Development of a formal [4 + 1] cycloaddition: Pd(OAc)2-catalyzed intramolecular cyclopropanation of 1,3-dienyl beta-keto esters and MgI2-promoted vinylcyclopropane-cyclopentene rearrangement.

A formal [4 + 1] cycloaddition of 1,3-dienyl beta-keto esters has been developed. This two step process involves Pd(II)-catalyzed intramolecular cyclopropanation to produce vinylcyclopropanes and a subsequent mild vinylcyclopropane-cyclopentene rearrangement promoted by MgI(2). The cyclopropanation method notably requires the use of Mg(ClO(4))(2), presumably to facilitate keto-enol tautomerization, and is greatly improved by the use of copper(II) isobutyrate as co-oxidant. A range of substrates with various substitution patterns is demonstrated.