Enantio‐ and Diastereoselective Access to Distant Stereocenters Embedded within Tetrahydroxanthenes: Utilizing <i>ortho</i>‐Quinone Methides as Reactive Intermediates in Asymmetric Brønsted Acid Catalysis

Abstract

A protocol for the highly enantioselective synthesis of 9-substituted tetrahydroxanthenones by means of asymmetric Brønsted acid catalysis has been developed. A chiral binol-based N-triflyphosphoramide was found to promote the in situ generation of ortho-quinone methides and their subsequent reaction with 1,3-cyclohexanedione to provide the desired products with excellent enantioselectivities. In addition, a highly enantio- and diastereoselective Brønsted acid catalyzed desymmetrization of 5-monosubstituted 1,3-dicarbonyl substrates with ortho-quinone methides gives rise to valuable tetrahydroxanthenes containing two distant stereocenters.