Beta elimination

The hydroxyl group is lost from a carbon that bears three equivalent ethyl sub stituents Beta elimination can occur in any one of three equivalent directions to l give the same alkene 3 ethyl 2 pentene ... 

Hydroxylation of the double bond of methyltestosterone by means of osmium tetroxide and hydrogen peroxide affords the 4,5 diol. This undergoes beta elimination on treatment with base to yield oxymestrone (83). ... 

Rossol I, A Ptihler (1992) The Corynebacterium glutamicum aecD gene encodes a C-S lyase with alpha-beta-elimination activity that degrades aminoethylcysteine. J Bacteriol 174 2968-2977. 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

FIGURE 3.10 Deprotection of functional groups by beta-elimination.17 (A) Removal of a labile proton beta to a good leaving group leads to release of the protector as the didehydro compound. (B) Recently developed protectors (Samukov et al., 1988) also designated untra-ditionally as 4-nitrophenyl- (C) Transformation of an O-protected serine residue into a dehydroalanine residue by hefa-elimination. 

All proteins and peptides display chemical and physical instability that affects the way they are distributed and cleared in the body and their delivery to the site of action. Physical and chemical instability is affected by primary sequences and secondary and tertiary structures and the degree of glyco-sylation of protein. Chemical degradation of proteins and peptides involves deamidation, racemization, hydrolysis, oxidation, beta elimination, and disulfide exchange. Physical degradation of proteins involves denaturation and aggregation. 

Proteins, peptides, and other polymeric macromolecules display varying degrees of chemical and physical stability. The degree of stability of these macromolecules influence the way they are manufactured, distributed, and administered. Chemical stability refers to how readily the molecule can undergo chemical reactions that modify specific amino-acid residues, the building blocks of the proteins and peptides. Chemical instability mechanisms of proteins and peptides include hydrolysis, deamidation, racemization, beta-elimination, disulfide exchange, and oxidation. Physical stability refers to how readily the molecule loses its tertiary and/or sec-... 

Proteins are subject to a variety of chemical modification/degradation reactions, viz. deamidation, isomerization, hydrolysis, disulfide scrambling, beta-elimination, and oxidation. The principal hydrolytic mechanisms of degradation include peptide bond... 

Beta-elimination reactions have been observed in a number of proteins. This reaction occurs primarily at alkaline pH conditions. Abstraction of the hydrogen atom from the alpha-carbon of a cysteine, serine, threonine, phenylalanine, or lysine residue leads to racemization or loss of part of the side chain and the formation of dehydroalanine (26). 

To ascertain the upper limit of protein thermostability and to evaluate the effect of additional disulfide bridges on the enhancement of protein thermostability, additional cysteine residues were introduced into several unrelated proteins by site-directed mutagenesis and deactivation behavior tested at 100°C (Volkin, 1987). All the proteins investigated underwent heat-induced beta-elimination of cystine residues in the pH 4—8 range with first-order kinetics and similar deactivation constants kj that just depended on pH 0.8 0.3 h-1 at pH 8.0 and 0.06 0.02 h 1 at pH 6.0. These results indicate that beta-elimination is independent of both primary amino acid sequence and the presence of secondary structure elements. Elimination of disulfides produces free thiols that cause yet another deleterious reaction in proteins, heat-induced disulfide interchange, which can be much faster than beta-elimination. 

Beta-elimination. Beta elimination of cysteine, serine, threonine, lysine, and phenylalanine residues proceed via a carbanion intermediate. This mechanism is influenced by metal ions and favored under basic conditions (190, 191). 

From this, it can be seen that the amount of KOH within the hydroxide mixture would probably be critical in removing organo-sulfur from coal. While the particular role of KOH has not been determined, evidence from the literature has shown that the size of the cation may be important in stabilizing intermediate carbanions. Wallace et al. (J ) conducted a series of base- catalyzed, beta-elimination reactions with isopropyl sulfide and measured the amount of olefin production. The proposed mechanism involved initial abstraction of a proton by the t-butoxide base, and formation of a carbanion, with subsequent elimination of the sulfur moiety (which can be considered a good leaving group) to form the olefin (Equation 5). 

This dehydrohalogenation is called an alpha elimination (a elimination) because the hydrogen and the halogen are lost from the same carbon atom. The more common dehy-drohalogenations (to form alkenes) are called beta eliminations because the hydrogen and the halogen are lost from adjacent carbon atoms. 

Alkali has long been used on proteins for such processes as the retting of wool and curing of collagen, but more recently it has received interest from the food industry. Alkali can cause many changes such as the hydrolysis of susceptible amide and peptide bonds, racemization of amino acids, splitting of disulfide bonds, beta elimination, and formation of cross-linked products such as lysinoalanine and lanthionine. 

Our own laboratory has studied these reactions and, in particular, beta eliminations involving disulfides (Figure 16) and... 

Under this heading might be placed many of the reactions already discussed, but there are several that fit more appropriately in such a classification. One of these, with which the author has been associated, is the formation of inactive derivatives of proteolytic enzymes by alkaline beta elimination of a derivative of the active site serine of trypsin (45) (Figure 22). This modification uses an affinity reagent followed by a second chemical modification, the alkaline beta elimination, to form the product. The products of the reaction with trypsin and chymo-... 

A second reaction would involve beta elimination—i.e., some basic group on the enzyme surface could abstract a proton from the beta carbon of dalapon to form a-chloroacrylate or the 1-chloropropene (Reaction 2). 

Klein, G. and Reymond, J.L. (1998) An enantioselective fluorimetric assay for alcohol dehydrogenases using albumin-catalyzed beta-elimination of... 

For the late metals where M-C bonds are less strong, CC bond activation seems always to need some special driving force, such as relief of strain, chelate assistance, or attainment of an aromatic product. For early metals, M-C bonds can be much stronger and simple CC bonds can be more easily cleaved. A classic early example from Watson (equation 1) requires chelate assistance but C-C cleavage can be competitive with the alternate CH bond cleavage (beta-elimination) that normally dominates ... 

In the bimolecular concerted beta elimination reaction, E2, heterolytic cleavage of the C—X and C—H bonds takes place within the same reaction step, without formation of an intermediate (see Vol. 9). It appears that the energy barrier for the concerted process is lower than each of the barriers for the separate steps involving either a carbanion or a carbonium ion intermediate. 

The mechanism of disulfide reduction by phosphines is hypothesized to involve a stable intermediate containing a sulphur-phosphorous bond (6). Beta elimination would yield the phosphine sulfide and dehydroalanine. The formation of relatively stable adducts between cystine-containing peptides and the reagent was confirmed by mass spectrometry for several peptides with the major adduct representing one reagent molecule per cystine residue. 

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