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.