One-Step Synthesis of β-Alkylidene-γ-lactones via Ligand-Enabled β,γ-Dehydrogenation of Aliphatic Acids.

Abstract

Ligand-enabled Pd-catalyzed regioselective α,β-dehydrogenation of carbonyl compounds via β-methylene C-H activation has recently emerged as a promising transformation. Herein, we report the realization of β,γ-dehydrogenation and subsequent vinyl C-H olefination reactions of free carboxylic acids, thus providing a unique method for the structural diversification of aliphatic acids containing α-quaternary centers through sequential functionalizations of two β-C-H bonds and one γ-C-H bond. This tandem dehydrogenation-olefination-lactonization reaction offers a one-step preparation of β-alkylidene-γ-lactones, which are often difficult to prepare through conventional methods, from inexpensive and abundant free aliphatic acids. A variety of free aliphatic acids, such as isosteviol and grandiflorolic acid natural products, and olefins are compatible with the reported protocol. The newly designed bidentate oxime ether-pyridone and morpholine-pyridone ligands are crucial for this tandem reaction to proceed. Notably, these ligands also enable preferential methylene C-H activation over the previously reported, competing process of methyl C-H bond olefination.