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

Frutescone A–G [(1–6), (+)-7, (−)-7], a new group of naturally occurring tasmanone-based meroterpenoids, were isolated from the aerial parts of Baeckea frutescens L. Compounds 1 and 4 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 4–7 were achieved in a concise manner by a biomimetically inspired key hetero-Diels–Alder reaction “on water”. Compounds 1, 4, and 5 exhibited moderate cytotoxicities in vitro.