Enamines transamination

Scheme 11.23. Pathways to Af-acetylglucosamine and A -acetylgalactosamine. The transamination process shown in the first step is presumed to involve an oxidation to a ketone, transamination, enamine formation, isomerization, and ring closure. Transamination, use of acetyl-CoA, and uridine will be discussed subsequently (vide infra). EC numbers and some graphic materials provided in this scheme have been taken with permission from appropriate links in a URL starting with http //www.chem.qmul.ac.uk/iubmb/enzyme/.

Sandmeyer reaction, 943 saponification. 809-810 SN1 reaction, 373-375 Sn2 reaction, 363-364 Stork enamine reaction, 897-898 transamination, 1167 Williamson ether synthesis, 655 Wittig reaction, 720-721 Wolff-Kishner reaction, 715-716 Meerwein-Ponndorf-Verley reaction, 746... 

A further PLP reaction involving enamine chemistry has to do with / -elimination reactions, that probably commences analogously to the well-characterized transamination reaction. Instead of hydrolysis of the Schiff base, these reactions result in proton abstraction, this time at the / rather than the a carbon of the amino acid. The intermediate iminium ion is the electron sink that helps to acidify the / proton (equation 14). 

The morpholine enamine reacts in an acid-catalyzed transamination with 2-siloxyaniline 7. 

The enantioselective total synthesis of 1 commences with the formation of the enamine of morpholine and cyclohexane-1,2-dione (6), which actually exists almost entirely in the enolic form. Constant removal of water shifts the equilibrium to the side of the product. 2-Siloxyaniline 7, " which can be easily prepared from 2-aminophe-nol, reacts with the morpholine enamine in an acid-catalyzed transamination to give enamine 8. 

The molecule is a chiral analogue of pyridoxamine and transamination occurs chemically by a mechanism similar to the biological one described in the chapter (pp. 1384-6). The zinc holds the molecule in a fixed conformation during reaction. The key step is the protonation of the enamine as that produces the new chiral centre. If the chain is across the top of the ring, protonation occurs preferentially from underneath. Hydrolysis gives the new amino acid (Phe) and the pyridoxal analogue, which can be recycled by reductive amination via the oxime. 

This reaction requires pyridoxal phosphate or pyridoxamine phosphate as a coenzyme (Scheme 8.8). If the former is used, it produces a Schiff base or enamine with the a-amino acid. The enamine undergoes an azaallylic transformation to form the alternative enamine. Hydrolysis of this produces pyridoxamine phosphate and the a-keto acid corresponding to the first amino acid. The pyridoxamine phosphate now forms an enamine with the first a-keto acid. Another azaallylic transformation takes place and the reaction is completed. It will be seen that there is no loss of ammonia or conversion of it into urea via the urea cycle. The transamination route simply shuffles the pack. 

Simple nitro-enamines have been reported to undergo transamination reactions via an addition-elimination mechanism. For example, 1-di-methylamino-2-nitroethene reacts with amines to release dimethylamine, accompanied by the formation of a new nitro-enamine (Scheme 6 where R R NH = piperidine, morpholine and pyrrolidine) [32,33]. 

Serine, like alanine, is converted into pyruvate by a PLP-dependent pathway, but the two reaction sequences are not the same. Whereas alanine catabolism involves a PLP-dependent transamination, serine catabolism involves a PLP-dependent dehydration to form an intermediate enamine that is then hydrolyzed. 

Catalytic asymmetric transamination of i-keto carboxylic esters 94 catalyzed by a chiral base was firstly reported by Soloshonosk et al. [40a] providing an original biomimetic approach to p-fluoroalkyl- 3-amino acids. This transformation involves the formation of A-benzyl enamines 96 in tautomeric equilibrium with A-benzyl... 

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