Synthesis of the ABC fragment of calyciphylline A-type Daphniphyllum alkaloids

The ABC-ring system of calyciphylline A-type alkaloids has been synthesized. The key steps of the synthetic approach are an atom transfer radical cyclization of a trichloroacetamide upon an enol acetate to generate the B ring, and a sulfone-based conjugated addition upon a β-methyl-α,β-unsaturated ketone to give the target azatricyclic ketone. Selecting the cation (K+ or Cs+) of the carbonate in the C ring-forming intramolecular Michael addition gives stereodivergent access to both epimers of the cyclized product.