Chiral 2,6-Bis(oxazolinyl)pyridine-Rare Earth Metal Complexes as Catalysts for Highly Enantioselective 1,3-Dipolar Cycloaddition Reactions of 2-Benzopyrylium-4-olates

摘要

Significant levels of enantioselectivity were obtained in 1,3-dipolar cycloadditions of 2-benzopyry-lium-4-olate generated from the RH_2(OAc)4-catalyzed decomposition of o-methoxycarbonyl-a-diazoacetophenone.This reaction utilized chiral 2,6-bis(oxazolinyl)pyridine(Pybox)-rare earth metal triflate complexes as chiral Lewis acid catalysts.The reactions with several benzyloxyacetaldehyde derivatives catalyzed by a Sc(III)-Pybox-i-Pr complex(10 mol %)proceeded smoothly to yield endo-adducts selectively with high enantioselectivity(up to 93% ee).For the reaction with benzyl pyruvate,the Sc(III)-Pybox-i-Pr complex(10 mol %)catalyzed the reaction effectively in the presence of trifluoroacetic acid(10 mol %)to yield an exo-adduct with both high diastereo-and enantioselectivity(94% ee).This catalytic system was efficiently applied to the reactions with several other a-keto esters with high exo-and enantioselectivities(up to 95% ee).In contrast to the reaction with carbonyl compounds,Yb(III)-Pybox-Ph complex(10 mol %)was found to be effective to obtain high enantioselectivity(96% ee)of diastereoselectively produced exo-cycloadduct in the reaction with 3-acryloyl-2-oxazolidinone.