Selective Synthesis of Non-Aromatic Five-Membered Sulfur Heterocycles from Alkynes by using a Proton Acid/N-Chlorophthalimide System

A multicomponent strategy to achieve two different regioselectivities from alkynes, isothiocyanates and H₂ O with a proton acid/N-chlorophthalimide (NCPI) system is described to selectively obtain non-aromatic five-membered sulfur heterocycles (1,3-oxathiol-2-imines/thiazol-2(3H)-one derivatives) through multiple bond formations. The process features readily available starting materials, mild reaction conditions, broad substrate scope, good functional-group tolerance, high regio- and chemo- selectivities, gram-scale synthesis and late-stage modifications. Mechanistic studies support the proposal that the transformation process includes a combination of H₂ O and isothiocyanate, free-radical formation, carbonation and intramolecular cyclization to give the products. Furthermore, the 1,3-oxathiol-2-imine derivatives possess unique fluorescence characteristics and can be used as Pd²⁺ sensors with a "turn-off" response, demonstrating potential applications in environmental and biological fields.