Addition to Carboxylic Acids and Esters

Since aryl radicals are less stable than alkyl radicals it was thought that 1,2 migration of an acetoxyl group in an o-acetoxyphenyl radical would be an easier process. Careful cidnp studies and product analysis, however, have revealed no evidence for this migration, which has been ascribed to the inability of the sp orbital of the aryl radical to become coplanar with the n orbital of the carbonyl group.  

Of possible biological interest is the report, based on a model reaction study with hydroxocobalamine, that the coenzyme-B, 2-dependent methyl-malonyl-CoA mutase reaction could involve intramolecular addition of a primary radical to a thioester group. The cyclopropyloxy radical so formed in a Cy3/Cy4 case would open to the carbethoxy-substituted and hence stabilized radical to give the rearranged product.  

Except for this last example which involves a Cy3/Cy4 case it is of interest to emphasize that the examples of intramolecular addition to carbethoxy groups so far reported deal exclusively with the formation of (Cy 5) radicals in Cy5/Cy4 cases. This cyclization was already observed with the carbonyl group (Section X.1), but other cyclization modes such as Cy6/Cy5 were also observed. What is particularly striking with the carbethoxy group is that a search for Cy5/Cy6 cyclizations, for example, with the next higher homolog of 205, failed. This is the opposite of the behavior of olefinic bonds and is in disagreement with Baldwin s rules.  

---