Ylides selenium

In all the above examples, the synthesis of nitronates (5) is rather chemose-lective. In any case, data on the formation of their structural isomers, viz, the corresponding nitrocyclopropanes (13), are lacking. However, the synthesis of five-membered nitronates (5) with the use of sulfur or selenium ylides is not chemoselective (see Scheme 3.16). 

Nitronates (5) can be synthesized by the Michael addition of in situ generated sulfur or selenium ylides (12) (58-60, 62, 63) to conjugated nitroalkenes... 

The selenium ylides (140) with dimethyl acetylenedicarboxylate afford moderate yields of tetrasubstituted furans (Scheme 31) (73KGS1447). 

Reaction of the dimethylselenium oxide adduct378 with benzoylfluoroacetone 379 gives the selenium ylide 380. ... 

Keywords Selenoxide elimination, [2,3]Sigmatropic rearrangement, Selenoxide, Selenimide, Selenium ylide... 

Asymmetric selenoxide elimination of the optically active vinyl selenoxides affords optically active allenes and cyclohexylidenes. On the other hand, asymmetric [2,3]sigmatropic rearrangement of allylic selenoxides, selenimides, and selenium ylides leads to the formation of the corresponding optically active allylic alcohols, amines, and homoallylic selenides, respectively. 

In sharp contrast to the well-studied [2,3]sigmatropic rearrangement of allylic sulfur ylides [42], the study of allylic selenium ylides is quite limited [43]. Furthermore, its asymmetric version using optically active allylic selenium ylides has not been developed until quite recently. In this section, some examples of the asymmetric [2,3]sigmatropic rearrangement via optically active allylic selenium ylides are described. 

After the first preparation of a selenium ylide by Hughes and Kuriyan in 1935 [46], only a few papers on the synthesis of optically active selenium ylides have appeared until recently [47]. However, their optical purities were low. The synthesis of an optically pure selenium ylide by fractional recrystallization of single diastereomeric selenium ylides was first reported by Kamigata and co-workers in 1992 [48]. They designed a selenium ylide (II) with an electron-withdrawing... 

Koizumi and co-workers used optically active allylic chloroselenuranes bearing a 2-exo-hydroxyl-lO-bornyl group to produce optically active allylic selenium ylides in situ [52]. The nucleophilic reaction of the corresponding chiral chloro-selenurane and selenoxide with an active methylene compound occurs in a highly stereoselective manner to give the corresponding chiral selenium ylides with retention of configuration [53] (Scheme 33). 

Nishibayashi and Uemura attempted the diastereoselective [2,3]sigmatropic rearrangement via a chiral selenium ylide bearing an optically active ferrocenyl-selenium moiety, but the selectivity was low (up to 25% de) [54]. The treatment of an allylic chiral ferrocenyl selenide with ethyl diazoacetate in the presence of a catalytic amount of Rh2(OAc)4 afforded a diastereomeric mixture (around 2 1) of ethyl 2-ferrocenylseleno-3-phenylpent-4-enoates (III) with low diastereo-selectivity (Scheme 35). 

Reactivity cf a-selenoalkyl metals, a-selenoxyalkyl- y-metals and selenium ylides with enals and enones... 

Procedures which utilize selenides are similar, but a-lithio selenides are not generally preparable via simple deprotonation chemistry, due to facile selenium-lithium exchange. - Selenium-stabilized anions are available, however, by transmetalladon reactions of selenium acetals and add readily to carbonyl compounds. The use of branched selenium-stabilized anions has been shown to result exclusively in 1,2-addidon to unhindered cyclohexenones, in contrast to the analogous sulfur ylides. The resulting 3-hydroxy selenides undergo elimination by treatment with base after activation by alkylation or oxidation (Scheme 10). An alternative method of activating either p-hydroxy selenides or sulfides toward elimination involves treatment of a chloroform solution of the adduct with thallium ethoxide (Scheme 11). A mechanism involving the intermediacy of a selenium ylide is proposed. 

Reactivity of a-selenoalkyl metals, a-selenoxyalkyl- y-metals and selenium ylides with enals and enones 2.6.333 Reactivity of l,l-bis(seleno)-l-alkyl metals and a-selenocarbonyl compounds with enals and enones... 

Krief et al. have shown that selenium ylides behave as their sulfur analogues and convert a variety of carbonyl compounds to oxiranes <89H(28)1203>. The latter compounds can be directly obtained by using R2Se=CHR /i-hydroxyalkylselenides (available from carbonyl compounds by addition of RSeCH2Li) may serve as suitable precursors as well, either in a two-step protocol, via the selenonium salt by alkylation with magic methyl (MeS03F), or directly by treatment with thallous ethoxide in chloroform. Oxidation of the /t-hydroxyalkylselenides with peracid, followed by treatment of the resulting selenone with base, results in oxirane formation (Scheme 60). 

The reaction of selenium ylides stabilized by a carbonyl function with a,jS-unsaturated ketones afforded cyclopropanes substituted with two ketone functions, e. g. formation of 19 ° (see also refs 161 and 162). 

Reaction of 2,3-diarylcyclopropenones with carbonyl stabilized nitrogen, sulfur, and selenium ylides produced 3,4-diphenyl-2//-pyran-2-ones. ... 

The first examples of reactions of non-stabilized selenium ylides with enolisable carbonyl compounds have been reported. Ylide (45) was generated in situ from Me2 ePh MeS04 and NaH in the presence of R COR [R = Ph, 4-O2NC6H4, Bu, or hexyl R = H, Me, or Et or R R = (CH2)s] to give the corresponding oxirans (76-94%). ... 

Epoxide formation. Aldehydes are attacked by the selenium ylides derived from the cation. The -selenonium oxide intermediates decompose hy selenide elimination, resulting in epoxide products. 

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