Efficient catalytic effects of Lewis acids in the 1,3-dipolar cycloaddition reactions of carbonyl ylides with imines.

[reactions: see text] 1,3-Dipolar cycloaddition reactions between imines and carbonyl ylides generated by tandem intramolecular carbenoid-carbonyl cyclizations were found to be effectively catalyzed by Lewis acids (10 mol %). The Rh2(OAc)4-catalyzed reactions of o-(methoxycarbonyl)-alpha-diazoacetophenone with imines such as N-[2-(benzyloxy)benzylidene]aniline in the absence of Lewis acid gave no 1,3-dipolar cycloaddition products, but rather the dimeric product of the corresponding carbonyl ylide. In contrast, in the presence of Lewis acids such as Yb(OTf)3, the 1,3-dipolar cycloaddition reactions of the corresponding 1-methoxy-2-benzopyrylium-4-olate proceeded smoothly with several imines, giving in most cases exo-selectivity and no formation of the dimeric product.