Alkaloid-Catalyzed Enantioselective [3 + 2] Cycloaddition of Ketenes and Azomethine Imines

A new
asymmetric synthesis of bicyclic pyrazolidinones through
an alkaloid-catalyzed formal [3 + 2] cycloaddition of in situ generated
ketenes and azomethine imines is described. The products were formed
in good to excellent yields (52–99% for 17 examples), with
good to excellent diastereoselectivity (dr 5:1 to 27:1 for 11 examples),
and with excellent enantioselectivity in all cases (≥96% ee).
This method represents the first unambiguous example of an enantioselective
reaction between ketenes and a 1,3-dipole.