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Elimination Reactions
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
The term stereospecific means that of two or more possible products, one stereoisomer gives one and only one product, whereas the other stereoisomer will give a different product. In the preceding example, stereoisomers 2R-bromo-3R-methylpentane or 2S-bromo-3S-methylpentane gives the E-alkene and whereas the diastereomeric 2R-bromo-3S-methylpentane or 2S-bromo-3R-methylpentane enantiomers will give the Z-alkene. The cyclohexane derivative 2S-bromo-1S-ethyl-3S-methylcyclohexane gives 3S-ethyl-1-methylcyclohex-1-eneas the alkene product of an E2 reaction. Why?
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.
Stabilization of α-ZrP ceramic nanosheets adsorbing quaternary ammonium ions in organic solvents and their application as a stable solid support for lipase catalyzing stereospecific synthetic reactions
Published in Journal of Asian Ceramic Societies, 2022
Akane Yamada, Gen Onodera, Masanari Kimura, Kai Kamada
Chiral allylic alcohols and their derivatives are valuable substrates for many stereospecific allylic substitution reactions used to prepare versatile intermediates and artificial building blocks for pharmaceuticals, agrochemicals, and natural products [7–11]. For instance, the asymmetric Tsuji–Trost reaction and stereospecific allylation using chiral allylic alcohol derivatives are especially important strategies in synthetic organic chemistry [12–15]. However, these chiral allylic alcohol derivatives have been frequently prepared using complicated reaction systems and improvement and/or simplification of the reaction conditions are still an ongoing challenge, even in state-of-the-art organic syntheses.
Preparation of corrosion-resisting superhydrophobic surface on aluminium substrate
Published in Surface Engineering, 2019
Xinyan Xiao, Wei Xie, Zhihao Ye
Click chemistry, introduced by Sharpless and co-workers, is an effective technique for surface modification due to the modular, stereospecific, mild reaction conditions, high selectivity and yields [22]. Thiol–ene coupling (TEC) is an attractive click process which has wide applications in organic synthesis, materials chemistry and bio-organic chemistry [23]. Hayase et al. [24] attached perfluoroalkyl groups to the rich vinyl groups on the porous surface of silicone monolith using TEC reaction to transform the superhydrophobic gel into the superamphiphobic one.
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
Because of the similarities between the chiral profiling in effluent and in receiving water, the EFs of some chiral PhACs, such as propranolol or venlafaxine, have been proposed to be used as markers of untreated or partially treated wastewater discharges (Fono & Sedlak, 2005; Li et al., 2013). Surface waters, affected by known or suspected untreated sewage discharges, contained propranolol with EF = 0.50 ± 0.02 whereas surface waters with large discharges of effluent wastewater contained EF <0.42, similar to those observed in effluent wastewater (Fono & Sedlak, 2005). In Figure S1, it can be seen a summary of chiral PhACs EF values (mean and standard deviation), in wastewater and surface water, reported in recent literature. However, WWTPs are heterogenous and complex environments difficulting data interpretation. For example, Nikolai et al. (2006) reported the enantioselective bahaviour of atenolol, metoprolol and propranolol in two WWTPs in Canada. Propranolol EF showed similar behavior to that previously reported by Fono and Sedlak (2005), while the results of atenolol and metoprolol were oppsite in two WWTPs, i.e. slightly decreased at one WWTP and increased at the other, exhibiting also variable behavior during different sampling events. The differences observed may be attributed to changes in microbial population and diversity and operating conditions of the WWTPs. In another attempt MacLeod et al. (2007) observed the changes in EFs from influent to effluent from WWTP in Canada of six beta-blockers, two selective serotonin re-uptake inhibitors and one b2-agonist and revealed non-racemic EFs for all PhACs in effluent samples, with the exceptions of atenolol and metoprolol. Baker and Kasprzyk-Hordern (2013) found that the degradation of 3,4-methylenedioxy-methamphetamine (MDMA) and amphetamine was stereospecific, with the S(+)-enantiomer preferentially degraded in both raw and treated wastewater, due to preferential metabolism of this form in human and microbial processes (particularly in activated sludge processes). In receiving waters, the effect was more pronounced with the course of the river due to microbial processes. These findings are environmentally relevant because the stimulant capacity of S(+)-amphetamine is two-fold higher than that of R(−)-amphetamine. Moreover, higher stereoselectivity was observed in spring and summer due to a higher microbial activity. Similar results were reported by Evans et al. (2017). Authors also highlighted that both stereoselective metabolism and non-stereoselective photochemical processes (abiotic) were responsible for the higher degradation rates of desmethyl metabolites of some parent compounds (venlafaxine, citalopram and MDMA). mmetabolites were often enriched with opposite enantiomers, which might be of important ecotoxicological relevance.