GC enantiomer separation

Nowadays, derivatized cyclodextrins are the most common chiral selector in the direct GC enantiomer separation of flavors, fragrances, essential oils, pheromones and other natural, nature identical and synthetic volatile COmpounds.20.22,23,47,49-57... 

At the present state of knowledge, the mechanisms of GC enantiomer separation have not been elucidated. Unusual chromatographic behaviour and reversal of the elution order of enantiomers have been observed. Consequently, the usefulness of a given chiral stationary phase as well as the order of elution of separated enantiomers cannot be predicted. References of definite chirality are essential to identify the separated isomers, no matter whether directly stereoanalyzed with chiral stationary phases or via derivatized stereoisomers [12-19]. 

The major breakthrough in the GC enantiomer separation has been the work of Bayer and associates [23,76], who synthesized a silicone-based chiral phase, stable up to 240°C. As shown in Fig. 3.14, a racemic mixture of 19 protein amino acids can be separated [23] on a glass capillary column coated with Chirasil-Val, a chiral polysiloxane phase. The phase was synthesized through coupling L-valine-tert-butylamide to a copolymer of dimethylsiloxane and carboxyalkylmethylsiloxane. 

The introduction of capillary columns for GC analysis produced a breakthrough in the analysis of environmental pollutants due to their high separation efficiency. For environmental applications, fused-silica wall-coated open-tubular columns with internal diameters from 0.1 to 0.32 mm and film thickness of 0.1-0.2 pm, and lengths from 25 to 60 m are currently used. The wide range of stationary phases commercially available with different polarities and high thermal stabiHty provides the tool required to maintain the prominent position of GC in environmental analysis. In addition, the availability of chiral stationary phases gives GC the capability to perform GC enantiomer separations. Table 1 gives the recommended columns used in routine analysis of some selected pollutants. 

A lot of published data on the separation of enantiomers of flavors and fragrances by GC is reviewed by Chirbase/Flavor database. Table 1. summarizes the enantiomer separation of oxygenated monoterpenes on chiral stationary phases of cyclodextrin derivatives by high resolution gas chromatography. 

Table 5. Some Examples25" of Analytic Enantiomer Separations by GC on Chirasil-VAL...

Two-dimensional GC in the direct enantiomer separation of menthone, isomenthone and menthol with Ni(HFC)2 as the chiral main column has been reported by Bicchi et al. [92]. Werkhoff et al. [68] isolated these compounds from peppermint oils before stereoanalysis with permethylated p-cyclodextrin. 

CDs have a longstanding tradition as versatile host systems in the molecular recognition field, and have seen extensive use in various enantiomer separation techniques [206, 207], including GC, LC, CE and CEC. The widespread applications of CDs for enantiomer separation of chiral drug compounds have been detailed in recent reviews [55, 208]. 

The sandfly Lutzomyia longipalpis is the vector of the protozoan parasite Leishmania chagasi, the causative agent of visceral leishmaniasis in South and Central America. Population control of L. longipalpis is therefore of urgent importance to prevent the disease. In 1994, Hamilton and coworkers isolated the male-produced pheromone of L. longipalpis from Jacobina, Brazil, and proposed its structure as 3-methyl-a-himachalene (96, Figure 4.47) with unknown stereochemistry. We first synthesized ( R, 3R, 1 S )-( )-96.81 Enantiomer separation (optical resolution) of a synthetic intermediate enabled us to prepare both the enantiomers of 96, and their bioassay and GC comparisons with the natural pheromone showed the latter to be (lS,3S,lR)-96. 

Here, only general categories of chiral stationary phases will be mentioned. One of the more popular types of GC and HPLC columns use donor-acceptor interactions such as those illustrated in Figure 2.16 for enantiomer separation. 

Although efforts were made to separate chiral substances in the late 1950s, the use of GC to separate enantiomers was not successfully started until the mid 1960s, at which time, the main developments in the general technique of GC had been completed and the methodology was well established. There were two reasons for this early lack of concern for chiral separations. Firstly, there was the lack of interest shown in the separation of chiral substances generally (the importance of their different physiological activities had, at that time, not been fully disclosed). Secondly, it was found very difficult to obtain adequate selectivity between such closely similar substances to effect a separation. 

M-Y. Nie, L.-M. Zhou, Q.-H. Wang and D.-Q. Zhu, Enantiomer separation of mandelates and their analogs on cyclodextrin derivative chiral stationary phases by capillary GC, Anal. Sci., 2001,17, 1183-1187. 

The group of Prof. Roussele in Marseille has been working for the past ten years on the assembly of a database for enantiomer separations in HPLC and GC called Chirbase [47]. So far, more than 100,000 enantiomer separations of more than 30,000 compounds have been described every three months another 5000 entries are being added, some of which have not been published in the open literature. 

A diiral GC column has the potential to separate enantiomers of epoxy pheromoies in the Type II class, but die applications are vay limited because no good column with a universal ability fw the resolution has been commercialized. On the odia hand, the resolution abilities of chiral HPLC columns have been examined in detail (77). The Chiralpak AD column qierated under a nmnal-phase oonditiiMi suffidaitly separates the two enantiomers of 9,10-epoxydienes, 6,7-epoxymonoenes and 9,10-epoxy-monoenes. Another normal-phase column, the Chiralpak AS column, is suitable fiir the resolution of the 3,4-epoxydienes. The Chiralcel OJ-R colunm operated unda a reversed-phase condition sufficiently accomplishes the enantiomeric separation of 6,7-epoxydienes and 6,7-epoxymonoenes 18). The stereochemistry of each enantiomer separated by chird HPLC has been studied after methanolysis of the epoity ring. Examining the H NMR data of esters of the produced methoxyalcohols with (S)- and (Ry)-a-methoxy-a-(trifluoro-methyl)phenylacetic acid Ity a modified Mosher s method (79), die parent epoxides widi shorter Rts have been indicated to be (3.9,47 )-, (69,77 )-, and (97 105)-isomers (77). 

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