Structural rearrangement cascade initiated by irradiation of but-3-enyl orotates

Upon irradiation at λ = 254 nm in acetonitrile solution, the title compounds formed 8-(2-formyloxyethyl)-substituted 2,4-diazabicyclo[4.2.0]oct-1(8)-en-3,5-diones, which underwent thermal cyclobutene ring opening to 6-substituted pyrimidine-2,4-diones. The reaction cascade results in an unprecedented formal 1,5-shift of the substituent at the pyrimidine-2,4-dione core. It was shown that the reaction is likely to proceed via the [2 + 2] photocycloaddition products of the orotates, which could be intercepted in good yields (85% and 98%) upon irradiation at λ = 300 nm. At shorter wavelength a cleavage of the bond between the lactone carbonyl carbon atom and the α-carbon atom is induced, which leads – as shown by deuterium labelling experiments – by intramolecular hydrogen abstraction to the above-mentioned cyclobutenes.