Cationic palladium(ii) complex-catalyzed [2 + 2] cycloaddition and tandem cycloaddition–allylic rearrangement of ketene with aldehydes: an improved synthesis of sorbic acid

Cationic palladium(II) complexes [PdL2(PhCN)2](BF4)2 efficiently catalyze the [2 + 2] cycloaddition of ketene with aldehydes to give the corresponding oxetan-2-ones, among which 4-vinyl-substituted ones are further isomerized under the conditions to give 3,6-dihydro-2H-pyran-2-ones in good yields.