ASPECTS OF THE ALGAR-FLYNN-OYAMADA (AFO) REACTION

The oxidation of 2'-hydroxychalcones 1 with alkaline hydrogen peroxide , commonly known as the Algar, Flynn and Oyamada (AFO) reaction is a c onvenient method for the synthesis of 2-aryl-3-hydroxy-4H-1-benzopyran -4-ones (flavonols). -aryl-2,3-dihydro-3-hydroxy-4H-1-benzopyran-4-one s (2,3-dihydroflavonols), 2-arylidenebenzo[b]furan-3(2H)-ones (aurones ) and other flavonoid compounds. Oxidation of substrates bearing a B-s ubstituent was initially believed to proceed through an epoxide interm ediate, giving aurones or benzofuran-3(2H)-one derivatives as the chie f reaction products. This was disputed when it was observed that AFO o xidation of (Z)-2'-hydroxy-alpha 4',6'-trimethoxychalcone gave the ery thro-alpha-methoxybenzofuran-3-one 7 (Scheme 3) instead of the threo-i somer, which is expected if an epoxide were an intermediate. Evidence to suggest that an epoxide can be an intermediate in this reaction whe n 2'-hydroxy-6'-substituted substrates are employed, is described here in. Copyright (C) 1996 Elsevier Science Ltd