Lewis Acid-Catalyzed Reactions of Anthrone: Preference for Cycloaddition Reaction over Conjugate Addition Depending on the Functionality of α,β-Unsaturated Carbonyl Compounds

The Lewis acid-catalyzed reactions of anthrone with a variety of ethylenic substrates under various conditions have been studied. It has been observed that depending on kinds of ethylenic substrates and catalysts, products were varied. In particular, the ZnCl2-catalyzed reaction of anthrone with α,β-unsaturated ester gave bridged compounds 3 (Diels-Alder adduct type) and mono-Michael adduct 4 exclusively, while the base-catalyzed reaction gave 10,10-bis-Michael adduct as a major product independent of the amount of ethylenic substrate and base. Bridged compounds 3 were easily converted to the corresponding mono-Michael adduct 4 by a catalytic amount of base. Further Michael reaction of mono-Michael adducts with different ethylenic substrates in the presence of a catalytic amount of alkoxide gave unsymmetrical 10,10-bis Michael adducts in good or moderate yields. Key Words: Anthrone, Lewis acid, Cycloaddition, Conjugated addition, Michael reaction Anthrone 1 and anthracenol 2 are typical examples of keto-enol isomerization in solution (eq. 1).1 Anthracenolate ion generated from the deprotonation of anthrone by base leads to a consecutive double Michael reaction2 which is often observed with substrates containing active methylen