Other Fries Reactions

With p-tolyl acetate, in which the position in para to the acetate group is blocked by the methyl group, no intermolecular reaction is observed and only 2-hydroxy- 

5-methylacetophenone, resulting from an intramolecular Fries rearrangement, and cresol, resulting from hydrolysis, are formed [8]. 

Phenyl benzoate (PB) was chosen as a model for the double Fries rearrangement of diphenyl benzoate into diphenol para monomers (Eq. 4) [4] and its transformation over several solid catalysts was investigated [4,8]. 

The reaction rate was shown to be related to the accessibility of the acid sites-H-MFI zeolite was practically inactive and Nafion was the most active catalyst. With all the catalysts, not only the desired p-hydroxybenzophenone (/ -HBP) product but also j3-benzoxybenzophenone (p-BXBP) and o-hydroxybenzophenone (o-HBP) were formed. The para products result from benzoylation of phenol (formed by hydrolysis of PB) or of PB and o-HBP from the intramolecular rearrangement of PB. The selectivity to p-HBP can, therefore, be significantly improved by adding phenol to the reactant mixture [4]. Unfortunately, thermodynamic limitations arising from the low polarity of the medium employed (even with very polar solvents such as sulfolane) seems to preclude the use of a solid acid for the production of diphenol monomers from diphenyl benzoate [4]. 

The synthesis of 2,4-dihydroxybenzophenone (compound 4 Eq. 5) was investigated over various solid catalysts by reacting a 1 1 molar mixture of resorcinol with benzoic acid in chlorobenzene as solvent, water resulting from ester formation being removed [3,22]. 

---

Photo-Fries rearrangement of aryl esters or its participation in addition to other photochemical reactions was reported in numerous papers concerning low-molecular78 or macromolecular79,63 compounds. 

In the photoageing of polycarbonates, photo-oxidation has been shown to be a more important process than the photo-Fries reaction. The photo-oxidation of epoxy-resins has been found to depend on the type of hardener and its concentration. A correlation has been observed between the changes in chemical structure and thermally stimulated current of poly(vinyl alcohol) during photo-oxidation. Other studies of interest include the weathering of polymers outdoors,effect of ozonation on polymers, effect of drawing, effect of biodegradation, and changes in mechanical properties of thermoplastics. ... 

The effects that initiator derived residues and functional groups have on polymer properties is an area that needs further study. For example, as discussed above, benzoyl peroxide leads to the formation of benzoyloxyphenyl groups in PS. It is known that the polymerization of p-benzoyloxystyrene [155] and its copolymerization with styrene [156] leads to the formation of photo-reactive polymers. Upon irradiation these polymers undergo a facile photo-Fries Rearrangement resulting in the conversion of the benzoyloxyphenyl groups to hydroxybenzophenone moieties [157]. Other side reactions also lead to the formation of phenolic groups and free benzoyloxy radicals [155]. 

The rearrangement of phenolic esters to either o-and/or p-phenolic ketones on being heated upon with anhydrous aluminium chloride or other Lewis acid catalysts is known as Fries Reaction or Rearrangement, as depicted below ... 

Two other photochemical reactions that appear to proceed through a bond homolysis are the photo-Claisen reaction of phenyl allyl ether (106, equation 12.75), which produces phenol (107) plus o- (108) and p-allylphenol (109) ° and the photo-Fries reaction of phenyl acetate (110,equation 12.76) to produce o- (111) and p-hydroxyacetophenone (112) plus phenol. In the case of the photo-Fries reaction, both concerted and dissociative mechanisms were proposed for the formation of the acylphenols. 

The term processed flavor is used in different contexts to describe (a) products where the natural raw materials lack a characteristic flavor profile and the desired aromatic profile is achieved only by deliberate processing (e.g., coffee) (b) flavorings created as a result of Maillard and other related reactions between amino acids and sugars (e.g., meat-like flavors) (c) flavorings resulting from controlled enzymatic reactions (e.g., enzyme-modified dairy products) (d) products made by fermentation (e.g., wines, vinegar) and (e) products of thermal reactions of lipids (e.g. French fry flavor). 

Many stereoselective reactions have been most thoroughly studied with steroid examples because the rigidity of the steroid nucleus prevents conformational changes and because enormous experience with analytical procedures has been gathered with this particular class of natural products (J. Fried, 1972). The name steroids (stereos (gr.) = solid, rigid) has indeed been selected very well, if one considers stereochemical problems. We shall now briefly point to some other interesting, more steroid-specific reactions. 

---