Transition-Metal-Free Synthesis of Imidazo[2,1‑b]thiazoles and Thiazolo[3,2‑a]benzimidazoles via an S‑Propargylation/5-exo-dig Cyclization/Isomerization Sequence Using Propargyl Tosylates as Substrates

A transition-metal-free route for the synthesis of several N-fused heterocycles, including thiazolo­[3,2-a]­benzimidazoles and imidazo­[2,1-b]­thiazoles, is reported. The reaction between propargyl tosylates and 2-mercaptobenzimidazoles under basic conditions results in 3-substituted thiazolo­[3,2-a]­benzimidazoles, in yields up to 92% in a single synthesis step. With 2-mercaptoimidazoles as the substrate, the corresponding imidazo­[2,1-b]­thiazoles were exclusively obtained. The transformation is considered to proceed as an intermolecular S-propargylation that is followed by 5-exo-dig ring closure and double-bond isomerization.