Stereo isomer separation

These techniques are of particular interest in that they provide a means of separating molecular species which are difficult to separate by other techniques and which may be present in very low concentrations. Such species include large molecules, sub-micrometre size particles, stereo-isomers and the products from bioreactors (Volume 3). The separations can be highly specific and may depend on molecular size and shape, and the configuration of the constituent chemical groups of the molecules. 

There is an increasing number of areas where bioreactors are serious alternatives to conventional chemical reactors, particularly when their mild conditions and high selectivity can be exploited. In the pharmaceutical industry micro-organisms and enzymes can be used to produce specific stereo-isomers selectively, a very desirable ability since it can be that only the one isomer (possibly an optical isomer) may possess the required properties. In such applications the limitations of bioreactors are clearly outweighed by the advantages in their use. The fact that the products are formed in rather dilute aqueous solution and at relatively low rates may be of secondary importance and it may then be economically feasible to employ multiple separation stages in their purification. 

To describe the composition of fatty acids it is sometimes useful to use a shorthand designation. In this convention the composition of a fatty acid can be described by two numbers separated by a colon. The first number indicates the number of carbon atoms in the fatty acid chain, the second number indicates the number of double bonds. Thus, 4 0 is short for butyric acid, 16 0 for palmitic acid, 18 1 for oleic acid, etc. The two numbers provide a complete description of a saturated fatty acid. For unsaturated fatty acids, information about the location of double bonds and their stereo isomers can be given as follows oleic acid (the cis isomer) is 18 lc9 elaidic acid (the... 

The enol ester or silyl enol ether route to enolates has advantages over direct deprotonation in certain cases. If direct deprotonation provides a mixture of regio- or stereo-isomers, it is often possible to trap the enolate mixture by esterification or silylation, separate the desired enol ester or silyl enol ether and regenerate the enolate by reaction with methyllithium. It is also useful for preparation of enolates from substances that are so electrophilic that direct deprotonation is complicated by self-aldolization. For example, aldehyde enolates have been prepared in this manner (equation 14). ... 

All the above-mentioned syntheses of accurate copies of natural pheromone compositions are based on the proper choice of cometathesis reaction conditions. It is possible to realize this approach only in some cases. In order to prepare pheromone compositions of any desired content, we have elaborated clatrate separation technique, allowing the isolation of individual stereo-isomers. 

Herbivores can typically sense suitable host plants using olfactory cues from long distance. Many volatile terpenoids bear the essential information in their molecular structure. Different stereo isomers of the same compound may result in different response when sensed by insect antennae or the olfactory sensors in the nose of vertebrate animals. Another important factor affecting signal perception and behavioral response in herbivore is the relative proportion of different volatile compounds, terpenoids or other volatiles, in the odor plume released by a plant. CombinatiOTi of certain monoterpenes and sesquiterpenes are very distinctive in certain plant families. Specialist herbivore species can separate these combinations from similar monoterpenes released by other plants, because of their strict ratio in the host species. 

Fujimoto and co-workers [37] studied the pyrolysis products of polysiloxanes including their stereo and structural isomers. Separation was achieved by GC on a fused silica capillary column. Mass fragmentation data was obtained on pyrolysis products formed at 600 °G. 

Reaction of the isohumulones with sodium borohydride leads to reduction of the carbonyl group in the 4-methyl-3-pentenoyl side chain to a secondary alcohol function. Since a new chiral centre is introduced, four diastereo-isomeric reaction products are possible. They are the rho-isohumulones or 2-(3-methylbutanoyl)-5-(3-methyl-2-butenyl)-3,4-dihydroxy-4-(1-hydroxy-4-methyl-2-pentenyl)-2-cyclopentenones (18,19). Separation by counter-current distribution (CCD) of the individual stereo-isomers is only possible when the reduction is carried out on cis and trans isohumulone separately. In the two-phase system iso-octane aqeous buffer pH 5.4, trans rho-1-isohumulone (99, Fig. 50) has a K value of 1.0 after 450 transfers trans... 

A palladium-cataly/ed 1,6-enyne cycli/ation of the Tmst20 type provides the desired bicyelic system as a mixture of hexahydro-5//-1-pyrindine 14-Z and its double-bond isomer 15-Z in a ratio of 97 3. The two can be separated by column chromatography In the process, the exocyclic alkylidene sidechain is created with Z selectivity, and the rings are stereo specifically joined cis. It should be noted that only with the use of BBEDA [A V -bis(benzylidene)ethylene diamine] as a powerful a-donor ligand is high Z selectivity achieved Other catalytic systems, such as Pd(OAc) or Pd(OAc) ifPPhOVHOAc produce EfZ mixtures of 14 and 15 in ratios of 45 25 27 3 or 9 69 6 16 (14-Z 15-Z 14-E 15-E). 

Michael addition of pentM-yn-1-thiol to an activated alkyne produces co-yne vinylsulfides 424 as a separable mixture of the E and Z isomers. On treatment with -Uu (SnI I and a radical initiator, these substrates undergo a double radical cyclization accompanied by [(-fragmentation of the stannyl radical. The process is regio-, chemo-, and stereo-selective and produces the -isomer of the 5-substituted 3,4-dihydm-2//-thiopyran (Scheme 134) <1997JOC8630>. 

At the instant Pasteur recognized the existence of stereoisomers (objects), he also accepted the existence of stereoprocesses (operations). For the notion of isomer carries with it criteria of distinguishability among these is the possibility that a given isomer can be formed, separated, or altered in a way which differentiates it from other isomers. This applies equally to isomers with many properties in common, e.g. optical antipodes, or to those with essentially all different properties, e.g. cis-trans, syn-anti, gauche-anti, erythro-threo, or axial-equatorial pairs. Now, the stereo-path may be part of an overall conversion which, if described in some detail, we term a mechanism. Our present task is to attempt to understand those elementary or single-step processes by which stereochemical choices are made. 

Oxiranes can be opened stereo- and regiospecifically under mild conditions with alcohols, thiols, amines, and acetic acid, with simultaneous activation of the nucleophilic and electrophilic centers on an alumina surface.Diols have been obtained on silica gel in the presence of water " and adsorbed FeCls. On an ion-exchange resin, the opening of oxirane isomers to diols and the separation of the latter, ethanolysis, the reaction with phenol,and catalytic ammonolysis have been described. Hydrolysis and methanolysis catalyzed by Nafion-H, a perfluorinated resinsulfonic acid have been reported. ... 

The authors have included a stereo-unselective dipolar cycloaddition to provide two diastereomers of the final product in order to assign the hitherto unknown stereochemistry at C-5 of the natural product. Therefore, separation of isomers was necessary once in the course of the synthesis to obtain diastereo- and enantiopure myriaporone 4 and its C-5 epimer. 

As mentioned before, this cycloaddition is deliberately stereo-unselective so as to produce two diastereomers of myriaporone 4, in order to assign the hitherto unknown stereochemistry of the natural product at C-5. However, it was not possible to separate 29a and 29b at this stage. Therefore, total synthesis was continued with the mixture of isomers before separating them via column chromatography after three additional steps. 

The IMER approach does not require that the enzyme be placed in close proximity to the detector if the transducer signal is generated by a soluble product or cosubstrate of the enzymatic reaction. In the latter case, a variety of flow systems and postreactor detectors can be utilized to produce simultaneous determinations of the concentrations of several analytes. For example, an IMER can be combined with a high-performance liquid chromatography (HPLC) instrument (perhaps also in combination with mass spectroscopy) for purposes of both qualitative and quantitative analysis. The chemo-, stereo-, and regio-selectivities of enzymes facilitate separation and/or identification of analytes that may be present as different isomers (e.g., in peptide analysis based on use of peptidase IMERs in combination with these techniques to obtain structural information about the sequence of amino acids in peptides). 

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