Highly Functionalized Spirobisnaphthalenes from <i>Roussoella</i> sp. KT4147

Highly functionalized spirobisnaphthalenes, preussomerins N (<b>1</b>) and O (<b>2</b>), and simpler compounds, such as 2,3-α-epoxypalmarumycin CP₁₈ (<b>3</b>), 3α-hydroxy-CJ-12,372 (<b>4</b>), and 16 known structurally related congeners, were isolated from a culture broth of <i>Roussoella</i> sp. KT4147. Structural analysis revealed that <b>1</b> was a dimer of preussomerin G (<b>6</b>), connected by a nitrogen atom, and <b>2</b> was a derivative of <b>6</b> with a macommelin substructure. Preussomerin N (<b>1</b>) was considered to be biosynthetically derived via the Michael-type 1,4-addition of ammonia to <b>6</b>, followed by another Michael addition to another molecule of <b>6</b>. Contrarily, <b>2</b> was suggested to be derived through an <i>endo</i>-Diels-Alder cycloaddition between a diene derived from the (<i>E</i>)-enol form of macommelinal via an <i>ene</i>-reaction and dienophile <b>6</b>. Compounds <b>1</b> and <b>2</b> exhibited potent cytotoxicity against COLO-201 human colorectal cancer cells.