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Określanie konfiguracji absolutnej za pomocą magnetycznego rezonansu jądrowego

Kołodziejska R., Jasińska L., Karczmarska A., Dramiński M.

Wiadomości Chemiczne

2008

[Z] 62, 7-8

709-732

In relation to a very limited scale of tolerance of organisms to different geometrical isomers it has been imperative to invent a method which would enable a precise and fast evaluation of a spatial structure of optically active compounds. Using a spectroscopic method of nuclear magnetic resonance (NMR) proved to be an excellent solution. In order to define an absolute configuration by means of NMR, the enantiomeric mixture must be transformed into diastereoisomeric one by adding chiral auxiliary substituents. We distinguish three types of chiral auxiliary reagents: CDAs (chiral derivatizing agents), CSAs (chiral solvating agents), CLSRs (chiral lanthanide shift reagents). Chiral derivatizing agents are the most frequently used in analyses. The condensation reaction of an auxiliary compound with enantiomer may be single or double derivatization. In case of a double derivatization, 1H NMR spectra of two diastereoisomers obtained as a result of condensation of (R)- and (S)-CDAs with the substrates are compared. The changes in the chemical shifts of the substituents L_1 (the most bulky substituent) and L_2 (the least bulky substituent) asymmetric carbon of the substrate in the two derivatives (R)- and (S)-CDAs is defined as ?[delta delta]^RS. The [delta]^RS value is the difference between the chemical shift in the (R)-CDAs derivative ([delta](R)) and (S)-CDAs derivative ([delta](S)) for the substituents L_1 ([delta delta]^RSL_1) and L_2 ([delta delta]^RSL2) (Figure 2). In case of a single derivatization, the tested enantiomer is combined with only one enantiomer ((R)- or (S)-CDA). In the single derivatization [delta delta]^AR ([delta delta]^AR = [delta](A)-[delta](R)) is the difference in the chemical shifts of the substituents L_1 and L_2 of a derivative and a free substrate (Figure 3) [1]. Among these auxiliary reagents are MPA (methoxyphenylacetic acid), MTPA (methoxytrifluoromethylphenylacetic acid), 9-AMA (9-anthrylmethoxyacetic acid), BPG (boc-phenylglycine), 9-AHA (ethyl 2-(9-anthryl)-2-hydroxyacetate), PGME (phenylglycine methyl ester), and PGDA (phenylglycine dimethyl amide). These reagents are currently being used to determine the absolute configuration of primary alcohols (Figure 4), secondary alcohols (Figure 5), tertiary alcohols, diols [2-5], triols [6], primary amines (Figure 6, 7), secondary amines (Figure 8), and carboxylic acids (Figure 9). Other methods of determining absolute configuration such as HPLC-NMR or "mix and shake" method are currently investigated - HPLC-NMR method allows determining the absolute configuration of enantiomeric mixture as well as a pure enantiomer, the use of semipreparative column allows to precisely distinguish the obtained derivatives, which undergo the spectroscopic analysis (Figure 11) [1]. The "mix and shake" method allows determining the absolute configuration in a few minutes and without any additional separation methods. The derivative/s is/are prepared by simply mixing a solid matrix-bond auxiliary reagent with a chiral substrate and NMR spectra of the resulting derivatives are obtained without any further manipulation (Figure 12) [7].