Thymine carboxylation: nucleophilic addition of carbon dioxide radical anion.

The nucleophilic addition properties of carbon dioxide radical anion (CO2*-) towards N1-substituted thymine derivatives in aqueous solution is studied for comparison with their one-electron reducing reactivity. N2O-Saturated aqueous solutions of 1-methylthymine, 1,3-dimethylthymine, and thymidylyl(3'-->5')-thymidine containing excess formate ions were gamma-irradiated at 1.0Gy min(-1). Several carboxylated thymines were isolated by preparative HPLC and identified by GC-MS, NMR and X-ray crystallography. Along with one-electron reduction yielding N-substituted 5,6-dihydrothymines and C5--C5'-linked dihydrothymine dimers, the addition of CO2 radical anion(s) to the C5--C6 double bond of N-substituted thymines produced several mono- and di-carboxylic acids, among which N-substituted derivatives of 5,6-dihydrothymine-6-carboxylic acid [5-methyldihydroorotic acid (5-methyl-DHO)] were produced in the highest yield. Similar carboxylation by CO2 radical anions was also observed for thymine dinucleoside monophosphate. The X-ray structure of cis-5,6-dihydro-1-methylthymine-6-carboxylic acid (cis-1,5-dimethyl-DHO) was determined to show a chair conformation in the crystal. The CO2 radical anion is a nucleophilic radical with rather low reduction potential, thereby possessing a dual reactivity of radical addition preferentially at C6 and one-electron reduction towards thymine-related compounds.