Novel Bisamide Alkaloids Enantiomers from Pepper Roots (<i>Piper nigrum L.</i>) with Acetylcholinesterase Inhibitory and Anti-Neuroinflammatory Effects

Abstract

The roots of <i>Piper nigrum L.</i>, a seasoning for cooking various types of broths, are renowned for their high nutritional content and potential medicinal benefits. In this study, nine pairs of novel cyclohexene-type bisamide alkaloids (<b>1a</b>/<b>1b-9a</b>/<b>9b</b>) were isolated from the pepper roots using molecular network analysis strategies. Their structures were determined by extensive spectroscopic data, electronic circular dichroism (ECD) calculations, and X-ray diffraction analyses. Using an intermolecular Diels-Alder reaction, a strategy for the synthesis of bisamide alkaloids from different monomeric amide alkaloids was developed. Furthermore, these compounds were chirally separated for the first time, and compounds <b>3a</b> and <b>5a</b>/<b>5b</b> showed significant anti-neuroinflammation effects in the models of lipopolysaccharide(LPS)-induced BV2 microglial cells. Meanwhile, compounds <b>6b</b> and <b>7a</b> displayed concentration-dependent inhibitory activities against acetylcholinesterase with IC₅₀ values of 6.05 ± 1.10 and 3.81 ± 0.10 μM, respectively. These findings confirmed that these bisamide alkaloids could be applied in functional food formulations and pharmaceutical products as well as facilitate the further development and usage of pepper roots.