B(C6F5)3-Catalyzed formal (4+1)-annulation of ortho-quinone methides with diazoacetates: access to 2,3-dihydrobenzofurans.

A novel (4+1)-cycloaddition reaction of alkyne-tethered ortho-quinone methides with diazoacetates has been well established by using B(C6F5)3 as the catalyst, providing 2,3-dihydrobenzofuran derivatives containing a newly formed quaternary carbon center in good yields and diastereoselectivities (up to >19 : 1 dr).