Guests enantiomers

There are several methods to enantiodifferentiate chiral ammonium ions by FAB-MS. One is the so-called enantiomer-labeled (EL) guest method. The method is based on the preparation of a mixture containing the enantiopure host (denoted as U) and the racemate of the guest. One of the guest enantiomers is isotopically labeled (e.g., [M5]+) and the other is not (e.g., [M ] ). Consequently, the signals for the two diastereomeric host-guest pairs (i.e., [U M/j] and [U-Ms] of equations (21) and (22)) appear at different miz ratios. 

Besides, information on intermolecular interactions has been derived in these studies from complexation-induced shifts (CIS). The chemical shift is an indicator for the shielding of a nucleus and thus for the electronic state of a specific proton. Since the electronic environment may change on complexation, CIS can be used to monitor where host-guest contacts may take place. If these interactions occur stereoselectively, the CIS will be different for the two guest enantiomers (AS distinct from 0) giving possibly some insight into the chiral recognition mechanism. 

Promotion of crystallization can be achieved, e.g., by inoculation with an appropriate enantiomorphic seed crystal, by the presence of further chiral compounds or by use of solvents which already contain a guest enantiomer in small excess. The latter method allows to win the other enantiomer which is now enriched in the filtrate of recrystallization by repetition of the process... 

This effective optical resolution is possible because the enantiomers mirror-related to those listed in Tables 7 and 8 do not allow inclusion formation with 72 and 73. The X-ray crystal structural study of the 1 1 inclusion compound of (R)-(—)-72a and 72 shows that the host enantiomer is accomodated in the channel space formed by 72, and that two kinds of hydrogen bonds, OH N of 72 and C sCH — O of 72 a, play an important role to hold the guest components together (Fig. 13). Replacement of the (R)-(—)-72a molecules in this lattice aggregate by the other guest enantiomer, (S)-(- -)-72o, would result in an unfavorable system of hydrogen bonds. 

It results from the approach presented in the preceding section that the qualitative manipulation of lock-and-key models does not provide adequate information about the relative energies involved in the diastereomeric host-guest interactions, hence it does not seem possible to predict the preferred guest enantiomer. However the relative complementarity of a pair of enantiomers to the shaj)e of the cage is... 

An important point to realize is that the enantioselectivity values, obtained from host-guest screening experiments, can be influenced by the concentration of the sample components used in the experiment. Shown in Figure 9.6, this phenomenon has been modeled and shown to be consistent with experimental results [87]. Here, enantioselectivity values were calculated as a ratio of normalized ion abundances (response of diastereomeric complex ion(s) normalized to total ion current) equimolar mixtures of selector and guest enantiomers were considered. It can be seen that as the association constant for the guest enantiomers diverge, enantioselectivity increases. However, for a single-point measurement of enantioselectivity at the defined concentration, the true enantioselectivity is only obtained in the limit of dilute solution. As concentration... 

A. Relative Peak Intensity Method Mix a given enantiopure chiral host with enantiopure guest and an achiral reference host record ESI mass spectrum of this sample and evaluate peak intensity ratio of both host-guest complexes repeat same experiment, but with a sample containing the same two hosts and the other guest enantiomer again, evaluate peak intensity ratio compare both ratios. 

B. Enantiomer-Labeled Method Synthesize one isotopically labeled guest enantiomer mix 1 1 with the second unlabeled enantiomer and add a small amount of enantiopure host record ESI mass spectrum and compare the intensities of both diastereomeric complexes directly to separate isotope and stereochemical effects, the other pseudoracemate needs to be subjected to the same experiment. 

A. Cooks Kinetic Method Generate a heterochiral 1 1 1 complex in the ion source from a guest pseudoracemate (one enantiomer isotopically labeled) and an enantiopure host mass-select this ion collide the heterodimer ion with a collision gas or fragment by any other method detect the intensities of the two 1 1 product ions after loss of one of the two guest enantiomers -> compare abundances of the two product ions. 

P-CD diastereomeric complexes. However, the extent of enantioselectivity tends to decrease when the guest enantiomers are modified to complex more strongly as the additional weak interactions induced diminish the chiral complementarity between the guest and the P-CD annulus. 

Fig. 11.14 Tetrahenzoxazines (/f)-3 and (S)-3 and tetrakis-aminomethylated Fesoicinarene (S)-17. Quaternary ammonium ions (18). Isotope pattern regions of the ESI-PTlCR (FTICR Fourier transform ion cyclotron resonance) mass spectra of (S)-17 with pseudo-racemates of the two guests. The bottom row represents control experiments with a reversed labelling of the two guest enantiomers as compared to the top row [35] (Image reproduced frinn [35] with permission from Springer)...

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