Photoreactions, asymmetric

Of further particular interest was that the crystallographic results on 2,5-DSP and poly-2,5-DSP had pointed out a very important future possibility that an absolute asymmetric synthesis could be achieved if any prochiral molecule, e.g. an unsymmetrical diolefin derivative, could be crystallized into a chiral crystal and if the reaction of the chiral crystal proceeded in the same manner as the 2,5-DSP crystal with retention of the crystal lattice (Wegner, 1972, 1973). Such types of absolute asymmetric synthesis with a high enantiomeric yield have now been performed by topochemical [2+2] photoreaction of unsymmetric diolefin crystals (Addadi etal., 1982 Hasegawa et al., 1990 Chung et al., 1991a,b). 

Topochemical [24-2] photoreactions of diolehn crystals has been reviewed. The reactions clearly depart from typical solution chemistry crystal-lattice control offers a unique synthetic route into photodegradable polymers, highly strained [24-2] paracyclophanes, stereoregular polymers, and absolute asymmetric synthesis. However, achieving the desired type of crystal... 

The structural changes that accompanied the [2 + 2] photodimerization of the metastable a -polymorph of ort/zo-ethoxy-tranx-cinnamic acid have been studied [93]. In this study, the photochemical reaction was carried out at 293 K, and observed in situ by single-crystal X-ray diffraction. In the structure of the title compound, the three molecules in the asymmetric unit are arranged to form two potential reaction sites, but only one of these was found to be photoreactive. Since only two out of three molecules in the asymmetric unit take place in the photodimerization reaction, the crystal of the final product contains an ordered arrangement of the photodimer and the unreacted monomer. 

Joy, A., Robbins, R.J., Pitchumani, K. and Ramamurthy, V. (1997). Asymmetrically modified zeolite as a medium for enantioselective photoreactions reactions from sping forbidden excited states. Tetrahedron Lett. 38, 8825-8828... 

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 Photoreactions of Conjugated Enones and Esters (Pete). . . Atmospheric Reactions Involving Hydrocarbons, ETIR Studies of (Niki and Maker). 

The P- and M-cis-syn-conformers give photoreactions, whose rates are proportional to el when left-handed CPL is used. Therefore, a high optical yield requires a high g-factor 69 After the excitation cyclization occurs leading to a DHP, which is readily oxidized to helicene. The precursor remains racemic as racemization in the ground state, is easy. Racemization in the excited state, however, may be slow, compared to cyclization. According to Kagan 69) this mechanism explains the asymmetric synthesis well. 

In some cases, achiral molecules are arranged in a chiral form in their own crystals. When the chirality can be fixed by photoreaction, this becomes a convenient asymmetric synthesis without using any further chiral source, so it can be described as an absolute asymmetric synthesis. This phenomenon is also important in relation to the mechanism of generation of chirality on Earth. Several such examples of absolute asymmetric synthesis in crystals have been reported so far. In this section, some of these found mainly in the author s laboratory are described. 

Enantioselective photoreactions in the solid state have many advantages as described in Section I. As reviewed in this chapter, highly enantioselective photoreactions can be accomplished in the solid state without using any chiral source. There are hopeful possibilities to find many other examples of similar absolute asymmetric photosynthesis in the solid state. Enantioselective photosynthesis in inclusion complex crystal with a chiral host compound will also be a new general method of asymmetric synthesis in future. By successful designing of many new chiral host compounds, the research field would also be developed widely. 

Table 9 Photoreactivity of Crystals With Two Molecules in the Asymmetric Unit...

On the other hand, since the concept of topochemically-controlled reactions was established, various approaches to asymmetric synthesis using solid-state photoreaction... 

Similar absolute asymmetric synthesis was demonstrated in the solid-state photoreaction of A-(P,y-unsaturated carbonyl)thiocarbamate 41. [27] Achiral 0-methyl AT-(2.2-dmeth ibut-3-enoyl)-iV-phenylthiocarbarnate 41 crystallized in chiral space group P2i, and irradiation of these crystals gave optically active thiolactone in 10-31% ee. A plausible mechanism for the formation of 42 is rationalized on the basis that photolysis of 41 undergoes [2 + 2] cyclization to thietane and is subsequently followed by rearrangement to thiolactone 42. 

Absolute asymmetric synthesis was observed in the solid-state photoreaction of benzoylbenzamide 59 to phthalide 60 however, the reaction mechanism was completely different from that of thioester 57. [35] Recrystallization of these amides 59a-c from the chloroform-hexane solution afforded colorless prisms in all cases. X-ray crystallographic analysis revealed that all prochiral amides 59a-c adopted orthorhombic chiral space group P2 2 2 and were frozen in chiral and helical conformation in the crystal lattice. 

In this reaction, achiral A-methacryloylthiobenzanilide 39 gave chiral crystals by spontaneous crystallization, and absolute asymmetric synthesis was performed by the solid-state photoreaction leading to optically active thietane-fused 3-thiolactam. The reaction mechanism for the cyclization was elucidated on the basis of the correlation between the absolute structures of both the prochiral starting monothioimide and the photoproduct. 

Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to die di-Jt-methane and Norrish type II photorearrangements, J. Am. Chem. Soc., 108, 5648-5649. (b) Chen, J., Pokkuluri, P. R., Scheffer, J. R., and Trotter J. (1990) Absolute asymmetric induction differences in dual pathway photoreactions, Tetrahedron Lett., 31, 6803-6806. (c) Fu, T. Y., Liu, Z., Scheffer, J. R., and Trotter, J. (1993) Supramolecular photochemistry of crystalline host-guest assemblies absolute asymmetric photorearrangement of the host component, J. Am. Chem. Soc., 115, 12202-12203. (d) Leibovitch, M.,... 

Sakamoto, M., Sekine, N., Miyoshi, H., and Fujita, T. (2000) Absolute asymmetric phthalide synthesis via the solid-state photoreaction of AW-disubstituted 2-benzoylbenzamides involving a radical pair intermediate, J. Am. Chem. Soc., 122, 10210-10211. 

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