Quantitative analysis of benzofuran neolignans from Piper rivinoides using nuclear magnetic resonance

In order to characterize a sample obtained from an important medicinal plant we applied nuclear magnetic resonance to access quantitative data of its bioactive compounds. Piper rivinoides has been a focus of our interests due to the disclosed vasodilator activity found for its extracts and purifies compounds. Following an usual trend for Piper species we were able to identify four benzofuran neolignans, namely Eupomatenoid V, Eupomatenoid VI, (2R,3R)-2,3-dihydro-2-(4-hydroxy-3-methoxyphenyl)-3-methyl-5-(E)-propenylbenzofuran and Conocarpan. In this paper we aim to characterize the bioactive fractions and the raw extract under quantitative analysis. A fundamental feature of NMR spectroscopy is that the intensity of a single resonance obtained will have direct relation with the number of spins and so with the concentration. Thus, since each of the intensity interfering parameter is optimized the integrated signal becomes directly correlated to the concentration of a certain compound. Then, key known resonances from Eupomatenoid VI (1 H, d 3JHH = 8.18 Hz, 7.66 ppm; H-6'), Eupomatenoid V (1 H, s, 7.37 ppm; H-6) and Conocarpan (1 H, d JHH = 8.10 Hz, 6.76 ppm; H-3) related to one proton each were found to be isolated, and so, capable of being quantified. The integrals of each of the selected resonances were used as their absolute values for quantitative calculations; m^analito=(I^analito/I^padrão) * (MM^analito/MM^padrão) * (N^analito/N^padrão) * (1/m^padrão). Dimethylbenzaldehyde resonance at 9.85 ppm (aldehyde proton, 1 H, s) was used as internal standard. The results found with basis on the equation (1) show that the evaluated extract contains 15.29% of Eupomatenoid VI, 29.81% of Eupomatenoid V, 5,52% of Conocarpan; percentages were calculated for the total powdered and dry plant material. By means of two signals from 6.15 ppm to 6.40 ppm related to two typical protons of benzofuran neolignans we could define to total benzofuran neolignans concentration in the sample.

Fig. 1