Regio- and stereo-selective aza-Diels–Alder reaction of ethyl glyoxylate 4-methoxyphenylimine with 1,3-dienes in the presence of BF3·Et2O. Evidence for a non-concerted mechanism

Cycloadditions of the glyoxylate imine 1 with 1-substituted and 1,4-disubstituted 1,3-dienes furnished tetrahydroquinoline compounds 4 and 5/15 with total regio- and stereo-control, except for one case where a mixture of isomers was formed. A stepwise mechanism is proposed in view of the stereochemistry of the products.