Photodimerization 4+2 photocyclization

The photocyclization reaction is in contrast to the photodimerization of 58 in 59. In the case of 61, the photodimerization of 58 is probably prevented by steric hindrance. It is almost certain that one optical conformer of 58 is included in 61, but a real proof of this fact requires an X-ray structural study in the future. However, the formation of the optically active 62 is valuable, because photoreaction of 58 in solution does not give any intramolecular photocyclization product34). 

Ito Y, Matsuura T. A simple method to estimate the approximate solid state quantum yield for photodimerization of trans-cinnamic acid. J Photochem Photobiol 1989 50 141-145. Ito Y, Matsuura T, Fukuyama K. Efficiency for solid-state photocyclization of 2,4,6-triisopropylbenzophenones. Tetrahedron Lett 1988 29 3087-3090. 

Durr et al. have described some novel complex cinnoline derivatives which show photochromic properties. Albini et al. have provided some new evidence on the mechanism for the photoisomerization of heterocyclic N-oxides. Simple pyridine N-oxides are exceptional. Thus irradiation of pyridine N -oxide in aqueous base affords the ring-opened product (23). Aoyama et al. have described the unprecedented photocyclization of the amide (24) to the lactam (25). The first examples of C2a+2Tr] photoreactions of a three-membered ring and an azo-compound have been described by Hunig and Schmitt. Nicolaou et al. have prepared the first stable example of a 1,2-dithiethane the procedure involves (2i +2Tr) photodimerization of C=S groups. 

The opposite behaviour, namely photodimerization, can take place when local concentrations of substrate are high, as for anthracene in Langmuir-Blodgett monolayers." In fact, this is a common reaction pathway for anthracene derivatives and occurs by both bimolecular and intramolecular routes. The efficiency of intramolecular photocyclization depends on the length and flexibility of the connecting chain. 

Photocyclization is a particularly valuable route to the formation of cyclic compounds. There is a wide variety of photocyclization reactions reported in the literature of organic photochemistry, but relatively few of these have been carried out in solid polymers. The earliest reports concern the photodimerization of cinnamic acid derivatives, leading to crosslinking in solid polymers. These polymers have important applications as commercial photoresists. The chemistry has been reviewed by Delzenne (46) and Williams (47). 

This chapter deals with the photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement reactions of aromatic compounds and related photoreactions. 

The photochemistry of aromatic compounds is classified into the same categories adopted in the previous reviews in the series. The photoisomerization of arylalkenes, photoaddition and cycloaddition to aromatic rings, photosubstitution, photorearrangement reactions have less appeared in the period (2010-2011) considered. On the other hand, the photo-chromism including photoisomerization of azobenzenes and intramolecular photocyclization and cycloreversion of 1,2-diarylethenes, and the photodimerization have been widely developed. Supramolecular Photochemistry as a series of Molecular and Supramolecular Photochemistry was edited by Ramamurthy and Inoue in the period. In addition, it should be noteworthy that so many photochemical reactions in solid and/or crystalline states have appeared and developed. 

Interratingly, irradiation for 6 h of the 1 1 complex 61 compc d of 8 a and 58 which had been prepared by keeping a solution of 8a and an equimolar amount of 58 in benzene-hexane at room temperature for 12 h (colorless needles of mp 133 to 136 °C) gave the optically active photocyclization product (—)-bk yclo[4.2.0]oct-7-en-2-one (62) Qa]p —60.6 (c 0.18, CHCI3)] in quantitative yield The photocyclization reaction is in contrast to the photodimerization of 58 in 59. In the case of 61, the photodimerization of 58 is probably prevented by steric hindrance. It is almost certain that one optical conformer of 58 is included in 61, but a real proof of this fact requires an X-ray structural study in the future. However, the formation of the optically active 62 is valuable, because photoreaction of 58 in solution does not give any intramolecular photocyclization product It has been reported that irradiation of bicyclo[5.1.0]octa-3,5-dien-2-one (63) in methanol leads to a mixture of racemic tricyclo[4.2.0.0 ]oct-7-en-2-one (65), 37, and cyclohepta-l,3,5-triene Control of the reaction was also tried in expectation... 

The [2-1-2]-cycloaddition reactions in inclusion crystals, or host-guest crystals, have been thoroughly reviewed. Coumarin 61a, thiocoumarin 61b, and cyclohex-2-enone 62 efficiently undergo single-crystal-to-single-crystal enantioselective photodimerization in inclusion complexes with chiral host compounds (R,R)-(-)-fra s 63a, (R,R)- -)-trans 63b, and (-)-64, respectively. The products are (-)-anri-HH 65a (100% EE), +)-anti-HH 65b (100% EE), and -)-syn-trans 66 (48% EE) dimers. An example for enantioselective intramolecular [2+2]-photocycloadditions in inclusion crystals is the reaction of guest 67 in chiral host 68. Irradiation of the powdered inclusion crystals in water suspension affords the optically active photocyclization product 69 with 100% EE in 90% yield. 

If the photodimerization occurs face-to-face between the stacked thymines, the photodimer should be the trans-syn isomer. However, the photodimer obtained was the trans-anti isomer, suggesting another mechanism of photodimerization. The photocyclization of a diene compound is known to occur by disrotatory motion, although the thermal cychzation occurs by a conrotatory motion. If the photodimerization of thymine bases in the crystal occurred by disrotatory motion, the product should be the... 

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