All-carbon N-heterocyclic carbene-catalyzed (3+2) annulation using donor-acceptor cyclopropanes

Donor-acceptor cyclopropanes are known to serve as dipole precursors capable of engaging in (3+2) annulations with electron-deficient π-systems. In 2013, the reaction of donor-acceptor cyclopropanes with α,β-unsaturated acyl fluorides in an all-carbon (3+2) annulation was discovered. The reaction proceeds in good yields using the IMes NHC to provide diastereomerically pure β-lactone-fused cyclopentanes bearing four contiguous stereocentres. Subsequent studies demonstrated that N-t-butyl substituted homochiral morpholinone NHCs allowed the reaction to be achieved in up to 98 % ee. In this account, a background to this reaction is introduced, along with a complete account of the strengths, limitations and challenges encountered while developing this chemistry.