Uracils photochemical reactions

Aromatic compounds can participate in both [2+2] and [4+2] photocycloaddition reactions with uracil derivatives to give either benzocyclobutane or ethenoquinazoline (barrelene) derivatives, which can then undergo a number of subsequent photochemical reactions. The products obtained are dependent upon the reaction conditions, and thus the photocycloaddition reaction between naphthalenes 470 and l,3-dimethyl-5-fluorouracil 471 in cyclohexane gave 4a-fluoro-5,10-ethenobenzo[/]quinazolines 472 as products as a result of a [4+2] photocycloaddition (photo-Diels-Alder) reaction <2002TL3113, 2003H(61)377>. 

When polyuridylic acid is photolyzed with radiation in the 250 p region, two types of photochemical reactions take place. One is the reversible formation of dimers between adjacent uracils and the other is a photohydration of the 5-6 double bond of the uracil. The position of the photochemical equilibrium of the first reaction is known to depend upon the wavelength of the radiation used and that of the latter is determined by... 

Photochemical reactions of the purines and pyrimidines assume special significance because of the high molar extinction coefficients of the nucleic acids present in cells. Light is likely to be absorbed by nucleic acids and to induce photoreactions that lead to mutations.190 Both pyrimidines and purines undergo photochemical alterations, but purines are only about one-tenth as sensitive as pyrimidines. Photohydration of cytidine (Eq. 23-25) is observed readily. The reaction is the photochemical analog of the hydration of a,P-unsaturated carboxylic acids. Uracil derivatives also undergo photohydration. 

In addition to the thermal decomposition the photochemical reaction of geminal diazide 62 was also studied. Irradiation of an acetone solution of 62 under an inert gas atmosphere afforded a complex mixture of products which could not be separated or identified. However, if the reaction was carried out in the presence of oxygen the uracil derivative 66 was obtained in 48 % yield. Surprisingly, in addition to the oxidation of the CH2 group, the 6-diazidomethyl function was completely lost during the reation [91JCS(P1)1342]. At the present time no mechanistic explanation for this unusual behavior can be presented. On the other hand, photooxidation of compound 63 leads straightforward to compound 67 [91 JCS(P1)1342],... 

Interference by certain phenylurea and tf-phenylcarbamate herbicides with the photochemical reactions of isolated chloroplasts was first reported in 1956 (2). Over the next few years, inhibition by the s-triazines, uracils, benzimidazoles, and ben-zonitriles was reported 3, 6). ... 

Trifluoromethylation of arenes.1 Arenes are trifluoromethylated by CF3Br on irradiation (315-440 nm) in acetonitrile. This photochemical reaction with uracil provides 5-trifluoromethyluracil in 11% yield. 

Benzoxazole, 2-trimethylsilyl-, -trimethylstannyl-, 56, 221 Benzoyl isocyanide, possible intermediate in fragmentation, 56, 80 Benzoylacetone, reaction with ethylenediamine, 56, 4 Benzoylation of uracils, photochemical, using diphenyl-l,3,4-oxadiazole, 55,... 

Another type of photochemical reaction involving a pyrimidine base is the addition of a molecule of water across the 5,6 double bond of C to yield a 5,6-dihydro-6-hydroxy derivative called the cytosine hydrate. The quantum yield for the formation of cytosine hydrates in UV-irradiated DNA is greater in single-stranded than in duplex-DNA (45). Hydrates of cytosine, deoxycytidine, CMP, or dCMP are unstable, readily reverting to the parent form by rehydration (45). However, their half-life is dramatically increased in DNA, and cytosine hydrate may be the major nondimer C photoproduct. Cytosine hydrate can undergo deamination and dehydration to yield uracil (1). The hydrate of 5-methylcytosine may undergo deamination to yield 5-thymine hydrate, which can convert to thymine upon dehydration (1). 

The 5,6-double bond in uracils is involved in several [2 + 2] photochemical reactions. The cyclo-octapyrimidine-2,4-dione (338) is formed by photocycloaddition of the A-substituted 6-chlorouracil (336) to benzenes in the presence of an acid, preferably TEA (Scheme 57). The reaction is assumed to proceed by orr//o-addition, with a cyclobutane (337) as intermediate. Without acid, 6-phenyluracil is formed in low yield. In toluene as solvent and participating reagent, a regioisomeric mixture with the methyl group in the 6-, 7-, 8-, or 10-position was formed. In fluorobenzene, all of the regioisomers... 

The observed sigmoidal dependence of rate constant upon pH is consistent with an ordinary photochemical reaction mechanism which includes an equilibrium between an excited uracil molecule and hydrogen ion. Consider the following set of reactions, a-f... 

Uracils undergo radical additions these and their photochemical reactions (section 11.8) are of possible relevance to mutagenesis mechanisms. 

It has been reported in 1980 s that the photochemical reactions can be conveniently carried out in aqueous phase. Thus, the photodimerisation of thymine, uracil and their derivatives could be carried out in water giving considerably better yields than in other organic solvents (Scheme 121). ... 

Photochemical reaction of uracil 7 with CFjBr also gave the corresponding 5-trifluoromethyl derivative, although in this case, the yield was unsatisfactory (11 %, 56 % conversion) (Scheme 121) [298],... 

Pyrimidine bases such as uracil and thymine are known to form photodimers with ejqmsure to UV light (2). It seemed interesting to e]q>lore the photochemical reaction of the pyrimidine bases to the photolithographic process in microelectronics fabrication technology. 

Uracil derivatives can also participate in cobalt-mediated [2-f2-f2] cycloaddition reactions with dialkynes under photochemical conditions, to give cobalt complexed dihydrobenzo[g]quinazoline derivatives 482 <1999CEJ3549>. 

JellinekT, Johns RB (1970) The mechanism of photochemical addition of cysteine to uracil and formation of dihydrouracil. Photochem Photobiol 11 349-359 Jiang Y, Lin W-Z, Yao S-D, Lin N-Y, Zhu D-Y (1999a) Pulse radiolytic study of electron transfer reaction for fast repair of the one-electron oxidized radicals of dAMP and dGMP by hydroxycinnamic acid derivatives. Radiat Phys Chem 54 349-353... 

Kong, F.F., Zhai, B.C., and Song, Q.-H. (2008) Substituent effects on the regioselective formation of the Patemd-Biichi reaction of 5- or/and-6-methyl substituted uracils with 4,4 -disubstituted benzophenones. Photochemical el Photobiological Sciences, 7, 1332-1336. 

Halogen atoms in nucleosides can be replaced photochemically by sulphur as has been demonstrated in the photoinduced alkylthiolation of halogenated purine nucleosides433,749, in the synthesis of 2-(methylthio)adenosine from 2-iodoadenosine750 and in the photochemical formation of a cysteine-uracil adduct from 5-bromouracil and A-acetylcysteine methyl ester435. Photochemical synthesis of phosphonopyrimidine and phosphonopurine nucleosides by reaction of bromonucleosides with triethyl phosphite has also been reported751. 

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 l-(2-iodovinyl)-2-atylbenzene derivatives (212) upon direct irradiation gave phenanthrene derivatives in good yields. This reaction was initiated hy photochemical homolysis of the C-I hond. The E-Z isomerization of the fluorinated l,3-dimethyl-5-propenyl uracils (213) upon >300 nm irradiation afforded mixtures of ElZ=1 1.5 and ElZ=7.5 1, respectively. ... 

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