Geometrical isomers with phase

In the latter publication, for example, the vapor-phase IR spectra of all the four isomers of pulegol and dihydrocarveol are shown, which have been extracted from a GC/FTIR run. These examples convincingly demonstrate the capability of distinguishing geometrical isomers with the aid of vapor-phase IR spectra, which cannot be achieved by their mass spectra. A broad application of GC-FTIR in the analysis of essential oils, however, is limited by the lack of suf dent vapor-phase spectra of uncommon compounds, which are needed for reference use, since the spectra of isolated molecules in the vapor phase can be signi cantly different from the corresponding condensed-phase spectra. 

Gas-phase vacuum pyrolysis of the substituted butadienes (81 X = 0, S, or NMe) leads to the bicyclic heterocycles (82) via electrocyclization, followed by [1,5] hydrogen shifts." Benzofuran, benzothiophen, and iV-methylindole react with ethyl diazoacetate to give the corresponding carbene adducts (83 X = O, S, or NMe) as mixtures of geometrical isomers with benzothiophen, the insertion product (84) is also obtained/ ... 

The theory of the separation of geometric isomers on stationary phases that have a number of sterogenic centers has not been developed to the point where a particular stationary phase together with an appropriate mobile phase can be deduced for the separation of a specific pair of isomers. A number of theories have been put forward to explain the resolution of geometric isomers (some of which have been quite "imaginative" and "colorful") yet a reliable theory to help in phase selection for a hitherto unresolved chiral pair is still lacking. Unfortunately, the analyst is left with only two alternatives. The first is to search the literature for a model separation similar to the problem in hand and start with that phase system or, alternatively, resort to the technique of the early days of LC, namely, find the best phase system by a trial-and-error routine. 

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. 

It is usual to convert fatty acids to their methyl ester derivatives before separation by GLC, although it may be possible to analyse those with short chain lengths (two to eight carbon atoms) as the free fatty acids. Polar or non-polar stationary phases can be used and capillary (open-tubular) or SCOT columns will separate positional and geometric isomers. The cis isomers have shorter retention times than the corresponding trans isomers on a non-polar phase and visa versa on a polar phase. 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

Fig. 15 Separation of the phenacyl derivatives of the geometrical isomers of (A) linoleic and (B) linolenic acids by HPLC in the silver ion mode. The column temperature was 38°C, and the mobile phase was 1,2-dichloroethane/dichloromethane/acetonitrile (49.75 49.75 0.5 v/v/v) at a flow rate of 0.75 ml/min, with detection at 242 nm. Note the change of scale on the time axis.

The HPLC separation into geometrical isomers was performed on a 5-yii.m particle size Hi-bar RP Clg LiChrosorb column (250 mm X 4-mm ID). The mobile phase was methanol-2-propanol (3 1 v/v), with AgN03 in a concentration of 0.085 M, at a flow rate of ca. 1 ml/min. The major PN fractions, at a concentration of 20 mg/ml benzene, were injected in volumes of 5-10 fil. 

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. 

The resolution of unsaturated and saturated TGs with the same PN is strongly increased in the subsequent separation using silver in the mobile phase, compared with the initial fractionation. For example, the separation of OOO and PPP increased dramatically. Consequently, an extended space between the first and the last eluted component, within a PN, becomes available for the geometrical isomers of that particular PN. 

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. 

The results obtained by the first method especially those of recent studies performed with a commercially available stable 0(-, fj- and )f-CD bonded phases ( 8 -15) and with their newest improved modifications (16T demonstrate the great practical value of the sorbents and procedure.These studies dealt with structural and geometrical isomers and diastereoisomers as well as enantiomers of numerous compounds of various hydrophobic or hydrophilic nature. 

The product from Step 2 (2.06 mmol) was dissolved in 10 ml THE and added drop wise to a stirred suspension of NaH (2.27 mmol) in 5 ml THE at ambient temperature. After 15 minutes, the product from Step 1 (2.1 mmol) dissolved in 5 ml THE was added, the mixture stirred for several hours and was then quenched with 1 ml water. The organic phase was evaporated and the residue purified by column chromatography on silica with CH2Cl2/methyl alcohol, 95 5, yielding a Z/E geometric isomer ratio of 76 24, respectively. The desired Z-isomer was isolated by re-crystallization, mp = 189-190 °C. H-NMR and mass spectrum data supplied. 

Separations involving cis/trans isomers can also be accomplished by employing NPC. An example of this application is the separation of tricyclic antidepressant doxepin, which is marketed as a mixture of geometric isomers in a cis/trans ratio of 15 85 [41], When a spherisorb silica column is used with a hexane-methanol-nonylamine mobile-phase system, the cis isomer of doxepin elutes first. The structures of the two isomers and the chromatographic separation are shown in Figure 5-7. NPC has also been successfully employed in the separation of cis/trans isomers of steroids. Four diastereomers... 

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