Microbial Biotransformation of 1-Methyl-<scp>L</scp>-tryptophan into Herbicidal Indole Alkaloids by Endophytic Fungus <i>Nigrospora chinensis</i> GGY-3

Indole alkaloids are privileged secondary metabolites, and their production may be achieved by the microbial biotransformation of tryptophan analogues. By feeding 1-methyl-L-tryptophan (1-MT) into the culture of endophytic <i>Nigrospora chinensis</i> GGY-3, six novel (<b>1</b>-<b>6</b>) and seven known indole alkaloids (<b>7</b>-<b>13</b>) were generated. Their structures were elucidated by means of NMR spectroscopy, experimental electronic circular dichroism (ECD) spectra, and X-ray crystallography analysis. A Friedel-Crafts reaction was proposed as the key reaction responsible for the formation of the new compounds. Racemates <b>4</b> and <b>6</b> were separated into isomers by chiral HPLC, with their absolute configurations determined by X-ray and ECD calculation. Compounds <b>3</b>, <b>4</b>, and <b>8</b> display good herbicidal activity against dicotyledon weed <i>Eclipta prostrata</i>, of which <b>4</b> and <b>8</b> exhibited 88.50% and 100% inhibition rates on the radicle at 200 μg/mL, respectively, a similar effect compared to the positive control penoxsulam.