Catalyst-Free Synthesis of 2,3-Dihydrobenzofurans via a Formal [4+1] Annulation of Propargylamines with Sulfur Ylides

A simple, general
route to the 2,3-dihydrobenzofurans substituted
at C3 by an aryethynyl or aryl group, starting from propargylamine
and its derivatives with benzoyl sulfonium salts, has been developed.
This reaction involved an in situ generated o-quinone methide (o-QM) intermediate followed
by [4+1] annulation with sulfur ylides. Notably, this protocol’s
features include moderate to excellent yields and remarkable diastereoselectivity
(>20:1 dr in general), easy performance, as well as applicability
to versatile 2,3-dihydrobenzofurans with aryethynyl or an aryl group
via C–C and C–O bond formation in one pot without any
catalyst in an aqueous mixed solvent.