Molecular Complexity-Inspired Synthetic Strategies toward the Calyciphylline A-Type <i>Daphniphyllum</i> Alkaloids Himalensine A and Daphenylline

In this report, we detail two distinct synthetic approaches to calyciphylline A-type <i>Daphniphyllum</i> alkaloids himalensine A and daphenylline, which are inspired by our analysis of the structural complexity of these compounds. Using MolComplex, a Python-based web application that we have developed, we quantified the structural complexity of all possible precursors resulting from one-bond retrosynthetic disconnections. This led to the identification of transannular bonds as especially simplifying to the molecular graph, and, based on this analysis, we pursued a total synthesis of himalensine A from macrocyclic intermediates with planned late-stage transannular ring formations. Despite initial setbacks in accessing an originally designed macrocycle, targeting a simplified macrocycle ultimately enabled investigation of this intermediate's unique transannular reactivity. Given the lack of success to access himalensine A based solely on molecular graph analysis, we revised our approach to the related alkaloid, daphenylline. Herein, we also provide the details of the various synthetic challenges that we encountered and overcame en route to a total synthesis of daphenylline. First, optimization of a Rh-mediated intramolecular Buchner/6π-electrocyclic ring-opening sequence enabled construction of the pentacyclic core. We then describe various attempts to install a key quaternary methyl group and, ultimately, our solution to leverage a [2 + 2] photocycloaddition/bond cleavage sequence to achieve this elusive goal. Finally, a late-stage Friedel-Crafts cyclization and deoxygenation facilitated the 11-step total synthesis, which was made formally enantioselective by a Rh-mediated dihydropyridone conjugate arylation. Complexity analysis of the daphenylline synthesis highlights how complexity-building/C-C cleavage combinations can be uniquely effective in achieving synthetic outcomes.