Formates Fries rearrangements

A photochemical variant, the so-called photo-Fries rearrangement, proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ort/zo-derivative is formed in an analogous way ... 

Chain cleavage with subsequent formation of phenolic products, rather than the photo-Fries rearrangement to form salicylates and dihydroxybenxophenones, has been identified as the major initial degradation pathway of PC exposed to natural weathering conditions. 

Fries rearrangement.1 Rearrangement of phenyl esters with Lewis acids results in a mixture of ortho- and para-phenolic ketones. In contrast, reaction of an o-bromophenyl ester with sec-butyllithium results in exclusive formation of the orf/jo-phenolic ketone by an intramolecular acyl rearrangement.2... 

If the photo-Fries reaction would occur via a concerted mechanism, the absence of solvent should be of minor importance for the formation of rearranged products. However, conclusive evidence supporting the radical pair mechanism arises from the experiments carried out with phenyl acetate (10) in the vapor phase. The major product in the irradiations of 10 is phenol (13), which accounts for 65% of the photoproducts. Under these conditions, less than 1% of ortho -hydroxyace-tophenone (11) appears to be formed [19,20]. Conversely, when a high cage effect is expected, as in rigid matrixes (i.e., polyethylene), the result is completely different, and phenol is practically absent from the reaction mixtures [29]. In the intermediate situation (liquid solution), both rearranged products and phenol are formed in variable amounts depending on solvent properties. These observations... 

Dialkylquinazolines 825 are available by microwave-assisted amination and ring closure of 2-acylamino phenyl-ketones 824 with ammonium formate <20070L69>. The 2-aminophenyl ketone precursors of the amides 824 are available by a photochemically induced Fries rearrangement of anilides 823, which enables a variety of different alkyl substituents to be incorporated at the 4-position of the quinazoline <20070L69>. 

Ortho- and para-rearrangement and phenol formation on uv-irradiation of aryl esters are accompanied in several cases by decarboxylation,37,60,62,64,80,81 represented for 3,5-di-t-butylphenyl benzoate by the equation 118 -> 119-122. It was shown that this reaction cannot be sensitized,64 but the dramatic differences in product distribution could be observed by changing of the solvent.60,84 The results in Table VI indicate that in polar solvents the decarboxylation process is minimized while the formation of the photo-Fries rearrangement 119 is enhanced. The reverse appears to be true when nonpolar ethereal solvents are used. A considerable amount of biaryls are formed, and hence this reaction may prove useful for the preparation of biaryls and alkylary Is. 

From the point of view of quantum yields calculation, photo-Fries rearrangement, including phenol and products formation, represents a photoreaction (expressed by Eq. 3) in which all products absorb intensively in the absorption region of the starting phenyl ester A. 

CPB4368, 87TL1565). Again, the formation of 4-acyloxy tetramic acids is favored. However, under the conditions of Fries rearrangement or alternatively, in the presence of triethylamine, conversion to 3-acyltetramic acids is observed (87CPB4368). 

A considerable amount of attention has also been paid to the photo-Fries rearrangement of polymer pendant groups. For example, the rearrangement of poly (phenyl acrylate) (10,11) in solution or in the solid-state, is usually incomplete and results in the formation of both the ortho and the para-hydroxyphenone rearranged products in amounts which vary with the conditions of the photolysis. A concurrent side-reaction, which we term the Fries degradation, also results in the liberation of small amounts of phenol (Scheme 2). Similar results have been obtained with poly (phenyl methacrylate) and other substituted aryl acrylates (4,9,12). 

Fries rearrangement of aromatic formate esters suggests that phenols are the major products (.24) obtained in the reaction. As poly(p-hydroxystyrene) is remarkably clear in the deep UV, it is likely that poly(p-formyloxystyrene) will not suffer from the same problem of photostabilization upon exposure as was the case with poly (p-acetoxystyrene). This expectation was confirmed by our study of the photo-Fries reaction of p-cresyl formate no ortho rearranged product was isolated after reaction while p-cresol and a small amount of starting material were obtained. 

Although there is evidence that chromone syntheses which proceed by the cyclization of phenyl esters under Friedel-Crafts conditions may involve a Fries rearrangement and hence require the formation of one bond adjacent to the heteroatom, syntheses of chromones from phenols will be considered together in this section. The Simonis reaction (530R(7)l)... 

It is pertinent to note that the ester (472) yields a chromone on treatment with hydrogen fluoride (54LA(587)16), suggesting the intervention of a Fries rearrangement once again. The same applies in the formation of 2-chlorochromone from phenyl 3,3-dichloropropenoate <60CR(250)2819). 

A minor variant of this, method makes use of the reaction between an anisole and a 3-halogenopropanoyl chloride (14CB2585). These same 3-substituted acid chlorides react with phenols to give esters and a Fries rearrangement is now a prerequisite of chromanone formation (58JCS1190). 

On HY, phenylacetate dissociates into phenol and ketene (reaction a). Ortho-hydroxyacetophenone is produced partly by the Fries rearrangement of phenylacetate (intramolecular reaction, reaction b) and by trans-acylation (reaction c) while para-hydroxyacetophenone is exclusively the result of trans-acylation (reaction d). Phenylacetate can also disproportionate into phenol and acetoxyacetophenones (reaction e). Para-acetoxyacetophenone can also be formed through transesterification between para-hydroxyacetophenone and phenylacetate (reaction f).The formation of secondary products like 2-methylchromone and 4-methylcoumarine is consecutive to the formation of... 

The details of the mechanism of the Fries rearrangement are uncertain but the reaction probably involves the formation and migration of the acylium ion (7). 

Phenolic ketones may be prepared by the Hoesch acylation reaction, which may be regarded as an extension of the Gattermann aldehyde synthesis (Section 6.10.1, p. 990). The procedure involves reaction of a nitrile with a phenol (or phenolic ether) in the presence of zinc chloride and hydrogen chloride best results are usually obtained with polyhydric phenols or their ethers, as for example in the preparation of phloroacetophenone (Expt 6.125). The formation of phenolic ketones by means of the Fries rearrangement of phenolic esters with aluminium chloride is discussed on p. 976. 

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