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.