Ring opening and expansion reaction mediated with samarium diiodide

Abstract

The usefulness of samarium diiodide as a versatile reagent in organic synthesis has been demonstrated by its widespread application for reduction of variety of functional groups, as well as its mediation of a range of reductive small ring cleavage and intramolecular coupling reactions. Another type of valuable reaction that alkyl-substituted epoxides rearranged into allyl alcohols was investigated. Alkyl-substituted α-halo epoxide reacted to 2.2mmol of $SmI_2$ in the presence of 6 equivalents of HMPA in THF to give the rearranged products in good yields. In the absence of HMPA, aryl-substituted α-halo epoxides also gave the rearranged allyl alcohols from the reaction with $SmI_2$. In contrast, they reacted with $SmI_2$ in the presence of HMPA in THF to give cyclopropanol which are very interesting molecules in organic synthesis due to their ability to be transformed into homoenolate derivatives. Two possible pathways could be considered for the mechanism of the reaction. Epoxy alkyl radical or epoxy alkyl anionic C-O bond cleavage could be suitable for the reaction of alkyl-substituted α-halo epoxides with $SmI_2$ and radical-radical coupling due to the effect of HMPA explain the results of the reaction of aryl-substituted α-halo epoxides with $SmI_2$. α-Iodomethyl cyclic β-ketoester reacted to $SmI_2$. 2 Equivalents of $SmI_2$ and 6 equivalent of HMPA are considered to be the mot adequate for ring expansion of α-iodomethyl cyclic β-ketoester. Ring expansion of 5-,7-membered ring to 6-,8-membered ring occurs smoothly in fairly good yields. This new procedure may provide many potential applications.