Houge, C., Frisque-Hesbain, A.-M., Mockel, A., Ghosez, L., Declercq, J.-P., Germain, G., & Vanmeerssche, M. (1982). Models for Asymmetric [2+2] Cyclo-additions. Journal of the American chemical society, 104(10), 2920-2921. https://doi.org/10.1021/ja00374a036 (Original work published 1982)