THE PHOTOCHEMISTRY OF PIPERAZINE-2,5-DIONE IN AQUEOUS SOLUTION

Abstract The aqueous photochemistry of deoxygenated solutions of piperazine-2,5-dione was investigated and three products were isolated in low yield using preparative high-performance liquid chromatography: 3,3'-bis(piperazine-2,5-dione) ( 1 ), oxamide and N-methyloxamide. The structure of 1 was established by infrared, mass and high-field proton magnetic resonance spectroscopy, and by the synthesis of its meso diastereoisomer. A 1:2 mixture of meso and racemic diastereoisomers of 1 was formed in good yield in thermally or photochemically initiated reactions of piperazine-2,5-dione with t -butyl peroxide in aqueous t -butanol.
The rate of photolysis of aqueous piperazine-2,5-dione was markedly enhanced in the presence of iron(II) ions, copper(II) ions or powdered titanium dioxide. A diastereomeric mixture of 1, oxamide and N-methyloxamide were isolated from the reaction with iron(II) ions. The possibility that oxamides were formed in these reactions via reaction with atmospheric oxygen was eliminated in a series of control experiments.
The results suggest α-carbon radicals are important in the photolysis of peptides in fluid aqueous media, possibly as a consequence of photoinduced electron transfer. No significant adduct formation could be detected when mixtures of piperazine-2,5-dione and uracil or thymine were irradiated in frozen aqueous solution.