Reagent Selector

Fig. 10. Reagent selector tool. Here we have chosen to look for all compounds that contain only one nitrile and none of the other functional groups.

Mosley R.T., Culberson J.C., Kraker B., Feuston B.P., Sheridan R.P., Conway J.F., Forbes J.K., Chakravorty S.J., Kearsley S.K., Reagent selector using synthon analysis to visualize reagent properties and assist in basis set selection. J. Chem. Inf. Model. 2005, 45, 1439-1446. 

Figure 9.17. Reagent Selector—an example of a chemical data mart. Various components of the system are shown, including the data sources, the daemon program that automatically updates the mart, the concordance database, and the client/server architecture, which is implemented in a three-tier system.

Products Index (CPI) database. To aid in reducing the size of a hit list, a Reagent Selector user can filter reagents and sort on properties, availability, presence or absence of functional groups, etc. (Fig. 9.18). Further list reduction can be achieved by clustering the structures by means of a cluster analysis using substructure keys as descriptors. 

The two enantiomers of a given amino acid have identical chemical and physical properties in a symmetrical environment. To resolve such a pair of amino acid by chromatography, diastereomers must be formed. Diastereomers can be formed if a chiral reagent (selector) is introduced to either the mobile or the stationary phase. In case of thin-layer chromatography (TLC), latter manner has largely been used for resolution of amino acids, their PTH-, dansyl, and other derivatives [2,4-6]. 

CE has been applied extensively for the separation of chiral compounds in chemical and pharmaceutical analysis.First chiral separations were reported by Gozel et al. who separated the enantiomers of some dansylated amino acids by using diastereomeric complex formation with Cu " -aspartame. Later, Tran et al. demonstrated that such a separation was also possible by derivatization of amino acids with L-Marfey s reagent. Nishi et al. were able to separate some chiral pharmaceutical compounds by using bile salts as chiral selectors and as micellar surfactants. However, it was not until Fanali first showed the utilization of cyclodextrins as chiral selectors that a boom in the number of applications was noted. Cyclodextrins are added to the buffer electrolyte and a chiral recognition may... 

Enantiomeric compounds cannot be separated directly in normal chromatographic systems since the groups attached to the chiral atom are equally accessible to binding. If the enantiomers are combined with a chiral selector (enantiomeric reagent), however, two diastereomers are formed that may... 

There are three possible approaches to the separation of chiral species by CE (1) addition of chiral selectors to the buffer, (2) use of a chiral stationary phase, and (3) precolumn derivatization. These correspond to the approaches in HPLC, and the separation mechanisms are described in Section 2.8. In the first approach, additives are added to CZE, CGE, or MECC buffers to effect the separation. In the second approach, chiral selectors can be immobilized on the capillary wall, although that is a difficult process. Alternatively, capillaries filled with enantiospecific packings can be employed for CEC. In the third approach, enantiomers are derivatized with chirally specific reagents prior to CZE or MECC. Addition of chiral selectors to the buffer is the most common approach. 

A high-throughput peptide synthesizer is shown in Fig. 9. Wash solutions and reagents common to all synthetic entities are automatically delivered through a multichannel distributor connected to a six-port selector valve. Building blocks and other specific reagents are individually delivered to their respective wells... 

Enantiomeric separations of amino acids and short peptides are performed using either a direct or the indirect approach [10]. The indirect approach employs chiral reagents for diasteromer formation and their subsequent separation by various modes of CE. The direct approach uses a variety of chiral selectors that are incorporated into the electrolyte solution. Chiral selectors are optically pure compounds bearing at least one functional group with a chiral center (usually represented by an asymmetric carbon atom) which allows sterically selective interactions with the two enantiomers. Among others, cyclodextrins (CDs) are the... 

Resolution of racemates by the selective liquid-liquid extraction (Figure 13.8) of diastereomers due to their different solubility in two-phase system is by far a less common technique than crystallization of diastereomers. In this process, a highly discriminative chiral ionic reagent transfers (preferably in one extraction step) a highly enriched enantiomer as an ionic pair from one phase to another, which dissolves selectively the formed diastereomeric associate but not the starting enantiomers. The following back-extraction with an appropriate acid or base produces the desired enantiomer and recovered chiral selector (SO). 

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