Syn-Elimination Reactions

Both E2 and El reactions require a base, and that base is usually a separate molecule added to the reaction medium that is., they are intermolecular processes. In those reactions, the hydrogen removed by the base 

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Sulfonium compounds (—C— SR2) undergo elimination similar to that of their ammonium counterparts (17-6 and 17-7) in scope and mechanism but this reaction is not of great synthetic importance. These syn-elimination reactions are related to the Cope elimination (17-8) and the Hofmann elimination (17-6). ... 

As a consequence of the selenoxide instability, the oxidation into selenones is only possible if the syn-elimination reaction becomes difficult, as in the case of strained structures. Under these conditions, the selenone is deprotonated and the a-selenonylalkyllithiums formed can react as other selenium-stabilized car-banions. It must be added that the seleninyl [PhSe(O)-] and selenonyl [PhSe(02)-j substituents behave as very good leaving groups. Vinylic selenoxides and selenones can undergo an intramolecular displacement of the selenium moiety after nucleophilic addition to the double bond under basic conditions. Cyclopropanes and oxetanes have been synthesized in this way [1,2]. 

In the case presented in Scheme SI the di erences in behavior of the p-hydroxyalkyl selenoxides of different origin may be due to different diastereoisomeric ratios the equilibration of which is hampered by steric crowding. Therefore, there is a pronounced stereochemical effect on the regiochemistry of sele-noxide syn elimination reactions. This elimination reaction leads to allyl alcohols where the a,3-disub-stituted double bond possesses exclusively the ( )-stereochemistry when reasonably bulky groups are present (Scheme 52 heme 105, c Scheme 174, Those substituted with stnaller groups... 

This chapter is concerned with a group of thermally induced elimination reactions widely used for the introduction of carbon-carbon double Irands into complex molecules. These reactions form a discrete group of elimination reactions in that they proceed with syn stereochemistry via concerted cyclic transition states. Related syn elimination processes are believed to be involved in other elimination reactions, e.g. alcohol dehydration using the Burgess reagent, but are not discussed here. One of the advantages of the syn elimination reactions discussed in this chapter is that they do not require the use of... 

Again a syn elimination reaction, involving now the carbanion present in the ylide moiety, has been invoked (Scheme 7). 

The mechanistic features of several enzyme-catalyzed syn-elimination reactions either clearly demonstrate or strongly imply the formation of a stable carb-anion intermediate. These include the previously discussed PLP-dependent syn-eliminations as well as several of the enzymes listed in Table X. 

N-oxides or nitro compounds. These groups figure prominently in many synthetic transformations. In section 2.9.C, N-oxides, sulfoxides and selenoxides were important intermediates for syn-elimination reactions. In this chapter (sec. 3.5.B), N-oxides were used to catalyze osmylation reactions with alkenes. In sections 8.6 and 8.8.B, sulfoxides and sulfones will be used to stabilize carbanions, which react to form carbon-carbon bonds. It is therefore fitting that this chapter conclude with a brief survey of the methods for oxidizing these important synthetic intermediates. 

Vinylic ether-containing Claisen rearrangement substrates may be generated using syn-elimination reactions of sulfoxides, selenoxides, and selenones. 2-(Arylsulfinyl)ethyl ethers are particularly useful substrates in these reactions because of their ready availability by nucleophilic addition of allylic alcohols to commercially available phenylsulfinylethene. Scheme 13.27 shows a typical synthetic context for this chemistry, involving the stereospecific introduction of quaternary centers from easily accessed allylic alcohol precursors." ... 

SCHEME 6.23 Some important syn elimination reactions through selenoxides. 

The dienediynes 38 were readily synthesized via condensation of 3 with the enynyl aldehydes 36 followed by a syn elimination reaction of 37 (Scheme 9) (22). A variety of substituents could be placed at one of the acetylenic termini. 

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