Indium-Mediated 1,2,4,5-Hexatetraen-3-ylation of 4-Acetoxy-2-azetidinones and Their Applications to the Diels−Alder Reactions for the Synthesis of 2-Azetidinone Derivatives

4-Acetoxy-2-azetidinones reacted with organoindium reagent generated in situ from indium and 1,6-dibromo-2,4-hexadiyne in the presence of LiCl in DMF to selectively produce 2-azetidinones possessing 1,2,4,5-hexatetraen-3-yl group on the C4-position. The Diels-Alder reactions of 4-(1,2,4,5-hexatetraen-3-yl)-2-azetidinones with a variety of dienophiles and subsequent aromatizations afforded valuable functional group-substituted 2-azetidinones in good yields.