Organic synthesis using samarium diiodide

Abstract

The usefulness of $SmI_2$ as a versatile reagent in organic synthesis has been demonstrated by its widespread application for reduction of various functional group, as well as its mediation of a range of unusual reductive cyclization and intermolecular coupling reactions. α- Bromoesters were transferred to β- ketoesters by $SmI_2$ under mild reaction conditions. This strategy can provide an alternative to other more common ester condensation reactions such as the claisen condensation, avoiding the strongly basic or carbaionic conditions normally employed to prepare β- ketoester. α- Bromo siloxy derivatives were cyclized to β-hydroxy cyclic silyl ethers by $SmI_2$ in the presence of HMPA. As another Babier type reaction, the cyclization gave useful heterocyclic compounds of which the syn isomers were obtained as major product of the two possible diastereoisomers. The syn configuration was determined by NOE experiment. Cyclic silyl ethers were obtained in good yields and could be transferred to various useful substrates which can be used in natural products synthesis. Isothiocyanates and isocyanates are also reactive to $SmI_2$. Isothiocyanates were reduced to thioform amides in excellent yields. While isocyanates were coupled with t-alcohol(proton source) and resulted in carbamates in the presence of catalytic amount of $SmI_2$. Iso(thio) cyanates were coupled with unsaturated ester or carbonyl compounds. When halo epoxides were treated with 2 equivalent of $SmI_2$, the cyclopropanol or allylic alcohol were obtained. Aryl substituted epoxides gave the cyclopropanols in the presence of HMPA. On the other hand, the same reactions without HMPA gave the rearranged allyl alcohols. However, alkyl substituted halo epoxides gave only rearranged allyl alcohols irrespective of HMPA. The reaction provide a new method of synthesis for cyclopropanol or allyl alcohols. Epoxides were transferred to the trans iodohydrines by $SmI_2$ with the additive α- bromoacetate or uns...