Regioselective Synthesis of Dihydrothiophene and Thiopyran Frameworks via Catalyst-Controlled Intramolecular C_γ/C_δ–S Fusion of α‑Allyl-β′-oxodithioesters

A highly efficient and atom-economic dual reaction manifold has been developed to synthesize 4<i>H</i>-thiopyran and 4,5-dihydrothiophene frameworks via regioselective intramolecular C–S fusion of α-allyl-β′-oxodithioesters. The ring size of the sulfur-heterocycles has been efficiently tuned by the use of two different catalytic systems. Palladium activates the C_δ–H of the allyl termini and facilitates the intramolecular C_δ–S coupling to furnish six-membered thiopyran skeletons exclusively. Conversely, the allylic double bond of the same substrate has been activated by BF₃·Et₂O to promote the C_γ–S cyclization leading to the formation of a five-membered dihydrothiophene nucleus