Alpha-alkenoyl ketene S,S-acetal-based multicomponent reaction: an efficient approach for the selective construction of polyfunctionalized cyclohexanones.

A versatile multicomponent reaction based on the new four-carbon synthons alpha-alkenoyl ketene S,S-acetals 1 has been developed. This three-component reaction of readily available alpha-alkenoyl ketene S,S-acetals 1 with aldehydes 2 and active methylene compounds 3 proceeds smoothly in acidic medium (glacial acetic acid in tetrahydrofuran) to give various polyfunctionalized cyclohexanones 4, 5, and 6 in a highly regio- and diastereoselective manner with good to excellent yields. The reaction can tolerate a broad range of substituents in the three components involved and is proposed to proceed via a tandem Knoevenagel-intermolecular Michael-intramolecular Michael sequence. As an extension of the synthetic application of polyfunctionalized cyclohexanones obtained, unsymmetrical biaryls 7 were synthesized in almost quantitative yields by simple transformations of the corresponding cycloadducts 6.