Amines biotransformation reactions

The cyclic metabolite 11.169 was also a substrate in further biotransformations, being (V-demethylated to the corresponding endocyclic imine, and oxidized to phenolic metabolites. Very little if any of the secondary amine metabolite (11.168) appeared to undergo direct (V-demethylation to the primary amine, in contrast to many other tertiary amines, presumably due to very rapid cyclization of the secondary amine facilitated by steric and electronic factors. The possibility for the iminium cation (11.169 H+) to become deprotonated (a reaction impossible for the iminium 11.166 in Fig. 11.20) should also drive the cyclization reaction. 

Oxidative reactions at carbon predominate in the biotransformation of cyclic amiiies, and an important consequence of this is often the cleavage of the carbon-nitrogen bond. For example, A-dealkylation of N- alkyl substituted pyrrolidine (or piperidine, morpholine, etc.) involves an initial oxidative attack at the a- alkyl carbon atom to yield an N hydroxyalkyl derivative (carbinolamine), which is then metabolized to a secondary amine and the corresponding aldehyde. The metabolic conversion of nicotine to nornicotine (30 see Scheme 3) probably involves this mechanism, although the iminium ion (31) has also been suggested as an intermediate in the biotransformation (76JMC1168). Carbinolamines are unstable intermediates and have been identified only in a few cases, e.g. A-hydroxymethylcarbazole... 

Mechanism of action Upon entry into the host cell, didanosine is biotransformed into ddATP through a series of reactions that involve phosphorylation of the ddl, amination to ddAMP and further phosphorylation. The resulting ddATP is incorporated into the DNA chain like AZT, causing termination of chain elongation. 

Figure 43-1 I Schematic view of the role of NAT enzymes in the metabolism of aromatic amines. N-acetylation might be a detoxification reaction in a number of cases however, after N-hydroxylation of aromatic amines (e.g., by CYP enzymes), NAT enzymes can bioactivate these intermediates by either 0-acetylation or intramolecular N,0-acety transfer, leading to the formation of nitrenium ions, which might react with DNA or alternatively be detoxified by, for example, GST enzymes. Importantly, it is shown that a number of other biotransformation enzymes are also involved in the metabolism of aromatic amines as well. (Redrawn from Wormhoudt LW, Commandeur jNM, Vermeuien NPE. Genetic polymorphisms of human N-acetyitransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes relevance to xenobiotic metabolism and toxicity. Crit Rev Toxicol 1999 29 59-124. Reproduced by permission from Taylor and Francis, Inc.)...

Screening kits/sets containing samples of the normal commercially available enzymes are also provided by other enzyme suppliers, such as Boehringer Mannheim/Roche (Chirazyme sets for lipases/esterases, aldol reaction kits), Altus Biologies (ChiroScreen Kits TE and EH (based on CLECs, see section 5) for the chiral resolution of alcohols, amines, and esters), Biocatalysts (kits with alcohol dehydrogenases), Enzymatix (lipase biotransformation research kit), and others. 

In industrial biotransformations, hydrolytic reactions occupy a prominent position for the production of optically active amines, alcohols, and carboxylic acids. Compared with other reactions, hydrolytic reactions are feasible to scale up because they are cofactor-free, relatively simple, and chemically tunable systems. In addition to home-made whole-cell biocatalysts, which are considered to be more cost-effective for specific syntheses, some commercially available hydrolases, including lipases/esterases, epoxide hydrolases, nitrilases, and glycosidases, are also employed for the enantioselective production of chiral chemicals. 

Suna developed a one-pot, two-step process for the intermolecular C—H amination of indoles and other electron-rich aromatic systems. Indole 229 is converted to indolyliodonium tosylate 230 (the structure of which was confirmed by X-ray crystallography) the intermediate is stable in MeCN,dichlo-romethane (DCM), and DMSO at room temperature for at least 72 h. Upon exposure to morpholine and catalytic copper salts, aminated indole 231 is isolated in good yield. The scope of competent amines is quite broad alkyl amines, benzylic amines, anilines, and allylic amines are all tolerated.The reaction sequence also works with pyrroles, azaindoles, pyrazoles, and related systems (14JA6920). Gross and colleagues reported the first one-pot synthesis of L-7-iodotryptophan firom 7-iodoindole and serine the biotransformation uses a bacterial cell lysate that can be stored lyophilized for several months and used catalyticaUy (19 examples, 9-81% yield) (140L2622). 

In a related fashion, asymmetric amination of ( )-cinnamic acid yields L-phenylalanine using L-phenylalanine ammonia lyase [EC 4,3,1,5] at a capacity of 10,000 t/year [1274, 1601], A fascinating variant of this biotransformation consists in the use of phenylalanine aminomutase from Taxus chinensis (yew tree), which interconverts ot- to p-phenylalanine in the biochemical route leading to the side chain of taxol [1602], In contrast to the majority of the cofactor-independent C-0 and C-N lyases discussed above, its activity depends on the protein-derived internal cofactor 5-methylene-3,5-dihydroimidazol-4-one (MIO) [1603], Since the reversible a,p-isomerization proceeds via ( )-cinnamic acid as achiral intermediate, the latter can be used as substrate for the amination reaction. Most remarkably, the ratio of a- vs, 3-amino acid produced (which is 1 1 for the natural substrate, R = H) strongly depends on the type and the position of substituents on the aryl moiety While o-substituents favor the formation of a-phenylalanine derivatives, / -substituted substrates predominantly lead to p-amino analogs, A gradual switch between both pathways occurred with m-substituted compounds. With few exceptions, the stereoselectivity remained exceUent (Scheme 2,215) [1604, 1605],... 

Hydrolytic enzymes such as proteases, esterases and lipases are ready-to-use catalysts for the preparation of optically active carboxylic acids, amino acids, alcohols, and amines. The area is sufficiently well researched to be of general applicabihty for a wide range of synthetic problems. Consequently, about two thirds of the reported research on biotransformations involves these areas. This is facilitated by the fact that a considerable collection of commercially available proteases and lipases is available in conjunction with techniques for the improvement of their selectivities. The development of simple models aimed at the prediction of the stereochemical outcome of a given reaction is still a challenge and will be the subject of future studies. A search for novel esterases to enrich the limited number of available enzymes and for lipases showing anti-Kazlauskas stereospecificities would be a worthwhile endeavor. 

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