Olefination reactions Wittig Reaction

Urns-Epoxides. This unstable ylide (1), when generated as formulated above, reacts with an aliphatic aldehyde at —78° to give a fram-epoxide with almost complete stereoselectivity. The stereochemical selectivity is markedly dependent on the base and also on the counterion of the arsonium salt. Optimum selectivity for the trans-epoxide is obtained with conditions similar to those that induce cis-olefination in Wittig reactions.2 Stereoselection is not so high with aromatic aldehydes. The reagent also reacts with ketones to form trisubstituted epoxides. 

Esterification reactions Wittig reactions to form olefin... 

Macrolides and Related Compounds.- The total synthesis of amphoteronolide B (166) and amphotericin B (167) has been reported in a series of papers by Nicolaou s group.The method uses the phosphonate-based olefination of (164) with (165) to form the basic carbon skeleton which is cyclised by an intramolecular phosphonate-based olefination. A Wittig reaction of the ylide (168) is a key step in the synthesis of the cytotoxic macrocycle riccardin C (169). 

The temporary presence of a carbon-carbon double bond can be advantageous for creating branches in a molecular skeleton by bond formation. This functionality can be obtained not only by olefin metathesis, but also by any carbonyl olefination reaction (Wittig [24], Homer-Wadsworth-Emmons [25], Peterson, or Julia-Lythgoe [9]) (Scheme 3.14). 

Ketals are converted to allyl vinyl ethers after Wittig olefination [14]. Wittig reactions permit the incorporation of various substituents into the allyhc terminal of the ether. Enol ethers have been conveniently prepared by cleavage of acetals with various Lewis acids. Cleavage of the ketals with the Lewis acid, for example, triethylsilyl triflate, in the presence of diisopropylethylamine in refluxing 1,2-di-chloroethane afforded the enol ethers (Eq. 3.1.9). The resulting enol ethers were then heated to effect the Claisen rearrangement without isolation. 

With "non-stabilized" ylides the Wittig Reaction gives predominantly Z-olefins. Seebach et al... 

An altematiye route to olefins is by an immediate disconnection of the double bond. This corresponds to the Wittig reaction ... 

A good approach to 1,2-diols is hydroxylation of an olefin with reagents such as OSO4 or KMn04. The olefin can be made by the Wittig reaction so the discoimections arc ... 

Note that is must be the trans olefin as it is the trans epoxide we want. Tins is all right as the Wittig reaction can easily be controlled to give mostl the more stable trans olefin. 

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

The Julia-Lythgoc olefination operates by addition of alkyl sulfone anions to carbonyl compounds and subsequent reductive deoxysulfonation (P. Kocienski, 1985). In comparison with the Wittig reaction, it has several advantages sulfones are often more readily available than phosphorus ylides, and it was often successful when the Wittig olefination failed. The elimination step yields exclusively or predominantly the more stable trans olefin stereoisomer. 

Another very important reaction initially involving nucleophilic attack on an aldehyde carbonyl is the Wittig reaction. An yUd adds to the carbonyl forming a betaine intermediate which then decomposes to produce an olefin and a tertiary phosphine oxide. 

The reaction has been extended to include carbanions generated from phosphonates. This is often referred to as the Horner-Wittig or Homer-Emmons reaction. The Horner-Emmons reaction has a number of advantages over the conventional Wittig reaction. It occurs with a wider variety of aldehydes and ketones under relatively mild conditions as a result of the higher nucleophilicity of the phosphonate carbanions. The separation of the olefinic product is easier due to the aqueous solubility of the phosphate by-product, and the phosphonates are readily available from the Arbusov reaction. Furthermore, although the reaction itself is not stereospecific, the majority favor the formation of the trans olefin and many produce the trans isomer as the sole product. 

Similar to phosphur ylides, sulfur ylides 1 and 2 possess the nucleophilic site at the carbon atom and the pendant leaving group at the heteroatom (sulfur). Different from the Wittig reaction, the Corey-Chaykovsky reaction does not lead to olefins. 

Nonclassical Wittig reaction, olefination by phosphoranes interaction with carboxylic acid derivatives, in transformation of heterocycles 99JCS(P1)3049. 

The Peterson olefination can be viewed as a silicon variant of the Wittig reaction, the well-known method for the formation of carbon-carbon double bonds. A ketone or aldehyde 1 can react with an a-silyl organometallic compound 2—e.g. with M = Li or Mg—to yield an alkene 3. 

The reaction of an alkylidene phosphorane 1 (i.e. a phosphorus ylide) with an aldehyde or ketone 2 to yield an alkene 3 (i.e. an olefin) and a phosphine oxide 4, is called the Wittig reaction or Wittig olefination reaction. ... 

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