The Synthetic Potential of Thiophenium Ylide Cycloadducts.

(3+2) cycloaddition reactions are undeniably one of the most robust and versatile synthetic tools in heterocyclic chemistry. The classically required 1,3-dipoles are however limited to three-atom sequences bearing stabilized formal charges in their Lewis structure. The scope of three-atom groupings possible in (3+2) cycloadditions can be greatly expanded by taking of advantage neutral three-atom components (TACs). These groupings result in zwitterionic (3+2) cycloadducts adaptable to multiple outcomes depending on structure and conditions. Herein, the intramolecular (3+2) cycloaddition reaction between alkynyl sulfides (neutral TAC) and alkynes to provide key thiophenium ylide intermediates is first reported. These reactive species provide access to highly substituted fused thiophenes following predictable chemical sequences. Structural features on the obtained thiophenes were highly configurable by judicious choice of both alkynyl sulfide substitution and reaction conditions.