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Usefulness of racemic mixtures

In practice, if both optical isomers are of similar potency and do have similar pharmacokinetic profiles, it may be [Pg.543]

FIGURE 26.6 The structures of hexobarhital, 5-ethyl-5-phenyl-hydantoin, and glutethimide. [Pg.543]

FIGURE 26.8 Stereoselective metabolic attack yielding toxic metaboUtes.  [Pg.544]

In both series almost eqnipotent activities were observed for thromboxane receptor antagonism and thromboxane synthase inhibition (IC50 = 2-30 nM). Upon oral administration to guinea pigs the enantiomers inhibited the ex vivo U-46619-induced platelet aggregation with potencies similar to that of the corresponding racemates. This indicates [Pg.544]

The recommendations of the European Community Working Party on drug quality, safety and efhcacy, take into account two situations. For already well-established racemates the clinical use can continue as such, no specific study of the isolated enantiomers is required. For newly introduced chiral drugs both enantiomers have to be prepared and studied separately with regard to their activity as well as their disposition in vivo. However the final decision to introduce the drug on the market as enantiomer or as racemate belongs to the producer. [Pg.544]


The distomer counteracts the eutomer Racemic switches The distomer is metabolized to unwanted or toxic products Deletion of the chiral centre Usefulness of racemic mixtures... [Pg.275]

Much more attractive seems to be the use of racemic mixtures of enantiomers, generally easily available, which could be polymerized through a sterically controlled mechanism and give optically active polymers. [Pg.111]

Microorganisms and their enzymes have been used to functionalize nonactivated carbon atoms, to introduce centers of chirahty into optically inactive substrates, and to carry out optical resolutions of racemic mixtures (1,2,37—42). Their utifity results from the abiUty of the microbes to elaborate both constitutive and inducible enzymes that possess broad substrate specificities and also remarkable regio- and stereospecificities. [Pg.309]

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. [Pg.89]

A better solution for preparative columns is the development of separation media with substantially increased selectivities. This approach allows the use of shorter columns with smaller number of theoretical plates. Ultimately, it may even lead to a batch process in which one enantiomer is adsorbed selectively by the sorbent while the other remains in the solution and can be removed by filtration (single plate separation). Higher selectivities also allow overloading of the column. Therefore, much larger quantities of racemic mixtures can be separated in a single run, thus increasing the throughput of the separation unit. Operation under these overload conditions would not be possible on low selectivity columns without total loss of resolution. [Pg.61]

For the separation of racemic mixtures, two basic types of membrane processes can be distinguished a direct separation using an enantioselective membrane, or separation in which a nonselective membrane assists an enantioselective process [5]. The most direct method is to apply enantioselective membranes, thus allowing selective transport of one of the enantiomers of a racemic mixture. These membranes can either be a dense polymer or a liquid. In the latter case, the membrane liquid can be chiral, or may contain a chiral additive (carrier). Nonselective membranes can also... [Pg.126]

The cyclic steady state SMB performance is characterized by four parameters purity, recovery, solvent consumption, and adsorbent productivity. Extract (raffinate) purity is the ratio between the concentration of the more retained component (less retained) and the total concentration of the two species in the extract (raffinate). The recovery is the amount of the target species obtained in the desired product stream per total amount of the same species fed into the system. Solvent consumption is the total amount of solvent used (in eluent and feed) per unit of racemic amount treated. Productivity is the amount of racemic mixture treated per volume of adsorbent bed and per unit of time. [Pg.235]

With this single example we have in tact described two uses of enzymes in alicyclic chemistry, the reduction of ketone groups and the resolution of racemic mixtures. [Pg.327]

Kcurentjes et al. (1996) have also reported the separation of racemic mixtures. Two liquids are made oppositely chiral by the addition of R- or S-enantiomers of a chiral selector, respectively. These liquids are miscible, but are kept separated by a non-miscible liquid contained in a porous membrane. These authors have used different types of hollow-fibre modules and optimization of shell-side flow distribution was carried out. The liquid membrane should be permeable to the enantiomers to be separated but non-permeable to the chiral selector molecules. Separation of racemic mixtures like norephedrine, ephedrine, phenyl glycine, salbutanol, etc. was attempted and both enantiomers of 99.3 to 99.8% purity were realized. [Pg.433]

My second question is this You mentioned ferrfluramine. I presume you used the racemic mixture, which would mean that in the brain you would have both R and S fenfluramine and R and S norfenfluramine present. And since these differ widely in their effects on dopamine versus serotonin neuronal systems, have you studied individual enantiomers of either fenfluramine or norfenfluramine ... [Pg.23]

We monitored the percent conversion of epichlorohydrin and enantiomeric excess of the recovered S-epichlorohydrin with time by using GC-FID. Approximately 54% conversion and >99% ee were obtained in about 4 h reaction time. After 4 h, the epichlorohydrin was removed under vacuum at room temperature and diol was removed at a temperature of 329 K. The recovered catalyst was further treated in the HKR of racemic mixture of fresh epichlorohydrin. In the second run, we observed a decrease in the conversion and ee compared to the fresh catalyst. The Co-salen was again recovered after the second run by removing all the products under vacuum and recycled two more times. With each subsequent HKR reaction, the conversion and ee were found to decrease with time (22). Table 43.1 summarizes the initial rates and ee s determined from the four runs without intermediate catalyst regeneration. Interestingly, the initial catalyst activity was resumed when the catalyst was regenerated with acetic acid prior to recycle. [Pg.392]

Aliphatic acids such as butyric acid have been previously implicated as being allelopathic compounds (46, 47, 23). Chou and Patrick (23) isolated butyric acid from soil amended with rye and showed that it was phytotoxic. Hydroxy acids have also been shown to possess phytotoxic properties (48) but have not been implicated in any allelopathic associations. Since SHBA is a stereo isomer, and the enantiomer was not identified because of impurity, all bioassays were run using a racemic mixture. The D-(-) stereo isomer of SHBA has been isolated from both microorganisms and root nodules of legumes and is suspected to be a metabolic intermediate in these systems (49). It is likely that only one enantiomer was present in the extract therefore, the true phytotoxic potential of this compound awaits clarification of the phytotoxicity of the individual enantiomers. [Pg.264]

The methods which have been used to determine the configurational stability of organotin compounds and those which have successfully been applied to synthesize such optically active molecules are reviewed, including the chromatographic resolution of racemic mixtures. [Pg.62]

D-Pantolactone and L-pantolactone are used as chiral intermediates in chemical synthesis, whereas pantoic acid is used as a vitamin B2 complex. All can be obtained from racemic mixtures by consecutive enzymatic hydrolysis and extraction. Subsequently, the desired hydrolysed enantiomer is lactonized, extracted and crystallized (Figure 4.6). The nondesired enantiomer is reracemized and recycled into the plug-flow reactor [33,34]. Herewith, a conversion of 90-95% is reached, meaning that the resolution of racemic mixtures is an alternative to a possible chiral synthesis. The applied y-lactonase from Fusarium oxysporum in the form of resting whole cells immobilized in calcium alginate beads retains more than 90% of its initial activity even after 180 days of continuous use. The biotransformation yielding D-pantolactone in a fixed-bed reactor skips several steps here that are necessary in the chemical resolution. Hence, the illustrated process carried out by Fuji Chemical Industries Co., Ltd is an elegant way for resolution of racemic mixtures. [Pg.86]

Stereoselective kinetic control of the 0-methylation of racemic mixtures of secondary alcohols has been reported using (S)-(+)-(2-methylbutyl)triethylammo-nium bromide as the catalyst [27]. However, the claim that the (/ )-(+)-methyl ether (48% ee) is produced from racemic 1-hydroxy-1-phenylethane leaving the (S)-alcohol unchanged has been shown to be totally spurious [28]. [Pg.535]

Trigonal ML3 metal complexes exist as optically active pairs. The complexes can show enantiomeric selective binding to DNA and in excited state quenching.<34) One of the optically active enantiomers of RuLj complexes binds more strongly to chiral DNA than does the other enantiomer. In luminescence quenching of racemic mixtures of rare earth complexes, resolved ML3 complexes stereoselectively quench one of the rare earth species over the other. 35-39 Such chiral recognition promises to be a useful fundamental and practical tool in spectroscopy and biochemistry. [Pg.88]

Levorotatory isomers of these drugs are much more powerful adrenoblockers than dextrorotatory isomers however, all of these drugs are made and used as racemic mixtures. [Pg.163]

Scheme 1. Structures of the native amino acids and their nonnatural a-substituted analogs. a-Trifluoromethyl phenylalanine as well as a-trifluoromethyl alanine have been used as racemic mixtures. Scheme 1. Structures of the native amino acids and their nonnatural a-substituted analogs. a-Trifluoromethyl phenylalanine as well as a-trifluoromethyl alanine have been used as racemic mixtures.
Racemate - Traditionally, a 1 1 mixture of enantiomers was denoted racemic mixture. In order to describe precisely the solid state according to distinct phases racemates (or racemic compounds), conglomerates and solid solutions were distinguished. Thus, the term race-mate had a very specific function. However, currently the term racemate is almost exclusively used in the general sense of racemic mixture. It appears sensible to comply to current usage and reserve the term racemic compound to a distinct solid phase. [Pg.74]

Production. Many industrial processes exist for the production of menthols. For (—)-menthol, isolation from peppermint oil (see Mint Oils) competes with partial and total syntheses. When an optically active compound is used as a starting material, optical activity must be retained throughout the synthesis, which generally consists of several steps. Total syntheses or syntheses starting from optically inactive materials require either resolution of racemic mixtures or asymmetric synthesis of an intermediate. Recently used processes are the following ... [Pg.53]

Apart from their obvious utility in separating mixtures of cations,68 crown ethers have found much use in organic synthesis (see the discussion on p. 363). Chiral crown ethers have been used for the resolution of racemic mixtures (p. 121). Although crown ethers are most frequently used to complex cations, amines, phenols, and other neutral molecules have also been complexed69 (see p. 133 for the complexing of anions).70... [Pg.83]

Another disadvantage of chemical synthesis is that when it is used to produce certain biologically important compounds, such as amino acids, a racemic mixture of D and L isomers results. Since organisms, by and large, metabolize the L-form selectively, as in the case of amino acids, the use of such racemates in biological investigations is somewhat unphysical and may lead to undesirable confusion. Methods for the resolution of racemic mixtures are available. Most of these are tedious and not suited for small-scale operation. [Pg.100]


See other pages where Usefulness of racemic mixtures is mentioned: [Pg.647]    [Pg.543]    [Pg.284]    [Pg.543]    [Pg.215]    [Pg.249]    [Pg.647]    [Pg.543]    [Pg.284]    [Pg.543]    [Pg.215]    [Pg.249]    [Pg.238]    [Pg.105]    [Pg.14]    [Pg.243]    [Pg.105]    [Pg.115]    [Pg.29]    [Pg.255]    [Pg.103]    [Pg.58]    [Pg.121]    [Pg.248]    [Pg.160]    [Pg.7]    [Pg.144]    [Pg.171]    [Pg.119]    [Pg.1266]   


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