Palladium-catalyzed decarboxylative [4 + 2] cycloaddition of δ-vinylvalerolactones for the synthesis of spirocyclohexane compounds

Abstract

A palladium-catalyzed decarboxylation of α,α-diester-δ-vinylvalerolactones was developed to in situ generate all-carbon π-allylpalladium species that can act as 1,4-dipoles. These intermediates were successfully applied in [4 + 2] cycloaddition reactions with 3-alkenyloxindoles and 2-benzylideneindan-1,3-diones for the construction of a diverse array of spirocyclohexane compounds in high yields with excellent diastereoselectivities. Furthermore, the same in situ generated 1,4-dipoles were also employed in [4 + 2] cycloaddition reactions with 2-benzylidenemalononitriles for the synthesis of polysubstituted cyclohexanes. Notably, the current cycloaddition reactions proceed via all-carbon π-allylpalladium intermediates acting as 1,4-dipoles with exclusive branch selectivity, which is different from previously reported reactions in which these intermediates served as 1,6-dipoles with linear selectivity. The practicality of this methodology was further demonstrated by a gram-scale reaction and further transformations of products.