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.