Cyclopropanation samarium

An example, where two C-C-bonds are formed and one C-C-bond is broken is the synthesis of the tricycle 3-285, which has some similarity with the eudesmane framework 3-286, developed by Kilburn and coworkers (Scheme 3.72) [113]. Thus, exposure of the easily accessible methylenecyclopropyl-cyclohexanone 3-281 to samarium(II) iodide led to the generation of ketyl radical 3-282, which builds up a six-membered ring system with simultaneous opening of the cyclopropane moiety. Subsequent capture of the formed radical 3-283 by the adjacent alkyne group afforded the tricycle 3-285 via 3-284 as a single diastereoisomer in up to 60% yield. It should be noted that in this case the usual necessary addition of HMPA could be omitted. 

Chiral fluorinated allylic alcohols obtained by enzymatic resolution (Table 5, Runs 13 and 14) were employed in diastereoselective cyclopropanation using the carbenoid from samarium and di-iodomethane [32]. This reaction proceeded smoothly at -78 °C to 0 °C, but not as smoothly as the case of non-fluorinated allylic alcohols at -78 °C. 

Two DFT studies (one in Chinese and one in English) of the cyclopropanation of ethene with samarium(II) carbenoids have been reported.53,54 The reaction path is likely to involve a methylene transfer rather than a carbometallation with or without coordination of up to two THF molecules to the rare earth. 

When a-halo ketones are treated with diiodomethane and samarium at 0 °C, cyclopropanols can be obtained in reasonable yields. Curiously, under the same conditions 1,2-dibenzoylethane also leads to cyclopropanol products (equations 34 and 35). Several pathways for conversion of a-halo ketones to the observed cyclopropanols can be envisioned. It has been proposed that the mechanism of this reaction involves reduction of the a-halo ketone by Sm (or Smh) to a samarium enolate. Cyclopropanation of this enolate with a samarium-based carbenoid subsequently provides the observed product. 

Zhao, C., Wang, D., Phillips David, L. Theoretical study of samarium (II) carbenoid (ISmCH2l) promoted cyclopropanation reactions with ethylene and the effect of THF solvent on the reaction pathways. J. Am. Chem. Soc. 2003, 125, 15200-15209. 

Certain vinylstannanes will undergo cyclopropanation with diiodomethane and samarium amalgam.69 71 Epoxidation of the double bond can be carried out with meta-chloroperoxybenzoic acid,71,72 and the kinetic resolution of a 3-stannylallyl alcohol has been achieved under Sharpless conditions (equation 8-38)73... 

A computational study of cyclopropanation reactions of the divalent samarium carbenoid ISmCH2I with ethylene has been presented. The ISmCH2I species was found to have a samarium carbene complex character with... 

The zinc metal in the Simmons-Smith reaction was replaced by samarium for the cyclopropanation of allylic alcohols with chloroiodomethane. High yields of cyclopropanes, often with high diastereoselectivities, were obtained using samarium amalgam/diiodomethane. Cyclo-hex-2-enol (115a) gave exclusively s y -bicyclo[4.1.0]heptan-2-ol (116a), no traces of the anti-... 

Geraniol (117) and nerol (118) underwent samarium-promoted cyclopropanation with complete stereoselectivity. A staggered (Houk) transition structure has been proposed to explain the observed diastereoselectivity for the Z-isomer. Geminal alkene substituents are detrimental to cyclopropanation under samarium-induced conditions. Tertiary allylic alcohols also give low yields of cyclopropanated product. 

Cyclopropanation of Allylic Alcohols Using Samarium/Dihalomethane General Procedure ... 

Samarium(II) iodide and diiodomethane efficiently cyclopropanated lithium enolates 119 under very mild conditions, providing a convenient route to cyclopropanols 120. 

This alternative procedure can also be applied to allylic alcohols, however, more samarium and dihalomethane must be used to achieve complete eonversion of allylic alcohol to cyclopropanated product. 

Stereoselective samarium-based Simmons-Smith cyclopropanation of cyclopentyl allylic alcohols is the key step in the synthesis of the precursor of 1,25-dihydroxy vitamin The... 

Hexa-2,4-dien-l-ol reacted under the same conditions selectively at the double bond proximal to the hydroxy group to give the monocyclopropanated product 135 in 91% yield.The hydroxy moiety is essential for an efficient samarium-mediated cyclopropanation. ... 

Samarium-based hydroxy-directed cyclopropanation was useful in obtaining the requisite cyclo-pentanol 136 to prepare several bicyclo[3.1.0]hexane-fused 2, 3 -modified dideoxycarbocyclic nucleosides. ... 

Samarium-initiated cyclopropanation of 3,3-bismetallic allylic alcohols was realized with high diastereoselectivity providing functionalized metal- and bismetal-substituted cyclopropanes. Z-Substituted allylic alcohols, e.g. 137 provided excellent levels of diastereoselectivity independent of the size of the R group, while Z-substituted alkenes afforded very good diaste-reomeric excess for R = cyclohexyl, moderate diastereoselectivities were obtained when R = propyl or methyl. ... 

The reductive cleavage of the proximal cyclopropane bond in bicyclo[3.1.0]heptan-2-ones with samarium(II) iodide involves a one-electron transfer and formation of a radical intermediate which can undergo intramolecular cyclization reactions. ... 

Stereoselective samarium-based Simmons-Smith cyclopropanation of cyclopentyl allylic alcohols is the key step in the synthesis of the precursor of 1,25-dihydroxy vitamin D3.128 The desired chirality was generated in one case under the influence of the two stereogenic centers on the (R,/ )-butane-2,3-diol acetal 121 and in the other case was directed entirely by the allylic hydroxy group of 122. 

The zinc-based Simmons-Smith type procedures frequently require rather harsh conditions in order to provide acceptable cyclopropane yields. Also, the discrimination between allylic alcohols, homoallylic alcohols and olefins without a hydroxyl group is often not very pronounced. These drawbacks are avoided by a new method which substitutes samarium metal (or samarium amalgam) for zinc (Table 4)43. This cnahlcs only allylic alcohols to be cyclo-propanated under very mild conditions, even for highly crowded substrates. The hydroxy-directed diastereofacial selectivity is good to excellent for cyclic olefins. Due to this property, the method has been applied to the stereoselective synthesis of 1,25-dihydroxycholecalciferol44. 

Table 4. Cyclopropylalkanols by Cyclopropanation of Allylic Alcohols using Samarium Melal or Samarium Amalgam...

Cyclopropanation of vinylorganometallic compounds. Molandcr cyclopropana-tion of allylic alcohols substituted by silyl or stannyl groups can show high diastereo-sclcctivity particularly when carried out with a large excess of the samarium reagent. 

Samarium diiodide has also proven to be a useful reagent for this type of transformation with cyclopropyl ketones [11], In this case a ketyl radical is generated adjacent to the cyclopropane ring. Ring opening takes place as readily as with simple cyclopropyl carbinyl radicals. 

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