Purity enantiomeric excess

It is interesting that the use of excess ligand DBFOX/Ph led to a decreased en-antioselectivity for the endo cycloadduct, especially when the enantiomeric purity of the ligand was low. This phenomenon is closely related with the chirality enrichment mechanism operating in the solution. 

Reaction of the chiral (45,5R)-oxazolidine 9. obtained from 3-pentanone and (-)-2-amino-l-phenylpropanol, with aldehydes gives predominantly a H -aldol adducts of high enantiomeric purity. The corresponding spn-adducts, formed in low enantiomeric excess, are isolated from the diaslereomeric mixture by chromatography 5. 

The addition of the lithium enolates of various acetic add esters to (S)-3-(4-methylphcnyl-sulfmyl)-2(5//)-liiranone and (,S)-5,6-dihydro-3-(4-methylphenylsulfinyl)-2//-pyran-2-one gives, after desulfurization with Raney nickel, 4-substituted dihydro-2(3//)-furanoncs and 4-substituted tetrahydro-2//-pyran-2-ones, respectively, in good to quantitative enantiomeric excess. Addition of the enolate occurs via the nonchelate mode. The enolate of methyl (phenylthio)acetatc is best overall in regards to chemical yields and enantiomeric purity of the final lactone product13. 

PPF catalyzed an enantioselective polymerization of bis(2,2,2-trichloroethyl) tra 5-3,4-epoxyadipate with 1,4-butanediol in diethyl ether to give a highly optically active polyester (Scheme 9). °° The molar ratio of the diester to the diol was adjusted to 2 1 to produce the (-) polymer with enantiomeric purity of >96%. The polymerization of racemic bis(2-chloroethyl) 2,5-dibromoadipate with excess of 1,6-hexanediol using lipase A catalyst produced optically active trimer and pentamer. The polycondensation of 1,4-cyclohexanedimethanol with fumarate esters using PPL catalyst afforded moderate diastereoselectivity for the cis/trans monocondensate and markedly increased diastereoselectivity for the dicondensate product. 

When dealing with reactions leading to stereoisomeric products we have the additional complication that descriptors such as enantiomeric (diastereomeric) excess and enantiomeric (diastereomeric) ratio are used to describe product purities. The evaluation of RME for a specific stereoisomer, say the R enantiomer, is exactly as above using the connecting relationships for the fraction of each product shown below. 

Monoisocampheylborane (IpcBH2) can be prepared in enantiomerically pure form by purification of a TMEDA adduct.202 When this monoalkylborane reacts with a prochiral alkene, one of the diastereomeric products is normally formed in excess and can be obtained in high enantiomeric purity by an appropriate separation.203 Oxidation of the borane then provides the corresponding alcohol having the enantiomeric purity achieved for the borane. 

Derivatization of the optically active aldehydes to imines has been used for determination of their enantiomeric excess. Chi et al.3 have examined a series of chiral primary amines as a derivatizing agent in determination of the enantiomeric purity of the a-substituted 8-keto-aldehydes obtained from catalysed Michael additions. The imine proton signals were well resolved even if the reaction was not completed. The best results were obtained when chiral amines with —OMe or —COOMe groups were used [2], The differences in chemical shifts of diastereo-meric imine proton were ca. 0.02-0.08 ppm depending on amine. This method has been also used for identification of isomers of self-aldol condensation of hydrocinnamaldehyde. 

Transannular interactions lead to ring closures and reductions to adamantane compounds when dienes of the bicyclo[3.3.1]nonane family are treated with Brpnsted acids and triethylsilane. Compounds 48-51 form reaction mixtures containing various amounts of products 52-54 (R = OH, O2CCF3, Cl) under such conditions.243 The best yields of hydrocarbon 52 occur when the dienes are treated with a 25% excess of sulfuric acid and a 50% excess of triethylsilane in dichloromethane at 20°.243 The stereospecific nature of these transannular reductions is demonstrated by the observation that the enantiomeric purity of the chiral diene 55 is retained in the chiral hydrocarbon product 56 (Eq. 98).243 Dienes of... 

Recent advances in gas chromatographic separations of enantiomers allow precise determination of the enantiomeric purity of the algal pheromones. The czs-disubstituted cyclopentenes, such as multifidene, viridiene, and caudoxirene, are of high optical purity [ 95% enantiomeric excess (e.e.)] whenever they have been found (32,33). The situation is different with the cyclopropanes and the cycloheptadienes, as shown in Table 2 and Figure 1. Hormosirene from female gametes or thalli of... 

The enantiomeric excess of the two samples should be >98.0% for the (R,R)-enantiomer and within 0.2% enantiomeric excess of each other. Otherwise, the product should be washed with more methanol to remove the undesired enantiomer. If this procedure fails to yield product of acceptable enantiomeric purity, the product can be further purified by recrystallization of the salt from water (1 10, w/v). This affords product in 60-70% overall yield and >99.5% ee. 

A final word needs to be said about the supposedly unique features of enzymes, namely, their ability to produce enantiomerically pure products. This is not the place to speculate about the stereospecificity of enzymes, a problem that has been discussed elegantly by Comforth (15). It cannot be denied that the high (>99.9%) enantiomeric purity achieved by enzymes may be uniquely useful in the case of liquid products. However, when crystalline products are obtained in an asymmetric synthesis and the e.e. exceeds 80%, crystallization to enantiomeric purity without excessive loss of material is routinely achieved. 

This definition refers to an enantiomeric mixture produced in an asymmetric synthesis. In some cases where a diastereomeric mixture is produced, the definition has to be altered accordingly. Percent optical purity is an operational term that depends on optical rotation measurements. It is not necessarily equal to the percent enantiomeric purity (13), which is a more meaningful term and is the extent to which one enantiomer is formed in excess over the other ... 

Clearly what we need are verifying experimental demonstrations to help us select among the various mechanisms outlined above for the appearance of a small enantiomeric excess (e.e.) within a mixture of two enantiomers. Only after such an initiating event can an e.e. be amplified into the state of homochirality and enantiomeric purity necessary to permit the emergence of self-replicating biopolymers. In what... 

How are the smafl-to-microscale excesses of one enantiomer over the other, produced by any of the scenarios outlined above, capable of generating a final state of enantiomeric purity In 1953 Frank [16] developed a mathematical model for the autocatalytic random symmetry breaking of a racemic system. He proposed that the reaction of one enantiomer yielded a product that acted as a catalyst for the further production of more of itself and as an inhibitor for the production of its antipode. He showed that such a system is kinetically unstable, which implies that any random fluctuation producing a transient e.e. in the 50 50 population of the racemic... 

Monobasic solutes that have no carbinyl hydrogens may also show nonequivalence. 3-Methyl-2-butanone, 4-methyl-2-pentanone, 2-methylpropanal, methyl 2-methylbutyrate, 2,2,6,6-tetramethyl-piperidine, methyldiisopropylcarbinol, and methylethyl-n-butyl-carbinol in TFAE-saturated benzene all show nonequivalence of sufficient magnitude (0.01-0,03 ppm) to allow nonequivalence sense determination at 220 MHz. An especially striking example is that provided by pyrazolines 26. With only a severalfold excess of (S)-TFAE, 3(5),5(S)-enriched samples of these compounds show nonequivalence in their methyl resonances (downfield sense for the singlets and upfield sense for the triplets) sufficient for enantiomeric purity determination at 90 MHz (52). 

Progress of the reaction was monitored using a GC equipped with a FID on an achiral CP 1301 capillary column (30 m x 0.25 mm x 0.25 m film) and N2 as carrier gas. Enantiomeric purity of 2-octanol was analysed after derivatization with acetic anhydride (see below) using a CP-Chirasil Dex-CB column (25 m x 0.32 mm x 0.25 pm film, column B) and H2 as carrier gas. Enantioselectivities (expressed as the enantiomeric ratio E) were calculated from enantiomeric excess of the product and conversion as previously reported. Retention times and methods are listed in Table 3.1. 

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