Summary: | A new lupane caffeoyl ester, lup-20(29)-ene 3β-caffeate-30-al (<b>7</b>), and a new oleanane-type triterpene, 3<i>β</i>-hydroxyolean-13(18)-en-12-one (<b>17</b>), were isolated from the aerial parts of <i>Dobera glabra</i> (Forssk), along with ten known triterpenes, including seven lupane-type lupeol (<b>1</b>), 30-nor-lup-3<i>β</i>-ol-20-one (<b>2</b>), ∆<sup>1</sup>-lupenone (<b>3</b>), lup-20(29)-en-3<i>β</i>,30-diol (<b>4</b>), lupeol caffeate (<b>5</b>), 30-hydroxy lup-20(29)-ene 3<i>β</i>-caffeate (<b>6</b>), and betunaldehyde (<b>8</b>); three oleanane-type compounds were also identified, comprising <i>δ</i>-amyrone (<b>15</b>), <i>δ</i>-amyrin (<b>16</b>), and 11-oxo-<i>β</i>-amyrin (<b>18</b>); together with six sterols, comprising <i>β</i>-sitosterol (<b>9</b>), stigmasterol (<b>10</b>), 7<i>α</i>-hydroxy-<i>β</i>-sitosterol (<b>11</b>), 7<i>α</i>-hydroxy-stigmasterol (<b>12</b>), 7-keto-<i>β</i>-sitosterol (<b>13</b>), and 7-keto-stigmasterol (<b>14</b>). Their structures were elucidated using a variety of spectroscopic techniques, including 1D (<sup>1</sup>H, <sup>13</sup>C, and DEPT-135 <sup>13</sup>C) and 2D (<sup>1</sup>H–<sup>1</sup>H COSY, <sup>1</sup>H–<sup>13</sup>C HSQC, and <sup>1</sup>H–<sup>13</sup>C HMBC) nuclear magnetic resonance (NMR) and accurate mass spectroscopy. Subsequently, the different plant extracts and some of the isolated compounds (<b>1–9</b>, <b>11</b> and <b>13</b>) were investigated for their possible cytotoxic activity in comparison to cisplatin against a wide array of aggressive cancer cell lines, such as colorectal cancer (HCT-116), hepatocellular carcinoma (HepG-2), and prostate cancer (PC-3) cell lines. Compound <b>11</b> displayed broad cytotoxicity against all of the tested cell lines (IC<sub>50</sub> ≅ 8 µg/mL in all cases), and a high safety margin against normal <i>Vero</i> cells (IC<sub>50</sub> = 70 µg/mL), suggesting that <b>11</b> may be a highly selective and effective anticancer agent candidate. Notably, the evidence indicated that the mode of action of compound <b>11</b> could possibly consist of the inhibition of phosphodiesterase I (80.2% enzyme inhibition observed at 2 µM compound concentration).
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