Frutescone A–G, Tasmanone-Based Meroterpenoids from the aerial parts of <i>Baeckea frutescens</i>

Frutescone A–G [(<b>1</b>–<b>6</b>), (+)-<b>7</b>, (−)-<b>7</b>], a new group of naturally occurring tasmanone-based meroterpenoids, were isolated from the aerial parts of <i>Baeckea frutescens</i> L. Compounds <b>1</b> and <b>4</b> featured a rare carbon skeleton with an unprecedented oxa-spiro[5.8] tetradecadiene ring system, existing as two favored equilibrating conformers in CDCl₃ solution, identified by variable-temperature NMR. The regioselective syntheses of <b>4</b>–<b>7</b> were achieved in a concise manner by a biomimetically inspired key hetero-Diels–Alder reaction “on water”. Compounds <b>1</b>, <b>4</b>, and <b>5</b> exhibited moderate cytotoxicities in vitro.