Bioactive A-Type Proanthocyanidins from <i>Cinnamomum cassia</i>

Two trimeric proanthocyanidins, cinnamtannin B-1 (<b>1</b>) and cinnamtannin D-1 (<b>2</b>), have been isolated from the bark of <i>Cinnamomum cassia</i> along with the known tetramer parameritannin A-1 (<b>3</b>) and a previously unreported tetramer, named cassiatannin A (<b>4</b>). The structures of <b>1</b>–<b>4</b> were elucidated on the basis of 1D and 2D NMR, MS, and CD analyses and compared to the reported data. Proanthocyanidins (<b>1</b>–<b>4</b>) possess significant in vitro inhibitory activity against cyclooxygenase-2 (COX-2) at micromolar concentrations.