Chiral photochemical reactions

Phosphine(s), chirality of, 314 Phosphite, DNA synthesis and, 1115 oxidation of, 1116 Phospholipid, 1066-1067 classification of, 1066 Phosphopantetheine, coenzyme A from. 817 structure of, 1127 Phosphoramidite, DNA synthesis and, 1115 Phosphoranc, 720 Phosphoric acid, pKa of, 51 Phosphoric acid anhydride, 1127 Phosphorus, hybridization of, 20 Phosphorus oxychloride, alcohol dehydration with. 620-622 Phosphorus tribromide, reaction with alcohols. 344. 618 Photochemical reaction, 1181 Photolithography, 505-506 resists for, 505-506 Photon, 419 energy- of. 420 Photosynthesis, 973-974 Phthalic acid, structure of, 753 Phthalimide, Gabriel amine synthesis and, 929... 

The literature presents a large number of examples concerning the use of known oxazolidinones as chiral auxiliaries in many kinds of reactions. Rare is the use of A-amino derivatives of oxazolidinones, which were used to synthesise new A-acylhydrazones 207. Radical addition reactions occurred with high diastereoselectivity <00JA8329>. The use of glycolate oxazolidinones 210 proved to be efficient for the enantioselective preparation of a-alkoxy carboxylic acid derivatives . Photochemical reaction of vinyl... 

We turn now to a presentation of our own research on the use of built-in or internal chiral auxiliaries for asymmetric induction in photochemical reactions in the crystalline state [28]. This work is a natural outgrowth of the work of Toda and coworkers on the use of external chiral host compounds for the same purpose discussed in Sect. 2.2. In both cases, the primary role of the chiral auxiliary is to guarantee the presence of a chiral space group for the ensuing solid-state photochemical reaction. 

The photochemical reaction that has been most thoroughly investigated from the ionic chiral auxiliary point of view is the well-known Norrish/Yang type II reaction. One example, taken from the work of Patrick, Scheffer, and Scott [36], deals with derivatives of 7-methyl-7-benzoylnorbornane-p-car-boxylic acid (37a, Scheme 8). This compound was treated with a variety of optically pure amines to afford the corresponding 1 1 salts (37b), and in an... 

In the solid-state photochemical reaction of N,N-disubstituted a,(3-unsatu-rated thioamides 24, a crystal-to-crystal nature was observed in 24c furthermore, absolute asymmetric transformation in the chiral crystalline environment was performed in the photoreaction of 24b, 24c, and 24e. 

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. 

Porous materials such as organic zeolites discussed in Section 8.4 are vigorously studied since they often exhibit catalytic activity [122]. An interesting type of zeolite materials is obtained by tucking chiral amines inside pores of a commercially available zeolite. Such an approach allowed Ramamurthy s group to enhance stereoselectivity of a photochemical reaction [122b]. 

There is also rapid development in the domain of standard silica-based zeolites. Their versatility can be extended by imprinting. For instance, Davis and Katz [15] recently successfully carried out imprinting and obtained a silica framework with pore walls anchoring three aminopropyl groups in cavities. Another achievement was reported by Ramamurthy, Schefer and coworkers [16]. The latter authors were able to obtain 90% diastereomeric excess of a product of the photochemical reaction in a commercially available zeolite containing chiral tropolone ether 433 in its pores. 

The range of alkenes that may be used as substrates in these reactions is vast Suitable catalysts may be chosen to permit use of ordinary alkenes, electron deficient alkenes such as a,(3-unsaturated carbonyl compounds, and very electron rich alkenes such as enol ethers. These reactions are generally stereospecific, and they often exhibit syn stereoselectivity, as was also mentioned for the photochemical reactions earlier. Several optically active catalysts and several types of chiral auxiliaries contained in either the al-kene substrates or the diazo compounds have been studied in asymmetric cyclopropanation reactions, but diazocarbonyl compounds, rather than simple diazoalkanes, have been used in most of these studies. When more than one possible site of cyclopropanation exists, reactions of less highly substituted alkenes are often seen, whereas the photochemical reactions often occur predominantly at more highly substituted double bonds. However, the regioselectivity of the metal-catalyzed reactions can be very dependent upon the particular catalyst chosen for the reaction. 

The purpose of this article is to briefly review those synthetic photochemical reactions with mono- and oligosaccharides used as specific substrates or as chiral synthons in synthesis. This article follows the excellent general and complete review published in 1981 by R. W. Binkely [1] in the Advances in Carbohydrate Chemistry and Biochemistry . 

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. 

Since photochemical reactions in inclusion compounds have been described in one chapter of the previous book [1], enantioselective (3-lactam formation reactions are summarized in this present chapter as a typical application of the inclusion technique for enantioselective photosynthesis. In addition, as a representative enantioselective single-crystal-to-single-crystal photoreaction, the photodimerization reactions of coumarin and thiocoumarin in their inclusion compound with a chiral host are also described. Furthermore, a host-catalyzed photodimerization reaction of chalcone and 2-pyridone in the solid state is also added to this chapter as a unique example of the application of inclusion techniques to selective photoreaction. 

Suzuki et al. reported the photochemical reaction of CT crystals, in which cycloaddition reaction of bis(l,2,5-thiadiazolo)tetracyano-quinodimethane 17 (electron acceptor) and 2-divinylstylene 18 (electron donor) is efficiently induced (Scheme 3). [17] A structural feature of the CT crystal is the asymmetric nature of the inclusion lattice because of the adoption of a chiral space group, P2. The [2 + 2] photoadduct 19 was formed via the single crystal-to-single crystal transformation, and the optically active product with 95% ee was obtained. 

The second example is photochemical reaction of 2-benzoylbenzothioesters 57 in the chiral crystalline environment. Solid-state photoreactions of, S -(o-tolyl),. S -phcnyl, and S-(ff)-tolyl) 2-benzoylbenzothioates 57a-c, which formed chiral crystals by spontaneous resolution, underwent an intramolecular cyclization involving phenyl migration to afford the corresponding optically active 3-phenyl-3-(arylthio)phthalide 58a-c in good chemo- and enantio-selectivities. (Scheme 11). [34]... 

T. (1998) Solid state photochemical reaction of achiral iV-((3,Y-unsaturated carbonyl)thiocarbamate to optically active thiolactone in the chiral crystalline environment, J. Chem. Soc. Chem. Commun., 2315-2316. 

Sakamoto, M., Takahashi, M., Fujita, T., Nishio, T., Iida, I., and Watanabe, S. (1995) Solid-State Photochemical Reaction of Phenyl-iV-(benzoylformyl)thiocarbamates "Absolute" Asymmetric Synthesis Using the Chiral Crystal Environment, J. Org. Chem., 60, 4682-4683. 

A solid-state photochemical reaction of A,iV-dialkylarylglyoxamide carrying an ionic chiral auxiliary yielded... 

For chiral induction in photochemical reactions see (a) Inoue, Y. and Ramapurthy, V. (eds) (2004) Chiral Photochemistry, in Molecular and Supramolecular Photochemistry, Vol. 11 (eds V. Ramamurthy and K.S. Schanze, Series), Marcel Dekker, New York ... 

Recent interest in the use of N-unsubstituted 2-quinolones stems from the fact, that they coordinate effectively to chiral lactam-based templates via two hydrogen bonds. The prototypical template to be used in photochemical reactions is compound 115, which can be readily prepared from Kemp s triacid [108]. The template is transparent at a wavelength X > 290 nm, and can be nicely used in stoichiometric amounts for enantioselective photochemical and radical reactions [109]. Conditions which favor hydrogen bonding (nonpolar solvent, low temperature) are required to achieve an efficient association of a given substrate. The intramolecular [2 + 2]-photocycloaddition of 4-alkylquinolone 114 proceeded in the presence of 115 with excellent enantioselectivity, and delivered product 116 as the exclusive stereoisomer (Scheme 6.41) [110]. Application of the enantiomer ent-115 ofcomplexing agent 115 to the reaction 111 —> 112 depicted in Scheme 6.40 enabled enantioselective access to (+ )-meloscine [111]. 

Herzog, H., Koch, H., Scharf, H.-D., and Runsik, J. (1986) Chiral induction in photochemical reactions V. Regio- and diastereoselectivity in the photochemical [2 + 2] cycloaddition of chiral cyclenone-3-carboxylates with l,l -diethoxyethene. Tetrahedron, 42, 3547-3558. 

Muller, C. and Bach, T. (2008) Chirality control in photochemical reactions enantioselective formation of complex photoproducts in solution. Australian Journal of Chemistry, 61, 557—564. 

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