Longphyllinesides A and B: natural Diels-Alder adducts from Daphniphyllum longeracemosum?

Two new alkaloids, longphyllinesides A (1) and B (2), were isolated from Daphniphyllum longeracemosum. Their structures were elucidated using 1D and 2D NMR spectra. The key step in their biogenetic pathway is indicated to be Diels-Alder cycloaddition via asynchronous concerted mechanism by density functional theory calculations. The possibility of involvement of a Diels-Alderase is also discussed.