| Summary: | A methanol extract of the flowers of <i>Mammea siamensis</i> (Miq.) T. Anders. (Calophyllaceae) showed anti-proliferative activity against human prostate carcinoma LNCaP cells (IC<sub>50</sub> = 2.0 µg/mL). Two new coumarin-related polysubstituted benzofurans, mammeasins P (<b>1</b>) and Q (<b>2</b>), and a known polysubstituted coumarin mammea B/AC cyclo F (<b>39</b>) were isolated from the extract along with 44 previously reported polysubstituted coumarin constituents (<b>3</b>–<b>38</b> and <b>40</b>–<b>47</b>). The structures of two new compounds (<b>1</b> and <b>2</b>) were determined based on their spectroscopic properties derived from the physicochemical evidence including NMR and MS analyses and taking the plausible generative pathway into account. Among the coumarin constituents, mammeasins A (<b>3</b>, IC<sub>50</sub> = 1.2 µM) and B (<b>4</b>, 0.63 µM), sugangin B (<b>18</b>, 1.5 µM), kayeassamins E (<b>24</b>, 3.0 µM) and G (<b>26</b>, 3.5 µM), and mammeas E/BA (<b>40</b>, 0.88 µM), E/BB (<b>41</b>, 0.52 µM), and E/BC (<b>42</b>, 0.12 µM) showed relatively potent anti-proliferative activity.
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