Is anthracene cofactor or spectator for the thermolysis of anthracenyl acylnitroso cycloadducts in the presence of a diene?

Acylnitroso species are very reactive dienophiles broadly used in organic synthesis. They are traditionally prepared from the oxidation of hydroxamic acids at low temperature. When this strategy fails, acylnitroso compounds may be generated from other stable sources, for example, their (9,10-dimethyl)anthracenyl cycloadducts. The thermolysis of such compounds performed in the presence of a diene leads generally to the corresponding hetero Diels-Alder (HD-A) cycloadduct in high yield with concomitant release of anthracene. The most widely accepted mechanism is the postulated slow release of acylnitroso via a retro Diels-Alder process, although the anthracenyl acylnitroso cycloadducts bear intrinsic structural features suggesting another pathway, that is, an exchange process. By using computational chemistry, we have hereby clarified the exact role of anthracene in this reaction.