ALKYLALUMINUM-CATALYZED CLAISEN EXPANSION REACTIONS - SCOPE AND STEREOCHEMISTRY

The triisobutylaluminum-catalyzed Claisen rearrangement of a series of bicyclic allyl vinyl ethers was examined in connection with a planned synthesis of the diterpene epoxydictymene. The exocyclic vinyl ethers studied undergo [3,3] sigmatropy via chairlike transition states to provide products having Z stereochemistry about the double bond of the eight-membered ring. Differences appear only in the extent of stereoselectivity in the ensuing carbonyl reduction step. Two companion endocyclic vinyl ethers were found to utilize boat transition states to deliver Z products exclusively. In all cases, the level of chirality transfer is excellent. An analysis is presented showing that the relative configurations of the newly generated stereogenic centers, established in several key examples by X-ray crystallography, follow directly from the boat-chair options, both of which have the latent ability to be influenced catalytically by (i-Bu)3Al.