1,3-Dipolar cycloaddition of hydrazones with α-oxo-ketenes: a three-component stereoselective entry to pyrazolidinones and an original class of spirooxindoles.

Stereodefined monocyclic, spirobicyclic, and bis-spirotricyclic pyrazolidin-3-ones can be prepared efficiently by a three-component reaction involving a 1,3-dipolar cycloaddition of azomethine imines obtained from hydrazones with α-oxo-ketene dipolarophiles generated in situ. The reaction allows the creation of four covalent bonds and two contiguous chiral quaternary centers with excellent diastereoselectivity in a single catalyst/additive-free, highly atom-economical transformation. From a fundamental point of view, the reaction introduces α-oxo-ketenes as effective dipolarophiles in 1,3-dipolar cycloadditions.