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  • filipin Experimentally the ECD spectrum of showed three ECD

    2024-04-02

    Experimentally, the ECD spectrum of 1 showed three ECD bands: positive ones at 260 and 200nm, and a negative one at 230nm (Fig. 4). Meanwhile, there is only one broad positive ECD band starting from 300nm in the ECD spectrum of 2. ECD spectra of 1 and its derivatives attracted great interest because of the particularity of peroxide bond. It was Professor Liang who first reported ECD spectrum of 1 and ascribed the positive maximum at 260nm to both lactone and peroxide groups. Then, Shen and co-worker [17] discussed ECD spectra of several artemisinin analogs and proposed that the contribution of peroxide chromophore was around 236nm. The theoretical oscillator and rotatory strengths of 1 and 2 were obtained at different levels using the optimized geometries and listed in Table 1, Table 2. It shown that the B3LYP functional could give similar results irrespective of the basis set level. However, the CAM-B3LYP functional would lose an important excitation state of the peroxide bridge at around 200nm. For filipin 2 with peroxide moiety as the only chromophore, two electron transition states with both positive rotatory strengths contributed to the broad positive ECD band in the far-UV range from 300nm to 200nm. There are two chromophores peroxide bridge and δ-lactone group in the chemical structure of 1. The positive rotatory strengths at 243 and 201nm are due to the peroxide group, and the negative Cotton effect at 221nm was originated from the n→π* transition of the carbonyl group of δ-lactone. The predicted ECD spectrum of 1 showed the same tendency as the experiment, with positive–negative–positive mode from the long wavelength. Therefore, it is undoubted that correct absolute configurations of 1 and 2 could be achieved by comparison of the experimental and theoretical ECD profiles. To clearly illustrate the effect of the peroxide bridge on ECD spectrum, 1-deoxyartemisinin (3, Fig. 1) with only a lactone chromophore was selected to investigate. ECD calculation of 3 gave a negative rotatory strength at 220nm, which corresponded to the negative Cotton effect at 220nm observed experimentally [17]. As shown in Fig. 2, the β-carbon atoms of 1 and 3 were below the δ-lactone plane, which would lead to a negative Cotton effect in absorption wavelength of the n→π* transition of the carbonyl group according to the Beecham rule [18]. Our results verified the suitability of this empirical rule in the case of 1 and 3, which stated the sign of the n→π* Cotton effect was solely determined by the position of the β-carbon atom relative to the lactone plane. It is interesting that the sum of the computed ECD spectra of 2 and 3 gave a better coincidence with the experimental data of 1 than the calculated data of 1, which reproduced all the key Cotton effects but a little blue-shifted (Fig. 5). It could attribute to the 5nm blue-shift of absorption wavelength of 1 compared with 2. This indicates that ECD spectra possess additivity, and it needs the development of new hybrid functional to calculate excitation energies of complex systems with higher accuracy. Molecular orbitals involved in key transitions for ECD spectra of 1 and 2 are shown in Fig. 6. For 2, the positive rotatory strength at 245nm is originated from the MO81 (HOMO)→MO82 (LUMO) transition, which is an n→σ* transition of the peroxide bridge. Meanwhile, the more intense positive rotatory strength at 201nm was contributed by two transitions from MO79 (HOMO–2) and MO80 (HOMO−1) to LUMO. These two transition were also n→σ* transition with lone pair electrons of all oxygen atoms participating in the molecule orbitals. As to 1, three lowest-energy rotatory strengths were all originated from the n→σ* transition of the peroxide bridge and n→π* transition of the lactone group. It seems that interaction might take place between the lone pair electrons of these two chromophores and lead to the delocalization of the unshared pair electrons of all oxygen atoms in the carbon–oxygen chain. This might interpret the high thermal stability of 1 and its analogs.