Synthesis of 4‑Fluoromethylsydnones and their Participation in Alkyne Cycloaddition Reactions

We
report the synthesis and some structural studies of 4-trifluoromethyl,
4-difluoromethyl-, and 4-monofluoromethylsydnones. All but the latter
compounds are stable and represent effective precursors to a range
of pyrazoles after cycloaddition reactions with alkynes. The cycloadditions
are generally highly regioselective and provide 5-fluoromethylpyrazole
products, although we have observed that Bn-substituted sydnones can
provide an unexpected alkyne insertion mode that generates the 3-fluoromethyl
isomer.