Radical-Induced Decarboxylation of a Lactone

Assessment of drug stability towards hydrolysis and radical-induced decomposition is an important preformulation exercise. Given the relative complexity of many modern pharmaceuticals, unexpected results from such studies are not uncommon, as illustrated by the following example. The antibiotic frenolicin-B, 1, was heated with AIBN in methanol in the presence of air for two days. This gave a number of uncharacterizable materials together with one major product which was shown to be the racemic pyranonaphthoquinone 2. 

Suggest a mechanism, or mechanisms, for the radical-induced degradation of 1 into 2. 

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