Studies towards the total synthesis of cochinchistemonine

The possibility of synthetizing a new class of Stemona alkaloids and the challenge of constructing this new pyrido[1,2-a]azepine framework provided sufficient motivation for our group to embark on a synthesis of cochinchistemonine. Our retrosynthetic analysis was based on two key transformations, namely an aldol spirocyclization and an intramolecular Stetter reaction. Two different retrosynthetic analyses, both based on the same strategy, were proposed.

Based on a similar strategy, a second variant of the retrosynthetic analysis was proposed. The major difference is the later installation of the methyl group at position C-11. Taking this difference into account, it was anticipated that the five-membered ring could be generated through an aldol spirocyclization but this time using precursor 43. The second key step of the synthesis was the intramolecular Stetter reaction of aldehyde 46 to generate the azepine ring of intermediate 45 creating, in contrast to variant A, only one stereogenic center. While variants A and B are similar, it was decided that the aldol spirocyclization and the Stetter reaction planned in both routes would be studied first using model systems.