Positional isomers, chromatographic separation

Principles and Characteristics Traditional analytical approaches include off-line characterisation of isolated components, and the use of several chromatographic separations, each optimised for a specific spectroscopic detector. Neither LC-NMR nor LC-MS alone can always provide complete structure determinations. For example, MS may fail in assigning an unequivocal structure for positional isomers of substituents on an aromatic ring, whereas NMR is silent for structural moieties lacking NMR resonances. Often both techniques are needed. 

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

U. Sidelmann, S.H. Hansen, C. Gavaghan, A.W. Nicholls, H.A.J. Carless, J.C. Lindon, I.D. Wilson, J.K. Nicholson, Development of a simple liquid chromatographic method for the separation of mixtures of positional isomers and anom-ers of synthetic 2-, 3- and 4-fluorobenzoic acid glucuronides formed via acyl migration reactions, J. Chromatogr. B Biomed. Sci. Appl. 685 (1996) 113-122. 

Almost complete retention of chirality was achieved in the alkylations of l-propionyl-l,4-dihydro-2//-3,1-benz-oxazines 242 bearing a stereogenic center in the substituent at position 2 (TBDPS = /< rt-butyldiphenylsilyl, KHMDS = potassium hexamethyldisilazide). The alkylations took place with high de s (70-92%) in favor of isomers 243, isolated after chromatographic separation. The allyl-substituted compound 243 (R = allyl) was reduced with LAH to yield the enantiopure (R)-3-methylpent-4-en-l-ol 244 and the N-unsubstituted 3,1-benzoxazine 245 as a 5 1 diastereomeric mixture (Scheme 45) <2000JOC6540>. 

On chromatographic separation of isomers 508 and 512, partial oxidation of azepine 508 leads to azepinone 506. Aminocarbinol 514, having the blocked /3-position, is cyclized into tetrahydronaphtho[/ c]azepine 515 in polyphosphoric acid (81H469). 

Chang, C.A., Wu, Q., and Tan, L., Normal-phase high-performance liquid chromatographic separations of positional isomers of substituted benzoic acids with amine and P-cyclodextrin bonded-phase columns, J. Chromatogr., 361, 199, 1986. 

The Beirut process as a method for preparing QDO and PDO is sometimes inconvenient. One of the main problems has been evidenced when substituted Bfxs were used as a reagent for the Beirut reaction. In this case, a mixture of positional isomers, in general non-separable by ordinary chromatographic techniques, of QDO or PDO have been obtained. This finding is the result of the well-known tautomerism that affects the Bfx reactants at room temperature (see example in Scheme 2) [41,42], In general, the tautomeric equilibrium energy barrier is very low at room temperature and therefore... 

The chromatographic separation of positional isomers (26-31), geometrical isomers (27,32-36) and enantiomers (37-49) has been achieved by utilizing the concerted action of inclusion complex formation, additional primary and secondary hydrogen-bond formation and steric hindrance effects between the solutes and the cyclodextrins (11,12,14-23,50). There is an abundant literature on the analytical applications of cyclodextrin-silicas (13-50), but not on their preparative chromatographic use. 

Structural Isomers. Chromatograms illustrating the separation of ortho, meta and para isomers of cresol (22) and and xylene ( O)on RP columns are shown in Figures 4 and 5. They enable a comparison of the chromatographic properties and selectivities due to <. - and -CD complexation between positional isomers of the above compounds.Similar behaviour was observed for ortho,meta and para isomers of fluoronitrobenzene, chloronitrobenzene, iodoni-trobenzene, nitrophenol, nitroaniline, dinitrobenzene (22), nitrocinnamic acid (22) some mandelic acid derivatives (19,21,34) and ethyltoluene (28). Both [Pg.225]

The intermediate 90 was treated with a small amount of PTS in acetone at room temperature followed by chromatographic separation of the reaction mixture. The enone 91 is the less polar epimer and its isolated yield, obtained by Vlattas et aL is two times more than 92. The stereochemical assignments in 91 and 92 were made by virtue of the difference between the resonances of their low field olefinic protons. In the isomer with 11/3, 12/3-configuration, 92, the low field olefinic proton appears 0,31 ppm downfield of the 11a, 12 -configurated isomer 91. The upfield olefinic protons were in approximately the same position. 

Figure 5 Chromatographic separation (RP-HPLC) of eight six-membered methylene sulfanes (CH2)6- S with n = 1, 2 (3 isomers), 3 (2 isomers), and 4 (2 isomers). The numbers give the positions of the S atoms in the ring, for example, 1,3 means 1,3-dithiane . (Reproduced by permission of Verlag der Zeitschrift fiir Naturforschung from Steudel and Strauss )...

L. Zhou, Y. Wu, B. D. Johnson, R. Thompson, and J. M. Wyvratt, Chromatographic separation of 3,4-difluorophenylacetic acid and its positional isomers using five different techniques, J. Chromatogr. A 866 (2000), 281-292. 

In this reaction, the formation of two series of compounds is proposed because in the chromatographic separations of polypeptide pyrolysates, an additional peak is noticed for each 3-alkenyl-5-alkyl-pyrrolidin-2,4-dione. This second peak is assigned to the corresponding 2,4-dialkyl-3,5-diketopyrroline (position isomers are not possible when R2 and R3 are identical) [1]. The list of different compounds from these two classes that may be formed during laser irradiation of different mammalian tissues [13a] due to peptide (protein) pyrolysis and the amino acid pair that can generate them is given in Table 12.2.3. 

The turbulent flow column switching (CS) approach has also been used for the bioanalysis of a single analyte, as well as for mixtures of multiple analytes. Jemal et al. used this scheme to effectively separate and quantify two positional isomers in plasma [114]. Multiple components in plasma were simultaneously quantified with good chromatographic separation and accuracy by Wu et al. [65]. Lim et al. used this approach to simultaneously screen for metabolic stability and profiling [115]. 

After neutralization of the nucleophilic reagent the position isomers of both series were separated chromatographically. Yields of compounds 180 varied within 16-89%, those of compounds 182 within 37-98% (99CPB1464) (Scheme 54). 

The circulins—As early as 1949, Peterson and Reineke characterized circulin as its sulphate. Total hydrolysis yielded D-leucine, L-threonine and L-K,y-diaminobutyric acid together with an optically active isomer of pelargonic acid. The existence of two components, found by Peterson and Reineke was later confirmed by the chromatographic separation of crude circulin into two major components, named circulin A and circulin B. In addition there was evidence for at least three other ninhydrin-positive, biologically active entities. In the hydrolysate of circulin A, L-isoleucine was found besides the amino acids previously reported . Quantitative amino acid analysis showed circulin A and B to be composed of L-a,y-diamino-butyric acid, L-threonine, D-leucine, L-isoleucine and ( + )-6-methyloctanoic acid in the molar ratio 6 2 1 1 1. After partial acid hydrolysis, fractionation and structure determination of the resulting peptides, circulin A and circulin B were formulated as cyclodecapeptides . Very recently, however, Japanese workers have revised the structure of circulin A. According to them, circulin A differs from colistin A only by a replacement of L-leucine in the latter by L-isoleucine Figure 1.7). 

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