Synthesis of (±)-Allocyathin B₂ and (+)-Erinacine A

(±)-Allocyathin B₂ (2) and (+)-erinacine A (4), the 1-β-d-xyloside of (+)-allocyathin B₂, the first cyathin diterpenes to be prepared, have been synthesized using a carbonyl ene reaction of 14a to construct an appropriately functionalized seven-membered ring and palladium-catalyzed carbonylation of dienyl triflates 10 and 39 as key steps. The entire cyathin carbon skeleton is constructed in seven steps, and allocyathin B₂ is synthesized in only 17 steps (>5% overall yield) from readily available enone 9.