Chirality compounds without asymmetric

Biological Discrimination of Enantiomers 189 5-6 Racemic Mixtures 191 5-7 Enantiomeric Excess and Optical Purity 192 5-8 Chirality of Conformationally Mobile Systems 193 5-9 Chiral Compounds without Asymmetric Atoms 195 5-10 Fischer Projections 197... 

On the basis of this discussion a particularly promising scheme for synthesizing chiral compounds with great isomer purity and high overall yield would be to start with a productive asymmetric synthesis (to assure high yield) and follow by a destructive one (to assure unlimited isomer purity of the desired product). Such a procedure has the essential advantage that under suitable conditions it leads to pure chiral compounds without cumbersome separation of stereoisomers. With proper choice of the reactants the procedure will also permit an effective recovery of the auxiliary chiral materials needed as chiral templates in the asymmetric synthesis. 

The way in which compounds with asymmetric carbon atoms are racemized is more complicated. One possibility would be for a tetrahedral chiral carbon attached to four groups to become planar and achiral without breaking any bonds. Theoretical calculations indicate that this is not a likely process for chiral tetravalent carbon but, as we will see, it does occur with chiral carbon and other chiral atoms that are attached to three groups ... 

Compounds synthesised in the laboratory without the use of chiral reagents (see asymmetric synthesis p. 15) are always obtained as the racemate. In order to separate the individual enantiomers, a resolution process needs to be adopted. This aspect is considered in more detail in Section 5.19. 

Spontaneous absolute asymmetric synthesis, i.e., the formation of an optically active compound without the use of chiral materials, has been proposed as one of the origins of biological homochirality in nature [14,15]. Spontaneous... 

The importance of the rigid aminoindanol backbone in asymmetric catalytic Diels-Alder reactions is a subject of continued interest.16 50 One example immobilized the copper-inda-box complex onto mesoporous silica in the context of continuous large-scale production of chiral compounds.16 Using 10 mol% of this catalyst (Figure 17.5), the Diels-Alder reaction between /V-acryoyloxazo-lidinone and cyclopentadiene proceeded in 99% yield, 17 1 endo. exo selectivity, and 78% ee of endo cycloadduct. The catalyst could easily be recovered and reused several times without significant loss of diastereoselectivity (15 1 endo. exo selectivity after the fifth reuse) or enantioselectivity (72% ee after the fifth reuse).16 The same remarkable reactivity was observed with a number of diene-dienophile partners. 

Diastereoisomerism is encountered in a number of cases such as achiral molecules without asymmetric atoms, chiral molecules with several centers of chirality, and achiral molecules with several centers of chirality (meso forms). Such cases can be encountered in acyclic and cyclic molecules alike, but for the sake of clarity these two classes of compounds will be considered separately. 

When two chiral compounds, racemic A and racemic B, react to form a covalent bond between them without affecting the asymmetric center, the stereochemical course of the reaction can be as follows [6] ... 

Pasteur s success in 1848 of the first enantiomer separation (optical resolution) of racemic acid as ammonium sodium ( )-tartrate tetrahydrate together with McKenzie s success in 1904 of the first asymmetric synthesis prompted many chemists to synthesize optically active compounds without recourse to the vital force of organisms, although by employing the capacity of a special species of organism, Homo sapiens, to discriminate left from right. Pasteur remarked in 1883 that The universe is dissymmetric. Since then chemists efforts have been focused on the control of asymmetry in this world of chiral and nonracemic materials. 

A more simple thiourea catalyst with amino functionality catalyses the asymmetric Michael addition of 1,3-dicarbonyl compound to nitroolefin [29,30]. In the reaction of malonate to nitrostyrene (Table 9.11) the adduct is satisfactorily obtained when A-[3,5-bis(trifluor-omethyl)phenyl]-A -(2-dimethylaminocyclohexyl)thiourea is used as a catalyst (ran 1), whereas the reaction proceeds slowly when the 2-amino group is lacking (ran2). In addition, chiral amine without a thiourea moiety gives a poor yield and enantioselectivity of the product (run 3). These facts clearly show that both thiourea and amino functionalities are necessary for rate acceleration and asymmetric induction, suggesting that the catalyst simultaneously activates substrate and nucleophile as a bifunctional catalyst. 

A strange method of preparation of optically active compounds without any chiral inductors was described using an electrochemical cell with electrodes of special asymmetric configuration made of barium titanate Reduction of fumaric acid resulted in (R)-(+)-malic acid with an ee of 17%, or... 

The high value of catalytically performed reactions as compared to non-catalytic variants is particularly evidenced in the field of enantioselective reactions. Chemists cannot complete enantioselective reactions without certain chiral information in the reacting system. This information is regularly derived from the chiral compounds present in nature, collectively named the chiral pool of the nature. Their availability is often limited, which is not an issue when they are used as catalysts, but causes significant costs of non-catalytic reactions when they are needed in equimolar quantities. The practical value of the catalytic approach to enantioselective processes cannot be overestimated. Asymmetric catalysis characterizes the amplification of chirality one chiral molecule of the catalyst generates an enormous number of chiral molecules of the product in the optically pure form. This results with high chiral economy of catalytically performed enantioselective syntheses. 

Whereas nucleophilic addition of alkyl-lithium compounds to the optically pure arene(tricarbonyl)chromium complex (8) proceeds without asymmetric induction, the chelates (9) react to give amines (10), after hydrolysis, with optical purity of up to 94%." Replacement of the phenyl groups on the azomethine function by alkyl groups should provide an efficient route to a large number of chiral amines. 

A final example shows a process called desymmetrization (Figure 15.25). We start with a raeso-compound (remember this is a molecule that contains asymmetric carbon atoms but is not chiral, because it has a plane of symmetry) with two identical functional groups. Under enzymatic hydrolysis, only one of these reacts, so a chiral compound is produced, and all the material is used, without any need to recycle. The enzyme used in this reaction is electric eel acetylcholinesterase (EEAc). Pig liver esterase is also commonly used, as a relatively crude extract is inexpensive and gives good results. An example is shown in Figure 15.26, in the synthesis of 7 -mevalonolactone, important in the biosynthesis of terpenes and steroids. 

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