A Concise Bioinspired Semisynthesis of Rumphellaones A–C and Their C-8 Epimers from β-Caryophyllene

The first semisynthetic route toward rumphellaones B ( 2 ) and C ( 3 ) and their C-8 epimers as well as the shortest synthesis of rumphellaone A ( 1 ) and its C-8 epimer from the most accessible sesquiterpene, β-caryophyllene ( 4 ), is presented. Synthetic routes involved caryophyllonic acid as a key intermediate, which was converted to rumphellaone A (and epimer) via acid-catalyzed lactonization and rumphellaone C (and epimer) using one-pot epoxidation–lactonization. Rumphellaone B ( 2 ) and its epimer were obtained from rumphellaone A ( 1 ) and its epimer, respectively, using Saegusa–Ito oxidation. The absolute configuration at C-8 was confirmed by single-crystal X-ray analysis of rumphellaone B ( 2 ) and an acylated derivative of rumphellaone C.