Enriched products

As was the case for kinetic resolution of enantiomers, enzymes typically exhibit a high degree of selectivity toward enantiotopic reaction sites. Selective reactions of enaiitiotopic groups provide enantiomerically enriched products. Thus, the treatment of an achiral material containing two enantiotopic functional groups is a means of obtaining enantiomerically enriched material. Most successful examples reported to date have involved hydrolysis. Several examples are outlined in Scheme 2.11. 

Enantioselective desymmetrization of achiral or meso compounds with formation of enantiomerically enriched products, among them heterocycles 99JCS(P1)1765. 

Hie use of chiral catalysts as an approach to enantiomer icaliy enriched products by means of coppet-mediated substitution reactions is covered in this chapter. Reactions in which a chiral auxiliary resides in the leaving group of the substrate w ill also he dealt with, since these reactions provide direct and efBcient routes to single enantiomers of the desired products. Most studies so far have been concerned with allylic substrates, with a new chiral center being produced in the course of a selec-... 

With this epoxidation procedure it is possible to convert the achiral starting material—i.e. the allylic alcohol—with the aim of a chiral reagent, into a chiral, non-racemic product in many cases an enantiomerically highly-enriched product is obtained. The desired enantiomer of the product epoxy alcohol can be obtained by using either the (-1-)- or (-)- enantiomer of diethyl tartrate as chiral auxiliary ... 

After filtering and evaporating the solvent under reduced pressure, the pasty residue, constituted by the enriched product, is diluted with 30 ml ether and in this way 0.225 g reserpine (which has not taken part in the reaction) is isolated by filtration. 

As well as the addition of achiral organometallic reagents to chiral aldehydes (see also Sections 1.3.2. and 1.3.3.), the addition of chiral organometallic reagents to carbonyl compounds is a well-known and intensively studied process which may lead to enantiomerically and/or diastereomerically enriched products. Chiral organometallic reagents can be classified into three groups ... 

Extension of these processes to provide enantio-enriched products was successfully applied after desymmetrization of the starting materials. An example is shown below (Reaction 76), where silane-mediated xanthate deoxygenation-rearrangement-electrophile trapping afforded the conversion of (+)-94 to (+)-95 in 56% yield. ... 

Development efforts in the nuclear industry are focusing on the fuel cycle (Figure 6.12). The front end of the cycle includes mining, milling, and conversion of ore to uranium hexafluoride enrichment of the uranium-235 isotope conversion of the enriched product to uranium oxides and fabrication into reactor fuel elements. Because there is at present a moratorium on reprocessing spent fuel, the back end of the cycle consists only of management and disposal of spent fuel. 

The range of whey products that are used include, for example, ultra-filtered and dried WPC, which contains between 20% and 89% protein ion exchange and membrane filtered WPI, which contains at least 90-95% protein (Tunick, 2008) and other whey fraction-enriched products such as p-lactalbumin. These enriched protein whey products can be texturized and used in the manufacture of high-protein content puffed com products (Onwulata et al, 2010). 

The stereogenic centers may be integral parts of the reactants, but chiral auxiliaries can also be used to impart facial diastereoselectivity and permit eventual isolation of enantiomerically enriched product. Alternatively, use of chiral Lewis acids as catalysts can also achieve facial selectivity. Although the general principles of control of the stereochemistry of aldol addition reactions have been well developed for simple molecules, the application of the principles to more complex molecules and the... 

Chiral Bronsted acid co-catalysts do not promote formation of optically enriched products in analogous couplings to pyruvates, although increased rate and conversion in response to the Bronsted acid co-catalyst is unmistakably apparent. For pyruvates, protonation likely occurs subsequent to the C-C... 

The following reasoning was used to eliminate the less probable mechanisms shown in Figure 4. A H atom is added to naphthalene to form an a-radical in reaction 1A and a /3-radical in reaction IB. Both are resonance-stabilized radicals. They can lose either a 2H atom or a H atom to regenerate naphthalene. We have shown a 2H atom lost to form a protium-enriched product in reactions 1A and IB. The fact that we observe a fourfold increase of protium in the a-position of spent naphthalene suggests that reaction IB is faster than reaction 1A and, therefore, is the predominant mechanism. 

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

In Scheme 17.2 palladium is coordinated from below, but it is also possible that it coordinates from above and forms the other enantiomer of the chiral syn,syn Jt-allyl complex. If palladium has another chiral ligand then these Jt-allyl complexes become diastereomers. Thus, from an unsymmetrically substituted allene (R R ), eight diastereomeric Jt-allyl complexes can be formed. If one of the diastereomers is preferred then further reaction of the Jt-allyl moiety leads to an enantiomerically enriched product. 

In order to elucidate the nature of this mechanism further, Gibson (nee Thomas) studied134,136 the conversion of an optically pure sample of vinylke-tone 121.fi to the corresponding vinylketene (221.e). The vinylketone was resolved by carbonyl ligand substitution with (+)-neomenthyldiphenyl-phosphine, followed by subsequent separation of the resultant diastereo-mers to yield an optically enriched product.136,137 When 222.e, of known e.e., was treated with methyllithium under an atmosphere of carbon monoxide, the expected vinylketene complex 221.e was isolated and was found... 

A number of separative units operating in parallel, all taking feed of identical composition, and discharging partially enriched product and partially depleted waste streams comprise a stage. A single unit may suffice for a stage, but more often several units or many units are required. Because the separation per stage is almost... 

Kim et al. [67] recently reported the synthesis of heterometallic chiral polymer (salen) Co-(Al, Ga, ln)Cl3 complexes 26-32 (Figure 10) and their use in the HKR of racemic epoxides. Polymeric salen catalysts showed very high reactivity and enantioselectivity at substantially lower catalyst loadings for the asymmetric ring opening of terminal epoxide to obtain the enantio-enriched products. The performance of catalysts is retained on multiple-use and do not suffer the problems of solubility and deactivation (Scheme 5). 

Enantiomer-selective deactivation of racemic catalyts by a chiral deactivator affects the enantiomer-selective formation of a deactivated catalyst with low catalytic activity (Scheme 8.2). Therefore, it is crucial for a chiral deactivator to interact with one enantiomer of a racemic catalyst (Scheme 8.2a). As the chiral deactivator does not interact with the other enantiomer of racemic catalyst, the enantiomeri-cally enriched product can be obtained. Therefore, the level of enantiomeric excess (% ee) could not exceed that attained by the enantiopure catalyst. On the other hand, nonselective complexation of a chiral deactivator would equally and simultaneously deactivate both catalyst enantiomers, thereby yielding a racemic product (Scheme 8.2b). Although this strategy tends to use excess chiral poison relative to the amount of catalyst, it offers a significant advantage in reducing cost and synthetic difficulty since readily available racemic catalysts and often inexpensive chiral poisons are used. 

Cholesterol samples biosynthesized from [5- C]- and from [3, 4- C2]-mevalonate, respectively, showed distributions in accordance with the accepted biosynthetic sequence (Scheme 1). N.m.r. spectra of the C-enriched products not only revealed the positions of the labels, but also showed the expected... 

Chirality plays a central role in the chemical, biological, pharmaceutical and material sciences. Owing to the recent advances in asymmetric catalysis, catalytic enantioselective synthesis has become one of the most efficient methods for the preparation of enantiomer-ically enriched compounds. An increased amount of enantiomerically enriched product can be obtained from an asymmetric reaction using a small amount of an asymmetric catalyst. Studies on the enantioselective addition of dialkylzincs to aldehydes have attracted increasing interest. After the chiral amino alcohols were developed, highly enantioselective and reproducible —C bond forming reactions have become possible. 

Hydrolase-catalyzed acylation can be used to purify a diastereo- and enantiomerically enriched product. For example dimethylzinc addition to the racemic aldehyde 77 furnishes the racemic phenylsulfanylbutanol 78 (Scheme 4.29) in a 95/5 (2R, 3R )/(2R, 3S )-mtio. When this is treated with Chirazyme L2 (CALB) and vinyl acetate in heptane it is resolved with a high E-value (>400) [91]. However the diastereomeric ratio in the remaining substrate and produced ester is virtually unchanged. To circumvent the problematic contamination with the undesired diastereomers, enantiomerically enriched aldehyde 77 was reacted with dimethyl-zinc to furnish one major stereoisomer of 78 contaminated with a small amount of a mixture of the other three (Scheme 4.29). Because the two major contaminants had the opposite configuration at position 2 relative to the major product, these contaminants were efficiently removed from the major product and the trace byproduct by treatment with the 2R-selective Chirazyme L2 (CALB) and vinyl acetate in heptane to furnish virtually diastereo- and enantiomerically pure acetate (2R,3R)-79 or the alcohol (2S,3S)-78 (Scheme 4.29) [91]. 

Cereal bran and bran-enriched products are the most important source of wall-bound cinnamates with up to 30 and 7 mg ferulate/10 g in maize and wheat bran, respectively. This would make these products the richest dietary source of ferulic acid. However, coffee brew could supply up to 10 mg ferulate (as feruloylquinic acid, FQA) per 200 ml cup [13], and it is the first for conjugated ferulic acid, followed by Citrus juices. 

Asymmetric synthesis is any synthesis that produces enantiomerically or diastereomeri-cally enriched products. This is the expected result if enantiomerically enriched chiral substrates are employed. Of interest here are asymmetric syntheses where the reactants are either achiral or chiral but racemic. Many examples of this type are collected in volumes edited by Morrison [33]. The first example of an asymmetric synthesis involved use of the chiral, optically pure base brucine in a stereoselective decarboxylation of a diacid with enantiotopic carboxyl groups [34] ... 

The reaction of Z enamides catalyzed by the (R)-BINAP-containing Ru complexes yields IR products predominantly, whereas lS -enriched products are obtained by hydrogenation with (S)-BINAP-based catalysts. As shown in Scheme 22, asymmetric hydrogenation followed by... 

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