New aporphine alkaloids from the aerial parts of <i>Piper semiimmersum</i>^§

<p>Two new aporphine alkaloids, semiimmersumines A (<b>1</b>) and B (<b>2</b>), along with 20 known compounds, were isolated from the aerial parts of <i>Piper semiimmersum</i> (Piperaceae). The structures of the new compounds were elucidated based on the analysis of 1D and 2D NMR, MS, and CD data. The absolute configuration of semiimmersumine A (<b>1</b>) was determined by single crystal X-ray diffraction analysis using anomalous dispersion with copper radiation. The effects of all compounds from the plant on rabbit platelet aggregation induced by thrombin (IIa) or PAF were also evaluated.</p