Abstract 1, 3-Dipolar cycloaddition reaction of diazo esters to electron-deficient dipolarophiles to yield the corresponding 1-or 2-pyrazolines was found to be significantly accelerated with Lewis acids (Yb (OTf) 3, Sc (OTf) 3, GaCl 3, EtAlCl 2). The use of GaCl 3 as the catalyst leads to the acceleration not only of the 1, 3-dipolar cycloaddition reaction, but also subsequent insertion of the CHCO 2 Me electrophilic fragment of methyl diazoacetate ...