Parameters regulating the α- and β-cyclization of chalcones

Abstract

Cyclizations of 2′-hydroxychalcones and their α-oxygenated counterparts (α-alkoxy- and peltogynoid chalcones) follow a logical sequence of α- or β-addition directed by the nature of the α-substituent and the pH or the reaction medium. For β-addition the reaction equilibrium and hence its course depends on the thermodynamic stability of, the presence of a 5-hydroxy function in, and the acidity of the 3-protons of the resulting flavanone. Cyclization of the chalcone is independent of possible α-directing steric effects iontroduced by 6′-methoxylation.

The latter reflects on similar cyclizations under oxidative conditions [alkaline ferricyanide, acidic lead tetraacetate, and alkaline peroxide (Algar–Flynn–Oyamada reaction)], where the directing effects of free radical and ionic mechanisms are invoked to rationalize their variable course.