Repeated resolution

Fig. 7. Time course of repeated resolution. The repeated resolution of 1 vol.% 1,2-butanediol was carried out in the presence of 1 mmol NAD+ ( ) or phenazinemethosulfate (o), or in the absence of an electron acceptor ( ) under aerobic conditions...

In cases where the E-value is rather low, repeated resolutions may be required to supply the desired enantiomers in high ees. The synthesis of both enantiomers of the marine natural product laurene 13 provides an illustrative example. Here, the racemic cyclopentylcarbinol 12 is acylated with vinyl acetate in hexane catalyzed by CRL (E 35) to provide the remaining substrate in high . Chemical hydrolysis followed by repeated CRL-catalyzed acylation and then chemical hydrolysis provides the other enantiomer of the alcohol (Scheme 4.11) [52]. 

Optical resolution did not provide pure enantiomers of 28 in any cases (Table 8) but repeated resolution of the complex forming enantiomeric mixture yielded (IR,2S,5R)-2H in 92-94 % ee. Treatment of the (1S,2R,5>S)-28 containing enantiomeric mixture with the same resolving agent (//.//-isomer of DBTA) did not affect dramatic change of the enantomeric ratio. Pure (lS,2R,5S)-28 could be prepared by the use of the mirror image isomer of DBTA during a repeated resolution (Table 9). 

TABLE -9. Repeated resolutions of enantiomeric mixtures of 28 in melt ... 

The aspects considered for a reliable evaluation of the performance of the miniaturized mass spectrometer are repeatability, resolution, and detection limit. Over the last years, these parameters could be improved by theoretical analyses and validated with successive chip generations [34, 35],... 

In practice, however, deracemization via repeated resolution is often plagued by low overall yields due to the harsh reaction conditions required for (chemical) racemization [71]. In view of the mild reaction conditions displayed by enzymes, there is a great potential for biocatalytic racemization based on the use of racemases of EC-class 5 [72, 73]. 

In such cases the enantiomeric excess is crystallized with a higher purity than that of in the initial composition, in a certain range of temperature. For example, the common intermediate of the synthesis of several prostaglandines is a lactone (PGL). Its enantiomeric mixtures (eeo) can be enriched by crystallization (eesoUd) from melt, while the racemic ratio (liquid residue, eeuquid) can be recovered for repeated resolution.20... 

In the ideal case, methods A, B and C provide equal amounts of (S)- and (i )-a-[ H/ C]-amino acids (or derivatives) upon separation. If complete transformation of racemic material is required, re-racemization of the undesired enantiomer can be accomplished chemically by acid catalysis in the presence of catalytic amounts of salicylaldehyde thermal treatment or enzymatically with the aid of transferases and subjection of the undesired product to repeated resolution cycles, as illustrated for (21 )-[3- C]tryptophan (16). 

Resolution methods using nonopticaHy active agents are also used by taking advantage of the fact that certain benzoic acid derivatives of (A)-menthol can be inoculated with crystals of one enantiomer to induce immediate crystallization of that enantiomer. Although repeated crystallizations and separations must be done, the technique has been successhil for (—)-mentho1 (157). 

Primary Irritancy Studies. These studies are employed to determine the potential of materials to cause local inflammatory effects in exposed body surfaces, notably skin and eye, following acute or short-term repeated exposure. In general, the approach involves applying the test material to the surface of the skin or eye, and observing for signs of inflammation, their duration, and resolution. Reviews have been written about the conduct of primary eye irritation (58,86,87) and primary skin irritation studies (88,89). 

The F-actin helix has 13 molecules of G-actin in six turns of the helix, repeating every 360 A. Oriented gels of actin fibers yield x-ray fiber diffraction patterns to about 6 A resolution. Knowing the atomic structure of G-actin it was possible for the group of Ken Holmes to determine its orientation in the F-actin fiber, and thus arrive at an atomic model of the actin filament that best accounted for the fiber diffraction pattern. 

The amplitudes and the phases of the diffraction data from the protein crystals are used to calculate an electron-density map of the repeating unit of the crystal. This map then has to be interpreted as a polypeptide chain with a particular amino acid sequence. The interpretation of the electron-density map is complicated by several limitations of the data. First of all, the map itself contains errors, mainly due to errors in the phase angles. In addition, the quality of the map depends on the resolution of the diffraction data, which in turn depends on how well-ordered the crystals are. This directly influences the image that can be produced. The resolution is measured in A... 

Yes, repeated laser shots sample progressively deeper layers depth resolution 50-100 nm... 

As we have repeatedly seen in this chapter, proponents of computer simulation in materials science had a good deal of scepticism to overcome, from physicists in particular, in the early days. A striking example of sustained scepticism overcome, at length, by a resolute champion is to be found in the history of CALPHAD, an acronym denoting CALculation of PHAse Diagrams. The decisive champion was an American metallurgist, Larry Kaufman. 

Two-phase suspension systems produce beaded products with broader particle-size distribution (e.g., 1-50 /rm). The microspherical particles usually need to be classified repeatedly to reduce the particle-size distribution in order to improve the resolution and efficiency in the separation for use in chromatography. The actual classification process depends on the size range involved, the nature of the beaded product, and its intended applications. Relatively large (>50 /rm) and mechanically stable particles can be sieved easily in the dry state, whereas small particles are processed more conveniently in the wet state. For very fine particles (<20 /rm), classification is accomplished by wet sedimentation, countflow setting, countflow centrifugation, or air classification. 

The methacrylic backbone structure makes the spherical Toyopearl particles rigid, which in turn allows linear pressure flow curves up to nearly 120 psi (<10 bar), as seen in Fig. 4.45. Toyopearl HW resins are highly resistant to chemical and microbial attack and are stable over a wide pH range (pH 2-12 for operation, and from pH 1 to 13 for routine cleaning and sanitization). Toyopearl HW resins are compatible with solvents such as methanol, ethanol, acetone, isopropanol, -propanol, and chloroform. Toyopearl HW media have been used with harsh denaturants such as guanidine chloride, sodium dodecyl sulfate, and urea with no loss of efficiency or resolution (40). Studies in which Toyopearl HW media were exposed to 50% trifluoroacetic acid at 40°C for 4 weeks revealed no change in the retention of various proteins. Similarly, the repeated exposure of Toyopearl HW-55S to 0.1 N NaOH did not change retention times or efficiencies for marker compounds (41). 

However, for quantities substantially less than this level, 7- to 10-mm i.d. analytical columns can often be used in a semipreparative mode. By repeatedly injecting 300 to 500 ju,l of up to 1% polymer, reasonable quantities of polymer can be isolated. An autosampler and automated fraction collector can be setup to perform such injections around the clock. Although the larger injections and higher concentrations will lead to a loss of resolution, in some situations the result is quite acceptable, with a considerable savings in time being realized over other means of trying to make the same fractionation. 

When packing of the porous materials is not sufficiently tight, repeated injection of a viscous solution leads to an increasing packing density, leaving a void space at the column inlet. When the void space was filled with extra porous materials, it was observed (1) that the resolution of HOPC decreased slightly compared with first-time use. When the column ceased to produce the void space, the separation performance became reproducible. 

Yamamoto et al. also coupled gel permeation HPLC and CE in an on-line fashion in 1990, where capillary isotachophoresis was again used in the second dimension. This technique was also not comprehensive due to the loss of resolution between the techniques. It was also not particularly fast, with a 23 min CE cycle, which was repeated 90 times throughout the HPLC run (14). Volume incompatibility between HPLC and CE was one problem not addressed in this study, in which a large HPLC column was coupled to an electrophoresis capillary. 

If further resolution is necessary one-third octave filters can be used but the number of required measurements is most unwieldy. It may be necessary to record the noise onto tape loops for the repeated re-analysis that is necessary. One-third octave filters are commonly used for building acoustics, and narrow-band real-time analysis can be employed. This is the fastest of the methods and is the most suitable for transient noises. Narrow-band analysis uses a VDU to show the graphical results of the fast Fourier transform and can also display octave or one-third octave bar graphs. 

Cyclodextrins, toroidal molecules composed of 6, 7 and 8 D-glucose units, are now commercially available at reasonable cost. They form inclusion compounds with a variety of molecules and often differentially include sulfoxide enantiomers29,30. This property has been used to partially resolve some benzyl alkyl, phenyl alkyl and p-tolyl alkyl sulfoxides. The enantiomeric purities after one inclusion process ranged from 1.1 % for t-butyl p-tolyl sulfoxide to 14.5% for benzyl r-butyl sulfoxide. Repeating the process on methyl p-tolyl sulfoxide (10) increased its enantiomeric purity from 8.1% to 11.4% four recrystallizations raised the value to 71.5%. The use of cyclodextrins in asymmetric oxidations is discussed in Section II.C.l and in the resolution of sulfmate esters in Section II.B.l. 

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