Preparation of Cyclopenta-fused N-, O-, and S-Heterocycles by Lewis Acid-Catalyzed Nazarov Reaction

The products of carbonylative coupling between lactam-, lactone- and thiolactone-derived vinyl triflates and alkenylboronic acids are suitable substrates for the Lewis acid catalyzed Nazarov reaction. The most efficient Lewis acids for the Nazarov reaction are scandium(III) and indium(III) triflates (3-15 mol%) in 1,2-dichloroethane, which provide the Nazarov products in moderate to excellent yield (53-86%). The electrocyclization rate depends also on the heteroatorn (N, O, S) and the N-protecting group. On the whole, the entire procedure is an expeditious synthesis of hexahydro[1]pyrindenes, -cyclopenta[b]pyrans, and -cyclopenta[b]thiopyrans of noteworthy interest as they form the structural core of several natural molecules.