Efficient Synthesis of the N‐(buta‐2,3‐dienyl)carboxamide of Isopimaric Acid and the Potential of This Compound towards Heterocyclic Derivatives of Diterpenoids

The N‐(2,3‐butadienyl)carboxamide of isopimaric acid, that is, compound 3, was prepared through a two‐step synthetic procedure starting from the natural diterpene isopimaric acid. The Pd‐catalyzed cross‐coupling and subsequent cyclization of terpenoid allene 3 with several aryl iodides and aryl bromides gave access to optically active diterpenoid–oxazoline derivatives in good to excellent yields. The functional group tolerance in the aryl iodides was demonstrated by several examples, including substrates with additional N‐tert‐butoxycarbonyl‐protected amino, hydroxy, and carboxy substituents in the ortho position. The cross‐coupling–cyclization reaction of those compounds with allene 3 proceeded selectively with the formation of cyclization products on the substituent in the aromatic ring. This transformation opens a potential route to the synthesis of hybrid compounds containing a tricyclic diterpenoid and several heterocycles.