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Ultraviolet irradiation of 1-methylthymine in frozen aqueous solution gives two photodimers which on methylation provide two dimethylthymine dimers. One of the latter is identical with the methylation product of thymine icedimer and therefore is the cis-syn isomer (IIb); the other is assigned trans-syn sterochemistry (Ib). 3-Methylthymine is but little dimerised under the same conditions to give the cis-syn isomer (IId).
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Blackburn et al. (1966) studied this question.
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