CIDNP and EPR study of phototransformation of lappaconitine derivatives in solution

Chemically induced dynamic nuclear polarization (CIDNP) and electron paramagnetic resonance (EPR) techniques have been used to study the paramagnetic species formed during the photolysis of the alkaloid lappaconitine and its synthetic analogues in solution. Lappaconitine is a photosensitive antiarrhythmic and hypertension drug, whose major photoproduct (N-acetyl anthranilic acid) is also a potent photosensitizer. Both these compounds are lipophilic and might bind efficiently to cell membranes thereby causing phototoxic damage. Photolysis of natural lappaconitine (I) as well as its N(20) des-ethyl derivatives (N-Bz (II), N-Me (III), N-H (IV), and N(O)-Et (V)) results in cleavage of the ester bond with the formation of N-acetyl anthranilic acid (VIII) and corresponding enamine. The lappaconitine derivative V shows maximum photostability which correlates with reference data about its low toxicity. It was shown that the primary reaction step is electron transfer from the amino group to the anthranilic fragment of lappaconitine resulting in an intermediate biradical. The final products are formed via fragmentation of the neutral lappaconitine radicals.