Rearrangement of Threonine- and Serine-Based N-(3-Phenylprop-2yn-1-yl) Sulfonamides Yields Chiral Pyrrolidin-3-ones

N-(3-Phenylprop-2-yn-1-yl)-sulfonamides derived from serine and threonine were synthesized using solid-phase synthesis and subjected to reaction with trimethylsilyl trifluoromethanesulfonate (TMSOTf). In contrast to the previously reported formation of 1,4-oxazepanes, this reaction afforded pyrrolidin-3-ones. A mechanistic explanation for this unexpected outcome is proposed, and the limitations and scope of the rearrangement are outlined.