Enantiomeric self-association

Figure 11 G 5 undergoes cation dependent enantiomeric self-association. Racemic (D, L)-G 5 self-assembles in the presence of Ba to give homochiral G-quadruplexes [ (D)-G -4Pic and [(L)-G l]]g -2Ba -dPic - Addi-...

Figure 12 Addition of Cs to racemic (D, L)-isoG 6 resulted in formation ofhomochiral hydrogen-bonded pentamers through enantiomeric self-association. The major species in solution and in the solid state is a meso decamer [(D)-isoG 6]s-Cs -[(L)-isoG6Js...

The determination of enantiomeric purity (ee) by NMR spectroscopy is usually carried out with the help of a nonracemic chiral auxiliary compound. NMR methods not requiring a chiral auxiliary compound are also known and are based on self-association of the enantiomers or on their reaction with a bifunctional achiral compound (see Section 3.1.4.7.). 

In the latter case, the applicability rests on a knowledge of the relative formation constants of homo- or heterochiral diastereomers. A special case arises when the formation of dimers occurs by self-association (being fast on the NMR timescale) under nonideal conditions leading to nonequivalence of resonance absorptions. When the enantiomeric composition differs from that of the racemic mixture, self-induced anisochrony is observed and the intensities of the signals are directly related to the enantiomeric ratio regardless of parameters such as temperature, concentration, and the ratio of homo- and heterochiral dimers116. 

NMR is a useful alternative to proton NMR. It is a reasonably sensitive nucleus which does not suffer from the relaxation problems associated with NMR. Phosphorus trichloride can be used as a reagent for self-recognition by a chiral substrate. Two molecules of an enantiomerically enriched alcohol react with each PCI3 molecule to form a phosphonate. Four stereochemically distinct species are possible (R,R) and (5, S) (a pair of enantiomers), (R,S) and (5, R) which are meso compounds. There are consequently three resonances in the 3ip NMR spectrum, whose non-equivalence is typically 0.5 ppm and integration gives results within 2% of those obtained by chiral GC methods. Subsequently methyl phosphoryl chloride has been employed in the same way giving improved chemical shift non-equivalences, A5 = 1 ppm (Scheme 3.3). 

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