Chromatography auxiliary phase

The main idea behind chromatography is the separation of a mixture of two or more compounds by using two auxiliary phases one of which is the static phase and the other the mobile phase. The physical state of the static or stationary phase can be solid or liquid whereas the mobile phase can be liquid or gaseous. All chromatographic methods can be classified into four main classes based on the combination of the physical state of the mobile and static phases ... 

Construction of the cyclopentane ring was accomplished by utilization of Trosf s Pd-mediated diastereoselective [3+2] trimethylenemethane (TMM) cycloaddition [4] on the cinnamate 5 having an Evans type chiral auxiliary [4b], The resulting diastereomeric mixture (3 1 at best) of 7a and 7b was separated by careful silica gel column chromatography (7a is less polar than 7b under normal phase). Puri-... 

Chromatography, the ratio of peak areas of enantiomers, which are separated on a non-racemic chiral stationary phase, is determined (see Sections 3.1.5. and A.3.1.6.). The chiral auxiliary can also be added to the mobile phase (see Section A.3.1.6.4.). The quantitative resolvability of the sample is the prerequisite for this method. 

Chromatography the enantiomers are converted into a pair of diastereomers by chemical reaction with an auxiliary (enantiomerically pure, chiral compound) and the ratio of the peak areas is determined by chromatography on an achiral stationary phase. 

A highly versatile method for enantiomer analysis is based on the direct separation of enantiomeric mixtures on nonraceinic chiral stationary phases by gas chromatography (GC)6 123-12s. When a linearly responding achiral detection system is employed, comparison of the relative peak areas provides a precise measurement of the enantiomeric ratio from which the enantiomeric purity ee can be calculated. The enantiomeric ratio measured is independent of the enantiomeric purity of the chiral stationary phase. A low enantiomeric purity of the resolving agent, however, results in small separation factors a, while a racemic auxiliary will obviously not be able to distinguish enantiomers. 

Resolution of chiral tetraorganotin compounds has been achieved, using chiral auxiliary groups (equation 35), as well as the use of chromatography on chiral stationary phases, e.g. cellulose triacetate for Me(PhCMe2CH2)Ph(Ph3C)Sn. 

A 0.05-0.10 M solution of (45,2S)-3-(2-azidoacyl)-4-benzyl-l, 3-oxazolidin-2-one in THF/H20 3 1 stirred at 0 rC under N2 is treated with 2.0 equiv of solid LiOH. After stirring for 20 min. excess 0.5 N aq NaHCO, is added and the THF is removed in vacuo. The residual mixture is extracted with four portions of CH2C12. The organic extracts are combined, dried with Na,S04, and evaporated in vacuo to afford the recovered chiral auxiliary in 95-100% yield. The aqueous phase is acidified to pH 1 -2 with 3 N aq HC1 and extracted successively with four portions of EtOAc. The combined organic extracts arc dried over Na2S04 and evaporated in vacuo to afford the a-azido acid 8 yield 90 100%. 8 is generally found to be pure by H-NMR spectroscopy and combustion analysis. If necessary, 8 can be purified by Hash chromatography (silica gel hexane/F.tOAc/HOAc 50 50 1). [The azido acid 8 (R = C H5) racemizes to the extent of 5-10% under these conditions.]... 

A stirred solution of 1.0 mmol of the (45,2 5)-3-(2 -azidoacyl)-4-benzyl-1,3-oxazolidin-2-one 7 in 15 mL of THF and 4.6 mL of H20, cooled to 0 "C, is treated with 0.40 mL (4.1 mmol, 4 equiv) of 31 % H202 followed by 48 mg (2.00 mmol, 2.0 equiv) of solid LiOH. After stirring at 0 "C for 30 min, the reaction is treated with a solution of 0.55 g (4.4 mmol) of Na,S03 in 3 mL of H20 followed by 10 mL of 0.5 N aq NaHCOj. After removal of the THF in vacuo on a rotary evaporator, the residue is diluted to 80 mL with H2G and extracted with four portions of CH2C12 (200 mL total). The aqueous phase is acidified to pH 1 -2 with 5 N aq HC1 and extracted with four portions of F.tOAc (400 mL total). The EtOAc extracts arc combined, dried over Na2S04, evaporated in vacuo to yield the pure a-azido acid 8. The CH,C12 extracts are combined, dried over Na2S04, and evaporated in vacuo to afford the chiral auxiliary 10, which can be purified further, if necessary, by recrystallization or chromatography. 

An extra group of special working procedures in gel chromatography are those separations where the steric exclusion mechanism is intentionally combined with an auxiliary additional separation mechanism to increase the selectivity. Such combinations can be realized either in one step, i.e., in one system gel-eluent, or in several steps, e.g., by the subsequent elution of the sample with two different mobile phases, or from two different column-filling materials. 

Indeed, one usually deals with retention of analytes on the stationary phase, which reduces the concentration of the analyte in the moving zone of the mobile phase and requires additional amounts of the mobile phase to elute the retained portion of the analyte from the stationary phase. Cases of peak compression in chromatography are mostly coupled with the displacement of the adsorbed portion of the analyte (or analytes) by an auxiliary component of the mobile phase (a displacer or mobile phase modifier). In order to act like this, the latter must be adsorbed on the stationary phase even stronger than the displaced analytes. Only in frontal analysis can several weaker retained components of a mixture be obtained with an enhanced concentration, at the expense of the stronger retained component that functions as a displacer and remains in the column [175]. 

An analysis of the techniques presented shows that in chromatography with a mobile gas phase the elution technique is used almost exclusively. The displacement technique suits chromatography with a mobile liquid phase in gas chromatography it may be an auxiliary method for the preliminary concentration of certain components. 

An attractive method for the analysis of mixtures of enantiomers is chiral gas chromatography (GC). This sensitive method is unaffected by trace impurities, and is quick and simple to carry out. The premise upon which the method is based is that molecular association may lead to sufficient chiral recognition that enantiomer resolution results. The method uses a chiral stationary phase which contains an auxiliary resolving agent of high enantiomeric purity. The enantiomers to be analysed undergo rapid and reversible diastereomeric interactions with the stationary phase and hence may be eluted at different rates. There are certain limitations to the method, some of which are peculiar to the gas chromatographic method. The sample should be sufficiently volatile and thermally stable, and, of course, should be quantitatively resolved on the chiral GC phase. Occasionally this... 

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