Synthesis of (±)-Allocyathin B<sub>2</sub> and (+)-Erinacine A

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