Asymmetric organocatalytic thio-Diels-Alder reactions via trienamine catalysis.

We report a new process to access highly enantioenriched sulfur-based heterocycles by an asymmetric catalytic thio-Diels-Alder reaction. Thiocarbonyls are challenging heterodienophiles in enantioselective Diels-Alder reactions, due to their inherent high reactivity and their poor ability to coordinate to chiral catalysts. We successfully circumvented these problems by employing a different strategy, which explores the use of in situ generated catalyst-bound dienes. Synthetically useful dihydrothiopyrans as well as other bi- and tricyclic sulfur-containing heterocycles are formed in high yields and high to excellent selectivities. DFT calculations were performed to examine the mechanism of the developed reaction. Furthermore, a series of synthetic transformations of the optically active sulfur-based heterocycles are presented.