Phloroglucinol Derivatives with Unusual Skeletons from <i>Cleistocalyx operculatus</i> and Their <i>in Vitro</i> Antiviral Activity

Four novel phloroglucinol derivatives (<b>1</b>–<b>4</b>) featuring a 2,4-dimethyl-cinnamyl-phloroglucinol moiety, along with their putative biosynthetic precursors <b>5</b> and <b>6</b>, were isolated from the leaves of <i>Cleistocalyx operculatus</i>. Compounds <b>1</b> and <b>2</b> are two pairs of new enantiomeric phloroglucinol dimers possessing an unprecedented polycyclic skeleton with a highly functionalized dihydropyrano­[3,2-<i>d</i>]­xanthene tetracyclic core. Compounds <b>3</b> and <b>4</b> are two new phloroglucinol-terpene adducts (PTAs) with a novel carbon skeleton. The structures of <b>1</b>–<b>4</b> including their absolute configurations were unambiguously accomplished by combination of extensive spectroscopic analyses, X-ray crystallography, and quantum chemical ECD calculations. A hypothetical biosynthetic pathway for <b>1</b>–<b>4</b> was also proposed. Compound <b>1</b> exhibited a promising <i>in vitro</i> antiherpes simplex virus type-1 (HSV-1) effect.