Signal separation and determination of the enantiomeric purity of primary amines with (−)-myrtenal – a 13C NMR study

Abstract

Abstract1R -((/)-myrtenal was tested as a chiral derivatising agent for the determination of the enantiomeric purity of aliphatic primary amines, b-aminoalcohols, b-diamines and a-amino-acids. Derivatisation procedure consists in mixing one equivalent each of the amine and 1R -((/)-myrtenal directly into the nuclear magnetic resonance (NMR) tube before addition of the appropriate deuterated solvent and, for a-amino-acids, one equivalent of NaOH was added to neutralise the acidic function. Diastereoisomeric imines were completely formed after 12 h and analysed by 1 H-NMR except for N -methyl b-diamines for which imidazolidines were obtained. The method gave satisfactory results only for a-and b-arylalkylamines for which CDCl 3 could be advantageously replaced by C 6 D 6 or pyridine-d 5 in order to increase the difference between the chemical shifts. The main advantage of the procedure is its simplicity as it does not involve steps such as activation, heating, solvent evaporation and isolation of the product. Its limitations are the limited series of compounds susceptible to be analysed and the small difference between the chemical shifts of the diastereoisomeric imines obtained. #

A lactate-derived chiral aldehyde for determining the enantiopurity of enantioenriched primary amines

“…[1][2][3][4][5][6] Ideally, the chiral derivatising reagent should be readily accessible in either enantiomeric form, and should undergo rapid and efficient reaction with the analyte in order to minimise errors which can occur due to the inherent diastereoselectivity of the reactions. This is typically achieved either via chiral HPLC analysis (which generally requires a sample of the opposite enantiomer or of the racemic material), or via the formation of a derivative with a chiral reagent which enables the resulting diastereomeric species to be observed via standard spectroscopic techniques.…”

“…[3][4][5] This strategy is potentially useful as imine formation is often rapid. If amides are prepared using a chiral acid such as 1, then the resulting compounds are frequently difficult to analyse via NMR due to the presence of rotational isomers caused by slow rotation around the C-N bond.…”

NMR Studies of Optically Active Schiff Bases

“…Imines being derivatives of some primary amines, for example, 2-butylamine, 2-aminobutanol and (À)-myrtenal have been also studied by 13 C NMR. 2 It has been shown that for all compounds studied, it is possible to establish the R:S ratio (ratio of the enantiomers R:S). Two regions of the 13 C NMR spectra were useful in determination of the enantiomeric purity: $ 160 ppm assigned to imine carbon and $ 70 ppm assigned to carbon being stereo centre of the former amine.…”

“…The imine proton signals were well resolved even if the reaction was not completed. The best results were obtained when chiral amines with À ÀOMe or À ÀCOOMe groups were used [2]. The differences in chemical shifts of diastereomeric imine proton were ca.…”

“…The 1 H NMR spectra of [Co 2 ((S)-L)2 Cl 2 ] 2þ complexes in CD 3 CN have shown signals paramagnetically shifted over a range of 110 ppm. The presence of the only one set of signals indicated its D 2 -symmetric structure and highly diastereoselective formation.…”