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Greener Synthesis of Natural Products
Published in Ahindra Nag, Greener Synthesis of Organic Compounds, Drugs and Natural Products, 2022
Renata Kołodziejska, Renata Studzińska, Hanna Pawluk, Alina Woźniak
The kinetic resolution is based on the difference in the reaction rate of enantiomers with the enzyme catalyst, resulting in an enantioenriched sample of the less reactive stereoisomer. The maximum yield of KR is 50%, provided the enzyme exhibits high selectivity. In order to improve the performance of biotransformation, a method of dynamic kinetic resolution (DKR) was developed in which the enzymatic process combines with a simultaneous in situ racemization of the less reactive enantiomer, thus, producing optically active products in up to quantitative yield (Figure 14.5). For DKR to be effective, certain requirements have to be fulfilled:The enzyme should display high specificity for one of the enantiomers R or S (kR >> kS or kS >> kR),The substrate must racemize at least as fast as the subsequent enzymatic reaction (krac(Sub) ≥ kR or kS),Racemization of the product should not occur under the reaction conditions or should be minimal.
Structural Design for Molecular Catalysts
Published in Qingmin Ji, Harald Fuchs, Soft Matters for Catalysts, 2019
Qingmin Ji, Qin Tang, Jonathan P. Hill, Katsuhiko Ariga
Azlactones have been attractive since they may be served as protecting amino acids in the synthesis of natural or synthetic bioactive molecules. The multi-functional heterocyclic azlactone structure confers to such molecules a diverse chemistry, acting as electrophiles via two distinct sites, while also suffering from nucleophile attack. A successful catalyst must bring about a selective kinetic resolution (i.e., kfast ≫ kslow). It must also be capable of catalyzing the epimerization equilibrium of the azlactone with an efficiency, which ensures that racemization occurs at a considerably faster rate than the addition of the alcohol to the less preferred substrate enantiomer. Connon and Palacio designed a series of C-5′-hydroxylated cinchona alkaloid catalysts for the dynamic kinetic resolution (DKR) of azlactones by thiolysis [100]. They achieved the thiolytic DKR of azlactones with excellent enantioselectivity (84–92% ee) for the first time, and surpassed the levels for all substrates previously evaluated. Eberlin and Amarante et al. showed camphorsulfonic acid (CSA) acting as an effective Brønsted acid catalyst for the azlactone ring (Fig. 2.15) [101]. With nucleophiles applied in the reaction system, protected amino esters and amides derivatives may be produced in good to excellent yields. The azlactone activation proceeds by forming an ion-pairing intermediate with CSA, and following by nucleophilic attack. CSA is responsible for the protonation and de-protonation step.
Enzymes—Kinetics of Enzymatic Reactions
Published in Jean-Louis Burgot, Thermodynamics in Bioenergetics, 2019
Recall that if any reaction leads predominantly or exclusively to only a set of stereoiomers, it is named stereoselective. In a stereospecific reaction, a given isomer leads to one product, while another stereoisomer leads to the opposite product. While all stereospecific reactions are stereoselective, the converse is not true. – Enzymes, in principle, react with only a single type of functional group. Hence, they show chemoselectivity;– They also exhibit regioselectivity and diastereoselectivity. This is due to their three-dimensional structure (see under). When a reaction can potentially give rise to two or more structural isomers but actually produces only one, the reaction is said to be regioselective. Any reaction in which only one of a set of stereoisomers is formed, exclusively or predominantly, it is named stereoselective.– Almost all enzymes are chiral. They are chiral catalysts. This is because they are constituted by L-amino acids (see appendix I-4). The consequence is that any kind of chirality present in the substrate molecule is recognized when the intermediary “complex” enzyme-substrate is formed. Hence, as example, a prochiral substrate may be transformed into an optically active product and both enantiomers of a racemic may react at different rates offering the possibility of kinetic resolution. This property is one of the most remarkable features of enzymes. Most of the time, enzymes are enantioselective.
Kinetic resolution of racemic naproxen methyl ester by magnetic and non-magnetic cross-linked lipase aggregates
Published in Preparative Biochemistry & Biotechnology, 2020
Sema Salgın, Mustafa Çakal, Uğur Salgın
Chiral separation of enantiomeric drugs has attracted great attention because the Food and Drug Administration is expressing a strong preference that all medicinal drugs were sold in enantiopure form. In view of this, enantiomeric separation of drugs has gained vital importance for the pharmaceutical companies. The chiral separation methods are diastereomeric or preferential crystallization, chemically or enzymatic kinetic resolution and column chromatography.[1–4] Kinetic resolution is defined as a method where the one enantiomer of racemate is transferred to the product much faster than the other one.[5,6] Enzymatic kinetic resolution is based on the ability of the enzyme to discriminate between the substrate enantiomers.[7,8] Lipases are the most widely used biocatalysts in organic synthesis due to high catalytic activity, stability, wide substrate specifity, and high stereoselectivity.[5,8,9] The stereoselectivity is exploited for the production of single enantiomers instead of racemic mixtures and becomes more important in the pharmaceutical industry since in most cases, only one of the two enantiomers has the desired activity, whereas no activity or even undesirable side effects remain in the other enantiomer.[10]