“…Regarding the mechanism, although there is debate in literature, it is believed that the reaction starts with the formation of Narylimines (azadiene) in the more stable E stereoisomer which are activated by the catalyst and might follow a formal [4 + 2] concerted cycloaddition through a trans-endo-favored transition state followed by a 1,3-hydrogen shift or a stepwise polar mechanism proceeding via zwitterionic intermediates (Figure 3). 31−34 Over the last two decades, this journal has published several Diels−Alder laboratory experiments to construct several carboand heterocycles were different topics of this reaction such as the effects of kinetic vs thermodynamic control on endo/exo adducts, 35,36 regioselectivity, 37 diastereoselectivity, 38,39 heat source, 40−42 and solvent free conditions, 43 have been addressed. These experiments have contributed to understanding and gaining experience with classic reactions in undergraduate organic chemistry courses; however, with the increasing interest in green chemistry, other topics like metrics and use of alternative solvents and renewable building blocks should be also considered.…”