Pd-catalyzed C-S activation for [3 + 3] annulation of 2-(methylthio)benzofuran-3-carboxylates and 2-hydroxyphenylboronic acids: synthesis of coumestan derivatives.

Pd-catalytic C-S activation was successfully applied to initiate the cross-coupling of (2-methylthio-3-ester)benzofurans with 2-hydroxyphenylboronic acids and sequential intramolecular transesterification process under Liebeskind-Srogl conditions. Thus, a novel [3 + 3] annulation strategy for efficient synthesis of coumestan derivatives has been developed from readily available starting materials.