0-Hydroxychalcones. oxidation

The conversion of 2 -hydroxychalcones to 2-aryl-3-hydroxy-4f/-lbenzopyran-4-ones (flavonols) by alkaline hydrogen peroxide oxidation is known as the Algar-Flynn-Oyamada (AFO) reaction or AFO oxidation. ... 

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). 

Oxidation of o-hydroxychalcones (90) using PIDA under basic conditions (KOH-MeOH) affords cis- or trans-3-hydroxyflavanone (91) selectively via its dimethylacetal [69] [Eq. (20)]. 

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>. 

Chalcone, chromone, flavone, /rans-cinnamaldehyde, cyclopentenone, etc. gave the corresponding products in satisfactory yields [28], The oxidation of some disub-stituted a,j8-unsaturated cyclic ketones, however, gave predominantly hydroxylated products [29], An interesting variation was the synthesis of ds-hydroxyflavanones from 2-hydroxychalcones, e.g. [30] ... 

Oxidation of pyrazolines, Hydroxyaryl pyrazolines (1) are oxidized by selenium dioxide in aqueous dioxane to 2 -hydroxychalcones (2) in about 50% yield. Use of other common oxidants (LTA, MnOj, KMn04, etc.) results in the corresponding pyrazoles. 

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. ... 

Oxidative heterocyclizations. 2-Hydroxychalcones are converted into flavones. 2-(3-Hydroxypropyl) sugars undergo photoinduced oxidation to give spiroacetals with PhI(OAc)2-l2. 

The dehydrogenation of 2 -hydroxychalcones and flavanones to flavones, of l-(2-hydroxyaryl)alk-2-en-l-ones and chroman-4-ones to chromones and of thiochroman-4-ones to thiochromones can be accomplished using iodine in hot DMSO <97HCM223>. E-3-Styrylchromones result from the oxidative rearrangement of 5-aryl-l-(2-hydroxyphenyl)penta-2,4-dien-l-ones with thallium(lll) nitrate <97LA2065>. Dimethyldioxirane converts flavanones into flavones by way of the 2-hydroxyflavanone. This approach enables flavans to be converted to a variety of flavonoids <97TL4651>. 

In particular, the key intermediate product 381 in the synthesis of wighteone was prepared by the oxidative rearrangement of 3 -iodotetraalkoxychalcone 380 [474] and the key step in the synthesis of ( )-indatraline involved the HTIB-promoted diastereoselective ring contraction of a 1,2-dihydronaphthalene 382 to construct the indane ring system 383 (Scheme 3.152) [475]. A similar oxidative rearrangement of 3-cinnamoyl-4-hydroxy-6-methyl-2/7-pyran-2-ones with HTIB in dichloromethane followed by cycliza-tion was used by Prakash and coworkers for the direct conversion of o-hydroxychalcones into isoflavone derivatives [476]. 

This reaction has been modified to convert 2 -hydroxychalcones into flavones via Se02 oxidation in addition, flavone has recently been prepared via a polystyrene-supported selenenyl bromide followed by H2O2 oxidation. 

Many products synthesized by Claisen-Schmidt reactions find applications in the pharmaceutical industry. In particular, chalcones and fiavanones are intermediates in the synthesis of flavonoids. The basic structure of fiavonoids, that is, 2 -hydroxychalcone can be obtained by condensation of substituted 2-hydroxyacetophenone and substituted benzaldehyde at 323 K using basic zeolites as catalysts (46) or Mg-Al mixed oxides (47). 

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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