Enantioselective Oxidation of Alkenylbenzoates Catalyzed by Chiral Hypervalent Iodine(III) To Yield 4-Hydroxyisochroman-1-ones

摘要

In this study, the enantioselective oxylactonization of orthoalk-1-enylbenzoates with chiral hypervalent iodine(III) reagents yielded 3-alkyl-4-hydroxyisochroman-1-ones with high enantiomeric purity (ca. 90% ee). The enantioselective oxidation was also performed under catalytic conditions, in which a catalytic amount (10 mol-%) of a chiral iodoarene was oxidized to the hypervalent iodine species in situ using a stoichiometric co-oxidant, m-chloroperbenzoic acid (mCPBA). The catalytic oxidation mediated by chiral hypervalent iodine(III) species yielded enantiocontrolled syn products, while the direct oxidation of the substrate with mCPBA occurred as background oxidation to give racemic anti products. Under optimized conditions, the catalytic oxylactonization led to high levels of enantioselectivity (ca. 90% ee) and improved syn/anti selectivities (ca. 80% syn). The product distribution indicated that the lactate side-chain on the chiral iodoarene precatalyst plays an important role in enhancing both the enantioselectivity and the catalytic efficiency in the oxylactonization.