Expanding the Synthetic Method and Structural Diversity Potential for the Intramolecular Aza Diels−Alder Cyclization

Abstract

New experimental facets have been examined to expand upon the known methods for an aromatic variant of the intramolecular Aza Diels-Alder cyclization. The specific transformation under study is one that uses functionalized anilines and an aldehyde-olefin tether to provide tetrahydroquinoline cycloadducts under mild acidic conditions. Variations investigated encompass the use of N-alkylated anilines, including one with ring-constrained nitrogen, in the context of glycine, phenylalanine, and glyoxyl ester bridging elements; bridge components with structural perturbations; modified dienophile segments; and different acid catalysts. Substituted tetrahydroquinolines obtained from many of the preceding experiments were obtained in good chemical yield, generally in excess of 80%. Designed as a platform for combinatorial chemical synthesis, this reaction manifold accommodates a range of starting materials with structurally and electronically distinct characteristics. The results of this report, in combination with the discoveries from previous work in this area, enhance the ability of the intramolecular Aza Diels-Alder transformation to generate a diverse array of quinolinic structures with multiple stereogenic centers, many of which resemble lignan and arylnaphthalene-type natural products.