Alkenes Wittig route

Either route is feasible and indeed styrene has been prepared from both combinations of reactants Typically there will be two Wittig routes to an alkene and any choice between them is made on the basis of availability of the particular starting materials... 

To use the Wittig reaction in synthesis, you must be able to determine what carbonyl compound and Wittig reagent are needed to prepare a given compound—that is, you must work backwards, in the retrosynthetic direction. There can be two different Wittig routes to a given alkene, but one is often preferred on steric grounds. 

This is the only convenient way to make these compounds, since elimination by any other route gives the thermodynamically more stable a,P-unsaturated isomers. This is an illustration of the utility of the Wittig method for the specific location of a double bond. Another illustration is the conversion of cyclohexanones to alkenes containing exocyclic double bonds, for example, ... 

Methoxybenzaldehyde and methyl diethoxyphosphonoacetate were reacted via the Wittig-Horner-Emmons route to give the corresponding alkene product [85] (see a more detailed description in [42]). 

Buchanan et al. (48) reported a new route to the synthesis of the chiral hydroxy-pyrrolidines 234 and 238 from D-erythrose (230) via an intramolecular cycloaddition of an azide with an alkene (Scheme 9.48). Wittig reaction of the acetonide 230 with (carbethoxyethylene)triphenylphosphorane gave the ( ) and (Z) alkenes 231 and 232. On conversion into the triflate followed by its reaction with KN3, the ( ) isomer 231 allowed the isolation of the triazoline 234 in 68% overall yield, which on treatment with sodium ethoxide afforded the diazo ester 235 in 86% yield. 

Other commonly used synthetic routes to alkenes are also affected by crown ethers. The Wittig (80TL4831), Wittig-Horner and Wadsworth-Emmons (81S117) reactions all give pure trans- alkene products in higher yields in the presence of crown ethers. 

But how are we to make the epoxide 61 The obvious route is by epoxidation of the alkene 63. The alkene 63 could be made by a Wittig reaction (chapter 15) on the ketone 64 or directly by sulfur ylid chemistry (chapter 30). 

Scheme 10.1 Comparison of routes to epoxides from carbonyl compounds. Top Asymmetric carbonyl epoxidation. Bottom Wittig olefination followed by asymmetric alkene epoxidation.

The strategy described here should find considerable use as a method for the stereoselective synthesis of alkenes. Although this olefination strategy involves one more step than the classic Wittig reaction, in many cases it may prove to be the more practical method. Finally, the scope, overall efficiency, and stereoselectivity of the 0-lactone route compares favorably to the Wittig, Julia-Lythgoe, and related established strategies for the synthesis of tri- and tetrasubstituted alkenes. 

There are usually two routes to a given alkene using a Wittig reaction ... 

What starting materials are needed to prepare each alkene by a Wittig reaction When there are two possibie routes, indicate which route, if any, is preferred (a) (CH 2C=CHCH2CH3 ... 

One of the main drawbacks of the Wittig reaction is the formation of unwanted triphenylphosphine oxide. A new route, which makes use of polymer-supported triphenylphosphine and microwave dielectric heating has been developed (Scheme 13), which yields the required alkene without the triphenylphosphine oxide. An alternative strategy for separation of the product alkene from unwanted phosphine oxide by-product is to carry out the Wittig reaction in a fluorous solvent using a perfluorinated ylide such as (45). One drawback of this... 

The best route to the allyl silane is the Wittig reaction suggested in the chapter (pp. 1296-7). The ylid is not stabilized by extra conjugation so the Z-alkene is favoured. 

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