A (6–4) photoproduct of 5-bromouracil

Abstract

The photoreactions of 5-bromouracil (BrUra) induced by 254 nm radiation have received a significant amount of scrutiny over the years, both when induced in liquid aqueous solution and in the frozen state. The characterized photoproducts in the liquid state include uracil, 5,5′-diuracil (I) and a variety of products arising from ring opening, while the photoreactivity of BrUra in ice using 254 nm radiation is very low. During examination of the photoreactions induced in BrUra in frozen aqueous solution by exposure to predominately UVB irradiation, we found that several products are formed. One of these products has an ultraviolet absorption spectrum similar to those associated with (6–4) adducts of the pyrimidine bases (λ_max = 326 nm). Studies with ¹H and ¹³C nuclear magnetic resonance spectroscopy and with mass spectrometry yielded data consistent with this compound indeed being a (6–4) product, namely 6-4′-(5′-bromopyrimidin-2′-one)-5,5-dihydroxy-5,6-dihydrouracil (IIa). The product IIa probably arises from an unstable precursor, namely 5-bromo-6-4′-(5′-bromopyrimidin-2′-one)-5-hydroxy-5,6-dihydrouracil (IV); this compound is a bromohydrin and, as such, is likely unstable to debromination to 6-4′-(5′-bromopyrimidin-2′-one)-5-oxo-5,6-dihydrouracil (V). Rehydration at the 5-position to form a gemdiol would lead to IIa, the isolated product.