Z alkenes

Hydroboration-oxidation of (E) 2 (p anisyl) 2 butene yielded an alcohol A mp 60°C in 72% yield When the same reaction was performed on the Z alkene an isomenc liquid alcohol B was obtained in 77% yield Suggest reasonable structures for A and B and describe the relation ship between them... 

N,N-Dimethy1aniline from Nakarai Chemicals was dried over calcium hydride and freshly distilled. Three molar equivalents of N,N-dimethylaniline are used to achieve complete conversion of the n-butyllithium, because In the present particular case free n-butyllithium, if present, causes the isomerization of the (Z)-alkene to the (E)-isomer. 

Entries 1 and 2 in Scheme 2.9 are typical of concerted syn addition to alkene double bonds. On treatment with peroxyacetic acid, the Z-alkene affords the cis-oxirane, whereas the -alkene affords only the iraws-oxirane. Similarly, addition of dibromocarbene to Z-2-butene yields exclusively l,l-dibromo-cw-2,3-dimethylcyclopropane, whereas only 1,1-dibromo-/ra 5-2,3-dimethylcyclopropane is formed from -2-butene. There are also numerous stereospecific anti additions. Entiy 3 shows the anti stereochemistry typical of bromination of simple alkenes. 

Allyl strain influences the conformation of Z-alkenes. A 4-substituted 2-alkene will prefer conformation C over D or E to minimize the steric interaction with the C—1 methyl group. ... 

Two methods shown below converted 175 to the Z alkene 205 in high yields. No expected E alkene with two bulky 1-adamantyl groups in cis positions was formed because of the quick isomerization at the intermediate stage (98BCJ1181). 

This reagent can be used for the enantioselective hydroboration of Z-alkenes with enantiomeric excess of up to 98%. Other chiral hydroboration reagents have been developed. ... 

A short enandoselecdve synthesis of f- -f/f,/f -pytenophorin, a naturaUy occurring and-fun-giil 16-membered macrohde dilactone, is prepared from fS -5-nitropentan-2-ol via the Michael addidon and Nef reacdon fScheme 4.23. The choice of base Is important to get the E-alkene in the Michael addidon, for other bases give a rruxture of and Z-alkenes. The reqidslte chiriil fS -5-nitropentan-2-ol Is prepared by enandoselecdve redncdon of 5-nitropentan-2-one v/ith baker s yeast. ... 

Sequence rules, 180-183, 297-298 E,Z alkene isomers and, 180-183 enantiomers and. 297-300 Serine, biosynthesis of, 1177... 

Vinylepoxides can be obtained by various strategies, all with their inherent limitations. Racemic epoxidation of olefins is a straightforward route to epoxides, as pure trans- or cis-epoxides can be obtained from ( )- or (Z)-alkenes, respectively. Various oxidants - such as mCPBA and other peracids, H2O2, or VO(acac)2/TBHP - can all be employed in this transformation [1],... 

Conjugated dienes can be epoxidized to provide vinylepoxides. Cyclic substrates react with Katsuki s catalyst to give vinylepoxides with high ees and moderate yields [17], whereas Jacobsen s catalyst gives good yields but moderate enantiose-lectivities [18]. Acyclic substrates were found to isomerize upon epoxidation (Z, )-conjugated dienes reacted selectively at the (Z)-alkene to give trans-vinylepoxides (Scheme 9.4a) [19]. This feature was utilized in the formal synthesis of leuko-triene A4 methyl ester (Scheme 9.4b) [19]. 

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

An analogous Lewis acid catalyzed cyclization of the oi-acctoxy derivatives 3 gives rise to one pair of diastereomers of 4 in the case of the ( )-alkenc, while the (Z)-alkene affords a diastereomeric mixture of (2S, 3/J, 4/J )-4 (major product) and (2,S, 3,S, 4/ )-4 (minor product)90. 

Three reaction sequences, all of a similar nature, have been published by Fujita and coworkers. The first175 is an alkene synthesis in which only three examples that involve sulphones are given. The most useful of these is a conversion of nerly phenyl sulphone (from neryl chloride) into aCn triene (75%), with the extra carbon atom being supplied by the iodomethylstannane reagent. The reaction proceeds spontaneously, with apparent retention of configuration at the Z alkene, as in equation (72). 

Some Wittig reactions give the (Z) alkene some the ( ), and others give mixtures, and the question of which factors determine the stereoselectivity has been much... 

E) alkenes. One explanation for this is that the reaction of the ylid with the carbonyl compound is a 2-1-2 cycloaddition, which in order to be concerted must adopt the [rt2s+n2al pathway. As we have seen earlier (p. 1079), this pathway leads to the formation of the more sterically crowded product, in this case the (Z) alkene. If this explanation is correct, it is not easy to explain the predominant formation of ( ) products from stable ylids, but (E) compounds are of course generally thermodynamically more stable than the (Z) isomers, and the stereochemistry seems to depend on many factors. 

A crucial issue for these reactions is the stereoselectivity for formation of E- or Z-alkene. This is determined by the mechanisms of the reactions and, as we will see, can be controlled in some cases by the choice of particular reagents and reaction conditions. 

The unique feature of the Horner-Wittig reaction is that the addition intermediate can be isolated and purified, which provides a means for control of the reaction s stereochemistry. It is possible to separate the two diastereomeric adducts in order to prepare the pure alkenes. The elimination process is syn, so the stereochemistry of the alkene that is formed depends on the stereochemistry of the adduct. Usually the anti adduct is the major product, so it is the Z-alkene that is favored. The syn adduct is most easily obtained by reduction of (3-ketophosphine oxides.269... 

Ipc)2BH adopts a conformation that minimizes steric interactions. This conformation can be represented schematically as in H and I, where the S, M, and L substituents are, respectively, the 3-H, 4-CH2, and 2-CHCH3 groups of the carbocyclic structure. The steric environment at boron in this conformation is such that Z-alkenes encounter less steric encumbrance in TS I than in H. 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

Partial reduction of alkynes to Z-alkenes is an important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that... 

Amine oxide pyrolysis occurs at temperatures of 100°-150°C. The reaction can proceed at room temperature in DMSO.323 If more than one type of (3-hydrogen can attain the eclipsed conformation of the cyclic TS, a mixture of alkenes is formed. The product ratio parallels the relative stability of the competing TSs. Usually more of the /f-alkene is formed because of the larger steric interactions present in the TS leading to the Z-alkene, but the selectivity is generally not high. 

Mixtures of alkenes are formed when more than one type of (3-hydrogen is present. In acyclic compounds the product composition often approaches that expected on a statistical basis from the number of each type of hydrogen. The E-alkene usually predominates over the Z-alkene for a given isomeric pair. In cyclic structures, elimination is in the direction that the cyclic mechanism can operate most favorably. 

Treatment of alkenyldialkylboranes with iodine results in the formation of the Z-alkene with migration of one boron substituent.26... 

Similarly, alkenyllithium reagents add to dimethyl boronate to give adducts that decompose to Z-alkenes on treatment with iodine.27... 

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