X=Y-ZH systems as potential 1.3-dipoles : Part 14. bronsted and Lewis acid catalysis of cycloadditions of arylidene imines of α-amino acid esters

Cycloadditions of arylidene imines of α-amino acid esters to a range of dipolarophiles show substantial rate enhancements in the presence of Bronsted and Lewis acids. For Bronsted acids the rate is related to the pK_a of the acid and cycloadditions to reactive dipolarophiles occur at room temperature. For the Lewis acids studied the rate acceleration decreases in the order Zn(OAc)₂ > AgOAc> LiOAc> MgOAc₂ but is also anion dependent with LiBr> LiOAc and AgOAc> AgOTs. The Lewis acid catalysed processes are believed to be examples of cycloadditions of metallo-1,3-dipoles. In both Bronsted and Lewis acid catalysed processes the cycloadditions are regio- and stereo-specific.