Fig. 1: Catalytic cycloaddition & cyclization methods.

a Construction of aromatic ring-fused cyclobutanones & unknown structure of furan-fused cyclobutanone. b Catalytic carbocyclization & carbene/alkyne metathesis (CAM). c This study: catalytic 4-exo-dig carbocyclization process for the direct construction of furan-fused cyclobutanones vs competitive 5-endo-dig cyclization. d Synthetic applications of synthesized furan-fused cyclobutanones.