Enolates Fries-type rearrangements

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. 

Tabuchi, H., Hamamoto, T., Ichihara, A. Modification of the Fries type rearrangement of the O-enol acyl group using W,W-dicyclohexylcarbodiimide and 4-dimethylaminopyridine. Synlett 1993, 651-652. 

The Lewis acid-catalyzed Fries rearrangement is performed under strong acidic conditions and therefore reqnires protection of sensitive functional groups. Conversely, the PFR can be achieved nnder milder conditions (organic solvents, nentral media, room temperature, etc.) and is therefore free from this type of problems. Moreover, polyacylation of aromatics is usually difficult to achieve. In the PFR, this problem can be solved by using acetals [144—146] or enol esters [147] as carbonyl protecting groups. Addition of anhydrous K CO is convenient to avoid deprotection [148]. 

Norrish type II cleavage Photoenolization (s. a. 3-Elimination, photo-enolization-induced) review 31,128 Photoflnorination 32, 484 Photo-Friedel-Crafts synthesis, intramolecnlar 32, 975 mem-Photo-Fries rearrangement, skeletal 31,685 Photoisomerization... 

---