The 1, 3-dipolar cycloaddition of an intermediary iminium ylide formed from N-benzyl-N-(methoxymethyl) trimethylsilylmethylamine to conjugated olefinic and acetylenic dipolarophiles in the presence of a catalytic amount of trifluoroacetic acid has been found to give 3-or 3, 4-substituted pyrrolidines and 2, 5-dihydropyrroles.