Labdane-Type Diterpenoids from <i>Leonurus japonicus</i> and Their Plant-Growth Regulatory Activity

Abstract

Thirteen new labdane-type diterpenoids <b>1</b>-<b>6</b>, <b>9</b>-<b>11</b>, <b>13</b>, <b>14</b>, <b>18</b>, and <b>19</b> and seven known ones were isolated from the aerial parts of <i>Leonurus japonicus</i>. Compounds <b>1</b>-<b>5</b> represent rare examples of labdane-type diterpenoids, of which compounds <b>1</b>-<b>4</b> carry an <i>N</i>-chain linked at C-7 in their B-ring and compound <b>5</b> featured an α,<i>β-</i>unsaturated<i>-γ-</i>lactam moiety. The structures and absolute configurations of these new diterpenoids were characterized by a combination of spectroscopic techniques, X-ray crystallography, electronic circular dichroism, and calculated specific rotations. The plant-growth regulatory activity of these compounds on the growth of the roots and shoots of <i>Lactuca sativa</i> and <i>Lolium perenne</i> seedlings were evaluated. Compound <b>3</b> showed a broad-spectrum inhibitory activity with the inhibition rates ranging from 60 to 83.5% at a concentration of 200 μg/mL, which were as active as those of glyphosate. Compound <b>8</b> had a selective inhibitory activity against the growth of the roots of <i>L. perenne</i> seedlings with an inhibition rate of 81.7%. However, compounds <b>11</b> and <b>16</b> exhibited significant stimulation effects on the roots of <i>L. sativa</i> with stimulation rates of 59.8 and 65.3%, respectively. In addition, compounds <b>3</b> and <b>8</b> exhibited inhibitory effects on the germination of <i>L. perenne</i> seeds.