Unusual photochemical transformation of 5-ethylorotate to 5-ethylidenehydroorotate.

Ultraviolet irradiation of 5-ethylorotate in aqueous medium led to formation of 5-ethylidenehydroorotate via a 1,3-hydrogen rearrangement from the ethylene group of the 5-substituent to the ring C6. The same product was also obtained by an alternate pathway via a hitherto unidentified intermediate. The structure of this unusual pyrimidine analogue, established in the solid state by X-ray diffraction, was shown with the aid of spectroscopic methods to be the same in solution, and some of its properties are described. Heating of 5-ethylidenehydroorotate in the solid state led by direct conversion to a mixture of the parent 5-ethylorotate and 5-ethyluracil. By contrast, ultraviolet irradiation in aqueous medium resulted in quantitative photochemical conversion of the photoproduct to 5-ethyluracil.