Merging [2+2] Cycloaddition with Radical 1,4-Addition: Metal-Free Access to Functionalized Cyclobuta[a]naphthalen-4-ols

A metal-free [2+2] cycloaddition and 1,4-addition sequence induced by S-centered radicals has been achieved by treating benzene-linked allene-ynes with aryldiazonium tetrafluoroborates and DABCO-bis(sulfur dioxide) in a one-pot procedure. The reaction provides a greener and more practical access to functionalized cyclobuta[a]naphthalen-4-ols with valuable applications. More than 50 examples are demonstrated with excellent diastereoselectivity and chemical yields. The reaction pathway is proposed to proceed by the following steps:[2+2] cycloaddition, insertion of SO2, 1,4-addition, diazotization, and tautomerization.