Emanuel Gil-Av

Resolution of optical isomers by high-performance liquid chromatography, using coated and bonded chiral charge-transfer complexing agents as stationary phases

Abstract Ten racemic helicenes ([5] to [14]) and two double helicenes {diphenanthro[4,3- a ; 3′,4′- o ]picene (I) and 8,20-dibromodiphenanthro[4,3- a ; 4′,3′- j ]chrysene (II)} were resolved using high-performance liquid chromatography. R(−)- and S(+)-2-(2,4,5,7-tetranito-9-fluorenylideneaminooxy)propionic acid (TAPA) and three R(−)-homologues derived from butyric (TABA), isovaleric (TAIVA) and hexanoic (TAHA) acids were used as chiral charge-transfer (C.T.) complex-forming stationary phases, in situ coated on silica microparticles. The bulkiness of the group at the chiral centre of the C.T. acceptor is critical for the ease of resolution of the C.T. donors. The [6]-[14]-helicenes and the double helicenes I and II were completely resolved on R(−)-TAPA. On the other hand, the optical isomers of [5]-helicene could be separated only on R(−)-TABA and baseline resolution required ten recycling steps. R(−)-TAPA was also incorporated as a bonded solid phase and as a salt. The resolution factors, r , are given, a mechanism of resolution is proposed and the scope of the method and its significance are discussed.

Separation of enantiomers on packed columns containing optically active diamide phases

Chromatography of fourteen protein amino acids has been studied on 2-4- m long packed columns containing chiral diamide phases. Twelve of the thirteen optically active compounds examined could be resolved, with R greater than or equal to 1 in many instances. Two of the phases, N-docosanoyl-L-valine tert.-butylamide and N-lauroyl-L-valine 2-methyl-2-hepatadecylamide, can be employed at temperatures as high as 190 degrees and 180 degrees, respectively, without losing their efficiency, even after prolonged use. The problem of peak overlap in the analysis of mixtures of different amino acids was examined and partially solved.

N-acyl derivatives of chiral amines as novel, readily prepared phases for the separation of optical isomers by gas chromatography

Abstract The results reported demonstrate that it suffices for a chiral stationary phase to contain an amide group and an asymmetric carbon atom, attached to the nitrogen atom [RCONHCH(CH3)R′], in order to show selectivity in its interaction with the enantiomers of amides such as N-trifluoroacetylamines, N-trifluoroacetylamino acid esters and α-methyl- and α-phenylcarboxylic acid amides. The best efficiency is obtained when R′ is aromatic, particularly α-naphthyl, as in N-lauroyl-S-α-(1-naphthyl)ethylamine. The highest resolution factors were found for aromatic solutes, such as N-trifluoroacetyl-α-phenylethylamine and α-phenylbutyric acid amides, which could be resolved readily on packed columns. The enantiomers of α-branched carboxylic acids were separated for the first time by gas chromatography. Based on the packing arrangement in the crystalline form of the N-acetyl homologue of N-lauroyl-S-α-(1-naphthyl)ethylamine, a mechanism for the resolution is proposed. It is assumed that the mode of association found in the solid state is at least partially retained in the melt through a network of hydrogen bonds. The mechanism proposed is developed with particular reference to the aromatic solute-solvent systems. It is suggested that the solute is intercalated (“sandwiched”) between two solvent molecules. Arguments, based on X-ray data, are given to explain the selectivity observed.

Present status of enantiomeric analysis by gas chromatography

SummaryThe development of selective chiral solute-solvent systems showing separation of enantiomers by gas chromatography is reviewed. The enantiomeric analysis of α-amino-acids and its application to various problems, including notably the analysis of meteorites, is discussed. Some recent results with the highly efficient diamide phases, which permit resolution on packed columns, are presented. Some new ideas on the mechanism of resolution are briefly mentioned.

Complex-forming stationary phases in high-speed liquid chromatography

Abstract Solid dimeric rhodium(II) acetate deposited on Corasil II gave good separation of the butenes using high-speed liquid chromatograpy. Essentially, the unique order of retention times observed with gas—liquid chromatography using rhodium(II) benzoate was found, namely 1-butene

Resolution of non-racemic mixtures in achiral chromatographic systems: A model for, the enantioselective effects observed

Abstract Resolution of non-racemic samples in achiral chromatographic systems can occur when the solute undergoes self-association, e.g. , typically to dimers. In the resulting diastereomeric homo- and hetero-structures, the enantiomers are distributed in a non-symmetric fashion when the sample or sample fraction has an enantiomeric excess (e.e.) larger than 0%. In this paper, a new model for the resolution mechanism is presented. It takes into consideration that all molecules in the sample have capacity factors that are not constant during resolution, but change rapidly with time, owing to the rapid and reversible interconversion of monomers to dimers. Simple equations are given expressing the difference in the capacity factors of the enantiomers in a non-racemic environment on the basis of the time fraction spent by each of the antipodes in the form of the possible molecular species and their respective capacity factors. The process is thus represented in terms of the migration of the enantiomers. This approach leads to a better understanding of the resolution and of certain aspects of the order of elution. Different scenarios are considered with the dimerization taking place either only in the mobile or the solid phase or in both phases. Assuming that association normally also proceeds in the solid phase, both the “dimer distribution” and the “chiral layer” mechanisms can occur simultaneously. Their relative importance will vary with the experimental conditions. Analysis of the equilibrium concentrations showed that resolution can occur only if the dimers differ in stability.

Amino acid racemization of fossil bone

Abstract A study of aspartic acid racemization in different fractions of 51 fossil bones of various ages, shows that the collagen-rich fractions do manifest increasing D-aspartic acid contents with increasing age. No such correlation exists for the fractions rich in non-collagenous proteins. More collagen-rich samples, particularly from bones in older sites, are needed to verify the generality of this trend.

Gas liquid partition chromatography of mixtures of methylenecyclohexane and the isomeric methylcyclohexenes

Abstract Smooth separation and quantitative analysis of mixtures of methylenecyclohexane and the three isomeric methylcyclohexenes can be accomplished by gas liquid partition chromatogrphy on a column of 2 m length, with saturated silver nitrate/glycol as the stationary phase. The mean deviation of the results was found to be 0.6-0.7% of the totla and the maximum deviation. 1.2%.

Application of dicarbonyl-rhodium-trifluoroacetyl-d-camphorate to special problems of olefin analysis by gas-liquid chromatography

SummaryThe title compound, dissolved in squalane, has been used as stationary phase in g.l.c. for purity determinations of olefins. High resolution open tubular columns have been prepared with the same phase. C2H4, C2D4 and the isomeric butenes were efficiently separated.

N-docosanoyl-l-valine-2-(2-methyl)-n-heptadecylamide as a stationary phase for the resolution of optical isomers in gas-liquid chromatography

Abstract The synthesis and chromatographic properties of N-docosanoyl- l -valine-2-(2-methyl)- n -heptadecylamide are described. It has been employed as a stationary phase at column temperatures up to 200°C and it shows excellent stereoselectivity for various classes of compounds, including, in particular, N-trifluoroacetyl (N-TFA) esters of α- and γ-amino acids, aromatic N-TFA amines, N-TFA-O-acyl amino alcohols and N-TFA-α-methylvaline isopropyl ester. The influence of lengthening the chains of R′ and R‴ on the properties of the diamides R′CONHCH(R″)CONHR‴ is discussed.