Photoadduct formation

Gel electrophoresis of P-end-labelled oligonucleotides irradiated with visible light in the presence of RufTAP)] showed that the principal photochemical product is a less electrophoretically-mobile species [96]. This is consistent with the formation of a photo-adduct and it is clear that the yield of this reaction is 

Further experiments will need to be carried out in order to determine the factors that induce cleavages versus adducts, as both reactions are apparently initiated by a photo-electron transfer process. 

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Upon photo-excitation Ru(II) complexes have been reported to induce three main kinds of reactions of DNA strand cleavage, nucleobase modification, which leads to strand breaks on subsequent treatment with base, and photoadduct formation. 

The Ru11 complexes that contain polyaza-aromatic ligands, such as TAP (1,4,5,8-tetra-azaphenanthrene) and HAT (1,4,5,8,9,12-hexa-azatriphenylene) manifest the very high oxidation potential of their 3MLCT states. Therefore, under illumination, they are able to abstract an electron even from rather poor electron donors, eg biopolymers such as DNA, oligonucleotides, and oligopeptides. Other effects of the photoinduced electron transfer may be photoadduct formation and photocrosslinking via metal complexes [102],... 

Oroskar, A., Olack, G., Peak, M.J., and Gasparro, F.P. (1994) 4 -Ami nomethyl-4,5",8-tri-methylpsoralen photochemistry the effect of concentration and UVA fluence on photoadduct formation in poly(dA-dT) and calf thymus DNA, Photochem. Photobiol., 60, 567-573. 

Aromaticity. New aromaticity indices for cinnoline and related heterocycles have been derived from 12 weighted experimental or theoretical data. Complexes. Anomalous variations in the absorption spectra for cinnoline in nonpolar solvents on temperature change appears to result from three phenomena hydrogen bonding interactions, microcrystalline complex formation at low temperatures, and facile photoadduct formation. An X-ray analysis of the complex, cinnoline.2ZnCl2, has been reported. 

Akasaka et al. reported that the photoexcited Ceo generates [2 + 3] cycloadducts with the disiUranes, such as 1,1,2,2-tetramesityl-l,2-disilirane via the electron-transfer process (Scheme 2) [61]. It is interesting to note that the adduct formation was not observed by the thermal reactions in the dark. The photoadduct formation occurs in the nonpolar toluene, suggesting that the exciplex is a plausible intermediate. When the reaction proceeds in benzonitrile, the generated radical cation of the disiUrane forms 1 1 or 1 2 adducts with benzonitrile in this case, Cgg acts as a photocatalyst... 

Psoralen Photolysis Photoadduct Formation with Biomolecules Apoptosis... 

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. 

Our journey begins with the photo-induced union of 3,4-dimethylfuran (19) and / -(benzyloxy)-propanal (18) (see Scheme 5). Irradiation of a solution of these two simple, achiral compounds in benzene with a 450 W Hanovia lamp equipped with a vycor filter results in the exclusive formation of head-to-head, exo photoadduct 17 in 63% yield. As a cw-fused dioxabicyclo[3.2.0]heptene system, intermediate 17 possesses a folded molecular framework to which access is obstructed on the concave face. In the presence of mCPBA, the less hindered convex face of the enol ether double bond is oxidized in a completely diastereoselective fashion, affording intermediate 16 in 80% yield after regioselective opening of... 

Both the reduction potential of 6-substitutcd cyclohex-2-enones and the ionization potential of the alkene control the product ratio of photocycloadducts vs. photoadducts on the one hand,90 and cyclobutane vs. oxetane formation, on the other91 (a similar dependence of the site of reaction as a function of the ionization potential of the alkene has been observed for 1,4-naphthoquinone92). 3- and 2-Alkynylcyclohex-2-enones on irradiation in the presence of alkenes afford both cyclobutane and cyclopentane derivatives.93-94 The regiochemistry of the photoadducts of simple cyclohex-2-enones to cycloalkenes seems to be dependent on the ring size of this latter species.95... 

Rafikov et al. [35] describe a correlation between the electron-donating capacity of substituted benzenes and the efficiency of adduct formation with maleic anhydride. This is only valid if similar compounds are compared. The ionization potentials of benzene and toluene are 9.246 and 8.820 eV, respectively the yields of adduct formation are 70% and 30%, respectively. In the series of halogenobenzenes, the ionization potentials are as follows fluorobenzene, 9.195 eV chlorobenzene, 9.080 eV bromobenzene, 9.030 eV the yields of adducts are 7%, 2%, and <1%, respectively. Anisole and diphenyl ether, with ionization potentials of 8.220 and 8.090 eV, respectively, do not give adducts with maleic anhydride. It thus seems that an increase in the electron-donating capacity of the benzene derivative leads to a decrease in the yield of photoadducts. 

Intramolecular photocycloaddition of naphthalene and anthracene has been studied by Chandross and Schiebel [327], At concentrations above 10 3 M, bi-molecular photodimerization of the anthracene occurs in deaerated methylcyclo-hexane solution. In contrast, irradiation of much more dilute ( 2 X 105 M) solutions resulted in the formation of intramolecular adduct 342 (Scheme 94). Bouas-Laurent et al. showed that the CH2—O—CH2 link is more efficient than the (CH2)3 chain in bridging the two chromophores [328], Irradiation of diethyl ether or methylcyclohexane solution of 343 (5 X 10 5 M) with a high-pressure mercury lamp and liquid filter (X > 335 nm) gave a single photoadduct 344, which was isolated quantitatively. The quantum yield of 344 is 10 times higher than that of 342. 

It is observed that the reactivity of the KA increases as the number of electron-donating substituents on the olefin increases. Steric hindrance leads to a decreasing reactivity. The authors rationalized the formation of the photoadduct as follows (see Scheme 12). First, the radical ion pair 49 is formed by single electron transfer between photoexcited 3C6o and the electron rich ketene silyl acetal fol-... 

Photochemical cycloaddition of 2-cyanofuran with 2-alkoxy-3-cyanopyridine results in the formation of [4+4] photoadducts 228 and 229. The latter compound is seen to arise through the intermediate 230 (Scheme 40) <2004TL4437>. Mechanistic studies show that the photoadditions proceed from the singlet-excited state of the pyridine. The preference for the formation of 228 over 229 is explained by the two heteraromatics approaching each other such as to avoid proximity of their electronegative heteroatoms. 

As mentioned in Section 4.1.2 there are also scattered examples of [2 + 2]-photo-cycloaddition reactions ofpyridones. 4-Methoxypyridone (159) was reported to form a 1 1-photoadduct 160 with allene (Scheme 6.57), with exclusive HTproduct formation being observed. Analogously, the reaction with diketene was reported in the same study to be regioselective, but resulted in a mixture of two epimeric products [152],... 

Suginome, H., Kobayashi, K., Itoh, M., and Seko, S. (1990) Photoinduced molecular transformations. 110. Formation of furoquinolinones via p-scission of cydobutanoxyl radicals generated from [2 + 2] photoadducts of... 

The concentration dependence of the photocycloaddition of cis- and trans-cyclo-cocetene 136 with aliphatic aldehydes was studied in order to test the spin-selectivity with respect to the cis/trans oxetane ratio and/or the eftdo/exo-selectivity relevant for the cis-photoadduct [156]. The results indicate a moderate but still significant spin correlation effect in the Paterno-Biichi reaction of cyclo-octene with aliphatic aldehydes. The exo-diastereoisomers were formed with similar probability as the e do-diastereo-isomers in the singlet carbonyl manifold, whereas the triplet excited aldehydes preferred the formation of the e Jo-diastereoisomer and trans fused products. 

Irradiation of hydroxy-substituted 1,6-heptadienes in the presence of CuOTf results in formation of bicyclo[3.2.0]heptanes with complete regioselectivity and impressive stereoselectivity [27]. For example, when the S isomer of 6-methyl-l,6-heptadien-3-ol was irradiated in the presence of CuOTf the endo-bicyclo photoadduct shown in Sch. 14 was obtained in 98% e.e. Interestingly, the endoselectivity of this reaction increases in polar solvents. The authors argue that the copper intramolecular coordination between the hydroxyl group and the alkenes is favored in increasingly polar media. 

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