Resolution by HPLC

AA Resolution by crystallization Resolution by IE chromatography Resolution by HPLC"... 

Chiral resolution by HPLC can by divided into three categories (1) a direct resolution using a chiral stationary phase (CSP) (2) addition of a chiral agent to the mobile phase, which reacts with the enantiomeric analytes (chiral mobile phase additive method (CMPA)) (3) an indirect method that utilizes a precolumn diastereomer formation with a chiral derivatization reagent (Misl anova and Hutta, 2003). 

Using optically pure 121, available from resolution by HPLC, the authors demonstrated a rare example of asymmetric catalysis involving metal vinylidenes (Equation 9.13). The synthesis of 126, enriched in a single olefin geometrical isomer, was achieved in 59% ee. 

Prazepam Bulk TLC Silica ammonium hydroxide (99 1) S, - EtOAc-MeOH- HPLC [3] Enantiomer resolution by HPLC [703, 707] hRn = 81, hRp = 85. [132]... 

The mandelate esters serve yet an additional role in chiral synthesis - facilitation of resolution by hplc. The diastereo-mers in entries 1,2, and 4 of Chart 1 are all readily resolved on a Waters Prep 500 column utilizing 2-105S ethyl acetate in hexane (7,8,10,13). The diastereomer labelled S of entry 2,... 

There are many parameters which control the enantiomeric resolution by HPLC. The most important of them include parameters of the stationary phase, such as particle size of CSP, pore size of column, and kind of chiral selector, composition, and pH of the mobile phase, flow rate of mobile phase, and temperature. Systematic variation of column temperature should be considered as one way to improve chiral separations in HPLC. From the practical point of view, it is easier to vary column temperatures than mobile phase composition. In addition, variable temperature runs can provide useful information concerning the thermodynamic parameters for the CSP-analyte interactions. The effect of temperature on the resolution... 

Direct resolution by HPLC is performed using sdica gel stationary phases coated with optically active tetranitrofluorenyhdene (TAPA) derivatives. This is a peculiar method for polyaromatic systems, in that it is based on... 

In Table 2, the structures and properties of some frequently employed CSPs are summarized. It should be noted that this table represents only a very small part of more than 1300 known chiral stationary phases. Nevertheless, these phases have facilitated more than 95% of all known racemate resolutions by HPLC. 

In the last few years, numerous chiral stationary phases have been developed for optical resolution by HPLC. Two enantiospecific HPLC methods have been described for the separation of pantothenic acid, panthenol, and pantolactone enantiomers (54). 

Blasclike G, Brocker W, Frankel W. Enantiomeric resolution by HPLC on sflica-gel-bound, optically active polyamides. Angew Chem Int Ed Engl 1986 25 830-1. 

Figure 11.3 Resolution by HPLC through covalent attachment and subsequent hydrolytic cleavage of a chiral reagent...

Table 2. Preparative Chromatographic Resolution by HPLC on a Chiral...

Gas chromatography (gc) is inferior to hplc in separating abiUty. With gc, it is better to use capillary columns and the appHcation is then limited to analysis (67). Resolution by thin layer chromatography or dc is similar to Ic, and chiral stationary phases developed for Ic can be used. However, tic has not been studied as extensively as Ic and gc. Chiral plates for analysis and preparation of micro quantities have been developed (68). 

In the context of chemometrics, optimization refers to the use of estimated parameters to control and optimize the outcome of experiments. Given a model that relates input variables to the output of a system, it is possible to find the set of inputs that optimizes the output. The system to be optimized may pertain to any type of analytical process, such as increasing resolution in hplc separations, increasing sensitivity in atomic emission spectrometry by controlling fuel and oxidant flow rates (14), or even in industrial processes, to optimize yield of a reaction as a function of input variables, temperature, pressure, and reactant concentration. The outputs ate the dependent variables, usually quantities such as instmment response, yield of a reaction, and resolution, and the input, or independent, variables are typically quantities like instmment settings, reaction conditions, or experimental media. 

Optical resolution of the dithiirane 1-oxides 2 and 3 was accomplished by HPLC equipped with a chiral column (97T12203). Absolute configurations of 2a and 2b were determined by X-ray crystallography. Tire stereospecific isomerization (epimerrzation) of 2a to 3b and 2b to 3a was observed during the resolution study. 

HPLC separations are one of the most important fields in the preparative resolution of enantiomers. The instrumentation improvements and the increasing choice of commercially available chiral stationary phases (CSPs) are some of the main reasons for the present significance of chromatographic resolutions at large-scale by HPLC. Proof of this interest can be seen in several reviews, and many chapters have in the past few years dealt with preparative applications of HPLC in the resolution of chiral compounds [19-23]. However, liquid chromatography has the attribute of being a batch technique and therefore is not totally convenient for production-scale, where continuous techniques are preferred by far. 

The luciferin produces a blue oxidation product during its purification process. In the DEAE chromatography of luciferin, this blue compound is eluted before the fractions of luciferin. The fractions of the blue compound were combined and purified by HPLC on a column of Hamilton PRP-1 (7 x 300 mm) using methanol-water (8 2) containing 0.1% ammonium acetate. The purified blue compound showed absorption peaks at 234, 254, 315, 370, 410, 590 (shoulder) and 633 nm. High-resolution FAB mass spectrometry of this compound indicated a molecular formula of C l C Nai m/z 609.2672 (M - Na + 2H)+, and mlz 631.2524 (M + H)+]. These data, together with the HNMR spectral data, indicated the structure of the blue compound to be 8. 

For monographs on the use of liquid chromatography to effect resolutions, see Lough, W.J. Chiral Liquid Chromatography, Blackie and Sons London, 1989 Krstulovic, A.M. Chiral Separations by HPLC Ellis Horwood Chichester, 1989 Zief, M. Crane, L.J. Ref. 122. For a review, see Karger, B.L. Anal. Chem., 1967, 39 (8), 24A. 

A regio- and stereoselective Beckmann rearrangement utilized diastereose-lective host guest interactions of the inclusion complexes 225 and 228 in a solid state reaction. Initially, a 1 1 mixture of the chiral host 223 and the racemic oximes 224 and 227, respectively, was treated with ultra sound in the solid state to induce the optical resolution. Then H2SO4 was added to start the Beckmann rearrangement, the corresponding c-caprolactams 226 and 229 were isolated in 68 % and 64 % yields and ee of about 80 % and 69 % (determined by HPLC analysis on chiracel OC) (Scheme 43) [46]. 

These authors further described the synthesis and resolution (by chiral HPLC) of a new C2-symmetric planar-chiral bipyridine ligand [43] (see structure 35 in Scheme 18). They obtained an X-ray crystal structure of the corresponding copper complex proving a bidentate complexation. This system led to high diastereo- (up to 94%) and enantioselectivity (up to 94%) in the... 

In 1996, Fu et al. reported the S3mthesis of the planar chiral heterocycles 64, formally DMAP fused with a ferrocene core [82]. While the original synthesis provided racemic 64a in only 2% overall yield requiring a subsequent resolution by preparative HPLC on a chiral stationary phase, a recently improved synthesis furnished the racemic complexes 64 in 32-40% yield over seven steps. A subsequent resolution with di-p-toluoyltartaric or dibenzoyltartaric acid gave access to the enantiomers with >99% ee (28 14% yield for each isomer in this step) [83]. 

Strasters et al. [6] applied this method to data sets obtained by HPLC-DAD and concluded that the pure factors can be very well recovered even at very low chromatographic resolutions R < 0.02). A critical step in PCA-based methods is the determination of the number of significant PCs, which indicates the number of pure factors which have to be searched. 

Starting from the corresponding hydroxymethyl-benzocrown, it has been possible to generate the immobilized system (186) by reacting the above precursor with chloromethylated polystyrene (which is available commercially as Merrifield s resin). Typically, systems of this type contain a polystyrene matrix which has been cross-linked with approximately 1-4% p-divinylbenzene. In one study involving (186), a clean resolution of the alkali metal halides was achieved by HPLC using (186) as the solid phase and methanol as eluent (Blasius etal., 1980). In other studies, the divalent alkaline earths were also separated. 

Two compounds were separated by HPLC with an Rs value of 0.75, the plate number for the second compound being 4500. Calculate the number of plates required to obtain resolutions of (a) 1.0 and (b) 1.5. 

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