Structure elucidation of compounds extracted from the Chinese medicinal plant Patrinia heterophylla

Abstract

For several hundred years, Patrinia heterophylla has been used in traditional Chinese medicine as a treatment for abscesses, hepatitis, tonsillitis, ulcers, etc. Recent research suggests that it may also have some anti-cancer activity. We have extracted five pure compounds from this plant; two known flavonols without bio-activity, one known isocoumarin glucoside that exhibits some cytotoxic activity toward HeLa cervical cancer cells, and two novel compounds that show considerable cytotoxic activity toward HeLa cells. (Submitted March 2007) In collaboration with: Steven A. Fleming Department of Chemistry and Biochemistry Brigham Young University Provo. Utah. 84602 In the previous paper, compounds A and B were found to be have very similar structures where the only difference lies in one of the side-chains (A = isopropyl and B = propyl). We have further investigated the possible source of the complexity observed in the NMR spectra of these two compounds A and B. The X-ray crystallographic data for the compounds show that both exist in the solid state as one stable structure, whereas in the liquid state it appears that multiple structures exist for both compounds. There are eight possible tautomers for both compounds (figure 1) and the relative energies of the tautomers were calculated using Dewar's AM1 semi empirical method. In both instances tautomer 1 (shown encircled) was found to be the most stable. Ab initio calculations using Gausian 98 B3LYP with a 631G basis set carried out on tautomers 1 and 4 (with both side groups replaced by methyl groups) gave O--H bond distances (Å) for the hydrogen bonded segments as shown in figure 2. Upon energy minimization, both tautomers relaxed to the same structure with bond parameters that were in close agreement with those derived from the X-ray data of the most stable structures. If tautomerism were the main cause of the spectral complexity, then decreasing the temperature should result in a preponderance of the most stable form at the expense of the higher energy tautomers, and consequently the NMR spectra should simplify. As described in the main paper the opposite was observed to occur for both compounds A and B. On the other hand, if the spectral complexity was due to rotational isomerism arising from the side-chains, then as the temperature is lowered, so rotation about C--C single bonds becomes more hindered and the possibility increases of stabilizing different conformations with a concomitant increase in spectral complexity. This conclusion is in harmony with the fact that the X-ray data strongly suggests large thermal motion on atoms associated with the side-chains in both compounds.