Crystal-stabilisation of an elusive 4,6-pyrimidinedione dimer

The formation of a disproportionation dimer of 4,6-pyrimidinedione 1, 2-(4,6-dioxo-5-pyrimidinyl)-4,6-dioxo-1,2,3,5,5-pentahydropyrimidine 2, results from the reactivity of the inner-salt tautomers or ions of 1, contrasting to the general properties of nucleobases which are all stable in the lactam or dilactam molecular forms. The observation of dimer 2, indefinitely stable in the crystalline state, reveals an unusual chemical reactivity strongly dependent on the H-tautomeric forms of the substrate and product.