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Liquid Chromatography
Published in Ernő Pungor, A Practical Guide to Instrumental Analysis, 2020
Enantiomers have identical physical and chemical properties in an isotropic environment except that they rate the plane of polarized light in opposite directions. Racemic mixture which contains equal amounts of enantiomers are not able to rotate the plane of polarized light. Separation of enantiomers is one of the most difficult tasks, because no difference exists between the two enantiomers such as melting point, boiling point, refractive index, spectroscopic properties, and so on. Separation of enantiomers can be divided into two main types: socalled direct and indirect ones. In direct separation either stationary phase or mobile phase (adsorption of optically active additive from mobile phase) must be unisotropic (chiral). Indirect separation is based on the reaction of racemic mixture with a chiral reagent to form a pair of diastereomers. Diastereomers have different physicochemical properties and can be separated with chromatography.
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
This preferential formation of one diastereomer over the other is called diastereoselectivity. Diastereomers contain more than one chiral center. Consider the reaction between anthrone and R(+)-N-α-methyl benzyl maleimide in the presence of triethyl amine as achiral base in water-ethanol medium, given in Scheme 2.52. The reaction has excellent yields, but is not diastereoselective. Instead of the achiral base, chiral Brownsted bases like cinchonine, quinine, etc. produce diastereoselective products.
Stereochemistry
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
When a molecule contains two or more stereogenic centers, it is possible to generate stereoisomers that are different molecules (nonsuperimposable) and are not mirror images. Such stereoisomers are given the term diastereomer. What is a diastereomer?
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]