Colchicine red-ox chemistry revisited: Cathodic behavior and EPR observation of an intermediate radical anion

Colchicine (1), a potently antimitotic alkaloid and useful laboratory tool in cancer research, undergoes cathodic reduction in DMF forming an ESR-observable radical anion (1r) which is characterized by the isotropic hyperfine coupling constants 8.9, 4.3, 0.75, 0.49 and 0.48 G for H-8, H-12, OCH3, H-11 and H-4, respectively, and a much flattened troponoid ring. Assignments are aided by selective deuteriation of colchicine at C-8, C-11 and COCH3, as well as by spectral simulation and ab initio calculations of electron spin densities. Whether the colchicine radical anion may exist in nature is also discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.