Formation of the double addition products of donor-acceptor cyclopropanes with 2-pyrazolines in the presence of Lewis acids

A reaction of excess of cyclopropane-1,1-dicarboxylates substituted at position 2 with 2-pyrazolines in the presence of 3 equiv. of GaCl₃ at 5 °C selectively resulted in the cyclopropanes double addition products, viz., N-substituted 1,2-diazabicyclo[3.3.0]octanes. In this case, the first molecule of the starting cyclopropane formed the bicyclic system, whereas the second added at the N-H bond of the adduct formed. When catalytic amount of Sc(OTf)₃ was used in this reaction instead of GaCl₃ at 40 °C, besides the corresponding 1,2-diazabicyclo[3.3.0]-octanes, another type of cyclopropane double addition products, viz., N-alkyl-2-pyrazolines, was formed, in which the alkyl chain was assembled from two molecules of the starting cyclopropane. A plausible mechanism for the transformations observed was suggested.