Catalytic Asymmetric Synthesis of Aromatic Spiroketals by SpinPhox/ Iridium(I)-Catalyzed Hydrogenation and Spiroketalization of α,α'-Bis(2-hydroxyaryIidene) Ketones

摘要

Recently, the chemistry of chiral aromatic spiroketals has received much attention because they are key substructures in biologically active natural products, pharmaceuticals, and chiral ligands in transition-metal-catalyzed reactions. Although several methods for the preparation of aromatic spiroketals have been reported, the direct catalytic, enantioselective synthesis of aromatic spiroketals has not yet been achieved. On the other hand, transition-metal-catalyzed asymmetric hydrogenations have long held a respected position in modern organic synthesis. In particular, the iridium-catalyzed asymmetric hydrogenation of the C=C bonds of either functionalized or unfunctionalized olefin derivatives has lately been an important topic in asymmetric catalysis. We have recently reported a class of highly efficient iridium(I) catalyst complexes with spiro[4.4]-1,6-nonadiene-based chiral phosphine-oxazoline ligands (Spin-PHOX, 1) for the enantioselective hydrogenation of N-aryl or alkyl imines and α-aryl-β-substituted acrylic acids. Herein, we report the first catalytic enantioselective synthesis of aromatic spiroketals 2 through the SpinPHOX/Ir~I-promoted asymmetric hydrogenation of α,α'-bis(2-hydroxyarylidene) ketones and spiroketalization of hydrogenation products.