Scopadulan diterpenes

Our laboratory has also taken advantage of multiple insertion processes to access the scopadulan diterpenes (Scheme 6-24) [52]. An early example was the economical conversion of aryl iodide 136 to tetracycle 137, an intermediate which was subsequently converted to ( )-scopadulcic acid B (138). In this conversion, migratory insertion of the neighboring exocyclic alkene provided a tricyclic intermediate having no /5-hydrogens, which subsequently underwent a transannular 5-exo Heck reaction with the trisubstituted... 

The bis-E ck. cyclization of trienyl iodides has been applied as a key reaction by Overman to total syntheses of stemodane diterpenes and scopadulcic acid. Treatment of the 6R, %R epimer of the trienyl iodide intermediate 259 with Pd(dppb) in the presence of Ag2C03 in DMA provided two tricyclic products 260 (40 %) and 261 (25 %). The major product 260 has the stemodane skeleton [110]. On the other hand, w-cyclization of the 65, 8i epimer of the trienyl iodide 262 proceeded with complete stereo- and regioselectivity as shown by 263 and 264 to afford the tricycle of the scopadulan skeleton 266, from which total synthesis of scopadulcic acid A (267) was achieved. In the second cyclization as shown by 264, generation of the cyclohexylpalladium intermediate 265, having Pd attached to the secondary carbon center is favored to give 266 selectively. Formation of a stemodane skeleton is expected from the 6R, %R epimer [111]. 

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