical asymmetric synthesis, in which intramolecular 2-1-2 cycloaddition occurs to give L-erythritol esters of cyclobutane-1,2-dicarboxylic acid the 2,3-di-O-methyl derivative of l-erythritol gave a high degree of asymmetric induction, whereas the 2,3-0-isopropylidene derivative was much less stereospecific, and gave the opposite cyclobutane enantiomer. ... 

Photochemical asymmetric synthesis ( photochirogenesis ) uses photoexcited short-lived, weakly interacting molecular states. Their short lifetimes makes it challenging to control electron transfer and subsequent product formation and asymmetric induction in particular [30]. Until recently the transfer of chiral information was restricted to photoreactions where the chirality transfer was assured... 

Recent studies on enantiodifferentiating photoisomerization reveal that the photosensitization of C Cg (Z)-cycloalkenes with optically active aromatic esters affords chiral ( )-cycloalkenes in good to excellent ees. The mechanism involves enantiodifferentiating rotational relaxation within the exciplex intermediate formed from the excited singlet state of chiral sensitizer and prochiral substrate. The exciplex structure and hence the stereochemical outcome are very sensitive not only to the internal factors, such as sensitizer energy and structure, but also to the external entropy-related factors such as temperature, pressure, and solvent. This leads to a novel idea of multidimensional control of product chirality by several environmental factors. The asymmetric photosensitization can be applied to the photochemical asymmetric synthesis and also be used as a powerful tool for exploiting the reaction mechanisms in the excited-state chemistry. 

Green, B.S., Hagler, A.T., Rabinsohn, Y, and Rejto, M., Photochemical asymmetric synthesis. Irradiation of ring and open-chain derivatives of L-erythritol 1,4-dicinnamate,/sr. /. Chem., 15,124,1977. 

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