A review of the syntheses of flavanones, thioflavanones, and azaflavanones from 2'-substituted chalcones

Abstract

Flavanones have been prepared by the cyclization of 2'-hydroxychalcones using acidic reagents, such as H2SO4, HX, CF3COOH, CH3SO3H, KF-celite, or SiO2. Treatment of 2'-hydroxychalcones with bases, such as LiOH, KOH, NaOAc, K2CO3, pyridine, Et3N, or N-methylimidazole afforded flavanones. The conversion of 2'-hydroxychalcones to flavanones has been conducted using Co(salpr), K3Fe(CN)6, CuO, or Ni(Tfacac)2. Thioflavanones have been prepared by deprotecting and cyclizing S-protected chalcones using AgNO3, HBr, CF3COOH, or NaOH and by adding NaSH or KCS2OEt to 2'-halochalcones and subsequent substitution. Azaflavanones have been prepared by cyclizing 2'-aminochalcones using acidic reagents, such as H3PO4, HCl, silica chloride, or K-10 clay. Furthermore, treatment of 2'-aminochalcones with Lewis acids, such as ZnCl2, SbCl3, Yb(OTf)3, InCl3, TaBr5, or CeCl3 afforded azaflavanones, and chiral azaflavanones were prepared by cyclizing 2'-aminochalcones in the presence of per-6-ABCD, chiral amine, BINOL-phosphoric acid, or thiourea catalyst.