Structure of the main components of modified terpenoid-maleic adducts
A method for the synthesis of reaction products of the main components of terpenoid-maleic adducts with zinc acetate is presented. Based on the obtained NMR spectroscopy data, it can be stated that chemical transformation as a result of the interaction of maleopimaric acid with the modifier is observed at C-18, and the anhydride ring is not affected. IR and atomic emission spectroscopy, elemental analysis and computer simulation by quantum chemistry confirm the structure of the synthesized product. Comparison of chemical shifts in the 13C-NMR spectra of maleopimaric acidand the salt shows that significant changes are observed for the carbon nuclei C-3, C-5, C-18 and C-19. The greatest difference in the chemical shift is observed for the C-18 nucleus, and this signal is strongly broadened. For the remaining nuclei, Δδ is less than 0,3 ppm. In the NMR spectrum of the 1H product, the signals are broadened, a fine structure is not observed, and the difference in chemical shifts is insignificant in comparison with the maleopimaric acid and for most protons is within the measurement error (0,02 ppm). The greatest changes were recorded for protons at C-24, which issomewhat unexpected and requires further study. Because of the considerablebroadening of the lines in the NMR spectrum of the 1H product, the spectral data of the COSY and HMBC experiments turned out to be poorly informative and the assignment of some signals was performed by comparison with the data obtained from the analysis of the spectra of the maleopimaric acid. To confirm the established structure, elemental analysis was carried out, %: С - 66,63; H - 7,19; О - 18,38; Zn - 7,8. The presence of zinc was proved by atomic-emission spectrometry with inductively coupled plasma. In the IR spectrum of the synthesized product, absorption bands characteristic of the -C=Оgroups are found: 1697 and 1558 cm-1. This led to the conclusion that the reactionproceeds along the carboxyl group at C-18. It follows from the calculations that the zinc atom is localized in a certain «middle» state between the four oxygen atoms at C-18 and C-18 ' atoms, while the Zn-O distances agree with the value of the sum of the ionic zinc and oxygen radii - 2,06Å. In this case, the calculated values of the magnitudes of the bond orders, characterizing the strength of each bond, indicate that the zinc atom is equally connected with all the oxygen in this structure. It was found that, under given conditions, the α-terpenene adduct does not react with zinc acetate.