Total synthesis of (+)-intricarene using a biogenetically patterned pathway from (-)-bipinnatin J, involving a novel transannular [5+2] (1,3-dipolar) cycloaddition

An asymmetric synthesis of the furanobutenolide-based macrocyclic diterpene (-)-bipinnatin J (4a) isolated from the gorgonian octocoral Pseudopterogorgonia bipinnata is described. The synthesis is based on elaboration of the chiral lactone-substituted vinyl iodide 26b from (+)-glycidol, followed by an intermolecular Stille coupling reaction with the stannylfurfural 27, leading to 28a, and then an intramolecular Nozaki-Hiyama-Kishi allylation reaction, 28b→4a. Treatment of (-)-bipinnatin J (4a) with VO(acac) ₂-tBuO₂H followed by acetylation of the tautomeric hydroxypyranone product 7/8, next gave the acetoxypyrone 30. When the acetoxypyranone 30 was heated in acetonitrile in the presence of DBU, it gave (+)-intricarene 1, which is found in P. kallos, via a novel transannular [5+2] (or 1,3-dipolar) cycloaddition involving the butenolide-oxidopyrylium ion intermediate 31. We believe that this total synthesis of (+)-intricarene 1 mimics its most likely origin in nature via oxidation of (-)-bipinnatin J (4a), presumably involving photochemically generated singlet oxygen or possibly a P450 monooxygenase enzyme system.