Towards a total synthesis of (−)-cephalotaxine: construction of the BCDE-tetracyclic core

The synthesis of the BCDE-tetracyclic core of (−)-cephalotaxine has been achieved in eight steps starting from N-Boc-l-proline methyl ester. An alkylidene carbene 1,5-CH insertion reaction was used as a key step in the stereocontrolled synthesis of the DE-spirocyclic fragment and an intramolecular Heck-type cyclisation was used to form the benzazepine ring, and hence complete the tetracycle formation.