Synthesis of vinca alkaloids and related compounds. Part 108: Efficient convergent synthetic pathway to the ibophyllidine skeleton IV. First synthesis of (±)-18-hydroxy-20-epiibophyllidine

The first total synthesis of the pentacyclic alkaloid (±)-18-hydroxy-20-epiibophyllidine was realized via an efficient preparation of the d-seco-pseudoaspidospermane molecule. The key step of the sequence involves an intramolecular [4+2] cycloaddition reaction of the dihydrosecodine intermediate, which was built up from the reaction of a tryptamine derivative with an aldehyde–ester. After full epimerization, the intramolecular N-alkylation of the tetracyclic ester gave the pentacyclic compound. Reduction of the latter molecule led to the title compound.