Thioethers sulfonium salt, formation

Let us turn to answering the second question first. Potential precursors of dehydroalanine, by way of -elimination, are the amino acids with functional groups at the /3-carbon atom. Such -elimination reactions are likely to be enzyme-catalyzed in nature. Additionally, the substrates may be suitably substituted at functions, such as the sulfhydryl group (thioether and sulfonium salt formation) or the hydroxyl group (carbohydrate attachment—glycopeptides and proteins phosphorylation). 

Homofamesyl iodide 7 was prepared by the reaction sequence shown in the margin. Of interest here is the two-step transformation of an alkyl halide into a Crextended alkyl halide.9 Compound 30 is first subjected to a nucleophilic substitution by an urganolithium species with formation of a homoallylic phenyl thioether This is then methylated in a second step to an intermediate sulfonium salt. The final SN2 reaction with an iodide ion releases thioanisol as a stable leaving group to give compound 7. 

Highly Lewis basic and nucleophilic functional groups are not compatible with zinc carbenoids. The methylation or ylide formation of heteroatoms is one of the most important side reactions of these reagents. For example, amines, thioethers and phosphines readily react with the zinc reagents to generate ammonium salts", sulfonium" and phosphonium ylides" ". Terminal alkynes generally lead to a large number of by-products". ... 

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