Copper-Catalyzed [4+2]-Cycloadditions of Isoxazoles with 2-Alkynylbenzaldehydes To Access Distinct α-Carbonylnaphthalene Derivatives: C(3,4)- versus C(4,5)-Regioselectivity at Isoxazoles

Cu(II)-catalyzed [4+2]-cycloadditions occur between Cu-benzopyryliums and substituted isoxazoles with the regioselectivity on the C(3,4)-carbons of isoxazoles. We postulate that a prior coordination of isoxazoles with Cu(OAc)(2) increases the pi-bond character of the C(3,4) carbons to become an effective 2 pi-donor. In this reaction sequence, 3,5-disubstituted isoxazoles yield alpha,gamma-dicarbonyl-naphthalenes whereas, beta-substituted isoxazoles deliver alpha,gamma-dicarbonyl-beta-aminonaphthalenes. For unsubstituted isoxazole, its cycloaddition chemoselectivity is switched to the C(4,5)-addition regioselectivity to yield alpha-carbonyl-gamma-cyanonaphthalene derivatives.