Enantiomers paired

Propylene glycol, dipropylene glycol, and tripropylene glycol all have several isomeric forms. Propylene glycol has one asymmetric carbon and thus there are two enantiomers (R)-I,2-propanediol and (3)-1,2-propanediol. 1,3-Propanediol is a stmctural isomer. Dipropylene glycol exists in three stmctural forms and since each stmctural isomer has two asymmetric carbons there are four possible stereochemical isomers per stmcture or a total of twelve isomers. These twelve consist of four enantiomer pairs and two meso- compounds. Tripropylene glycol has four stmctural isomers and each stmctural isomer has... 

It is seen that if the enthalpies and entropy s differ for two enantiomer pairs, there will always be a temperature where they elute coincidentally and cannot be separated. From the curves and intercepts given in Figure 15, the temperature for coincident retention of the two phenyl ethanol enantiomers is 432°K or 159°C and for the methylpiperidine enantiomers is 433°K or 160°C, which agrees excellently with the... 

Crystallization methods are widely used for the separation, or resolution, of enantiomer pairs. Enantiomer mixtures may essentially crystallize in two different ways. In around 8 per cent of cases, each enantiomer crystallizes separately, giving rise to a mechanical mixture of crystals of the two forms, known as a conglomerate. Conglomerates may usually be separated by physical methods... 

By far the eommonest adduetion method is via an aeid-base reaetion, in whieh an enantiomer pair eontaining an aeid funetion (e.g. a earboxylie aeid) is reaeted with a base resolving agent, or viee versa. 

Figure 2.3 An example of a stereospecific biotransformation resolution of bicyclic lactams. A and Ai are an enantiomer pair as are B and Bi.

FIGURE 9.13 Optimized ioscratic separation of the enantiomer pair for a drug candidate. Mobile phases 30% methanol containing 0.2% isopropyl amine/70% CO2 on a Chiralcel OJ-H column, flow rate 5 mL/min, outlet pressure 12.0 MPa and temperature 313 K. (Reprinted from C. White, J. Chromatogr. A, 1079 163 (2005). With permission.)... 

When from initial experiments, conditions that indicate the enantioselectivity of the system towards a given enantiomer pair or towards a limited series of substances are known, one might optimize their separation. To obtain optimal conditions, the different chemometric techniques used for method optimization in classic chromatographic or electrophoretic separations can also be applied for the chiral ones. Different experimental design approaches, using both screening and response surface designs can be In Reference 331, for... 

FIGURE 8 Differences in enantiomeric separation of the eight enantiomer pairs of a tetrapeptide under the same conditions. Sample Tyr-Arg-Phe-Phe-NH2. Conditions 64.5 (S6.0)cm X 50pm ID fused silica capillary BGE lOmM DM-/ -CD, lOOmM phosphoric acid, 88 mM triethanolamine (pH 3.0) V= 25 kV T = 30°C (with permission from reference 64). 

A wide range of bidentates containing one or more asymmetric phosphorus or arsenic donor atoms is now available due to the exploitation of a resolution technique involving the fractional crystallization of pairs of diastereomeric complexes formed by the chiral bidentates with pal-ladium(II) complexes containing optically active dimethyl(a-methylbenzyl)amine or dimethyl(l-ethyl-a-naphthyl)amine. Indeed, in recent work the two enantiomer pairs of l-(methylphenyl-arsino)-2-(methylphenylphosphino)benzene, (29a) and (29b), have been separated and isolated as optically pure air-stable crystalline solids with [a]o values of 79° (R, R ) and 15.5° (R, S ). 95... 

Figure 8 S-syn and R-syn enantiomer pair of 4-(tetrahydrothiopyran 1 -oxide-4-ylidene)cyclohexanone oxime.

The chances of the chromatographic resolution by inclusion chromatography were systematically studied on a series of enantiomer pairs of indole alkaloids as model compounds (e.g. Table V) and promising results were achieved on both analytical and preparative scale (26). 

Fig. 5. Structure of the C6oF4s enantiomer pair (reproduced with permission from J. Am. Chem. Soc., 116 (1994) 820 [28]).

Draw the structures of all possible isomers for the following complexes. Indicate which isomers are enantiomer pairs. 

Mechanistic studies on the interaction of quinine or quininyl-9-O-carbamates 69 and 70 with selected enantiomer pairs, employing sophisticated NMR experiments and molecular modeling, have been carried out by the Salvadori group. These studies revealed that due to the multifunctional character of the CSA molecules and their specific conformational preferences, each type of analyte induced its own specific interaction pattern [134, 137]. The main driving forces leading to enantiodiscrimination are due to hydrogen bonds between hydroxy group in QN or... 

Enantiomer separation becomes very easy when a column with a solid chiral stationary phase (CSP) able to separate a given enantiomer pair is available. Unfortunately, there is no universal phase for solving all separation problems and it is impossible that there ever will be. However, an immense number of CSPs have been developed and many of them are commercially available. A good deal of literature has also been published, from which a promising phase can usually be selected. Sometimes, the sample can be adapted to the phase by nonchiral derivatization. The most important CSPs are listed in Table 22.1. They are based on very different separation principles and can be grouped as follows ... 

---