Elemental-Sulfur-Incorporated Cyclizations of Pyrrolidines Leading to Thienopyrroles

We report, herein, the synthesis of thieno­[3,2-b]­pyrroles from the direct oxidative [4 + 1] cyclization of 2-alkynyl pyrrolidines with elemental sulfur. This transformation likely originates from electrophilic attack at the β-position of pyrrolidine followed by an intramolecular thienannulation to deliver the desired product. Mechanistic investigation suggests that the present reaction involves the formation of dihydrothieno­[3,2-b]­pyrrole as an intermediate.