Page 98 - Chiral Separation Techniques

74 3 Combinatorial Approaches to Recognition of Chirality: Preparation …

DNB group appeared to be essential for high selectivity. Similarly, a large group
affording steric hindrance at the same amino acid also improved the resolution. In
contrast, proline at that position did not lead to CSPs with good selectivities.
Although the substituted amino acid-based selectors were thoroughly studied earlier
using various techniques including X-ray diffraction and NMR, this study brought
about unexpected results. For example, the glutamine residue (gln) at the initial posi-
tion was beneficial for selectivity. Similarly, homochiral (D–D) dipeptides afforded
better selectivity than many heterochiral sequences. The best selectivity of this selec-
tor library was observed for (L)-gln-(L)-val-DNB. Although successfully demon-
strated, the ”in-batch“ screening is less sensitive than the direct separation in HPLC
mode and its use appears limited to the discovery of selectors with selectivity factors
of at least 1.5. In addition, this evaluation allows only relative comparisons and exact
numerical values for the selectivity factors cannot be calculated easily.
To further extend this study, the authors expanded their selection of amino acids
including hydrogen bonding residues (L-isomers of glutamine, asparagine, serine,
histidine, arginine, aspartic acid, and glutamic acid) in position 1 closest to the sur-
face of the support and both D and L amino acids with bulky substituents (leucine,
isoleucine, t-leucine, valine, phenylalanine, and tryptophan) in the position 2 [86].
The terminal amine functionalities of these dipeptides were again capped by dini-
trobenzyl groups. One sublibrary of 39 attached selectors could be prepared directly,
while the second sublibrary involving 32 selectors required the Fmoc lateral chain
protection during its preparation. Hence, the complete library used in this study
incorporated 71 dipeptide selectors out of 98 possible structures. All of these CSPs
were tested for the resolution of 8 using the batch approach. Evaluation of results
shown in Fig. 3-7 indicated that glutamic acid, aspartic acid, and histidine in posi-
tion 1 and leucine, isoleucine, and phenylalanine in position 2 afforded selectors
with enantioselectivity far better than that of the gln-val-DNB selector lead identi-
fied in the original library [84].
The usefulness of this solid-phase synthesis/screening was finally validated by
synthesizing 5 g of beads with the (L)-glu-(L)-leu-DNB selector. These were packed
into a 250 × 4.6 mm i.d. HPLC column and evaluated using normal-phase chro-
matographic conditions. The separation of racemic 8 shows Fig. 3-8. This separation
was remarkable for several reasons: first, for its excellent selectivity factor (α =
20.74) enabling an outstanding separation of both enantiomers with an isocratic mix-
ture of 2-propanol-hexane; second, the k value for the second peak was 78.99 and
the peak did not elute until after almost 2 h, indicating that a rather strong interac-
tion is involved in the recognition process; and finally, the column afforded a high
selectivity factor of over 18 even in pure ethyl acetate that might be a better solvent
for many racemates than the hexane mixture and can easily be recycled. The large
“distance” between the peaks of both enantiomers resulting from the high selectiv-
ity was found extremely useful for separations under overload conditions. For exam-
ple, Fig. 3-9 shows a remarkable enantioseparation of 100 mg of the model racemate
on the analytical size column that produced both enantiomers in optical purity of
98.4 and 97 % ee, respectively [86].