Cytosine hydrogen abstraction

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

The methyl group of thymine is susceptible to hydrogen abstraction resulting in a carbon-centred allyl radical. Further oxidation of this radical intermediate can yield thymine-specific products, 5-hydroxymethyluracil and 5-formyluracil. Cytosine adducts also yield unique products under further... 

Figure 3.10 Formation of cytosine adducts (43) by reaction of reactive aldehyde arising from C4 hydrogen abstraction in DNA.

The reactions between the triplet excited state of l,8-dihydro>yan-thraquinone and cytosine, thymine or uracil have been investigated by nanosecond laser flash photolysis. The transient absorption spectra and the decay dynamics are consistent with electron transfer and hydrogen abstraction from the pyrimidine bases. ... 

In aqueous Bep2 glasses of base derivatives, hydroxyl radicals were found to add to the C5C6 double bond in cytosine and uracil, abstract a hydrogen atom from the methyl group in thymine and add to C2 in adenine [90]. In aqueous... 

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