Irradiation at 254 nm of aqueous solutions of 5-ethyl-, 5-propyl-, and 5-isopropyluracils (or their nulcleosides) leads to cleavage of the 5-alkyl substituents, via an intramolecular electrocyclic photoaddition intermediate, with formation of uracil (or its nucleoside). The plhotoaddition intermediates represent a new class of dihydropyrimidines, namely analogs of 5,6-dihydro-5,6-cyclobutanyluracil and its nucleosides; the biological significance is discussed.
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