(Z)- and (E)-2-phenyl-4-benzylidene-5(4H)-oxazolones as dienophiles. Improved selectivity by the use of heterogenous catalysts

Abstract

Silica gel treated with AlEt2Cl or TiCl4 efficiently promotes the reaction of cyclopentadiene with (Z)-2-phenyl-4-benzylidene-5(4H)-oxazolone (1a) with very little Z/E isomerization. Silica gel treated with TiCl4 and ZnCl2 supported on silica gel leads to very high yields for the reaction of cyclopentadiene with (E)-2-phenyl-4-benzylidene-5(4H)-oxazolone (1b). Furthermore, E/Z isomerization is almost suppressed, which is not achieved with homogeneous Lewis acids. Whereas for the reaction of 1a with cyclopentadiene the exo cycloadduct (3a) is always the major product, for the reaction of cyclopentadiene wth 1b the endo(2b)/exo(3b) selectivity is reversed as a function of the catalyst. Thus, in the reactions catalyzed by silica gel, alumina and exchanged clays, endo (2b) is the major product However, the exo cycloadduct (3b) is preferably obtained for reactions catalyzed by silica gel and alumina treated with TiCl4 and ZnCl2 supported on silica gel.