The two optical isomers of bromochlorofluoromethane

Asymmetric synthesis, also called chiral synthesis, enantioselective synthesis or stereoselective synthesis, is organic synthesis which introduces one or more new and desired elements of chirality.^[1][2] This is important in the field of pharmaceuticals because the different enantiomers or diastereomers of a molecule often have different biological activity. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... Organic synthesis is the construction of organic molecules via chemical processes. ... Chirality is a manga by Satoshi Urushihara Chirality (Greek handedness, derived from the word stem χειρ~, ch[e]ir~ - hand~) is an asymmetry property important in several branches of science. ...

Approaches

There are three main approaches to asymmetric synthesis:

• chiral pool synthesis
• asymmetric induction
• asymmetric catalysis

In practice, a mixture of all three is often used in order to maximize the advantages of each method. Chiral pool synthesis is a strategy that aims to improve the efficiency of chiral synthesis. ... Asymmetric induction in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment [1]. Asymmetric induction is a key element in asymmetric synthesis. ...

Chirality must be introduced to the substance first. Then, it must be maintained. Care needs to be taken when planning the synthesis: the chirality might be removed by a chemical change that makes the substance isotropic. This process is called epimerization. For example, a S_N1 substitution reaction converts a molecule that is chiral by merit of non-planarity into a planar molecule, which has no handedness. (To visualise, draw the outlines of both of your hands on paper, and cut the images out. You can now superimpose the images, even if the hands themselves do not superimpose.) In a S_N2 substitution reaction on the other hand the chirality inverts, i.e. when you start with a right-handed mixture, you'll end up with left-handed one. (A visualization could be inverting an umbrella. The mechanism looks just the same.) Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ... In chemistry, an epimer is a stereoisomer that has a different configuration at only one of several stereogenic centers. ... The SN1 reaction is an substitution reaction in organic chemistry. ... The SN2 reaction is a type of nucleophilic substitution, where a nucleophile attacks an electrophilic center and bonds to it, expelling another group called a leaving group. ... In chemistry Walden inversion is the inversion of configuration of a chiral centre in a molecule in a chemical reaction. ...

Chiral pool synthesis

Chiral pool synthesis is the easiest approach: a chiral starting material is manipulated through successive reactions using achiral reagents which retain its chirality to obtain the desired target molecule. This is especially attractive for target molecules having the similar chirality to a relatively inexpensive naturally occurring building block such as a sugar or amino acid. However, the number of possible reactions the molecule can undergo are restricted, and tortuous synthetic routes may be required. Also, this approach requires a stoichiometric amount of the enantiopure starting material, which may be rather expensive if not occurring in nature, whereas chiral catalysis requires only a catalytic amount of chiral material. Chiral pool synthesis is a strategy that aims to improve the efficiency of chiral synthesis. ... Magnification of grains of sugar, showing their monoclinic hemihedral crystalline structure. ... Phenylalanine is one of the standard amino acids. ... In chemistry two stereoisomers are said to be enantiomers if one can be superimposed on the mirror image of the other. ...

Asymmetric induction

What many strategies in chiral synthesis have in common is asymmetric induction. The aim is to make enantiomers into diastereomers, since diastereomers have different reactivity, but enantiomers do not. To make enantiomers into diastereomers, the reagents or the catalyst need to be incorporated with an enantiopure chiral center. The reaction will now proceed differently for different enantiomers, because the transition state of the reaction can exist in two diastereomers with respect to the enantiopure center, and these diastereomers react differently. Asymmetric induction in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in the substrate, reagent, catalyst or environment [1]. Asymmetric induction is a key element in asymmetric synthesis. ... In chemistry, enantiomers are stereoisomers that are mirror images of each other. ... Diastereomers (or diastereoisomers) are stereoisomers that are not enantiomers (mirror images of each other). ...

Asymmetric induction can also occur intramolecularly when given a chiral starting material. This chirality transfer can be exploited, especially when the goal is to make several consecutive chiral centers to give a specific enantiomer of a specific diastereomer. Aldol reaction, for example, is inherently diastereoselective; if the aldehyde is enantiopure, the resulting aldol adduct is diastereomerically and enantiomerically pure. A typical experimental setup for an aldol reaction. ...

One such strategy is the use of a chiral auxiliary which forms an adduct to the starting materials and physically blocks the other trajectory for attack, leaving only the desired trajectory open. Assuming the chiral auxiliary is enantiopure, the different trajectories are not equivalent, but diastereomeric. A chiral auxiliary is a chemical compound or unit that is temporarily incorporated into a organic synthesis so that it can be carried out asymmetrically with the selective formation of one of twoenantiomers. ...

Asymmetric catalysis

Small amounts of chiral, enantiomerically pure (or enriched) catalysts promote reactions and lead to the formation of large amounts of enantiomerically pure or enriched products.^[3][4][5]Mostly, three different kinds of chiral catalysts are employed: 1) metal ligand complexes derived from chiral ligands 2) chiral organocatalysts and 3) biocatalysts. The first method was pioneered by William S. Knowles (Nobel Prize in Chemistry 2001) in the 1960s.^[6]. He replaced the achiral triphenylphosphine ligands in Wilkinson's catalyst by the chiral phosphine ligands P(Ph)(Me)(Propyl) thus creating the first asymmetric catalyst ever. This experimental catalyst was employed in an asymmetric hydrogenation with a modest 15% enantiomeric excess result. The methodology was ultimately used by him (while working for the Monsanto company) in an asymmetric hydrogenation step in the industrial production of L-DOPA: William S. Knowles (born June 1, 1917) is a American chemist. ... This is a list of Nobel Prize laureates in Chemistry from 1901 to 2006. ... Triphenylphosphine (in Europe: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. ... Wilkinsons catalyst 1 is the common name for chlorotris(triphenylphosphine)rhodium(I), named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson. ... Hydrogenation is a class of chemical reactions which result an addition of hydrogen (H2) usually to unsaturated organic compounds. ... In chemistry two stereoisomers are said to be enantiomers if one can be superimposed on the mirror image of the other. ... The Monsanto Company (NYSE: MON) is a multinational agricultural biotechnology corporation. ... // Therapeutic use L-DOPA is used to replace dopamine lost in Parkinsons disease because dopamine itself cannot cross the blood-brain barrierwhere its precursor can. ...

Examples of asymmetric catalysis include: Image File history File links Size of this preview: 800 × 569 pixelsFull resolution (1052 × 748 pixel, file size: 17 KB, MIME type: image/png) Asymmetric L-DOPA synthesis I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document under the terms of...

• BINAP, a chiral phosphine, used in combination with compounds of ruthenium or rhodium. These complexes catalyse the hydrogenation of functionalised alkenes well on only one face of the molecule. Part of the 2001 Nobel Prize in Chemistry was awarded to Ryoji Noyori for this discovery, which is commercialized as the industrial synthesis of menthol using a chiral BINAP-rhodium complex.
• The other part of that Nobel prize concerned the Sharpless bishydroxylation
• Naproxen is synthesized with a chiral phosphine ligand in a Hydrocyanation reaction
• asymmetric catalytic reduction and oxidation

(S)- and (R)-BINAP Ball and stick model of BINAP viewed as above In organic chemistry, BINAP, an acronym used for 2,2-bis(diphenylphosphino)-1,1-binaphthyl, is an important chiral ligand widely used in asymmetric synthesis. ... Phosphine is the common name for phosphorus hydride (PH3), also known by the IUPAC name phosphane and, occasionally, phosphamine. ... General Name, Symbol, Number Ruthenium, Ru, 44 Chemical series transition metals Group, Period, Block 8, 5, d Appearance silvery white metallic Atomic mass 101. ... General Name, Symbol, Number rhodium, Rh, 45 Chemical series transition metals Group, Period, Block 9, 5, d Appearance silvery white metallic Atomic mass 102. ... Synthesis of copper(II)-tetraphenylporphine, a metal complex, from tetraphenylporphine and copper(II) acetate monohydrate. ... Hydrogenation is a class of chemical reactions which result an addition of hydrogen (H2) usually to unsaturated organic compounds. ... The chemical structure of ethylene, the simplest alkene. ... This is a list of Nobel Prize laureates in Chemistry from 1901 to 2006. ... Ryoji Noyori (野依良治) (born September 3, 1938) won the Nobel Prize in Chemistry in 2001. ... Menthol is a covalent organic compound made synthetically or obtained from peppermint or other mint oils. ... General Name, Symbol, Number rhodium, Rh, 45 Chemical series transition metals Group, Period, Block 9, 5, d Appearance silvery white metallic Atomic mass 102. ... Sharpless bishydroxylation or asymmetric dihydroxylation (AD) is a chemical reaction of an alkene with osmium tetroxide to form a diol (dialcohol). ... Naproxen (INN) (IPA: ) is a non-steroidal anti-inflammatory drug (NSAID) commonly used for the reduction of mild to moderate pain, fever, inflammation and stiffness caused by conditions such as osteoarthritis, rheumatoid arthritis, psoriatic arthritis, gout, ankylosing spondylitis, injury (like fractures), menstrual cramps, tendonitis, bursitis, and the treatment of primary... Hydrocyanation is, most fundamentally, the process whereby H+ and –CN ions are added to a molecular substrate. ... Proline-based CBS catalyst to reduce ketones William Standish Knowles ruthenium-based catalyst, similar to Wilkinsons catalyst, which reduces alkenes BINAP-based Noyori asymmetric hydrogenation catalysts which reduce ketones and alkenes      Concepts in asymmetric synthesis Analysis: Optical rotation, Enantiomeric excess, Diastereomeric excess, Chiral derivitizing agents Chiral resolution: Crystallization, Kinetic... Jacobsen epoxidation of alkenes using manganese-salen complex and NaOCl Sharpless epoxidation of allyl alcohols using Ti(O-iPr)4, diethyl tartrate, and t-butyl hydroxyperoxide Sharpless dihydroxylation of alkenes using dihydroquinine or dihydroquinidine ligands on an osmium metal center      Concepts in asymmetric synthesis Analysis: Optical rotation, Enantiomeric excess, Diastereomeric...

Biocatalysis & organocatalysis

Biocatalysis makes use of enzymes to effect chemical reagents stereoselectively. Some small organic molecules can also be used to help accelerate the desired reaction; this method is known as organocatalysis. If the organic molecule is chiral, it may react preferentially with the substrate of a certain chirality. Biocatalysis can be defined as the utilization of natural catalysts, called enzymes, to perform chemical transformations on organic compounds. ... Nov. ...

Alternatives

Apart from asymmetric synthesis, racemic mixtures of compounds may be separated by various techniques in chiral resolution. Where the cost in time and money of making such racemic mixtures is low, or if both enantiomers may find use, this approach may remain cost-effective. Chiral resolution in chemistry is a process for the separation of racemic compounds into their enantiomers [1]. Two common methods are: Derivatization of racemic compounds with optically pure reagents forming pairs of diastereomers which can be separated by conventional techniques in physical chemistry. ...

See also

• Aza-Baylis-Hillman reaction, for the use of a chiral ionic liquid in asymmetric synthesis.

The Aza-Baylis-Hillman reaction or aza-BH reaction in organic chemistry is a variation of the Baylis-Hillman reaction and describes the reaction of an electron deficient alkene usually an α,β-unsaturated carbonyl compound with an imine in the presence of a nucleophile. ...

References