Enantioselective Divergent Syntheses of Cephalotaxus Alkaloids: (−)-Cephalotaxine, (−)-Cephalotine B, and (−)-Fortuneicyclidins A and B

A new strategy focusing on the last-stage asymmetric assembly of the ring D, which inherently possesses the densest part of stereogenic centers and functional groups in the A/B/C/D ring system of (−)-cephalotaxine, has been developed, in which a novel Rh-catalyzed asymmetric (2 + 3) annulation of tertiary enamides with enoldiazoacetates is designed and explored for enantioselective construction of the crucial cyclopentane ring D bearing a unique spirocyclic aza-quaternary stereocenter. Based on the expeditious access of chiral functionalized building block with the tetracyclic A/B/C/D ring system, a concise enantioselective total synthesis of (−)-cephalotaxine starting from readily available homopiperonyl alcohol has been achieved in nine steps with only two column chromatography purifications. Following the tactical introduction of the Meinwald rearrangement, enantioselective divergent syntheses of (−)-cephalotine B with an additional C3–O–C11 oxo-bridged bond (14 steps), (−)-fortuneicyclidin B with an unprecedented C3–C10 bond (14 steps), and its 2-epimer (−)-fortuneicyclidin A (16 steps) have been also accomplished for the first time.