Eragrostis plana Nees (Poaceae) are invasive plants and it is believed that allelopathic interactions have guaranteed the dominance of this species on other plants. In addition, it is difficult to be controlled by commercial herbicides,1 which raise the interest for its phytochemical study aiming to achieve a generation of a new bio-herbicide. The objective of this project is to evaluate the allelopathic potential of root extracts of E. plana Nees, besides isolating and identifying the secondary metabolites of the extract with better allelopathic activity. The allelopathic tests showed that the petroleum ether extract from the roots increased the TMG (mean germination time) and decreased the rootlet growth, causing a negative effect in the seeds of both receptor species (Ipomoea grandifolia and Euphorbia heterophylla).2 Based on these results,the petroleum ether extract was purified by column chromatography with gradient solvent (petroleum ether, ethyl acetate and methanol), which resulted in 155 fractions that were grouped in 14 new ones. Until this moment, four novel diterpenes were identified, neocassane-type skeleton,3 and their structures were established by using 1D, 2D and NOE NMR spectroscopy. The 1H NMR spectrum of 1 showed four singlets attributed to methyl groups (1.07; 1.09; 1.10 and 2.04 ppm), one terminal double bond at 6.40 ppm (dd, 11.64 and 17.79 Hz), 5.39 ppm (dd, 2.12 and 11.64 Hz) and 5.46 ppm (dd, 2.12 and 17.92 Hz) and in the 13C NMR spectrum, two carbonyl groups (199.88 and 216.27 ppm). The HMBC, COSY and HSQC correlations confirmed the skeleton and the NOE experiments established the relative configuration as trans, trans, trans for ring junctions. The absolute configuration of 1 was determined as 5R,8R,9S,10R, using vibrational CD and DFT calculations at the B3PW91/6-311G(d,p) level. The other three compounds are very similar to 1 by comparison of the respective NMR data, differing by the number of carbonyl groups or by the replacement of the carbonyl by a hydroxyl group. The allelopathic activity of these diterpenes will be evaluated.
1FIORENZA, et al. Série Botânica, v. 71, p. 193-200, 2016.
2 HENDGES, et al. 6TH BCNP, Anais Eletrônicos, 2017.
3 FAVARETTO, et al. J. of Agric. and Food Chem., v. 67, p. 1973-1981, 2019.