Samarium diiodide regioselectivity

Moreover, propargyl oxiranes 202 were found to react with samarium diiodide and ketones to form a,a -dihydroxyallenes 203 with moderate to high anti-diastereo-selectivities (Scheme 2.62). Aurrecoechea and co-workers [99] reported this reductive coupling to proceed smoothly in the absence of a palladium catalyst, i.e. a direct electron transfer from the samarium(II) to the substrate has to take place in order to generate an allenyl/propargyl samarium intermediate of type 184/185, which is then regioselectively trapped by the electrophile. 

There exists also a synthesis of cyclopentadecanone (VII/81) and ( )-mus-cone, based on a three-carbon annulation of cyclic ketones followed by the regioselective radical cleavage of the zero bridge of the so formed bicyclic system [44], The synthesis of cyclopentadecanone is summarized in Scheme VII/16. The cyclization of VII/78 to the bicyclic alcohol VII/79 proceeds best (94 % yield) with samarium diiodide in the presence of hexamethylphosphoric acid triamide and tetrahydrofuran [45], The oxidative cleavage of VII/79 to the ring expanded product VII/80, was performed by treatment with mercury(II)-oxide and iodine in benzene, followed by irradiation with a 100 Watt high pressure mercury arc. Tributyltinhydride made the de-iodination possible. 

The reduction of benzo-fused bicyclic sulfonium ions with samarium diiodide also results in the cleavage of the —C bonds. However, as shown in Scheme 9, the regioselectivity here depends on... 

---