Studies on a Type III Atropisomeric Colchicine Analog Enabled by an Intermolecular Oxidopyrylium (5 + 2) Cycloaddition/Reductive Ring-Opening Sequence

Colchicine is one of the oldest known microtubule-targeting agents, and the development of clinically viable colchicine binding site inhibitors (CBSIs) remains of high interest to the medicinal chemistry community. Colchicine’s binding to tubulin is the result of its biaryl methoxytropone-trimethoxybenzene system (called the AC ring), which binds in a preferred aR conformation. In the natural source, this axial chiral form is favored thermodynamically by approximately 3 kcal/mol as a result of nearby point chirality on its B ring, and without this point chirality the AC ring will racemize within minutes to hours at room temperature (∆G ~ 22 kcal/mol, t1/2, 298K ~ 30 min). Herein, we describe the synthesis and evaluation of a novel acyclic AC analog of colchicine that is highly stable to epimerization based not on thermodynamic stabilization, but rather a high rotational barrier energy (∆G‡ = 31.2 kcal/mol, t1/2, 298K ~ 300 y). Access to this molecule, as well as several structural homologs, was enabled through the development of a convenient intermolecular oxidopyrylium (5 + 2) cycloaddition/reductive ring-opening approach from 3-hydroxy-4-pyrones. Profiling and characterization of the dihedral angles were carried out computationally and experimentally using vibrational circular dichroism (VCD), demonstrating that the ground state dihedral angles of the new molecules differ significantly from colchicine and MTC. However, despite this difference, the molecule retained anti-cancer, tubulin-binding, and tubulin polymerization inhibitory activity.