Steric Reversal of the endo-Selectivity Effect in 1,3-Dipolar Cycloadditions of Phthalazinium-2-ylides with N-Substituted Maleimides: endo- and exo-1,2-(Dicarboxy-N-substituted imido)-1,2,3,10b-tetrahydropyrrolo[2,1-a]phthalazines†

Abstract

N-Methyl- and N-aryl-maleimides undergo cycloadditions with phthalazinium-2-dicyanomethanide and -2-unsubstituted methanide 1,3-dipoles to give exclusive or predominant endo-cycloadducts but with N-tert-butylmaleimide this endo effect is reversed to favour the exo-cycloadducts exo-1,2-(dicarboxy-N-tert-butylimido)-1,2,3,10b-tetrahydropyrrolo[2,1-a]phthalazines 11 and 15.