The general synthesis of flavones by the construction of a benzopyran ring is achieved through the cyclodehydration of 1-(2-hydroxyphenyl)-3-phenylpropane-1, 3-diones, the oxidative cyclization of 2 0-hydroxychalcones, and Wittig olefination of phosphoranes. 3 The intramolecular rearrangement of 2-acetophenyl benzoates with bases4 such as KOH, 4a DBU, 4b and t-BuOK4c and the reaction of the dianions of 2 0-hydroxyacetophenones with benzoylating reagents5 such as benzoyl chlorides, 5a methyl benzoates, 5b and N-methoxy-N-methyl benzamides5c afforded 1-(2-hydroxyphenyl)-3-phenylpropane-1, 3-diones. These intermediates were cyclodehydrated with catalytic H2SO4, 4a, b, 5 TsOH, 4c and CuCl2 6 by heating to produce flavones. The reaction of 2’-hydroxyacetophenones and benzaldehydes in the presence of base afforded 2 0-hydroxychalcones, which were cyclized with Lewis acids such as FeCl3 7b and Na2PdCl4 7c by heating to give flavones. 7 2 0-Hydroxychalcones were also cyclized with iodine8a or NH4X8b, c (X= Br, I) via α, β-dihalo-2 0-hydroxychalcones, which underwent cyclization and subsequent elimination of hydrogen halides, to give flavones. 8 Alternatively, the olefination of benzoates of o-hydroxyphenacylidenetriphenylphosphoranes on heating afforded flavones from 2 0-hydroxyacetophenones in multiple steps. 9

The cyclization of o-(alkynon-1-yl) phenols has attracted much attention as a new type of flavone synthesis. However, the intramolecular cyclization of o-(alkynon-1-yl) phenols with K2CO3 occurred competitively by 6-endo and 5-exo ring cyclization to give flavones and aurones, respectively. 10 The use of triflic acid11 and a catalytic 4-(dimethylamino) pyridine12 predominantly induced 6-endo cyclization to provide flavones forming γ-benzopyranones. The 1, 4-addition of an excess of secondary amines to O-TBS protected alkynones in CH3OH afforded the