X=Y−ZH systems as potential 1,3-dipoles. Part 1. background and scope

X=Y−ZH Systems are considered in general terms and divided into four classes according to the number of constituent atoms that possess lone-pair electrons. Those systems in which the central Y atom possesses a lone pair are shown to be capable of participating in formal 1,2-H shifts generating 1,3-dipolar species. The scope of the reaction, including its possible relevance to the biochemistry of pyridoxal enzymes is discussed and the influence of structure on reactivity is demonstrated with rate data for the cycloaddition of a series of aryl imines of phenylglycine and alanine methyl esters to N-phenylmaleimide.