Isomer separation, geometrical

There is no reported separation by Johnson and coworkers of geometric or optical isomers from their reaction mixtures. Connick, Pepperman, and coworkers (10,24) and Cook et al. (22) separated geometric isomers into their two diastereomeric racemates and noted differences in activity for the geometric isomers. Many of the reported activities of strigol-related compounds have been obtained with mixtures of isomers. 

Competing amines such as triethylamine and di-rc-butylamine have been added to the mobile phase in reversed-phase separations of basic compounds. Acetic acid can serve a similar purpose for acidic compounds. These modifiers, by competing with the analyte for residual active sites, cause retention time and peak tailing to be reduced. Other examples are the addition of silver ions to separate geometric isomers and the inclusion of metal ions with chelating agents to separate racemic mixtures. 

From these considerations it is clear that complexes in spin equilibrium do not exist at the crossover point between high-spin and low-spin configurations represented on a Tanabe-Sugano diagram. The two states are electronic isomers with geometric and electronic structures well separated on either side of the crossover point. The energy required to reach the crossover point represents at least part of the activation energy for the spin state interconversion. 

Silver ion chromatography is a useful technique for separating geometrical isomers of fatty acids (as the methyl ester derivatives) for subsequent analysis by GC. On the other hand, a stable ion-exchange column loaded with silver ions has been developed for HPLC that has proved of value in the simplification of complex mixtures of fatty acids (FAs) of natural origin for subsequent identification by GC-MS and for separating molecular species of triacylglycerols (41). 

Because the retention times of the different TGs within a PN differ, it is important to collect the whole peak at fractionation. One total fractionation, with an elution time of ca. 1.5 h, will provide enough material for the subsequent separation procedure based on argentation-HPLC (utilizing silver ions in the mobile phase), in order to separate geometrical isomers. 

Normal-phase HPLC is a good complementary technique to reversed-phase HPLC in that it often gives different selectivity. It is also more effective in separating geometric isomers than reversed-phase HPLC. The main problem with normal-phase HPLC is that aqueous samples are not normally compatible with the technique. Since many of the stress-testing samples contain water, normal-phase HPLC is rarely used as the primary analytical technique for stress test samples. Nonetheless, normal phase can be a useful complementary technique to reversed-phase HPLC. A detailed discussion on the development of normal-phase HPLC methods is beyond the scope of this chapter. 

IMS is a relatively new technique in which ions are separated based on size and shape using an electric field. IMS was utilized by Dong et ah (2010) to separate all-trans -lycopene from cw-lycopene and all-irans -(3-carotene from cw-(3-carotene. Unfortunately, the various cis isomers could not be separated from each other using IMS alone. The authors provided evidence to suggest that cis/trans isomerization of carotenoids occur in-source (ESI positive mode was used in these experiments). Because of this isomerization, it does not appear likely that IMS will replace HPLC as a means of separating geometrical isomers of carotenoids in the near future (Dong et ah, 2010). 

The strigol analogs prepared by Johnson and co-workers are normally obtained almost exclusively as the natural E-isomers. Each geometric isomer can exist as two diastereomers (39). Compounds 68 and 70 have been separated into their diastereoisomeric forms. In each case, the two diastereomers were almost equally active as germination stimulants (36). 

Thin Layer Chromatographic Isomer Separation. The geometrical isomers were separated by spotting a chloroform solution of the product on a silica gel plate and eluting with a 15 1 v/v chloroform/methanol solution. The spots or bands were dissolved in methanol and the solutions were evaporated to dryness. 

The mobility coefficient is dependent on shape and charge distribution and not only on mass thus, structural and geometric isomers that are isobaric (and nnresolved in most MSs) may be separated by IMS. This was danonstrated early in development of IMS and seen then as an attraction of mobility-based measurements. This feature of isomer separation can be controlled by the composition of the supporting gas... 

MacCrehan and Schonberger used reversed-phase HPLC to separate geometric isomers of retinol (with electrochemical detection), and found methanol water mobile phases to be superior to acetonitrile water for resolution (113,114) addition of n-butanol reduced elution times without sacrificing resolution. The order of elution from this reversed-phase system (either Vydac 201TP or Zorbax ODS columns) was di-cis < ll-cis < 9-cis < 13-cis < 1-cis < dll-trans. 

Biesalski and Weiser separated a -trans from l3-cis, W-cis, and 9-cis isomers of retinyl esters (retinyl palmitate, stearate, oleate, palmitoleate, and linole-ate) by isocratic adsorption HPLC (126). Bridges et al. used step gradients to separate geometric isomers of retinyl esters, retinal, retinal oximes, and retinal (both vitamin Ai and vitamin A2 forms), also by adsorption ( normal-phase ) HPLC (139,140). 

The value of many chemical products, from pesticides to pharmaceuticals to high performance polymers, is based on unique properties of a particular isomer from which the product is ultimately derived. Eor example, trisubstituted aromatics may have as many as 10 possible geometric isomers whose ratio ia the mixture is determined by equiHbrium. Often the purity requirement for the desired product iacludes an upper limit on the content of one or more of the other isomers. This separation problem is a compHcated one, but one ia which adsorptive separation processes offer the greatest chances for success. 

Table 3 fists cycloaliphatic diamines. Specific registry numbers are assigned to the optical isomers of /n t-l,2-cyclohexanediamine the cis isomer is achiral at ambient temperatures because of rapid interconversion of ring conformers. Commercial products ate most often marketed as geometric isomer mixtures, though large differences in symmetry may lead to such wide variations in physical properties that separations by classical unit operations are practicable, as in Du Font s fractional crystallisation of /n t-l,4-cyclohexanediamine (mp 72°C) from the low melting (5°C) cis—trans mixture. 

---