Samarium iodide, with alkenes

Thus cyclopentene reacts with tribromomethyl phenyl sulfone to give a high yield of addition product (equation 106). Samarium iodide catalysis gives a good yield of addition product upon reaction of l-chloro-2-iodoethanes with 1-alkenes (equation 107)690. The product from this reaction has been used to prepare a cyclic target molecule in several further steps. a-Bromoesters also undergo a similar reaction, the product from which upon treatment with KOH gives lactones directly. 

Samarium iodide has also been used for reducing epoxides to alkenes. Compared with other low-valent metal systems this reagent requires prolonged reaction time at room temperature, but the stereoselectivity, with c -epoxides giving m-alkenes, is high. " ... 

Samarium iodide promotes this addition reaction. " " In a related reaction, simple alkene units add to esters in the presence of sodium and liquid ammonia to give an alcohol.Structural variations in the alkene lead to different products. Homo-allylic alcohols react with aldehydes in the presence of Montmorillonite KSF clay to give 4-hydroxytetrahydropyrans. " A variation of this reaction converts an aryl aldehyde and a homoallylic alcohol to a 4-chlorotetrahydropyran in the presence of Homoallylic alcohols, protected as —O(CHMeOAc) react with... 

Synthesis of Alkenes by Reductive Elimination. The treatment of 2-halo-3-hydroxy esters and amides with samarium iodide leads to the corresponding di- or trisubstituted E)-a,p-unsaturated derivatives in high yields and diastereoselectivities (eqs 39 and 40). The precursors are readily accessible by condensation of the lithium enolate of a-haloesters or amides. If the substrate contains y,i5-unsaturation, the /3,y-unsaturated ester is generated in the process (eq 41). 

Samarium iodide can also be used as an alternative to sodium/ mercury amalgam for the reductive elimination of 1,2-acetoxy-sulfones in the Julia-Lythgoe olefination. The alkene is generated in a two-step process that first involves DBU or LDA treatment to generate a vinyl sulfone that is then reductively cleaved with samarium iodide (eq 44). The diastereoselectivity of both transformations is usually quite good and the method is compatible with the synthesis of monoalkenes as well as dienes and trienes. 

Addition of Vinylsamarium to Aldehydes. Treatment of (Z)-a-chloro-a,)3-unsaturatedketones with samarium iodide leads to the vinylsamarium reagent that can be trapped with aldehydes or ketones to produce Baylis-HUlman type adducts with inversion of stereochemistry at the alkene (eq 55). ... 

Intermolecular coupling of ketones and alkenes, promoted by SmH, occurs with excellent stereochemical control. In one such reaction, samarium(II) iodide has been used to prepare cyclobutanones and cyclobutanols from chiral, 6-oxohex-2-enoates (equation 137)520. The reaction is performed in THF in the presence of HMPT and occurs in good yield with excellent stereocontrol. If appropriately located carbonyl and alkene moieties are present in a molecule, then Sml2-HMPT can be used to form cyclooctanols by a radical cyclization process in some cases there is a reasonable degree of diastereoselectivity (equation 138)521,522. 

Since the intramolecular reactions are so much the best, others have linked the alkene and the alkyne by a weak bond that can later be sacrificed. This is the tether strategy you will meet in chapter 36. An N-0 bond is ideal and with the alkyne additionally blocked with a silyl group 124, good yields of Pauson-Khand product 125 and of the amino alcohol 126 could be achieved even with an amine A-oxide as promoter. Samarium(II) iodide was used as the reducing agent.35... 

Homolytic cycUzation of a bromo-alkene yields dthydropyrroles but bromo-alkynes by the same mechanism form a mixture of pyrrole and 3-methyIene-pyrrole in high yield (the ratio is not stated) [3434]. Aryl radicals produced by the action of samarium di-iodide on an aryl Iwomide react with an o-propynyl-amino (or propynyloxy—see Section 1.3) side-chain to form a new fus pyrrole (or furan) under mild conditions. The corresponding allylamino bromides give... 

Samarium(ir) iodide in the presence of HMPA effectively promotes the intramolecular coupling of unactivated alkenic ketones by a reductive ketyl-alkene radical cyclization process (eq 25). This protocol provides a means to generate rather elaborate carbocycles through a sequencing process in which the resulting organosamarium species is trapped with various electrophiles to afford the cyclized product in high yield. ... 

Promoted by samarium metal in the presence of a catalytic amount of iodine, the MBH adduets underwent reductive elimination to form ( )-methylcin-namic ester derivatives 368. However, allylic iodide derivatives, (2Z)-2-(iodo-methyl)alk-2-enoates 369, were obtained when the iodine was used in 1 1 ratio with metallic samarium. This method gave a new approaeh to the seleetive construction of stereo-defined trisubstituted alkenes with the simple Sm/l2 system (Scheme 3.164). ... 

The reductive cyclization of N-(w-iodoalkyl)succinimides induced by samarium(II) iodide was disclosed by Ha et al. as a novel method for making pyrrolizidines and indoUzidines (Scheme 46). " In the apphcation of the method to the synthesis of (+)-lentiginosine (127), reaction of N-(4-iodo-butyl)tartarimide (+)-335 with samarium(II) iodide in the presence of the iron(III)—tris(dibenzoylmethane) complex as catalyst produced the unsaturated indolizidin-3-one (+)-336 in 82% yield. Reduction of the bridgehead alkene was accomplished with triethylsilane and trifluoroacetic acid via an intermediate acyliminium ion, giving (+)-337 as the sole product in 93% yield. Routine hydrolysis of the silyl ethers produced the known diol (+)-177, after which reduction of the lactam with Hthium aluminum hydride then completed this short synthesis of (+)-127. 

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