Photoaddition

3 Photoaddition - Micellar aggregates of alkylated -stilbazolium salts in water form only head-to-head photodimers, with a preference for the jj/n-isomer, suggesting a significant degree of order within the aggregates. Preferential 

The total synthesis of psoralen-thymidine monoadducts is of considerable importance in studies of nucleoside damage arising from radiation, chemical carcinogens and chemotherapeutic agents. Within helical DNA, psoralens (155) photoreact with thymidine to form a cis-syn [2+2]adduct. Outside the DNA helix, achievement of the required cis-syn addition has been very difficult and is complicated by the ease of psoralen photodimerisation. Efficient routes to a psoralen-thymidine adduct have used benzofuran derivative (156) which provides a tethered arrangement to ensure the correct cyclobutane stereochemistry. Irradiation in acetonitrile containing acetone, followed by methanol/silica gel 

Examples of arene-azo [6+2]addition have been reported for a series of 2,3-diazabicyclo[2.2.1]hept-2-enes (DBH) whereas closely related 2,3-diazabicy-clo[2.2.2]oct-2-enes (DBO) fail to cyclise. Compounds (171) are smoothly converted to (172), whereas compounds (173) fail to cyclise, the different strains experienced by an azo group within a DBH system compared to one within a DBO system being responsible for the differences in behaviour. Two more examples of cyclobutane formation involving arene/arene [6+6]photocycloaddi-tions in situations where rigid poylcyclic frameworks ensure a face-to-face orientation of the two chomophores at less than van der Waals separations have been reported. Irradiation of (174) and (175) at 254 nm in acetonitrile causes their 

Low temperature irradiation of the glycoside thiohydroxamates (178) in the presence of 2-nitropropene gives clean formation of adducts (179). Conversion of the geminal nitro thioether function to a ketone and removal of the chiral auxiliary yields the free aldol product (180) in 90% enantiomeric excess. Single stranded aminoethylglycine dimers (181) and (182) differ only in whether 

10-dihydroacridine (197) and the corresponding benzylic alcohols (200) are alternatively produced in the presence of added perchloric acid (path b). Protonation of the initially-formed radical (192) gives radical cation (194). Reaction with benzylic radical (195) yields (197) and benzylic cation (198). In the presence of oxygen the benzylic radicals (196) are converted to peroxy radicals (196) (path c). Back electron transfer between radicals (192) and (196) regenerates 10-methyl-acridinium ion (190) and protonation of the accompanying peroxy anion yields hydroperoxide (199). ° 

Intermolecular and intramolecular photocycloadditions to heterocyclic systems, including the photodimerization of individual heterocycles, are considered in this section. Two types of cycloaddition can readily be effected photochemically, namely [ 2 +, 2] and [,4 + 4]. 

Numerous examples of [ 2 -I- 2] photocycloaddition of five-membered oxygen-containing heterocycles to alkenes have been reported, many of which have useful synthetic applications. The regiochemistry and stereochemistry of each adduct has not always been fully established. The lactone (311) undergoes photoaddition to cyclopentene to afford the cis,anti,cis adduct (312), whereas three stereoisomeric adducts were obtained with cyclohexene. Analogous additions of substituted lactones to alkenes have been employed in the syntheses of ( )-dihydrofomannosin acetate and cyclobutane analog of chrysanthemumic acid. The photoaddition of 

2-dimethyl-3(2H)-furanone (313) to alkenes such as cyclopentene (314) takes place in high yield to afford the adduct (315). Further elaboration of these and related adducts has proved useful in synthesis. - Cycloaddition of 5-methyl-3(2H)-furanone (316) to methyl acrylate, vinyl acetate. 

New examples of [ 2 + 2] photocycloaddition of maleic anhydride (318) to alkenes have been reported. The major product of addition to ketene (319), for example, is the spiro cyclobutane (320). The stereoselective addition of dichlorovinylene carbonate to phenanthrene has been described, and the photoaddition of this carbonate (321) to the alkene (322) 

Tsuchiya, Chem. Pharm. Bull. 28, 327 (1980). 

Boulc and J. Lemaire, J. Chim. Pins. Phys.-Chim. Biol. 75. 776 (1978). 

Addition to six-membered oxygen heterocycles is also common. The photocycloaddition of 5,7-dimethoxycoumarin to tetramethylethylene has been described,269 and 4-hydroxycoumarin (326) undergoes facile addition to cyclohexene on direct irradiation to give the cyclobutane (327)270 analogous additions to a variety of other alkenes have been reported, and the cycloaddition of 4-methoxycoumarin to 2-methylpropene has been employed in a synthesis of l,2-dihydrocyclobuta[c]coumarin.271 Photoaddition of the 1,2-bisenol lactone (328) to tran.s-stilbene yields propellane (329),272 and [ 2 + 2] cycloaddition is observed along with other competing photoreactions on irradiation of chromone in the presence of alkenes.273 

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Many [2 + 2] photocycloadditions have not been assigned a definitive mechanism, but they serve well as synthetic methods. Thiones add vinyl ethers to give thietanes in very good yields (Section 5.14.4.1.2), and interesting wavelength-stereochemistry relations were found in the photoaddition of 2-adamantanone to dicyanoethylene (Section 5.14.4.1.2). Diheterocyclobutanes can also be prepared by [2 + 2] photocycloadditions (Section 5.13.3.3). 

There are few examples of formation of simple azetidines from [2 + 2] fragments, although this type of approach is important for certain azetidine derivatives, e.g. azetidin-2-ones (see Section 5.09.3). Nitrogen analogues of the Paterno-Buchi route to oxetanes are rare an example involves the photoaddition of 3-ethoxyisoindolone (37) to the enol ether (38) (75JA7288, 72CC1144). 

Azepine, 3,5-bis(t-butyl)-l-ethoxycarbonyl-photoaddition reactions, with oxygen, 7, 523... 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Paal-Knorr synthesis, 4, 118, 329 Pariser-Parr-Pople approach, 4, 157 PE spectroscopy, 4, 24, 188-189 photoaddition reactions with aliphatic aldehydes and ketones, 4, 232 photochemical reactions, 4, 67, 201-205 with aliphatic carbonyl compounds, 4, 268 with dimethyl acetylenedicarboxylate, 4, 268 Piloty synthesis, 4, 345 Piloty-Robinson synthesis, 4, 110-111 polymers, 273-274, 295, 301, 302 applications, 4, 376 polymethylation, 4, 224 N-protected, 4, 238 palladation, 4, 83 protonation, 4, 46, 47, 206 pyridazine synthesis from, 3, 52 pyridine complexes NMR, 4, 165... 

Photoaddition of acetylene, 349 Photoaddition of allene, 349 Photoaddition of dichloroethylene, 349 Photoaddition of ethylene, 348 Photoaddition of hexafluoroacetone, 345 Photoaddition of maleic anhydride, 348 Photochemical addition of ethylenes and acetylenes to steroidal enones and dienones, 343... 

The Methylene Blue sensitized photoaddition of singlet oxygen to ethyl l//-azepine-l-car-boxylate was reported originally to yield only a C2-C5 adduct 269 however, a reinvestigation with the methyl ester, and using tetraphenylporphine as a sensitizer, revealed that in addition to adduct 46 the [6 + 2] cycloadduct 47 is also produced.270... 

The methanol adduct may arise by photoprotonation of the double bond. A similar photoaddition of methanol has been reported on photolysis of 4973. In a closely related study, King and coworkers74 have shown that photolysis in methanol of the monocyclic unsaturated sulfone (50) gives the methanol adduct (51) and the acyclic sulfonate, CH2= C(Ph)CH—CHCH2S020Me, in 43 and 30% yield, respectively. Formation of the sulfonate ester in this case strongly supports the intermediacy of the sulfene (52) produced on cycloreversion. 

Recent synthetic applications of the photochemical [2 + 2] cycloaddition of unsaturated sulfones have been noted. Musser and Fuchs84 have effected an intramolecular [2 + 2] addition of a 6-membered ring vinyl sulfone and a five-membered ring vinylogous ester in excellent yield, as part of a synthetic approach to the synthesis of the mould metabolite, cytochalasin C. The stereospecificity of the addition was only moderate, however, and later problems with this synthetic approach led to its abandonment. Williams and coworkers85 have used the facile [2 + 2] photoaddition of 73 and... 

Diels-Alder Reaction Facilitated by Physical and Chemical Methods 1,4-Photoaddition of -morphoiinoacryionitriie to 1-acyinaphthaienes [105]... 

A [2 + 2] photoaddition-cycloreversion was applied to the enantioselective synthesis of the natural product byssocMamic add (Figure 6.11). Desymmetrization of a meso-cyclopentene dimethyl ester with PLE in pH 7 buffer-acetone (5 1) provided a monoacid, one of the photopartners. It is noteworthy that both enantiomers of this natural product were synthesized from the same monoacid [58]. 

The photoaddition reaction of 2-phenylbenzimidazole with Michael acceptors was investigated <96JHC1031> as was the preparation and cycloaddition-reactivity of benzimidazole-2-carbonitrile oxide <96AJC199>. The nitration of 1-methylbenzimidazole was found to give only the 5- and 6-nitrated products as a mixture of isomers in 87% yield... 

The photoadditions proceed through 1,4-diradical intermediates. Trapping experiments with hydrogen atom donors indicate that the initial bond formation can take place at either the a- or (3-carbon of the enone. The excited enone has its highest nucleophilic character at the (3-carbon. The initial bond formation occurs at the (3-carbon for electron-poor alkenes but at the a-carbon for electron-rich alkenes.191 Selectivity is low for alkenes without strong donor or acceptor substituents.192 The final product ratio also reflects the rate and efficiency of ring closure relative to fragmentation of the biradical.193... 

Flegel, M. Lukeman, M. Huck, L. Wan, P. Photoaddition of water and alcohols to the anthracene moiety of 9-(2 -hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer. J. Am. Chem. Soc. 2004, 126, 7890-7897. 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

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