By p-Elimination Reactions

A, B) The observation that the dehydroamino acids most frequently encountered are dehydroalanine and dehydroaminobutyric acid gives rise to the supposition that these are principally formed from serine, cysteine and threonine units by P-elimination reactions. A large number of P-eliminations have been carried out in vitro on S-methylcysteine, O-tosylserine and 0-phosphorylserine units incorporated in a peptide chain (cf. Section IV.A.2). 

Athene formation requires that X and Y be substituents on adjacent carbon atoms By mak mg X the reference atom and identifying the carbon attached to it as the a carbon we see that atom Y is a substituent on the p carbon Carbons succeedmgly more remote from the reference atom are designated 7 8 and so on Only p elimination reactions will be dis cussed m this chapter [Beta (p) elimination reactions are also known as i 2 eliminations ] You are already familiar with one type of p elimination having seen m Section 5 1 that ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane Both reactions involve (3 elimination of H2... 

Alkenes are prepared by P elimination of alcohols and alkyl halides These reactions are summarized with examples m Table 5 2 In both cases p elimination proceeds m the direction that yields the more highly substituted double bond (Zaitsev s rule)... 

Mechanism of the initial reaction, known as alkaline peeling, is shown in equation 4. EnoHzations and tautomerizations take place easily because of the contiguous hydroxyl groups. The hydroxyl or substituted hydroxyl on the second, ie, P-carbon, from a carbonyl group is released from the molecule by P-elimination. 

Except for the biochemical exanple just cited, the stnactures of all of the alcohols in Section 5.9 (including those in Problem 5.13) were such that each one could give only a single alkene by p elimination. What about elimination in alcohols such as 2-methyl-2-butanol, in which dehydration can occur in two different directions to give alkenes that are constitutional isomers Here, a double bond can be generated between C-1 and C-2 or between C-2 and C-3. Both processes occur but not nearly to the sane extent. Under the usual reaction conditions 2-methyl-2-butene is the major product, and 2-methyl-1-butene the minor one. 

Early investigations have indicated that sulfinyl radicals apparently do not add, at least in the usual way, to olefmic double bonds24. However, some recent results by lino and Matsuda25 obtained by studying the thermal decomposition of benzhydryl p-tolyl and benzhydryl methyl sulfoxides in the presence of cis-/2-deuteriostyrene lead one to believe that sulfinyl radicals add reversibly to CH2 =CHPh. The molar ratio of trans to cis /3-deuteriostyrene that they observed at nearly 50% conversion was explained by addition-elimination reaction of sulfinyl radicals. 

ProLynx LLC has developed another type of releasable PEGylation technology based on the p-elimination reaction shown in Scheme 4. In this system, the release rate of the native drug is determined by the acidity of the proton adjacent to the... 

A stoichiometric dehydrogenation reaction has been utilized for the synthesis of the natural product Rhazinilam in which the key step was a platinum-mediated C-H activation, followed by P-elimination (213,214). This step is shown in Scheme 50. 

These P elimination reactions have been used in an olefine synthesis called the Peterson olefination reaction which is analogous (and sometimes superior) to the Wittig reaction. The Peterson olefination reaction involves the addition of an a-silyl carbanion to an aldehyde or ketone to give P-hydroxysilane, followed by P-elimination to give the olefine. 

FIGURE 7.5 Preparation of a protected dipeptide by the mixed-anhydride method, employing a chloroformate that generates a cleavable urethane.13 The urethane impurity is destroyed by a P-elimination reaction. NMM = A-methylmorpholine, Msc = methane-sulfonylethoxycarbonyl. 

A. Heterolysis of the Metal-Carbon a-Bond Homolysis of the Metal-Carbon a-Bond Oxidation of Lm iM +1-R Followed by Homolysis P-Hydride Shift Reactions P-Elimination Reactions P-Elimination of Carboxylates CO Insertion/Methyl Migration... 

The second important chain termination reaction characteristic of the catalysis of anionic polymerization of epoxy compounds by the TA consists in the abstraction of the hydrogen atom from the p-carbon atom in the tetraalkylammonium cation by the growing alkoxy anion (P-elimination reaction)I58 164). 

The chemical reactions used to degrade these aromatic compounds are numerous and complex. As was mentioned in Chapter 16, some fungi initiate the attack on lignin with peroxidases and produce soluble compounds that can be attacked by bacteria. In other cases elimination reactions may be used to initiate degradation. For example, some bacteria release phenol from tyrosine by P elimination (Fig. 14-5). However, more often hydroxylation and oxidative degradation of side chains lead to derivatives of benzoic acid or of the various hydroxybenzoic... 

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