Marko, I., Ronsmans, B., Frisque-Hesbain, A.-M., Dumas, S., Ghosez, L., Ernst, B., & Greuter, H. (1985). Intramolecular [2 + 2] Cyclo-additions of Ketenes and Keteniminium Salts To Olefins. Journal of the American chemical society, 107(7), 2192-2194. https://doi.org/10.1021/ja00293a073 (Original work published 1985)