Photochemical Ring Expansion of 4-Azidouracil: a Route to 5<i>H</i>-1,3,5-Triazepin-2,4-dione in the Nucleoside Series

Under aqueous conditions, 4-azidouracil/tetrazolo[1,5-c]pyrimidin-5(6H)-one nucleosides undergo a very efficient photochemical nitrogen elimination and ring expansion to 1,3,5-triazepin-2,4-dione nucleosides whose structure has been confirmed by X-ray crystallography. In contrast, when the photolysis was attempted under anhydrous conditions in the presence of a nucleophile, a ring contraction reaction occurred, affording 2-oxoimidazolone nucleosides. A mechanism to account for the formation of ring expansion and contraction reactions and involving a carbodiimide intermediate is proposed which is reminiscent of the known photochemical behavior of 2-azidopyridines/tetrazolo[1,5-a]pyridines.