Structure Reassignment of Melonine and Quantum-Chemical Calculations-Based Assessment of Biosynthetic Scenarios Leading to Its Revised and Original Structures

Melonine is a basic monoterpene indole alkaloid (MIA) skeleton from Melodinus philliraeoides that was reported in 1983. The scarcity of its spectroscopic data questioned the validity of its structure. This prompted us to reisolate this molecule and to revise its structure into an unprecedented MIA scaffold. DFT-validated biosynthetic paths to both this new core and the originally reported form are proposed. The pathway to the original structure of melonine seems to be thermodynamically feasible, and that compound may exist as a natural product.