O-acyl phenol

A special case of aromatic acylation is the Fries rearrangement, which is the conversion of an ester of a phenol to an o-acyl phenol by a Lewis acid. 

Keywords acyloxybenzene, Fries rearrangement, microwave irradiation, flava-none, o-acyl phenol... 

The base-induced transfer of the ester acyl group in an o-acylated phenol ester, which leads to a 1,3-diketone. This reaction is related to the Claisen Condensation, and proceeds through the formation of an enolate, followed by intramolecular acyl transfer. 

An example of more complex structural photoisomerization may be the photo-Fries rearrangement leading to a photorearrangement of O-acyl-phenols or X-acyl-anilines to give the [1,3]- or [l,5]-rearranged products, as shown in Figure 6.5. 

Carboxylates Among various 0-substituted electrophiles, O-acylated phenols share a few distinct advantages over other pseudohahdes (i) they are readily available and affordable, (ii) they are relatively stable substrates to various reaction conditions, and (iii) they can serve as directing groups for aromatic substitution reactions such as directed ortho-metallations (DoMs) [58]. Moreover, these carboxylates are much more environmentally friendly, generating acetate salts as the only by-product in the Suzuki-Miyaura cross-coupling reaction [59,... 

Scheme 8.42. Electrophilic aromatic substitution of phenol with an acyl group. The acylation of phenol with acetyl chloride (CH3COCI) in the presence of aluminum trichloride (AICI3) can apparently occur via a direct addition of the aluminum trichloride complexed acetyl chloride (in competition with O-acylation) or by a subsequent rearrangement of O-acylated phenol. The former is presented in the upper portion of the scheme, while the latter is shown in the lower portion.

Acylatmg agents such as acyl chlorides and carboxylic acid anhydrides can react with phenols either at the aromatic ring (C acylation) or at the hydroxyl oxygen (O acylation)... 

As shown in the sixth entry of Table 24 4 C acylation of phenols is observed under the customary conditions of the Friedel-Crafts reaction (treatment with an acyl chloride or acid anhydride m the presence of aluminum chloride) In the absence of aluminum chloride however O acylation occurs instead... 

The O acylation of phenols with carboxylic acid anhydrides can be conveniently catalyzed m either of two ways One method involves converting the acid anhydride to a more powerful acylatmg agent by protonation of one of its carbonyl oxygens Addi tion of a few drops of sulfuric acid is usually sufficient... 

The preference for O acylation of phenols arises because these reactions are kmetically controlled O acylation is faster than C acylation The C acyl isomers are more stable how ever and it is known that aluminum chloride is a very effective catalyst for the conversion of aryl esters to aryl ketones This isomerization is called the Fries rearrangement... 

On reaction with acyl chlorides and acid anhydrides phenols may undergo either acylation of the hydroxyl group (O acylation) or acylation of the ring (C acylation) The product of C acylation is more stable and predominates under conditions of thermodynamic control when alu mmum chloride is present (see entry 6 m Table 24 4 Section 24 8) O acylation is faster than C acylation and aryl esters are formed under conditions of kinetic control... 

Hydroxy derivatives of type (503) show more phenolic character thus 4-hydroxy-isothiazoles are normally O-methylated and O-acylated (72AHC(14)l). 

Auch Li t hi urn a I ana t reduziert die o- und p-standig mit elektronenliefemden Gruppen (z.B. Amino, Dial-kylamino, Methoxy) substituierten aromatischen Aldehyde und Ketone mcist nur unter cncrgischen Bedingun-gen zu Kohlenwasserstoffen (z. B. in Dibutylather mehrere Tage bei 80-90°). Formyl- und Acyl-phenole werden nicht reduziert1. 

As in the case of benzimidazole, a parallel synthesis of benzoxazoles was described. The authors report that mixing directly differently substituted o-amino phenols 193 with acylating agents 194 and heating at 200 °C for 10-15 min under microwave irradiation, a collection of benzoxazoles 195 was obtained (Scheme 70). With this reaction, a 48-member library of benzoxazoles with different substituents on the aromatic rings was obtained [125]. 

Syntheses of 2- and 3-substituted chromones normally start from 2-hydroxyphenyl ketones. In the first of two examples, a route to flavone is shown in Scheme 5,10 using 2-hydroxyacetophenone (2-hydrox-yphenylethanone) and benzoyl chloride as starting materials. Initially, the phenolic group of the acetophenone is O-acylated by benzoyl chloride, using pyridine as a base (a Schotten-Baumann-type reaction). 

Amin Alkohol, Phenol bzw. O-Acyl-Derivat Mol-Verhaltnis Reaktionsbedingungen Produkt Ausbeute [%] Lite- ratur... 

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