Pentacyclic homoproaporphine alkaloids, which contain a 6/6/6/6/6-spiro-bridged framework and multiple stereocenters, present synthetic challenges. Here, we report a divergent synthesis of four pentacyclic homoproaporphine alkaloids, (-)-robustamine, (-)-robustamine cis-N-oxide, (+)-regelinine, and (+)-regeline, through sequential C-H elaboration that proceeds via a two-stage cyclization-functionalization strategy. The first stage of the synthetic strategy is the efficient construction of the 6/6/6/6-tetracyclic core framework via two sequential Csp2-H cyclizations. The second stage is precise regulation of the backbone stereochemical diversity through a series of Csp3-H functionalization steps, including a locally desymmetrizing spirocarbocyclic ketone transposition, tunable Csp3-H oxidation at C11, and stereoselective Csp3-H epimerization at C6a. This work establishes a practical C-H elaboration platform that enables concise access to structurally complex natural product architectures.
Pu et al. (Fri,) studied this question.