Total Synthesis of Ningalin B Utilizing a Heterocyclic Azadiene Diels−Alder Reaction and Discovery of a New Class of Potent Multidrug Resistant (MDR) Reversal Agents

A concise, efficient approach to the total synthesis of ningalin B (1) based on a heterocyclic azadiene Diels−Alder strategy (1,2,4,5-tetrazine → 1,2-diazine → pyrrole) ideally suited for construction of the densely functionalized pyrrole core found in the natural product is detailed. Examination of the natural product and a number of synthetic intermediates revealed that while lacking inherent cytotoxic activity, many reverse the multidrug-resistant (MDR) phenotype, resensitizing a human colon cancer cell line (HCT116/VM46) to vinblastine and doxorubicin at lower doses than the prototypical agent verapamil.