Rh(I)-Catalyzed Formal [6 + 2] Cycloaddition of 4-Allenals with Alkynes or Alkenes in a Tether

Rh(I)-catalyzed formal [6 + 2] cycloaddition of allenal 6 having an alkyne or alkene in a tether proceeded smoothly, giving 5–8- and 6–8-fused bicyclic ketone derivatives 7 in good to excellent yields. It was also found that cyclization of enantiomerically enriched (S)-6a (94% ee) gave cyclic ketone derivative (S)-7a in high yield with reasonable chirality transfer (86% ee). This result indicates that this cyclization proceeds through stereoselective formation of rhodacycle H′ followed by insertion of a multiple bond.