Nucleophilic acylation of esters by acid chlorides mediated by samarium diiodide: Formation and use of samarium enediolates

Synthesis of α-Aminosilanes by 1,2-Metalate Rearrangement Deoxygenative Silylation of Aromatic Amides

An efficient nickel-catalyzed deoxygenative silylation reaction of aromatic amides with silylboranes in the presence of a Sm/SmI₂ system for the construction of α-aminosilanes is described. This strategy provides a direct method for synthesizing α-aminosilanes with high efficiency and good functional group compatibility and includes readily accessible starting materials and valuable products.

Total synthesis of (-)-kainic acid and (+)-allo-kainic acid through SmI2-mediated intramolecular coupling between allyl chloride and an α,β-unsaturated ester

A 3,4-disubstituted pyrrolidine ring was effectively cyclized through SmI₂-mediated reductive coupling between allyl chloride and an α,β-unsaturated ester, although little has been reported about SmI₂-promoted C-C bond formation of an allyl chloride with an α,β-unsaturated ester. Selection of either the 3,4-cis- or 3,4-trans-selective cyclization can be accomplished simply by changing the additives from NiI₂ to HMPA during reductive cyclization conducted in H₂O-THF. Total synthesis of (-)-kainic acid and (+)-allo-kainic acid, which are pyrrolidine alkaloids used in neuroscience and neuropharmacology as useful molecular probes, was successfully achieved by using the stereo-complementary ring closure reactions promoted by SmI₂ for the construction of the 2,3,4-trisubsituted pyrrolidine scaffold of kainoids.

Catalytic Ni(II) in reactions of SmI2: Sm(II)- or Ni(0)-based chemistry?

The addition of catalytic amounts of Ni(II) salts provide enhanced reactivity and selectivity in numerous reactions of SmI(2), but the mechanistic basis for their effect is unknown. We report spectroscopic and kinetic studies on the mechanistic role of catalytic Ni(II) in the samarium Barbier reaction. The mechanistic studies presented herein show that the samarium Barbier reaction containing catalytic amounts of Ni(II) salts is driven solely by the reduction of Ni(II) to Ni(0) in a rate-limiting step. Once formed, Ni(0) inserts into the alkyl halide bond through oxidative addition to produce an organonickel species. During the reaction, the formation of colloidal Ni(0) occurs concomitantly with Ni(0) oxidative addition as an unproductive process. Overall, this study shows that a reaction thought to be driven by the unique features of SmI(2) is in fact a result of known Ni(0) chemistry.

Exerting control over the acyloin reaction

A synthetic method for conducting the acyloin reaction using electron transfer in solution is reported. By linking two esters via their oxygen atoms, it was possible to perform crossed acyloin reactions between two different ester functionalities and display a high degree of preference for an intramolecular coupling process.

Samarium diiodide mediated reactions in total synthesis

Introduced by Henri Kagan more than three decades ago, samarium diiodide (SmI(2)) has found increasing application in chemical synthesis. This single-electron reducing agent has been particularly useful in C-C bond formations, including those found in total synthesis endeavors. This Review highlights selected applications of SmI(2) in total synthesis, with special emphasis on novel transformations and mechanistic considerations. The examples discussed are both illustrative of the power of this reagent in the construction of complex molecules and inspirational for the design of synthetic strategies toward such targets, both natural and designed.

One-pot, high yield synthesis of alpha-ketols from Delta5-steroids

Alpha-hydroxy ketones (alpha-ketols) are present in many compounds with biological activity. Several methods are available for the preparation of alpha-ketols but only a few of them describe the synthesis of steroid alpha-ketols from olefins. In this work, a new system consisting of KMnO(4)/Fe(ClO(4))(3).nH(2)O was used in order to achieve the direct conversion of Delta(5)-steroids to their corresponding alpha-ketols, in high yields. Consideration of the probable reaction mechanism is provided. 2D homo- and heteronuclear correlation NMR spectroscopic techniques were used to assign (1)H and (13)C resonances of some synthesized compounds. This method has potential for the preparation of alpha-hydroxy ketones of biological interest.

Highly regioselective cleavages and iodinations of cyclic ethers utilizing SmI2

Various functionalized cyclic ethers such as oxiranes, oxetanes, and tetrahydrofurans have been prepared, and the regiochemistry of their ring opening with samarium diiodide and acyl chloride or anhydride has been investigated. Alkyl-substituted oxetane 5 and tetrahydrofurans 1 and 2 show almost no regioselectivity. However, high regioselectivities from the branched cyclic ethers (3, 8, 9, and 10) containing ethereal or hydroxyl moieties have been observed. This is probably the result of the bidentate chelated species between samarium and oxygen.