Stereoselective palladium-catalyzed carbocyclization ...



Title Stereoselective palladium-catalyzed carbocyclization of allenic allylic carboxylates
Author(s) J. Franzen, Jeanne Lofstedt, J. Falk, J. E. Backvall
Journal Journal of the American Chemical Society
Date 2003
Volume 125
Issue 46
Start page 14140
End page 14148
Abstract Palladium(0)-catalyzed reaction of allene-substituted allylic carboxylates 3-8 employing 2-5 mol % of Pd(dba)(2) in refluxing toluene leads to the carbocyclization and elimination of carboxylic acid to give bicyclo[4.3.0]nonadiene and bicyclo[5.3.0]decadiene derivatives (12-17). The carbon-carbon bond formation is stereospecific, occurring syn with respect to the leaving group. Addition of maleic anhydride as a ligand to the above-mentioned procedures changed the outcome of the reaction, and under these conditions 3-5 afforded cycloisomerized products 21-23. The experimental results are consistent with a mechanism involving oxidative addition of the allylic carboxylate to Pd(0) to give an electron-deficient (pi-allyl)palladium intermediate, followed by nucleophilic attack by the allene on the face of the pi-allyl opposite to that of the palladium atom. Furthermore, it was found that the Pd(dba)(2)-catalyzed cyclization of the trans-cycloheptene derivative (trans-8) can be directed to give either the trans-fused (trans-17) or the cis-fused (cis-17) ring system by altering the solvent. The former reaction proceeds via a nucleophilic trans-allene attack on the (pi-allyl)palladium intermediate, whereas the latter involves a syn-allene insertion into the allyl-Pd bond of the same intermediate. The products from the carbocylization undergo stereoselective Diels-Alder reactions to give stereodefined polycyclic systems in high yields.
DOI 10.1021/ja037398u

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