Stereodivergent synthesis of fused bicyclopyrazolidines: access to pyrazolines and pyrrolidines.

The reaction of hydrazinoethyl 1,1-cyclopropanediesters with aldehydes in the presence of catalytic Yb(OTf)(3) allows access to structurally complex fused bicyclo pyrazolidines. Either 2,5-cis or 2,5-trans adducts can be obtained in good to excellent yields in most cases by simply controlling the order of addition of the catalyst and aldehyde.