Photoreactions of Thymines etc

A study of the photodimerization of the pyridones (245) in a micellar environment has been reported.The results show that there is an alignment of the molecules in the micelle. The pyrazinone derivative (246) is photochemically unreactive when it is irradiated in solution at room temperature. However when it is irradiated in the solid state at room temperature a [4 + 4]-dimer (247) is formed. 

The acetone-sensitized irradiation of the bis-pyrimidine (248) yields the [2 -t- 2]-cyclized product (249). A chair-like conformation is adopted by the cycloheptane part of the molecule (249) in the solid and in solution. The dimerization of the bichromophores (250) to yield the cycloadducts (251) was slow by comparison with the dimerization in analogous less heavily substituted systems. The slowness of the process is, it is thought, due to steric factors.  

The regiospecific formation of the cyclobutene-type adduct (261) has been reported as a result of the photoaddition of diphenylacetylene to the cyanouracil (262). The cycloaddition of 1-phenylprop-l-yne to the same substrate is reported to yield the cyclobutene (263) as the main product when Pyrex-filtered irradiation is used. The irradiation of the same compounds using 254 nm light afforded the novel adduct (264), presumably formed by the secondary cyclization of the initially produced adduct (265). Indeed the sensitized irradiation of the uracil (262) and the propyne yields this adduct (265) as a Z- -mixture as well as the cyclobutene (263). Direct irradiation of (265) affords the cyclized compound 

Yamazaki, S. Hirokami, and M. Nagata, Tetrahedron Lett., 1980, 3067. 

Shimozono, and M. Teruo, Fukusokan Kagaku Toronkai Koen Yoshishu, 12th, 1979, 161 (Chem. Abstr., 1980, 93, 45 547). 

The addition of the thio-radical derived from cysteine (160), produced by its irradiation in acetone-water, to cytosine (161) results in the formation of 5-S-cysteinyl-5,6-dihydrouracil (162 24%).98 An analogous product is obtained when 

A full report of the photoaddition of olefins to the imine linkage of the azathymine (173b) and the azauracil (173a) has appeared,104 following initial 

Irradiation ( 280 nm) of aqueous acetone solutions of uracil (163a) affords the oxetan (180).183 108 

The photoreaction of 3-methyl-4-thiouracil (181) with 2-methacryIonitrile in CH2C at 0 °C affords two products (182) and (183).109 110 The indications are that the thietan (184) is formed initially, and then ring-opens to an intermediate which can be trapped as (185) when the irradiation is carried out in methanol at room temperature. At —10 °C in methanol (183) is the sole product. In CH2C12 the ring-opened thietan-derived intermediate dimerizes to afford the thermally unstable intermediate (186) which yields the two photoproducts (182) and (183). Other thiouracil derivatives have been reported to yield thietans (187)—(189) when irradiated in the presence of 2-methacrylonitrile.111 

Fombert, J. L. Fourrey, P. Jouin, and J. Moron, Tetrahedron Letters, 1974 3007 

2 Photoreactions of Thymines etc. - A detailed study of the photoreduction of thymine and uracil to (134) on irradiation at 254 nm in the presence of hypophosphite has been reported. Addition of methanol occurs when (135) is irradiated at 254 nm. The initially formed products such as (136) are unstable and readily, either thermally or photochemically, eliminate HF to give (137) or CH3OF to yield (138). A further product (139) is also obtained. A study of the outcome of irradiation at 302 nm of 5-iodouracil containing deoxyoctanu-cleotides has been carried out.  

6-Chloro-l,3-dimethyluracil undergoes 1,2-Addition to benzene on irradiation. A 1,3-addition path has been discounted. Other studies by the same group have demonstrated that the irradiation of the pyrimidine dione derivative (140) in acidic media (a large excess of trifluoroacetic acid) brings about its conversion into the tricyclic product (141).  

Single crystals of thymine derivatives with long alkyl-chain substituents are photochemically reactive and undergo (2 + 2) photodimerisation to yield solely the trans-anti dimer. In solution, however, the photoreaction affords the usual four cycloadducts. Irradiation of the bis-thymine PNA dimer (142) brings about intramolecular cycloaddition to give the adduct (143) in 50% yield. The reaction is carried out in water using irradiation at 254 nm. The results from a study of the photochemical cycloaddition within the thymidilyl system (144) has been reported. Photoadducts have been obtained from the 

Photochemical monomerisation of the cyclobutane dimers (145) can be brought about effectively using tetra-O-acylriboflavins as the sensitiser. The reaction is efficient when carried out in aqueous solution with surfactants such as sodium dodecyl sulfate and sodium hexadecyl sulfate. A review has highlighted the many methods available for the photocleavage of nucleic acids.  

The thymidine derivatives (146) and (147) undergo cleavage of a C-C bond on irradiation. These reactions are typical Norrish Type I processes and provide a route to study C-3 -DNA radicals. Hydrogen abstraction by the radicals yields a 1 1 mixture of the threo and erythro derivatives (148). The reactions from the P-isomers (146) are generally more efficient than from the a-isomer (147). A study of the photochemical reactivity of the deoxyuridine derivative (149) has been reported. This novel compound is an electron-accepting nucleo base. It has been used as a means of cleaving DNA. The photochemical fission occurs specifically at the 5 -G of 5 GG3 sequences. 

2 Photoreactions of Thymines etc. - The uracil derivative (184) undergoes photochemical transformation when it is irradiated in frozen benzene with added trifluoroacetic acid. Cyclodimerization occurs yielding derivatives of diaza-pentacyclo[6.4.0.0 . 0 . 0 ]dodecane derivatives such as (185). Other addition products (186) and (187) were also identified. The photoreduction of 5-bromo-uracil (188) has been studied.  

De Keukeleire and co-workers have reported a further example of the intramolecular addition of a pyrimidone to a pendant benzene ring. In this example the cyclobutane adduct (189) formed from (190) on irradiation could not be isolated. Instead it underwent rearrangement on attempted isolation. Cyclodextrin is often used as a template upon which specific photochemical reactions 

2 Photoreactions of Thymines, etc. - Irradiation at 254 nm of the pyrimidine derivative (153) induces a Norrish Type II hydrogen abstraction from a methyl group of the t-butyl substituent. The resultant 1,4-biradical (153a) undergoes cyclization to afford an unstable cyclobutanol. Elimination of water from this species affords the final product identified as the cyclobutane derivative (154). The structure of this product was verified by X-ray diffraction techniques. The Norrish type II reactivity of the pyrimidine derivative (155) at 254 nm in water follows the analogous path to that observed for (153) and yields the cyclized product (156) in 52 % yield. - ° 

As a consequence of photodamage to DNA there is still considerable interest in the photochemical dimerization of pyrimidine derivatives. Thus, the synthesis of the pyrimidine dimers (157), (158) and (159) has been carried out by irradiation of the pyrimidine derivative (160). Sugiki et al. have studied the photochemical dimerization of the long-chain substituted thymine derivatives (161). The results of the measurement of the reduction potentials of thymine and cytosine cyclobutane dimers have been reported. In addition the electron-transfer-induced ring cleavage by electron donation from suitable sensitizers of the adducts (162) and (163) has also been examined. Calculations have been carried out on the photo-induced repair mechanism in DNA both by direct irradiation and by the use of SET induced photocleavage.  

Irradiation of (164) in a matrix of frozen benzene and THE brings about different photochemical results from those obtained from irradiation in solution phase. The solution phase reaction was carried out in benzene with added trifluoroacetic acid. This treatment yielded only the cyclooctatriene derivative (165). In the frozen system meta addition to the benzene ring results in the formation of the three products (166), (167) and (168). The photochemical reaction of the same chlorouracil (164) in the presence of p-xylene results in the 

Intramolecular addition to aromatic systems also occurs and De Keukeleire and coworkers have described a fascinating example of such a process. In their example the pyrimidone unit adds to the 1,2-positions of a benzene ring. This reaction takes place in the molecule (170) and occurs with full chemo-, stereo-and regio-selectivity affording the single stable adduct identified as (171). Intramolecular addition is also reported for the pyrimidine derivative (172) which on irradiation at 300 nm in acetone/acetonitrile yields two adducts in a ratio of 4 1 of the general structure (173). The major isomer was isolated by transesterification and was identified as (174). This result is claimed to be the first synthesis of a cis-jy/i-furanoside (2+2)-cycloadduct.  

Interest in features that can control (2+2)-intermolecular cycloadditions is of considerable interest. Mori et have examined the influence that hydrogen bonding can have on such reactions. The system studied is the cycloaddition between the substituted coumarin (175) and the pyrimidone (176). Irradiation 

2 Photoreactions of Thymines, etc. - The photochemical decay paths for some pyrimidines (thymine, 1,3-dimethylthymine, 1,3-dimethyluracil) in the gas phase following laser excitation have been studied. Apparently, in contradiction to how they behave in aqueous solution, the excited state molecules do not decay back to the ground state immediately. The molecules exist in a dark state for tens 

Acetone-sensitized irradiation of (77) in acetonitrile solution affords the intramolecular adduct (78) in 36% yield. The cycloaddition is quite specific and affords the [5 S, 6 S, 7 S]-lactone. Acetone-sensitized irradiation of the thymine derivative (79) results in the formation of the cis,syn-dimQr (80). A patent has been filed dealing with the synthesis of (2 + 2)-adducts such as (81). A study of the conformational effects within the sugar moieties of (82) has been investigated. Systems such as (82) are photochemically reactive and undergo (2 + 2)-cycloaddition to afford the adduct (83) on irradiation at 254 nm.  

Photodimerization of thymine in the crystalline form has been reported/ A study has examined theoretically the dimerization of pyrimidine units in DNA. The results illustrate that thymine-thymine dimerization is more likely than other combinations.  

Mixtures of coumarin and thymine undergo cycloaddition on irradiation in the presence of a molecular recognition catalyst. The reaction affords two cross adducts identified as the cis,syn- and the cis,anti-adducts 1,3-Dimethyluracil 

1-substituted alkenes. The products formed have head-to-tail regiochemistry. Cycloaddition of 6-cyano-l,3-dimethyluracil also takes place with 1,3-dimethyluracil to yield a dimer with traus,syn-stereochemistry. Studies based on the vinylpyrimidine (106, Z = O or NH, X, Y = COOH, CN or amide) have been directed towards the photochemical synthesis of photoconnective nucleosides.  

Acetone-sensitized irradiation of uracil (107) with ethene affords the adduct (108) in 75% yield. This compound can be transformed into the cyclobutane derivative (109) in an overall yield of 52%. The intramolecular cyclization of the dinucleotide model (110) has been investigated. The reaction affords the cycloadduct (111) by irradiation using wavelengths 300 nm. The use of an anionic template for photochemical dimerization of a thymine system has been demonstrated. The thymine forms an assembly (112) with pyrophosphate. Irradiation of this brings about syn- 2 + 2)-cycloaddition of the thymine units.  

The phototautomerism of the uracil (113) has been studied in a quantum mechanical investigation. The study suggests that enols will be formed only in the excited state. Irradiation of (114) in frozen benzene with added tri-fluoroacetic acid affords the cyclized product (115) as well as other related compounds. 

5-Fluoropyrimidine (116) undergoes attack by primary amines when irradiated at 254 nm in water with the pH controlled to 8. This results in 

A photophysical study of the interaction of 2-amino-4,6-dimethylpyrimidine in p-CD has been carried out. Calculations have been carried out in an examination of the isomerism of methyl urocanate. Density functional theory techniques have been used to study the dimerization of thymine units in DNA. The effects of both low intensity irradiation (254 nm) or laser photolysis at 266 nm on adenosine have been reported. The photoionization of various purine and pyrimidine derivatives can be brought about by 266 nm nanosecond laser photolysis. Irradiation of alloxan in the presence of alkenes with allylic hydrogens affords adducts of 5-hydroxy-5-alkenyl barbituric acid. When the alkene is 

The A -alkyl derivatives (207) also react by the Norrish Type II 

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