Enantiomers reactions producing

In an enantioselective reaction, one enantiomer is produced predominantly over its mirror image. 

Abstraction of a hydrogen atom from C2 produces a trigonal planar radical that is achiral. This radical is achiral then reacts with chlorine at either face [by path (a) or path (b)]. Because the radical is achiral the probability of reaction by either path is the same therefore, the two enantiomers are produced in equal amounts, and a racemic form of 2-chloropentane results. 

Whilst this may appear a problem in stereochemistry, it is actually mechanism based and depends upon the nature of the reaction intermediates. The loss of optical activity is because we form a racemic product, i.e. a mixture of enantiomers, or produce a compound that is no longer chiral. 

Desimoni and co-workers were able to produce either enantiomer (2R or 25) of the Diels-Alder cycloadduct using the same isomer of phe-box ligand ent-6 under different reaction conditions (Fig. 9.25, Table 9.11)." " They found that when using magnesium(II) perchlorate as the metal source, the reaction produced cycloadduct in >98% yield with an endo/exo ratio of 93 7 and an endo ee of 70% for the (25) isomer. In contrast, when magnesium(II) perchlorate was used in the presence of 2 equiv of water, the reaction afforded the cycloadduct again in >98% yield with an endo/exo ratio of 93 7, but in this instance, the endo ee was 65% for the (2R) isomer. This selectivity difference was explained by a change in... 

Stereoselective reactions are those that result in the selective production of one of the stereoisomers of the product. The extent of the selectivity may be recorded as the enantiomeric excess (e.e.) when the reaction produces a mixture of enantiomers and the diastereoisomeric excess (d.e.) when it produces a mixture of diastereoisomers. These quantities are defined by the expression ... 

Intramolecular cyclization of 54 catalyzed by 53a produced (+)-55 in 94% ee and 61% yield (see Scheme 10.26) [101]. If 53b was used instead, the opposite enantiomer was produced in 94% ee and 48% yield. The addition of 40 mol% NaBr or Nal improved reaction times from 4-5 days to 1 day, and was necessary to obtain high enantioselectivities. 

The reaction produces a racemic mixture of the product shown, (lR S)-2-bromo-1-phenyl-1-propanol and its enantiomer, the (I S,2R)-stereoisomer, in 92% yield. It is regiospecific, because none of the isomeric I-bromo-1-phenyl-2-propanol is formed. It is stereospecific, because none of the (I S,2S)-diastereomei or its enantiomer, the (I R,2R)-stereoisomer, is produced. 

The starting material for the acylase process is a racemic mixture of N-acetyl-amino acids 20 which are chemically synthesized by acetylation of D, L-amino acids with acetyl chloride or acetic anhydride in alkaU via the Schotten-Baumann reaction. The kinetic resolution of N-acetyl-D, L-amino acids is achieved by a specific L-acylase from Aspergillus oryzae, which only hydrolyzes the L-enantiomer and produces a mixture of the corresponding L-amino acid, acetate, and N-acetyl-D-amino acid. After separation of the L-amino acid by a crystallization step, the remaining N-acetyl-D-amino acid is recycled by thermal racemization under drastic conditions (Scheme 13.18) [47]. In a similar process racemic amino acid amides are resolved with an L-spedfic amidase and the remaining enantiomer is racemized separately. Although the final yields of the L-form are beyond 50% of the starting material in these multistep processes, the effidency of the whole transformation is much lower than a DKR process with in situ racemization. On the other hand, the structural requirements for the free carboxylate do not allow the identification of derivatives racemizable in situ therefore, the racemization requires... 

Successive work of the asymmetric reduction of prochiral ketones shows that (S)-enantiomers are produced by using the combination of chiral diamine 1 with zinc(II) chloride whereas (R)-enantiomers are obtained by using the combination of chiral diamine 1 with tin(II) chloride under the same reaction conditions (eq 15). ... 

Sometimes this reaction produces two stereoisomers, as in the case of c/s-2-butene, which forms an equal amount of two enantiomers. Sometimes it produces a single compound, as in the case of frans-2-butene, where a meso compound is formed... 

As described in this chapter, there are many reactions that can be performed by chemists to create new chiral centers. When these reactions are performed in such a way as to create one enantiomer in greater amounts than the other the process is called asymmetric or stereoselective synthesis. The term en-antioselectivity refers to the efficiency with which the reaction produces one enantiomer. This efficiency is quantitatively described as the enantiomeric excess (ee) of the product, which is the percentage by which one enantiomer is produced in excess of the other. Thus a 45 8 mixture of two enantiomers will have an enantiomeric excess of [(45 - 8)/(45 + 8)] X 100, which equals 70%. It should be noted that if neither the startingmaterial or reaction system is chiral and non-racemic, then the product will be formed as an equal mixture of the enantiomers (i.e., a racemate). 

Fonnation of Isoindoks. In 1971 Roth described a sensitive analytical method for amino adds based on their reaction with o-phthaldia dehyde (OPA) and a thiol, 2-mercaptoethanol (117). The reaction produces an intensely fluorescent isoindole derivative of the amino acid and is specific for the primary amino group (118,119). Subsequently, the OPA method was extended to the enantiospedfic LC analysis of amino adds by substituting an optically active (single enantiomer) thiol for 2-mercaptoethanol in the derivatization reaction (120-122). This results in the formation of two... 

This reaction proceeds via a cyclic bromonium ion. The anti-addition of the bromide anion may take place at either of the two carbon atoms that form part of the ring, and so two products are formed in a 1 1 ratio. These are the threo d,l pair of stereoisomers. The resultant mixture is not optically active, because each enantiomer is produced in equal amounts. 

The enantioselective hydrolysis of racemic N-acetylated a-amino acids d,l-1 at De-gussa represents a long established large-scale process for the production of L-ami-no acids, l-2 [4]. This enzymatic resolution requires an L-aminoacylase as the biocatalyst. The starting materials for this process are readily available, since racemic N-acetyl amino acids d,l-1 can be economically synthesized by acetylation of racemic a-amino acids with acetyl chloride or acetic anhydride under alkaline conditions via the so-called Schotten-Baumann reaction [5]. The enzymatic resolution reaction of N-acetyl d,L-amino acids, d,l-1, is achieved by a stereospecific L-aminoacylase which hydrolyzes only the L-enantiomer and produces a mixture of the corresponding L-amino acid, l-2, acetate, and N-acetyl D-amino acid, d-1 (Fig. 4) [6],... 

It is apparent from the preceding chapter that the analysis of enantiomers (by whatever means) addresses only part of the problem often, a stereoselective reaction produces a mixture of diastereomers, and polarimetry is an inappropriate technique. Thus, asymmetric synthesis requires the means for the analysis of both enantiomeric and diastereomeric mixtures. Ultimately, the ratio of isomers and the cofifiguration of each new stereocenter should be determined. 

The final definition concerns formation of enantiomers, and the terms enantioselective and enantiospecific are used. If a reaction produces an unequal mixture of enantiomers it is enantioselective. If it generates only one enantiomer of two possibilities, it is enantiospecific. The baker s yeast reduction (see sec. 4.10.F) of 134 gave 135 with >99% ee (S). (Here % ee means percent of enantiomeric excess.) A 0% ee means a 50 50 mixture (racemic mixture), 50% ee means a 75 25 mixture and 90% ee means a 95 5 mixture. The predominance of the (S) enantiomer makes this reaction highly enantioselective. 

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