A New Insight into the Stereoelectronic Control of the Pd(0)-Catalyzed Allylic Substitution: Application for the Synthesis of Multisubstituted Pyran-2-ones via an Unusual 1,3-Transposition.

Pyran-2-ones 3 undergo a novel Pd(0)-catalyzed 1,3-rearrangement to afford isomers 6. The reaction proceeds via an η2-Pd complex, whose pyramidalization (confirmed by quantum chemistry calculations) offers a favorable antiperiplanar alignment of the Pd-C and allylic C-O bonds (C), thus allowing the formation of an η3 Pd intermediate. Subsequent rotation and rate-limiting recombination with the carboxylate arm then gives isomeric pyran-2-ones 6. The calculated free energies reproduce the observed kinetics semi-quantitatively.