Synthesis of 2 H -Chromenes via Unexpected [4 + 2] Annulation of Alkynyl Thioethers with o -Hydroxybenzyl Alcohols.

A novel Brønsted acid-catalyzed reaction of alkynyl thioethers with o -hydroxybenzyl alcohols via an unexpected formal [4 + 2] annulation has been developed. This metal-free protocol leads to the facile and practical synthesis of valuable polysubstituted 2 H -chromenes in mostly good to excellent yields under mild reaction conditions and features a wide substrate scope and excellent functional group tolerance.