Wittig reaction organic synthesis

Intermediates 18 and 19 are comparable in complexity and complementary in reactivity. Treatment of a solution of phosphonium iodide 19 in DMSO at 25 °C with several equivalents of sodium hydride produces a deep red phosphorous ylide which couples smoothly with aldehyde 18 to give cis alkene 17 accompanied by 20 % of the undesired trans olefin (see Scheme 6a). This reaction is an example of the familiar Wittig reaction,17 a most powerful carbon-carbon bond forming process in organic synthesis. 

The acidity of such R2B—CH2—R systems has been utilized as the basis for developing the so-called boron-Wittig reaction for organic synthesis.52-54 One of the distinct advantages of such reagents, the direct conversion of an aliphatic aldehyde to a ketone, is illustrated in Eq. (19).55... 

Krauueer, M., Hummel, W. and Groeger, H. (2007) Enantioselective one-pot two-step synthesis of hydrophobic allylic alcohols in aqueous medium through the combination of a Wittig reaction and an enzymatic ketone reduction. European Journal of Organic Chemistry, (31), 5175—5179. 

In the organic synthesis (Roche, BASF) two C2o molecules, retinal and retinulidene-triphenylphosphorane, are coupled according to the Wittig reaction to give /i-carotene in high yield [9]. A disadvantage is that triphenylphosphine oxide, formed in a stoichiometric amount, cannot be recycled economically to triphenylphosphine. 

Remember, however that CHAOS does not intend to be a database of "synthetic methods" presently available to organic chemists. CHAOS is aimed at finding the intermediate precursors of the "synthesis tree" by means of selected basic disconnections (either "consonant" or "dissonant"). In the author s view, it is of minor importance, for instance, how a "carbene" -resulting from a -(2 + 1) cycloelimination- is generated, or whether a double bond is the direct result of a Wittig reaction or the dehydration of an alcohol formed in a Grignard reaction. In... 

Enamines are highly valued intermediates in organic synthesis. Almost invariably they are prepared by reaction of a carbonyl compound with a secondary amine. In principle, another attractive route to enamines could be based on construction of the double bond by a Wittig- or Horner-Wittig reaction. The enamines J thus obtained could be easily converted into the corresponding homologous aldehydes, if desired fitted with an extra electrophilic substituent at the original carbonyl carbon atom. 

Among other important reactions that build up a molecule rapidly are aldol and conjugate addition. Together with Diels-Alder and Wittig reactions they are major players in organic synthesis. Another synthesis of lycoranes uses these reactions.11 Similar preliminary disconnections with the addition of a carbonyl group 89 lead to the amino acid 90. 

Wittig olefination reaction ( the phosphorus way ) has been a very popular reaction in organic synthesis. However, it is now in competition with Peterson/Chan olefination reaction327 ( the silicon way ). Formally, this latter involves the formation of a (3-silyl heteroatomic anion, which in the absence of an electrophile undergoes a (3-shift of the silyl moiety to the heteroatom (usually oxygen) with final elimination of silylated heteroatomic anion and formation of the olefin. 

The remainder of this chapter will detail examples of the main types of Wittig reaction using examples to highlight (i) those factors that influence the stereoselectivity of the above methods, (ii) modifications to the standard procedures, and (iii) uses of these reactions in organic synthesis. 

The Wittig alkenation has found widespread application in synthetic organic chemistry, and numerous papers and reviews have detailed the progress of the Wittig reaction. A principal advantage of alkene synthesis by the Wittig reaction is that the location of the double bond is absolutely fixed in contrast to the mixture often produced by alcohol dehydration. With simple substituted ylides Z-alkenes are favoured. 

The discovery of the Wittig reaction in 1953 (which merited a Nobel Prize) greatly enriched the arsenal of organic synthesis. In fact, the Wittig reaction and its later modification turned out to be the first general method for establishing a double bond in a predetermined position with controlled stereochemistry. As a result, retrosynthetic cleavage of a double bond to lead to alkyl halides and carbonyl components became a reliable option to identify suitable precursors in the search for the optimum constructive pathway. 

Devise a synthesis of each alkene using a Wittig reaction to form the double bond. You may use benzene and organic alcohols having four or fewer carbons as starling materials and any required reagents. 

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