Elimination—addition deamination

The lyases comprise enzyme class 4. They are enzymes cleaving C-C, C-0, C-N and other bonds by elimination, not by hydrolysis or oxidation. Lyases also catalyse addition to donble bonds. The types of reactions catalysed by lyases are decarboxylation (decarboxylase), hydration/dehydration (hydratase/dehydratase), ammonia addition/deamination (ammonia-lyase), cyanohydrin formation/cleavage (oxynitrilase),... 

Thus, N-nitrosamides also undergo deamination (hydrolysis) by an addition-elimination pathway involving nucleophilic rather than... 

The proposed mechanisms are similar in both cases and involve in particular an (aryl)(hydrido)ruthenium intermediate in which the ruthenium is additionally coordinated by an in. ( ////-generated /V-phenylimine moiety tethered to the same Ru-bound aromatic ring. The C-C bond-forming step for the construction of the corresponding heterocyclic framework proceeds via insertion of the C=N double bond into the C-Ru bond with transfer of the (hydrido) ruthenium complex to the now phenylamine nitrogen. The desired heterocycles 158 and 159 were obtained after successive reductive elimination, deamination, and dehydrogenation. 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

Ring closure with concomitant amino group elimination is actually a replacement reaction, as depicted below, strictly connected with the replacement reaction described in Sec. B. Similarly, the cyclization mechanism may or may not involve the formation of a vinyl intermediate (vinylketone, methylcnequinone, etc.), originated by the deamination of the initial Mannich base, which then undergoes addition. 

The chemistry of C deamination has been extensively studied (6, 7). It has been suggested that at neutral pH hydroxyl ions can directly attack the C-4 position of C (Fig. 2), especially in single-stranded DNA. An alternative explanation offered is an addition-elimination reaction during which dihydrocytosine is generated as an intermediate. This reaction may involve the formation of dihydrocytosine and dihydrouracil as intermediates... 

Protein catabolism and anabolism are often out of sync - either no additional protein is needed or the amino acid composition of the synthesized proteins is not identical to the protein being hydrolyzed. Neither protein nor amino acids are stored as such. Thus organisms must frequently degrade excess amino acids. Two paths may be available deaminate the unneeded amino acids and breakdown the carbon skeletons for energy or for storage as fat or carbohydrate and eliminate the nitrogen or the nitrogen may be transferred to another carbon backbone to make a needed amino acid. 

Treatment of a primary aliphatic amine with nitrous acid or its equivalent produces a diazonium Ion which results in the formation of a variety of products through solvent displacement, elimination and solvolysis with 1,2-shift and concurrent elimination of nitrogen. The stereochemistry of the deamination-substitution reaction of various secondary amines was investigated as early as 1950, when an Swl-type displacement was suggested. Thus, the process can hardly be utilized for the preparation of alcohols except in cases where additional factors controlling the reaction course exist. Deamination-substitution of a-amino acids can be utilized for the preparation of chiral alcohols. 

Transamination is just one of a wide range of amino acid transformations that are catalyzed by PLP enzymes. The other reactions catalyzed by PLP enzymes at the a-carbon atom of amino acids are decarboxylations, deaminations, racemizations, and aldol cleavages (Figure 23.13). In addition, PLP enzymes catalyze elimination and replacement reactions at the p-carbon atom (e.g., tryptophan synthetase p. 696) and the y-carbon atom (e.g., cystathionine p-synthase, p. 693) of amino acid substrates. Three common features of PLl catalysis underlie these diverse reactions. 

Supporting evidence for the series of reactions derived from study with extracts is obtained with resting cells of P. vulgaris. In the oxidation of L-cysteinesulfinate by washed, resting cells there is an initial lag period in the oxidation which is not eliminated by the addition of Mn++, although the latter increases the over-all oxidation. From this it is reasoned that the lag period is associated with the transamination reaction, whereas the acceleration of oxidation by Mn++ is concerned with the removal of the deaminated reaction product, the 3-sulfinylpyruvate. 

With educts containing an axially oriented LG at C-3, in addition to an intramolecular Sn2 reaction with an axially oriented hydroxy or acetamido group in a vicinal position (IS, Table 3, entries 1 [26] and 2 [30]), elimination and/or hydride shift (E or M/E, entries 3 [30],4 [31] and 6 [32]), as well as substitution with retention of configuration (AS, entries 7 and 8 [27b]) have been observed. For the transformation outhned in entry 4, 2,6-di-ferf-butyl-4-methylpyridine was used as acid scavenger, since pyridine per se in a similar reaction had caused Sn2 displacement (entry 5 [33]). Of special interest are the results from the deamination of methyl 3-amino-3-deoxy-/5-D-allopyranoside (entry 6), where the main reaction consists of direct Sn2 displacement with formation of methyl... 

LXXX. Nucleophilic addition of an acetate anion gives diacetyl-pseudodiosgenin (LXXXII) and 3j3,25-diacetoxyfurosta-5,20(22)-diene (LXXXIII). In the aqueous medium applied by Sato et al. (246) there may occur the same deamination mechanism, except that under these conditions the nucleophilic addition of water leads to the deacet-ylated derivatives of LXXXII and LXXXIII which are able to recyclize to the spiroketals diosgenin (LXXIV) and the 22,25-epoxyfurostene LXXVI, respectively. This mechanism, which involves the intermediate elimination of the chirality center 22, permits no conclusions with regard to the possible identity of the spirosolane side chain stereochemistry with that of the corresponding spirostanes. 

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