Separation by Chromatography

Liquid chromatography can be operated under mild conditions in terms of pH, ionic strength, polarity of liquid, and temperature. The apparatus used is simple in construction and easily scaled up. Moreover, many types of interaction between the adsorbent (the stationary phase) and solutes to be separated can be utilized, as shown in Table 11.1. Liquid chromatography can be operated isocratically, stepwise, and with gradient changes in the mobile phase composition. Since the performance of chromatography columns was discussed, with use of several models and on the basis of retention time and the width of elution curves, in Chapter 11, we will at this point discuss some of the factors that affect the performance of chromatography columns. 

In order to estimate resolution among peaks eluted from a chromatography column, those factors that affect N must first be elucidated. By definition, a low value of Hs will result in a large number of theoretical plates for a given column length. As discussed in Chapter 11, Equation 11.20 obtained by the rate model shows the effects of axial mixing of the mobile phase fluid and mass transfer of solutes on Hs. 

Many chemically different stationary phases are used in liquid chromatography of proteins. Ion exchange chromatography uses an ion exchange resin, and the proteins are eluted with buffer solutions differing in ionic strength 

CHAPTER 3 Protein Isolation and Determination of Amino Acid Sequence 

The pores of the gel particle exclude the larger solutes but admit smaller ones. Individual solute molecules are represented by the open circles. 

Gel filtration for separation of solutes by. size. Solute molecule.s smaller than the diameter of the pores of the gel particles enter them and are retained for a longer time. Larger solute molecules cannot penetrate the pores of the gel particles and are eluted off the column first. 

Affinity chromatography takes advantage of specific affinities between protein molecules and analogues of biological molecules that are covalently bound to the column 

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Plus other three isomeric diisocyanides, separable by chromatography. 

In contrast to the stability exhibited by tlie copper reagents contaimng an a-fluorine, the corresponding a-bromocopper reagent is not stable and decomposes to give an excellent yield of the triene (cumulene) product [254] (equation 167) The ( ) and (Z) cumulenes were separated by chromatography, and the structures were assigned by X ray analysis [254]... 

When the reaction of S2CI2 with ammonia is carried out in a polar solvent, e.g., DMF, the hydrolysis of the reaction mixture with aqueous HCl produces a mixture of the cyclic sulfur imides S7NH, 1,3-, 1,4- and 1,5-S6(NH)2 and 1,3,5- and 1,3,6-S5(NH)3, which can be separated by chromatography on silica gel using CS2 as eluant (Section 6.2.1). °... 

Ebumamonine was assembled utilizing a Pictet-Spengler cyclization of hydroxy-lactam 52 in the presence of trifluoroacetic acid at low temperature to give a mixture of diastereomers 53 in 95% yield. These compounds were readily separated by chromatography and the a-epimer was further elaborated to give the natural product. 

Likewise, synthetic 2//-azepines isomerize to 3//-azepines in refluxing chloroform (2-3 h) or in tert-butyl methyl ether at room temperature.291 The isomers can be readily separated by chromatography on silica gel, as the more basic 2//-azepines30 have lower Rf values. In contrast, 7-butyl-2//-azepin-2-acetic acid (11), obtained by heating the tert-butyl ester 10 with iodotrimethylsilane, is stabilized by intramolecular hydrogen bonding and shows no tendency to rearrange to the 3//-isomer.291... 

The Vilsmeier formylation of copper deuteroporphyrin dimethyl ester (6) in which unsubstituted /3-positions are present yields a complex mixture of mono- and disubstituted formylation products which can be partially separated by chromatography on neutral alumina.106... 

A similar rcgioselccti vity is observed in the Friedel-C rafts acylation of copper deuteroporphyrin dimethyl ester (6) which gives a mixture of two /Fmonoacylated products 9 when the reaction time and temperature are carefully controlled or diacylated products 10 on prolonged reaction time.85b-100 106 As with the four / -monoformylated deuteroporphyrin derivatives 7a. the acylated products can be separated by chromatography.100106... 

Using oxathiane 11, ( + )-(i )-2-methoxy-2-phenylpropanoic acid was obtained in 97% ee, however, the synthesis contains some inconvenient reaction steps. Thus, reduction of ( + )-10-camphorsulfonic acid (8) leads in low yield to a mixture of 10-mercaptoisoborneol (9 A) and 10-mercaptoborneol (9B) which must be separated by chromatography. The oxathiane 10 resists deprotonation with butyllithium and, therefore,, y -butyllithium had to be employed. Furthermore, after addition of methylmagnesium iodide, cleavage of the oxathiane moiety 12, with iodomethane did not proceed as well as with the simpler oxathianes 3. 

Another class of configurationally stable a-mctallo amines is derived from the N-tert-butoxy-carbonyl-protected piperidines 32 and 3516, l7. Addition of the lithiated piperidines to aldehydes leads to mixtures of the anti- and. yin-diastereoiners. Although the diastereoselectivity is low, the diastereomers can be readily separated by chromatography since the. vyn-isomer is often in a cyclized form 34. The stereochemistry of the products obtained from piperidines 32 are consistent with an equatorial a-lithiation followed by addition to the aldehyde with retention of configuration. However, with piperidine 35 selective axial lithiation is observed. 

Tributyl[( )-l-(methoxymethoxy)-2-butenyl]stannane (4) is available by the addition of tri-butyltin lithium to 2-butenal followed by treatment with chloromethoxymenthane 03. The resulting diastereomers were separated by chromatography to give stereochemically homogeneous a-alkoxystannanes 5 and 6104. 

In contrast, transmetalation of the lithium enolate at —40 C by treatment with one equivalent of copper cyanide generated a species 10b (M = Cu ) that reacted with acetaldehyde to selectively provide a 25 75 mixture of diastereomers 11 and 12 (R = CH3) which are separable by chromatography on alumina. Other diastereomers were not observed. Similar transmetalation of 10a (M = Li0) with excess diethylaluminum chloride, followed by reaction with acetaldehyde, produced a mixture of the same two diastereomers, but with a reversed ratio (80 20). Similar results were obtained upon aldol additions to other aldehydes (see the following table)49. 

The diastereomeric a-alkoxy complexes (1 )-15 and (S)-15, separable by chromatography, were each converted to the corresponding aluminum enolates and reacted with 2-methylpropanal (17)49. Enolate (/ )-16 selectively provided a mixture of two diastereomers with the (Fe/ ,2, 3 i )-complex (/ )-18 identified as the major constituent of a 94 6 mixture. The two chiral auxiliaries of complex (S j-lS exerted antagonistic effects and an undefined mixture of all four possible diastereomers was obtained. 

Condensation of a series of methyl glycopyranosiduloses with nitromethane provides isomeric nitroaldol adducts, precursors of branched-chain amino sugars30. The isomers are separable by chromatography. 

The diastereomeric lactones could be separated by chromatography and converted into optically active lactones by desulfurization with sodium amalgam or by pyrolysis to the corresponding butenolides. 

The diastereomeric aldehyde adducts are not readily separated by chromatography. Upon direct reduction to the secondary alcohols, the optical purities (25-46%) reflect the poor degree of asymmetric induction in the addition step. 

Reaction mixtures can be separated by chromatography. Individual isomers can be identified by their vibrational spectra. 

Mixtures may be separated by chromatography or by using solubility differences. Isomerization often occurs on heating solutions of m-Pt(RCN)2Cl2 give mixtures of the cis- and trans-forms, while solid m-Pt(PhCN)2Cl2 gives the trans-isomer. 

Tertiary amine N-oxides may also be used to convert sulphoxides to sulphones16. The reaction proceeds by initial attack by the N-oxide oxygen atom on the sulphoxide moiety, followed by subsequent elimination of the amine. In order to obtain good yields, the reaction must be carried out at 190°Cfor 20 hours with a 20-fold excess of N-oxide in the presence of acid catalysts. The sulphone must then be separated by chromatography, thus making the method less attractive than other procedures and so it has not been employed synthetically. 

In Ghosh s enantioselective total synthesis of the cytotoxic marine macrolide (+)-amphidinolide T1 (318) [143], the C1-C10 fragment 317 was constructed by CM of subunits 315 and 316 (Scheme 62). The reaction mediated by catalyst C (5 mol%) afforded in the first cycle an inconsequential 1 1 mixture of (E/Z)-isomeric CM products 317 in 60% yield, along with the homodimers of 315 and 316. The self-coupling products were separated by chromatography and exposed to a second metathesis reaction to provide olefins 317 in additional 36% yield [144]. 

As reported by Shani and Sondheimer,1 the dehydrohalogenation of the tetrabromide by means of potassium hydroxide in ethanol at 50-55° affords a mixture, which is readily separated by chromatography on alumina, of l,6-oxido[10]annulene and the isomeric 1-benzo-xepin. The latter compound is also formed during chromatography of l,6-oxido[10]annulene on silica gel.7... 

Interestingly, both invertomers of the obtained M-chloroaziridines 16 were clearly observable in the H-NMR spectrum and they could even be separated by chromatography. The dehydrochlorination was investigated with a variety of bases however, the resulting yields were disappointingly low. Only for R = Ph, a yield of 39% of azirine 17 was obtained using DBU as the base, in all other cases the yields were lower [22]. Davis et al. [23] successfully applied the -elimination of the sulfinyl group in chiral non-racemic N-sulfinylaziridines (Scheme 9), whereby the eliminated sulfenate was trapped by an excess of methyl iodide, which facilitated the isolation of the desired product (18). 

By heating 2-benzyloxycyclohexanone 208 and (R)-l-phenylethylamine in refluxing toluene for 4 days in a Dean-Stark apparatus, the imine 209 was formed, then a rearrangement occurred to give first the a-aminocyclohexanone derivative 210 and finally the Q, o -disubstituted imine 211 with moderate diastereoselectivity. Reduction of this imine with sodium borohydride gave a mixture of two trans diamines (S,S)-212 and (R,R)-212, which were separated by chromatography. The enantiomers of 1-benzyl-1,2-diaminocyclohexanes 213 were then obtained by hydrogenolysis [99] (Scheme 31). 

Decarboxylation of (44) on heating gave a mixture of < ts and trans acids (39) which were separated by chromatography. Each was converted to the phenyl ketone (38). 

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