FREE-RADICAL REARRANGEMENTS IN URACIL DERIVATIVES

As part of an effort to develop general probes for radical reactions involving DNA bases, several uracil derivatives were synthesized. The rates of the cyclopropyl carbinyl rearrangement in these systems were evaluated by means of competition experiments. The results indicate that 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 10(4) s(-1). On the other hand, the analog of the 5-hexenyl radical cyclization onto the 5,6-double bond of uracil derivatives occurs with rates which were similar to the parent process: (4.0-8.9) x 10(4) s(-1). The experimental results along with semiempirical calculations show that radicals 23 and 25 are unusually stable species. These results explain why no rearrangements are observed when a cyclopropyl-substituted thymine dimer is cleaved by reductive single electron transfer.