Synthesis of 3,6-dihydro-2H-pyran-2-ones via cationic palladium(II) complex-catalyzed tandem [2+2] cycloaddition-allylic rearrangement of ketene with α,β-unsaturated aldehydes and ketones

Treatment of ketene with α,β-unsaturated aldehydes and ketones in the presence of [Pd(dppb)(PhCN)2](BF4)2 leads to the formation of 4-vinyloxetan-2-ones, which rearrange under the conditions to give 3,6-dihydro-2H-pyran-2-ones in a variety of yields, depending on the substituents. Asymmetric induction with up to 57% de has been achieved by using α,β-unsaturated aldehydes bearing an asymmetric carbon center at the β-position. A zwitterion generated by heterolytic cleavage of the C(4)–O bond of the 4-vinyloxetan-2-one is considered to be an intermediate in the allylic rearrangement. Ethanolysis of 3,6-dihydro-6-methyl-2H-pyran-2-one under acid conditions, followed by saponification of the resulting ethyl 2,4-hexadienoate (ethyl sorbate), gives sorbic acid in 90% yield.