Application of the palladium(0)-catalyzed Ullmann cross-coupling reaction in a total synthesis of (±)-aspidospermidine and thus representing an approach to the lower hemisphere of the binary indole-indoline alkaloid vinblastine

As part of ongoing studies directed towards the construction of the anti-cancer agent vinblastine (1), the related but structurally less complex natural product aspidospermidine (3) has been synthesized. Two approaches to target 3 were pursued. In the first, which was unsuccessful, the amine-tethered enone 6 was prepared but this failed to engage in the pivotal intramolecular conjugate addition reaction to give the bicyclic system 5. In contrast, the related compound 46, incorporating tethered enone and azide moieties, engaged in an intramolecular 1,3-dipolar cycloaddition reaction to give, presumably via an intermediate triazoline, the isolable and ring-fused aziridine 47. This was then converted, over two steps, into the previously reported tetrahydrocarbazole 4. Application of established protocols to this last compound allowed for the installation of the E-ring of the title alkaloid 3 and completion of the total synthesis.