Rearrangements, acyl group migration

The mechanism of the Fries reaction is not known with certainty. One mechanism regards it as a true intramolecular rearrangement in which the acyl group migrates directly from the oxygen atom to the carbon atoms of the ring. Another scheme postulates that the ester is cleaved by the reagent... 

When treated with acid chlorides and acid anhydrides, phenols form esters. Under Friedel-Crafts conditions, phenolic esters undergo a Fries rearrangement in which the acyl group migrates to the 2- and 4-positions. Thus, treatment of the ester 11 with aluminium chloride in an inert solvent gives a mixture of 2- and 4-hydroxyacetophenones [(hydroxy-phenyl)ethanones] C-acylation has occurred (Scheme 4.7). The two isomers are separable and this is a useful method for the production of phenolic ketones. The mechanism remains uncertain, but it would appear that the acylium ion (RCO" ) is generated and that a Friedel-Crafts mechanism operates. 

When esters of phenols are heated with aluminum chloride, the acyl group migrates from the phenolic oxygen to an or/Ao or para position of the ring, thus yielding a ketone. This reaction, called the Fr/es rearrangement, is often used... 

Acyl group migration can be effected photochemically in nitrogen-containing systems. Regioselective photo-Fries rearrangements in 5- and 6-acetoxyindoles have been observed in... 

The quantum yield 0 is 0.03. The mechanism of this [l,2,3,4]-rearrangement [53] is unknown. Formally, the reaction can be considered an H migration from the OH group to the end of a four-membered chain with a concomitant acyl group migration from position 2 to 3 within that chain. According to the current canon of photochemistry, an oxa-di- r-methane rearrangement with subsequent... 

Synthesis of a cyclic 1,3- dicarbonyl compound from an acyclic 1,2-dicarbonyl compound may be achieved in two steps by a photochemical cyclisation followed by a cationic rearrangement in which the cyclobutanone acyl group migrates preferentially [equation (40)].141... 

Acyl rearrangement s. Acyl group migration Acylsilanes... 

O-acylation in peptide synthesis 18, 435 suppl. 26 O-acyl group migration 13,255 suppl. 26 allyl rearrangement 27, 578 coupling of halides 11, 731 suppl. 27 epimerization during oxidation 16, 319 suppl. 26 N-quaternization s. under N-Quaternization radical addition 27, 600 reduction of ketones in Grignard syntheses 19, 46 suppl. 26 ring closure, intramolecular 27, 799... 

Acylation. Reaction conditions employed to acylate an aminophenol (using acetic anhydride in alkaU or pyridine, acetyl chloride and pyridine in toluene, or ketene in ethanol) usually lead to involvement of the amino function. If an excess of reagent is used, however, especially with 2-aminophenol, 0,A/-diacylated products are formed. Aminophenol carboxylates (0-acylated aminophenols) normally are prepared by the reduction of the corresponding nitrophenyl carboxylates, which is of particular importance with the 4-aminophenol derivatives. A migration of the acyl group from the O to the N position is known to occur for some 2- and 4-aminophenol acylated products. Whereas ethyl 4-aminophenyl carbonate is relatively stable in dilute acid, the 2-derivative has been shown to rearrange slowly to give ethyl 2-hydroxyphenyl carbamate [35580-89-3] (26). 

Triazoles are acylated with acyl halides, usually initially at the 1-position, but the acyl group may migrate to the 2-position on heating or on treatment with base. Thus acetylation with acetyl chloride often gives 1-acetyl derivatives, which rearrange to the 2-isomers above 120 °C (74AHCil6)33). 

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