The use of Lewis acids in the synthesis of 5-arylhydantoins

Different Lewis acids are able to promote the Friedel–Crafts reaction between 5-bromohydantoin and aromatic compounds. In the case of phenol, mixtures of ortho and para isomers are always obtained, with Mg(ClO4)2 leading to the best selectivity. However, the best overall yield of 5-(hydroxyphenyl)hydantoin is obtained with YbCl3. This method can be extended to other aromatic systems such as anisole and thiophene. These reactions give similar yields but proceed with total selectivity to 5-(4-methoxyphenyl)hydantoin and 5-(2-thiophenyl)hydantoin, respectively. The cationic exchange of MgII and YbIII on anionic solid supports allows the preparation of very efficient heterogeneous catalysts for this reaction (productivity up to 600 mol of hydantoin per mole of Mg). These catalysts have practical advantages in that they can be recycled and reused.