Influence of Substituents on the Lappaconitine Acetylenic Derivatives Photodegradation

A photochemical approach for evaluating the reactivity of substituted lappaconitines

“…7. The stages 1-4 and 6-7 have been established earlier by the analysis of spin effects, 10,[13][14][15][16] while steps 5.1-5.3 are proposed based on the analysis of the NMR spectra of photolysis products in this work. The photolysis of N-acetyl-AA is considered to involve a cleavage and photo-Fries rearrangement.…”

“…We attribute this to the fact that the rate constant of ET has to be substantially high (10 8 -10 9 s −1 ), as follows from the manifestation of the initial LC CIDNP effect. 10,[13][14][15][16] It allows us to observe only a little effective yield of the triplet state.…”

“…In addition, the shortening of the LC fluorescence lifetime (2.15 ns), in comparison with N-acetyl-AA (4 ns), is most likely to reveal the process of the fluorescence quenching of the anthranilic fragment through the previously described ET from the amine group of the alkaloid to the anthranilic fragment. 10,[13][14][15][16] In this connection, the difference between LC and MA fluorescence quantum yields and lifetimes is caused by two factors: the influence of the acetyl group leading to the loss of the molecular planarity and a decrease of the electron-donating character of the amine group towards the aromatic moiety and the process of intramolecular ET. Table 2 summarizes the fluorescence data obtained for all derivatives of LC.…”

“…1, a detailed mechanism of photolysis has been established by CIDNP and EPR analysis in previous studies. 10,[13][14][15][16] According to these studies, a photoreaction begins from the partially reversible intramolecular electron transfer (ET) between nitrogen located in the diterpenoid skeleton and the AA fragment with a subsequent cleavage of the ether bond. Since the anthranilic fragment is a chromophore in these molecules as well as a potential electron acceptor in redox processes, substituents have a significant impact on the efficiency of ET and subsequent photodegradation, respectively.…”

“…It has been suggested that the photocleavage of LC derivatives depends on the proportion between singlet and triplet excited states. 16 To understand the nature of the influence of substituents and their impact on the formation of singlet and triplet excited states, this report is focused on the examination of the photophysical properties of LC and its substituted analogues and their photodecomposition quantum yields.…”