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

Significant levels of enantioselectivity were obtained in 1,3-dipolar cycloadditions of 2-benzopyrylium-4-olate generated from the Rh2(OAc)4-catalyzed decomposition of o-methoxycarbonyl-α-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 α-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.