Kagan-Molander Samarium Diiodide-Mediated Coupling

KAGAN-MOLANDER SAMARIUM DIIODIDE-MEDIATED COUPLING 

Samarium diiodide is a one electron reductant that is capable of reducing both alkyl halides and carbonyl compounds. The rate of the reduction depends on the nature of the substrate and the reaction conditions. The mechanism of the addition of alkyl halides to carbonyls was extensively studied. In case of the samarium Grignard processes, it was concluded that the reaction proceeds through an organosamarium intermediate. However, the mechanism of the samarium Barbier processes is not fully understood and there is no unambiguous evidence in favor of any of the possible pathways. 

The ABC ring system of the carbocyclic skeleton of variecolin, a sesterterpenoid natural product was accomplished by G.A. Molander and co-workers. In their approach, they utilized two samarium diiodide mediated processes. First, a primary alkyl iodide was reacted with a ketone substrate in the presence of two equivalents of samarium diiodide and catalytic nickel(ll) iodide under samarium Grignard conditions. Subsequent oxidation and lactone formation provided the chlorolactone substrate. As alkyl chlorides are less reactive than alkyl bromides and iodides, the second samarium diiodide mediated process, an intramolecular nucleophilic acyl substitution, required visible light irradiation. 

The research group of T. Nakata developed a convergent synthesis for the construction of a frans-fused 6-6-6-6-membered tetracyclic ether ring system, a subunit, which is present in several polycyclic marine ether natural products. Late in their synthesis, they utilized a samarium diiodide mediated nucleophilic acyl substitution as the key step to form one of the tetrahydropyran rings. 

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KAGAN-MOLANDER SAMARIUM DIIODIDE-MEDIATED COUPLING... 

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