Dithiane-Enabled Divergent Ring Rearrangements for the Construction of Sulfur-Containing Semisaturated Fused Pyridines and Pyrroles

Sulfur-containing semisaturated N-heterocycles represent valuable three-dimensional drug-relevant scaffolds, yet general divergent synthetic approaches remain scarce. Herein, we report a metal-free cis-selective [3 + 2] cycloaddition between alkynyl 1,3-dithianes and 2H-azirines to construct strain-encoded bicyclic aziridine intermediates. The 1,3-dithiane unit functions as a sulfur shuttle that enables two reagent-controlled rearrangement pathways: Brønsted-acid-mediated ring expansion through 1,2-sulfur migration affords sulfur-functionalized semisaturated fused pyridines, whereas oxidative Lewis-acid-promoted ring contraction furnishes sulfur-functionalized semisaturated fused pyrroles.