Stereospecific protonation of pyrrole-2-carboxaldehyde Z-oximes as a result of through-space cation stabilization with oxime hydroxyl

The protonation of the E/Z mixture of pyrrole-2-carboxaldehyde oxime (CF₃COOH, Et₂O or CDCl₃, room temperature) gives Z-configured pyrrole-2-carboxaldehyde oxime cations exclusively. The NMR (¹H, ¹³C and ¹⁵N) analysis and quantum chemical calculations [B3LYP/6-311G(d,p)] imply the stereospecific through-space stabilization of these cations by the oxime hydroxyl.