Steric reversal of the endo-selectivity effect in 1,8-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

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.