Cascade rearrangement in the reaction of methyl trifluoropyruvate sulfonylimines with terminal alkynes

Methyl trifluoropyruvate benzene- and methanesulfonylimines react with hex-1-yne and phenylacetylene to give methyl N-sulfonyl-4-oxo-2-trifluoromethyl-4-R-but-2E-enimidates. The reaction mechanism includes the formation of a six-centered bipolar ion followed by its cascade rearrangement.