The intramolecular [2 + 2] cycloaddition of keteniminium salt 5 derived from (2S, 5S)-dimethylpyrrolidine is the key step of a short synthetic route toward prostaglandins.
Chen, L.-Y., & Ghosez, L. (1991). Intramolecular Cycloadditions of Keteniminium Salts - a Practical Asymmetric-synthesis of Prostaglandins. Tetrahedron: Asymmetry, 2(12), 1181-1184. https://doi.org/10.1016/S0957-4166(00)80017-8 (Original work published 1991)