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Twelve Principles of Green Chemistry
Published in Vera M. Kolb, Green Organic Chemistry and Its Interdisciplinary Applications, 2017
Many catalysts have been developed to either enable or enhance selectivity of the reaction. Stereoselectivity and achievement of enantiomeric excess are necessary in the syntheses of fine chemicals, especially drugs. Enantiomers exhibit the opposite handedness, like the left and the right hand. Often only one of the enantiomers has the desired biological properties, and the other one may either be toxic or place burden on the organism’s detoxification system. The production of the undesired enantiomers creates waste, but the more serious problem is the required separation of enantiomers for the purpose of isolating the biologically active one. This process is often difficult, lengthy, and expensive. It is often a major factor that drives up the cost of drugs. It is therefore very important that the chemical synthesis yields the desired enantiomer either exclusively or in excess. This is often achieved with special catalysts. We shall address such catalysts in Chapters 9 and 10, especially on the examples of green pharmaceutical syntheses.
Lipase Immobilization
Published in Sulaiman Al-Zuhair, Hanifa Taher, Supercritical Fluids Technology in Lipase Catalyzed Processes, 2016
Sulaiman Al-Zuhair, Hanifa Taher
Selectivity improvement is a critical requirement for industrial applications of lipase. This includes substrate selectivity, stereoselectivity, regioselectivity, and enantio-selectivity. Adsorption of Candida rugosa on celite was reported to enhance the stability of lipase and improve its enantioselectivity up to 3-fold (Ogino, 1970). Entrapment in cellulose acetate–TiO2 gel fiber improved the selectivity of Rhizomucor miehei lipase in the hydrolysis of 1,2-diacetoxypropane (Ikeda and Kurokawa, 2001). Also the enantioselectivity of pegylated P. cepacea lipase was increased 3-fold by entrapment in Ca-alginate gel beads (Palomo et al., 2003). Physical immobilization of C. antarctica lipase B by adsorption onto octadecyl-Sepabeads (hydrophobic support) did not show any appreciable enantioselectivity. However, the covalent immobilization onto the glutaraldehyde (hydrophilic support) derivative showed high enantioselectivity (Palomo et al., 2002b). This is mainly because glutaraldehyde is soluble in an aqueous media and can form inter- and intrabonds. If immobilized, it alters the rigidity of the lipase resulting in a conformational change from closed form to open form.
Atom Economy
Published in Aidé Sáenz-Galindo, Adali Oliva Castañeda-Facio, Raúl Rodríguez-Herrera, Green Chemistry and Applications, 2020
Kunnambeth M. Thulasi, Sindhu Thalappan Manikkoth, Manjacheri Kuppadakkath Ranjusha, Padinjare Veetil Salija, Nisha Vattakkoval, Shajesh Palantavida, Baiju Kizhakkekilikoodayil Vijayan
Stereoselectivity in a reaction allows preferential formation of one stereoisomer in the reaction. Stereoisomers differ only in the spatial orientation of constituent atoms in space. Stereoisomers can be optically active or inactive. Stereoisomers that are mirror images are called enantiomers and those that are not mirror images are called diastereomers. Stereoselectivity have been achieved in many cases between enantiomers and diastereomers. They are considered separately below.
Serendipitous discovery of a regioselective synthesis of novel benzoyloxy substituted phenyl/benzyl-sulfanyl/selenylbisesters, 3-benzoyloxy-3-(phenylsulfanyl)-β-lactams and their antimicrobial evaluation
Published in Journal of Sulfur Chemistry, 2023
Dipika Narula, Shamsher S. Bari, Gulab Singh, Rajni Sharma, Ankita Garg, Aman Bhalla
In conclusion, a series of novel benzoyloxy substituted sulfanyl/selenylbisesters and β-lactams have been synthesized by the benzoyloxylation of appropriate sulfanyl/selenyl-monoesters and trans-3-phenylsulfanyl-β-lactams. The regioselective syntheticbenzoyloxylation protocol opens new possibilities to access sufficiently stable benzoyloxy compounds. Antimicrobial evaluation studies performed on benzoyloxy/m-chlorobenzoyloxy substituted benzylselenyl-bisesters suggest these to have significant antibacterial and antifungal properties. Further, we have shown that a change of stereochemistry at the C3 and C4 positions of β-lactam ring can exert a major influence on antimicrobial activity of 3-benzoyloxy-3-phenylsulfanyl-β-lactams, wherein the trans- isomer was active against all tested strains and the cis- isomer was completely inactive. These findings demonstrate the positive effects of diversely substituted benzoyloxy compounds as promising candidates for biological applications. In addition, preliminary investigations that utilize diversified chiral organocatalysts for accomplishing asymmetric α-benzoyloxylations on our substrates have already been initiated in our laboratory to access the benzoyloxy derivatives with high stereospecificity/stereoselectivity (de/ee).
The generation and reactions of sulfenate anions. An update
Published in Journal of Sulfur Chemistry, 2022
Adam B. Riddell, Matthew R. A. Smith, Adrian L. Schwan
As sulfenate chemistry has the potential to produce enantiomerically or diastereomerically pure sulfoxides, the lack of stereoselectivity in the past two examples was clearly apparent. However, Poli, Madec and coworkers [54] conducted research to determine if the use of chiral ligands could lead to the stereoselective arylation of sulfenate anions. The researchers found success when investigating JosiPhos type ligands with the use of ligand 10 (Scheme 11) resulting in an 83% enantiomeric excess (e.e.) [54]. When using Cs2CO3 (4 equiv), Pd2(dba)3 (1 mol%), 10 (2 mol%) in toluene, the researchers synthesized 10 aryl sulfoxides from sulfenates and both aryl and heteroaryl iodides in yields and e.e.’s ranging from 67 to 98% and 0–83% respectively (Scheme 11). The researchers noted that the use of ligand 10 led to a preference for arylation at the pro-R face of the sulfenate anion which was confirmed by X-ray crystallography [54]. While this work represents the first example of enantioselective arylation of sulfenate anions and the metal and ligand loading are the lowest of all related studies discussed in this review, the enantioselectivity wavered significantly. It was found that certain structural features such as ortho substituents on the aryl iodide and the use of benzyl sulfenates led to poor enantioselectivity with these particular conditions [54].
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]