Pd II ‐Catalyzed Oxidative Dimeric Cyclization–Coupling Reaction of 2,3‐Allenoic Acids: An Efficient Synthesis of Bibutenolide Derivatives

Three sets of convenient catalytic systems have been developed for the oxidative dimeric cyclization coupling of differently substituted 2,3‐allenoic acids catalyzed by Pd II , affording bibutenolides that are not otherwise readily available. The advantages and disadvantages of these systems are discussed. Although the diastereoselectivity for the bicyclization of racemic 2,3‐allenoic acids is low, excellent diastereoselectivity was realized in the bicyclization reaction of optically active 2,3‐allenoic acids, leading to the optically active bibutenolides in high yields and ee . Based on a mechanistic study, it is believed that the reaction may proceed by means of a double oxypalladation and reductive elimination to yield butenolide 3 and Pd 0 species, which may be reoxidized to the catalytically active Pd II species in the presence of alkyl iodide/air, metallic iodide/air, or benzoquinone.