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| - Chiral 2,6-Bis(oxazolinyl)pyridine−Rare Earth Metal Complexes asCatalysts for Highly Enantioselective 1,3-Dipolar CycloadditionReactions of 2-Benzopyrylium-4-olates
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| - 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 metaltriflate complexes as chiral Lewis acid catalysts. The reactions with several benzyloxyacetaldehydederivatives 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 oftrifluoroacetic 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-ketoesters with high exo- and enantioselectivities (up to 95% ee). In contrast to the reaction with carbonylcompounds, Yb(III)−Pybox-Ph complex (10 mol %) was found to be effective to obtain highenantioselectivity (96% ee) of diastereoselectively produced exo-cycloadduct in the reaction with3-acryloyl-2-oxazolidinone.
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