Chalcones oxidative rearrangement

Oxidative rearrangements resulting in the formation of heterocyclic compounds are relatively less explored, although some of them are quite important. A basic reaction is the conversion of a chalcone to 3,3-dimethoxy-l-phenylpropanone (Eq. 2) (85TL2961). This process may be adapted to heterocyclic synthesis. 

The oxidative rearrangement of chalcones is a valuable route to isoflavones which has been thoroughly investigated. Initially, the conversion was achieved in two distinct steps. Epoxidation of a 2 -benzyloxychalcone, carried out by conventional techniques, is followed by treatment with a Lewis acid, such as boron trifluoride etherate, which brings about the rearrangement. 

Polymer-supported (diacetoxyiodo)benzene (PSDIB) can promote an oxidative rearrangement of 2 -benzyloxy-chalcones to form the intermediate acetal 748, which can be isolated or directly treated with aqueous base to afford... 

The reaction between the acid chloride of chromone-2-carboxylic acid and ethyl ethoxymagnesioacetoacetate probably leads to the expected fi-diketone which enolizes and cyclizes spontaneously to spirofuranone(52).127 A different approach was made by Hungarian workers in their synthesis of tachrosin (53), an unusual kind of flavone isolated from Tephrosia poly-stachyoides and one of the earliest natural furanones to be isolated. They subjected an unsaturated ketone (Scheme 32) to oxidative rearrangement by thallium(III) salts, a reaction well known in chalcone chemistry, and eliminated methanol from the product to obtain the necessary starting material.128... 

Oxidative rearrangement of chalcones. 4-Methoxychalcone (1) on treatment with thallium triacetate in boiling methanol is converted into the 1,2-diaryl-3,3-dimethoxy-... 

Farkas et al., [123] synthesised different natural isoflavanones (255) and isoflavans (256) by oxidative rearrangement of 2-hydroxy chalcones (254) with thallium nitrate in methanol followed by acid-catalyzed cyclization as shown in Fig. (11). 

The ready availability of chalcones, from aldol condensation of acetophenones and benzaldehydes, makes this oxidative rearrangement a useful synthetic entry to isoflavone targets. The isoflavone products may be further elaborated to isoflavanones, isoflavans, pterocaipans and coumestones, broadening the scope of this method. 

The precursors of flavonoid biosynthesis include shikimic acid, phenylalanine, cinnamic acid, and p-coumaric acid. Shikimic acid acts as an intermediate in the biosynthesis of aromatic acid. The basic pathways to the core isoflavonoid skeletons have been established both enzymatically and genetically [16]. The synthesis of isoflavones can be broadly divided into three main synthetic pathways the formylation of deoxybenzoins, the oxidative rearrangement of chalcones and flavanones, and the arylation of a preformed chromanone ring. In leguminous plants, the major isoflavonoids are produced via two branches of the isoflavonoid biosynthetic pathway, and the different branches share a majority of common reactions [1]. Unlike the common flavonoid compotmds, which have a 2-phenyl-benzopyrone core structure, isoflavones, such as daidzein and genistein, are 3-phenyl-benzopyrone compounds. Biochemically, the synthesis of isoflavones is an offshoot of the flavonoids biosynthesis pathway. Several attempts have aimed to increase... 

E. C. Taylor, A. McKillop, and coworkers. Thallium in organic synthesis 49. Oxidative rearrangement of chalcone dimethylketals to methyl-2,3-diaryl-3-methoxypropanoates with thallium(m) trinitrate in trimethylorthoformate. J. Org. Chem., 1977, 42, 4167 50. A convenient synthesis of thallium(i) cyanide, a useful reagent in organic synthesis (for conversion of aroyl chlorides to aromatic a-ketonitriles). J. Org. Chem., 1978, 43, 2280 51. Oxidation of enolizable ketones to a-nitratoketones by thallium(in) nitrate in acetonitrile. 

Oxidation of chalcones with Tl(III) salts gives the aryl group rearranged ketals via... 

Reactions of [bis(acyloxy)iodo]arenes with alkenes in some cases can give products of an oxidative, carbocationic rearrangement. Such a rearrangement of pentaalkoxychalcones 376 upon treatment with [bis(trifluoroacetoxy)iodo]benzene has been applied toward the preparation of the rearranged chalcones... 

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