Chiral NHC-catalyzed 1,3-dipolar [3+2] cycloaddition of azomethine imines with α-chloroaldehydes for the synthesis of bicyclic pyrazolidinones

A chiral N-heterocyclic carbene (NHC)-catalyzed 1,3-dipolar [3 + 2] cycloaddition reaction of 3-oxopyrazolidin-1-ium-2-ides and -chloroaldehydes was developed for the synthesis of pyrazolo[1,2-a]pyrazole-1,7-diones in moderate to good yields (up to 76%) with good enantioselectivities (up to 98% ee) and diastereoselectivities (up to 8:1 dr).