Ring expansion and reductive cyclizations using $SmI_2$

Abstract

The usefulness of $SmI_2$ as a versatile reagent in organic synthesis has been demonstrated by its widespread application for the reduction of various functional groups. Development of novel reactions using $SmI_2$ is noteworthy. Treatment of cyclic α-halomethyl β-keto ester with $SmI_2$ in the presence of HMPA or catalytic amount of $NiI_2$ affords one carbon-expanded γ-keto ester. Synthesis of large-sized rings is also possible via this method using existing template. Only one example was observed for the chain extension reaction. Synthesis of thiolactams is usually relied on thionation of corresponding lactams with thionating reagents. Intramolecular reductive cyclization of β-ketoisothiocyanate by samarium diiodide affords the method of direct synthesis of α -hydroxythiolactams or thiolactam in high yields. The reaction proceeded from the addition of ketyl to isothiocyanate group to give α-hydroxythiolactams, which is followed by deoxygenation of tertiary alcohol by $SmI_2$ to afford thiolactams. The cyclizations took place with high stereoselectivities. Cyclization of α-phosphonovinyl radicals using $nBu_3SnH$ or $SmI_2$ affords the 5-membered ring and the 6-membered ring via 5-exo/6-endo cyclization. When $SmI_2$ was used in the cyclization, the reactivities and selectivities are increased than using $nBu_3SnH$ for 11a and 11b. However, in the case of 11c the cyclization did not occur and only deiodination was observed.