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The (5-4) and (6-4) Adducts of 1-Methylthymine and Their Dewar Valence Isomers

Authors


Corresponding author email: shetlar@cgl.ucsf.edu (Martin D. Shetlar)

Abstract

Previous studies of the photochemical reaction of 1-methylthymine (MeT) in frozen aqueous solution have indicated that four cyclobutane type dimers are formed. We have restudied this system and have found that, in addition to cyclobutane dimers, both a (5-4) adduct and a (6-4) adduct of MeT are formed in significant amounts. Upon standing in aqueous solution, the (5-4) adduct is susceptible to reaction to form an isomeric form of the parent adduct, possibly via ring-opening and closure reactions at C-6 of the saturated pyrimidine ring component of the adduct. Irradiation of each of these three adducts with UVB light produces a pair of Dewar-type adducts. The nine products were individually characterized by mass spectrometry, proton NMR spectroscopy and UV spectroscopy. A less comprehensive study showed that irradiation of thymidine in frozen aqueous solution produces a diastereomeric pair of (5-4) adducts, along with the previously known diastereomeric pair of (6-4) adducts.

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