Algar

The oxidation of 2′-hydroxychalcones 1 with alkaline hydrogen peroxide, commonly known as the Algar, Flynn and Oyamada (AFO) reaction is a convenient method for the synthesis of 2-aryl-3-hydroxy-4H-1-benzopyran-4-ones (flavonols), 2-aryl-2,3-dihydro-3-hydroxy-4H-1-benzopyran-4-ones (2,3-dihydroflavonols), 2-arylidenebenzo[b]furan-3(2H)-ones (aurones) and other flavonoid compounds. Oxidation of substrates bearing a 6′-substituent was initially believed to proceed through an epoxide intermediate, giving aurones or benzofuran-3(2H)-one derivatives as the chief reaction products. This was disputed when it was observed that AFO oxidation of (Z)-2′-hydroxy-α,4′,6′-trimethoxychalcone gave the erythro-α-methoxybenzofuran-3-one 7 (Scheme 3) instead of the threo-isomer, which is expected if an epoxide were an intermediate. Evidence to suggest that an epoxide can be an intermediate in this reaction when 2′-hydroxy-6′-substituted substrates are employed, is described herein.