2-Hydroxychalcones, oxidative cyclization

A selenium dioxide induced oxidative cyclization of the 2 -hydroxychalcone 745 is a key step in the total synthesis of ( )-5 -methoxyhydnocarin-D (Scheme 189) <2005T4149>. DMSO containing catalytic amounts of iodine also effectively promotes the oxidative cyclization of 2 -hydroxychalcones to afford flavones <1996CHEC-II>. The DMSO-iodine mediated cyclization of the bis(2 -hydroxychalcone) 746 is a key step in the synthesis of the natural atropisomer4 4 ",7,7 -tetra-O-methylcupressuflavone (Equation 298) <1997TL1087>. Likewise, silica gel supported indium(in) halides catalyze the oxidative cyclization of 2 -hydroxychalcones to afford flavones in excellent yield (Equation 299) <2005TL253>. 

FUlvones. 2 -Hydroxychalcones are oxidized in good yield to flavones in boiling amyl alcohol or xylene. This was the first example of use of the reagent for oxidative cyclization and is still one of the two best methods for the synthesis of flavones. ... 

Anodic addition of an oxygen function to a C = N or C = C double bond may be exemplified by the oxidative cyclization of aryl semicarbazones to 1,3,4-oxadiazoles in acetonitrile-acetic acid containing H2SO4 or, under strictly anhydrous conditions (in the presence of acetic anhydride), to triazolinones [40]. Other examples are the anodic oxidation of benzaldehyde-2-hydroxyanil to 2-phenyl-1,3-benzoxazole derivatives [41] and the indirect oxidation of 2-hydroxychalcones to flavonoids using tris(4-bromophenyl)amine in MeOH-CH2Cl2 as mediator [42] ... 

Selenophenes. Ring contraction of dihydroselenopyrans is effected by NalO. 2 -Hydroxychalcones - flavones. The oxidative cyclization proceeds in DMSO at 100-120°C. 

Oxidative cyclization. Treatment of 2 -hydroxychalcones (1) with mercuric acetate in DMSO gives cyclized adducts (2) and aurones (3) the adducts are converted into (3) by further treatment with CaO in DMSO. ... 

Conversion of 2 -hydroxychalcones to 2-aryl-3-hydroxy-4/f-l-benzopyran-4-ones (flavonols) by an oxidative cyclization. 

It is of interest to note that the thallium(III) acetate oxidation of a 2 -hydroxychalcone yields a mixture of a flavone, through participation of the 2 -hydroxy group in the cyclization, and a coumaranone, which presumably arises by further oxidation of an intermediate acetal <70JCS(C)119). 

With the structurally similar 2 -hydroxychalcones, Lier et al. reported that either InCl3 or InBr3 supported on silica gel catalyze the intramolecular cyclizations in the absence of solvent at elevated temperatures [62]. However, the expected flavanones were not isolated, but the corresponding flavones were obtained instead. The supported In(III) catalyst had not only catalyzed the 1,4-addition, but had proceeded to catalyze the oxidation. This was proven from the authors oxidation of flavanones to flavones under the same reaction condition (Figure 8.35). 

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