Rhodium(III)/Copper(II)-Promoted trans-Selective Heteroaryl Acyloxylation of Alkynes: Stereodefined Access to trans-Enol Esters
Enol esters are versatile synthetic building blocks which can be elaborated by a wide variety of transformations. The classical synthesis by O-selective enolate acylation often hampers control of the E/Z selectivity with highly substituted substrates. A rhodium(III)/copper(II)-mediated process is reported to provide tetrasubstituted enol esters in a transselective fashion. Overall, the reaction consists of a heteroaryl acyloxylation of alkynes. The process is initiated by a rhodium(III)-catalyzed C2-selective activation of electron-rich heteroarenes, such as benzofuran, furan, and thiophene. Upon addition across an alkyne, a transmetalation to copper(II) enables reductive C-O bond formation. The transformation allows the three-component couplings of heteroarenes, alkynes, and carboxylic acids. Application of the method in the functionalization of bioactive furocoumarin natural products is also described.