FREE-RADICAL REARRANGEMENTS IN URACIL DERIVATIVES

As part of an effort to develop general probes for radical reactions i nvolving DNA bases, several uracil derivatives were synthesized. The r ates of the cyclopropyl carbinyl rearrangement in these systems were e valuated by means of competition experiments. The results indicate tha t when a cyclopropyl group is substituted in the 5-position of uracil, the rearrangement occurs very slowly-with a rate constant of <2.5 x 1 0(4) s(-1). On the other hand, the analog of the 5-hexenyl radical cyc lization onto the 5,6-double bond of uracil derivatives occurs with ra tes which were similar to the parent process: (4.0-8.9) x 10(4) s(-1). The experimental results along with semiempirical calculations show t hat radicals 23 and 25 are unusually stable species. These results exp lain why no rearrangements are observed when a cyclopropyl-substituted thymine dimer is cleaved by reductive single electron transfer.