Single samarium diiodide

Into a solution of residue 59 (101 mg, 0.1 mmol) in 20 mL of dry toluene, kept at 60°C, was syringed, during 18 h and under argon, a freshly prepared solution of samarium diiodide in benzene-HMPA (9 1, v/v 6.3 mL, 0.51 mmol) which has been diluted with 3.8 mL of dry benzene. The solvents were distilled off under reduced pressure, and the residue was taken up in 10 mL of diethyl ether. The ether solution was washed with 10% aqueous solution of sodium bisulfite, then water, dried (MgS04), and concentrated. The crude product was dissolved in 1.5 mL of tetrahydrofuran and treated during 30 min at room temperature with 1.5 mL of a 40% aqueous solution of HF. The solution was neutralized with solid sodium carbonate, and concentrated. Flash chromatography on silica gel (cyclohexane-ethyl acetate, 3 1 to 1 2) afforded the product 80 (40.6 mg, 50%), a single isomer, as an amorphous solid. It was characterized by its diacetate [a]D +36° (c 4.0, CHClj). 

Exposure of the dioxanobornane 1055 to samarium diiodide in the presence of samarium metal at low temperature leads to the formation of a mixture of the tetrahydropyran-4-one 1056 and the ring opened product 1057. Treatment of this mixture with tosic acid leads to a single tetrahydropyran-4-one product (Scheme 269) <2004OL3735>. 

Compared with the numerous efficient syntheses based on nucleophilic/electrophilic or concerted (c/. Section 4.2.4) cyclization reactions of carbonyl derivatives, very little synthetic applications have been described that rely on radical-induced ring closure reactions, such as the reductive cyclization of 8-un-saturated iminium salts like (129) using samarium diiodide, which affords (132) as the only bicyclic product (Scheme 62). 2 The reaction is believed to proceed by single electron transfer, going through the a-amino radical (130), which cyclizes in an exocyclic mode to give the denoted product (132) via the benzylic carbon radical (131). 

Cyclopentanoids have been prepared by samarium diiodide-catalysed reaction of 8 (R-Tbdms) with either aldehydes or ketones. The stereochemical outcome was dependent on the carbonyl compound used, with cyclohexanone giving 9 as a single product, while cyclohexane carboxaldehyde gave an isomeric mixture containing predominantly 10. ... 

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