Synthesis of azaanthraquinones: homolytic substitution of pyridines

Abstract

Synthesis of specific di- and tri-hydroxyazaanthraquinones by Friedel-Crafts procedures is limited by orientational ambiguity and by the lack of reactivity of pyridine derivatives in electrophilic acylation processes; however, suitable pyridines have been made to undergo radical benzoylation and benzylation at unsubstituted positions 2, 4 and 6. In particular, derivatives of pyridine-3-carbo-nitrile have been benzoylated at positions 2 and 4. Ring closure by intramolecular Houben-Hoesch reaction has then led to specifically substituted 1-and 2-azaanthraquinones and thence to the antibiotic bostrycoidin (1).