Acyl groups, 1,3-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... 

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). 

Other forms of tautomerism are much less commonly encountered in heterocyclic six-membered rings. Like protons, acyl groups can occupy alternative positions, on a ring atom or a substituent (e.g. 92 93), and their migration from one position to another is frequently... 

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). 

A well-known example of non-prototropic tautomerism is that of azolides (acylotropy). The acyl group migrates between the different heteroatoms and the most stable isomer (annular or functional) is obtained after equilibration. In indazoles both isomers are formed, but 2-acyl derivatives readily isomerize to the 1-substituted isomer. The first order kinetics of this isomerization have been studied by NMR spectroscopy (74TL4421). The same publication described an experiment (Scheme 8) that demonstrated the intermolecular character of the process, which has been called a dissociation-recombination process. 

HgO), but which regenerates the N-benzoyl derivative on recovery of the base with sodium hydroxide solution.. Similar migrations of the acyl group in this type of alkaloid have been recorded by Nagai and Kanao. ... 

Karrer thought that the dihydro-1,3-oxazine skeleton can possibly be formed in proteins during their transformation to polypeptides. Fodor - and Fiescr explained the stereospecific migration of acyl groups from N to 0 and vice versa in tropine alkaloids by an intermediate formation of a 1,3-oxazine ring. 

A noteworthy feature of this sequence of reactions is the 0 N migration of the acyl group and the final stabilization as the N-acylaminoketone. 

When acylated arylamines are photolyzed, migration of an acyl group takes place" in a process that resembles the photo-Fries reaction (11-30). 

Photochemical treatment of an N-acylimidazole results in a 1,2-shift (N-C migration) of the acyl group to give a mixture of 2- and 5-acylimidazoles (photo-Fries products) [1],[2]... 

Deuteration experiments suggested an intra- as well as an intermolecular migration of the acyl group. / -Methoxybenzoyl- and / -nitrobenzoylimidazole did not undergo acyl migration upon irradiation. Af-Crotyl- and TV-geranylimidazoles are stable to irradiation. 

Acylation of the nitronate 9-71 leads to the iminium ion 9-72 which, by the addition of an isocyanide, forms the cation 9-73. Following two acyl group migrations, the compound 9-75 is obtained via 9-74. The best results were obtained when allylic nitro derivatives were used, as these can form the corresponding enolate in the presence of NEt3. Aliphatic nitro compounds could also be employed, but in these cases it was necessary to use the more basic DBU. 

The CO reductions generally could likely proceed through formyl intermediates, probably at a multinuclear site (420) hydride migration to a coordinated CO [e.g., as in the hypothetical scheme outlined in Eq. (72)] has not yet been observed, although metal formyl complexes have been synthesized via other methods (422-425). A ir-bonded formyl also seems plausible (426), since 7r-bonded acyl groups have been demonstrated (427). A stoichiometric hydrogen reduction of CO to methanol under mild conditions via a bis(pentamethylcyclopentadienyl)zirconium complex is considered to go through a formyl intermediate (428, 429) ... 

Carbohydrate chemistry contains many examples of intramolecular migration of substituent groups. The largest number of these involve acyl groups, but migrations in phosphoric esters, and certain ethers have also been reported. Surprisingly, the topic has not been the subject of a comprehensive review, but there are several useful summaries thereof.539-542 In this Section, mainly those examples that have... 

The value of methylation studies in structural determination of carbohydrates is well known. Methylation of sucrose has generally been achieved by the use of dimethyl sulfate-sodium hydroxide,34,35 methyl iodide-silver oxide-acetone,20 sodium hydride-methyl io-dide-N,N-dimethylformamide,35 or diazomethane-boron trifluoride etherate.36,37 The last method (already applied to monosaccharides38,39) has been found particularly useful for sucrose, because it proceeds without concomitant migration of acyl groups. The reaction of 2,3,6,T,3, 4, 6 -hepta-0-acetylsucrose (21) and 2,3,4,6,1, 3, 4 -hepta-O-acetylsucrose (22) with diazomethane in dichloromethane in the presence of a catalytic proportion of boron trifluoride etherate for 0.5 h at —5° gave the corresponding 4-methyl (23) and 6 -methyl (24)... 

Other types of nucleophiles can also react with the electrophically activated triple bond. An O-acyl group has been shown to be an excellent participating group. Interestingly, this process, which is reminiscent of the well-known palladium(ll)-mediated 1,3-migration of allylic acetates327 represents a highly efficient means for the formation of... 

Fig. 11.1. Regio-isomerization of acyl glucuronides. Migration of the /3-1-acyl group from C(l) to C(2) by intramolecular nucleophilic attack. Migration can continue to C(3) and then C(4). Such reactions are usually competitive with chemical hydrolysis [25][26],... 

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