Degradation by p-elimination

Aspinall, G. O., and A. S. Chaudhari Base-catalyzed Degradations of Carbohydrates. X. Degradation by p-Elimination of Methylated Degraded Leiocarpan A. Can. J. Chem. 53, 2189 (1975). 

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... 

Degradation studies. Degradation limits by endopolygalacturonase and by p-elimination were determined as described by Thibault (17) and by endopectate lyase as described by Rombouts a] (4). For pectin lyase the reaction conditions were 0.25 (w/v) pectin, 0.01 M sodium phosphate buffer, pH 5.2 and 0.52 U/ml of pectin lyase, 30°C, 24 h. The pectin lyase reaction was monitored by measuring nm of aliquots, diluted thirtyfold with 0.1 N hydrochloric aciQ and the degradation limit was calculated, using a molar extinction coefficient of 5500 (18). 

Pectin is relatively stable under acidic conditions and its stability is highest in the pH range of 3-4. Hydrolysis of methoxyl and acetyl groups and hydrolysis of glycosidic bonds occurs in strongly acidic solutions. Free D-galacturonic acid can degrade to furan-2-carbaldehyde and other products. At elevated temperatures, hydrolysis of pectin occurs even in weakly acidic solutions (pH > 5) the polysaccharide chain is cleaved by P-elimination. 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

Smoking of a single cigarette yields peak plasma levels in the range of 25-50 ng/mL. The effects described on p. 110 become evident When intake stops, nicotine concentration in plasma shows an initial rapid fall, reflecting distribution into tissues, and a terminal elimination phase with a half-life of 2 h. Nicotine is degraded by oxidation. 

Reduction (lithium aluminium hydride/tetrahydrofuran) of the tetraester 34 to the tetraol 35, followed by chlorination (thionyl chloride), afforded 36 in good yield. This tetrachloride was then subjected to base-promoted P-elimination (potassium rerf-butoxide) giving the desired bisdiene 37 in quantitative yield without purification. The sensitivity of 37 toward both thermal and photochemical degradation and its propensity to polymerize necessitated its immediate use following its preparation. 

These are adapted from the ampicillin molecule, with a side-chain derived from urea. Their major advantages over the carboxypenicillins are higher efficacy against Pseudomonas aeruginosa and the fact that as monosodium salts they deliver on average about 2 mmol of sodium per gram of antimicrobial (see above) and are thus safer where sodium overload should particularly be avoided. They are degraded by many p-lactamases. Ureidopenicillins must be administered parenterally and are eliminated mainly in the urine. Accumulation in patients with poor renal function is less than with other penicillins as 25% is excreted in the bile. An unusual feature of their kinetics is that, as the dose is increased, the plasma concentration rises disproportionately, i.e. they exhibit saturation (zero-order) kinetics. 

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