Sainte, F., Serckx-Poncin, B., Frisque-Hesbain, A.-M., & Ghosez, L. (1982). A Diels-alder Route To Pyridone and Piperidone Derivatives. Journal of the American chemical society, 104(5), 1428-1430. https://doi.org/10.1021/ja00369a049 (Original work published 1982)