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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.
P
Published in Joseph C. Salamone, Polymeric Materials Encyclopedia, 2020
The detailed synthesis of the pentamer, (GVGVP), is given below, followed by the syntheses of other pentamers, of the tricosapeptides (30mers), and of the polytricosapeptides, poly(30mers). Here the importance of purity, particularly optical purity, of the components of the pentamer is stressed. The pentamer synthesis goes forward by a dimer plus trimer strategy. The dimer, VP, is synthesized, carefully purified, and crystallized. The synthesis of the trimer, GVG, occurs by synthesizing, purifying, and crystallizing VG and then adding G to give GVG, which is purified and coupled to VP to give GVGVP. In general, on activation of the carboxyl to give the peptide bond, racemization can occur. Since Gly is not optically active, it cannot racemize; and because of the cyclic side-chain of Pro, this residue does not appreciably racemize. Thus, racemization is a problem during the coupling to form VP and VG, and the purification and crystallization of these dipeptides are critical to the optical purity of the pentamer. With activation of the Pro of GVGVP, the polymerization of GVGVP can proceed with an expectation of an optically pure, protein-based polymer.3,4
Polymeric Materials Obtained through Biocatalysis
Published in Severian Dumitriu, Valentin Popa, Polymeric Biomaterials, 2020
Florin Dan Irimie, Csaba Paizs, Monica Ioana Tosa
The solution is to find a system that could racemize the monomer, at a rate higher than its incorporation into the polymer. This system is known as (enzymatic) dynamic kinetic resolution and it is applied in enzymatic biotransformations of organic substrates. In the absence of racemization system, the preferred enantiomer (MR) is consumed, and the relative concentration of the enantiomer (MS) increases. When the racemization system is present, the enantiomer (MS), which is in a higher concentration, is racemized to (MR) and (MS); the (MR) enantiomer is further incorporated into the product, while (MS) enantiomer is transformed to (MR) and so on (Scheme 18.23, dynamic kinetic resolution).
Enantioselective behavior of environmental chiral pollutants: A comprehensive review
Published in Critical Reviews in Environmental Science and Technology, 2022
Marina Arenas, Julia Martín, Juan Luis Santos, Irene Aparicio, Esteban Alonso
In sum, different events may contribute to EF changes during enantioselective degradation processes of pesticides including enantiomerization, racemization, or the preferential degradation of one of the enantiomer. Pesticides racemization is a point of emerging concern and if racemization occurs when a pesticide is applied it would make no sense to use chiral switch (pesticides containing only or mostly the active enantiomer) or develop enantiopure pesticides.
Synthesis, structure and reactivity of some chiral benzylthio alcohols, 1,3-oxathiolanes and their S-oxides
Published in Journal of Sulfur Chemistry, 2020
R. Alan Aitken, Philip Lightfoot, Andrew W. Thomas
In order to access the full range of target benzylthio alcohols B we adopted the more direct approach of initial nucleophilic displacement using sodium benzylthiolate (Scheme 2). Thus the bromo acids 1–3 afforded the benzylthio acids 8–10 in good yield. Of these three compounds, only 9 has been previously reported [6] and it showed good agreement with literature NMR data although, as noted in the experimental section, there is a slight error in the literature interpretation. The three acids were then readily esterified to give the benzylthio esters 11–13 in almost quantitative yield. Again two of these three compounds are previously unknown and only compound 11 is briefly mentioned in the literature [7] with no characterization data. The final reduction to the target benzylthio alcohols 7, 14, and 15 was achieved in high yield with lithium aluminium hydride in diethyl ether. Of these three compounds, the valine-derived example 14 has been reported by Evans [8] and used to form mixed P/S ligands for palladium catalysis. In his study, installation of an additional stereocentre adjacent to OH was followed by reaction with Ar2PCl to give a phosphinite much as suggested in Figure 1. Importantly however, our optical rotation value for 14 is roughly half that reported by Evans and also for the isoleucine derived compounds 10, 13 and 15 with a second and presumably invariant stereocentre present, diastereomeric mixtures were evident by NMR, thus pointing to a degree of racemization, most likely at the stage of the initial sodium benzylthiolate substitution. Clearly this problem would need to be addressed for effective implementation of the strategy outlined in Figure 1. However, in the mean time, progress in this direction was halted by the discovery that treatment of the benzylthio alcohols 7, 14 and 15 with sodium hydride followed by diethyl chlorophosphite gave not the expected phosphites F but rather the isomeric chlorides G formed via an intermediate thiiranium salt (Scheme 3). This will be described in detail elsewhere.