Rearrangements, electrophilic Fries

Accompanied by 169 3.5-disilylated material other electrophiles low yield. " 4-Pyridone product via anionic Fries rearrangement, f Formed in situ from 4-pyridone. 

This type of duality of action is presumably present in other situations, such as the Fries rearrangement (78), the Friedel-Crafts reaction with acid chlorides (65) or acid anhydrides (21), and the catalytic chlorination of nitrobenzene (17). In these reactions it appears that the uncoordinated Lewis acid is the effective catalyst. The same situation is illustrated by recent work on aromatic amination (32, 33) and halogenation (57, 58, 71) and seems to be general feature of Lewis acid-catalyzed electrophilic reactions of aromatic compounds containing suitable donor groups. 

Hydroxythiocoumarins are, of course, enolic and react as acetoacetate analogues. Electrophiles such as Mannich reagents, aromatic aldehydes and enones react readily (Scheme 22), while acylation can be direct, or via a Fries-type rearrangement of 4-acyloxy precursors. 

The O-carbamate, the most powerful of DMGs in competition experiments [13], is similarly plagued by stability as the N,N-diethyl derivative 13 (Scheme 6). However, the N-cumyl -N-methyl counterpart 14 undergoes unproblematic metalation-electrophile quench to give 15 and anionic Fries rearrangement (16) reactions. The very mild conditions for hydrolysis to 17, and hence to 18, as well as to 19 bode well for further useful chemistry of the N-cumyl DMG in context of more substituted aromatics [14]. 

Solid acid catalysts are, in principle, applicable to a plethora of acid-promoted processes in organic synthesis [27-29]. These include various electrophilic aromatic substitutions, e.g. nitrations, and Friedel-Crafts alkylations and acylations, and numerous rearrangement reactions such as the Beckmann and Fries rearrangements. 

Phenols are C-acylated either by electrophilic substitution under acidic conditions or by nucleophilic acylation under basic conditions. Advances in the chemistry of strong acids and Lewis acids provided novel aspects to catalytic Fries rearrangement and Friedel-Crafts acylation. Effenberger and Gutermann used a catalytic amount of... 

The keto tautomers of monohydric phenols are frequently invoked as reactive intermediates in many reactions, such as the Reimer-Tiemann and Kolbe-Schmitt reactions , electrophilic substitution (e.g. bromination i) s well as the photo-Fries rearrangement (see Section IV.D and also Refs. 16-18 cited in Reference 7). In certain cases the keto... 

In the presence of a stoichiometric amount of SbCls, Friedel-Crafts acylation proceeds with acyl hahdes and acid anhydrides [47], SbCls also promotes the Fries rearrangement of phenyl acetates [48], The electrophilic acylation of fluoro-olefins with acetyl fluoride or benzoyl fluoride is promoted by SbFs in liquid SOy [49], The Friedel-Crafts acylation of benzene and electron-rich arenes is successfully catalyzed by SbCl5-AgClO4 [50], SbCl, Ar.BCl [51], SbCl5-LiClO4 [52], or CaCI, AgSbFg [53] (Scheme 14,19), Acyl chlorides, acid anhydrides, and acyl enolates are used as sources of acyl groups. 

It is important to recognise the element of choice that exists in the anionic Fries rearrangement. After the initial lithiation of 127 we can either let it warm up and do the rearrangement or, if we are careful, we can keep it cold so that the rearrangement does not take place and we can introduce another electrophile to react with the intermediate lithiated compound instead.11... 

Subsequent work by Ruwet and co-workers proposed something similar to the proposal 1 mechanism, albeit they did not think that the phenolate ester rearranged via a Fries reaction. Ruwet and co-workers studied the heteroatom displacement of the phenol oxygen with S and Se. They suggested that the first step could be esterification followed by nucleophilic attack of the phenol on the more electrophilic carbon of the ketone moiety. Ruwet and co-workers prepared the phenolate 22a-c from the diketene and the corresponding phenols la-c. Treatment of la-c (X = O, S or Se) and diketene 21 with PPA (polyphosphoric acid) did indeed provide the chromone 23a-c in about 40% yield.This was the first reported use of PPA for this transformation. 

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