Pirkle phases, chiral HPLC

Popova and colleagues47 carried out TLC of oxidation products of 4,4 -dinitrodiphenyl sulphide (the sulphoxide and sulphone) on silica gel + a fluorescent indicator, using hexane-acetone-benzene-methanol(60 36 10 l) as solvent mixture. Morris130 performed GLC and TLC of dimethyl sulphoxide. For the latter, he applied a 6% solution of the sample in methanol to silica gel and developed with methanol-ammonia solution(200 3), visualizing with 2% aqueous Co11 thiocyanate-methanol(2 1). HPLC separations of chiral mixtures of sulphoxides have been carried out. Thus Pirkle and coworkers131-132 reported separations of alkyl 2,4-dinitrophenyl sulphoxides and some others on a silica-gel (Porosil)-bonded chiral fluoroalcoholic stationary phase, with the structure ... 

Chiral stationary phases for the separation of enantiomers (optically active isomers) are becoming increasingly important. Among the first types to be synthesized were chiral amino acids ionically or covalently bound to amino-propyl silica and named Pirkle phases after their originator. The ionic form is susceptable to hydrolysis and can be used only in normal phase HPLC whereas the more stable covalent type can be used in reverse phase separations but is less stereoselective. Polymeric phases based on chiral peptides such as bovine serum albumin or a -acid glycoproteins bonded to... 

More than 30 years ago, Bill Pirkle, the recognized inventor of modern chiral HPLC, realized that it may be possible to effect a chromatographic separation of enantiomers by use of chiral selectors (or ligands) bound to a silica matrix 3 3,3 4]. There has been a phenomenal amount of development in chiral stationary phases over subsequent years but, a relatively small number of... 

Modified-C02 mobile phases excel at stereochemical separations, more often than not outperforming traditional HPLC mobile phases. For the separation of diastereomers, silica, diol-bonded silica, graphitic carbon, and chiral stationary phases have all been successfully employed. For enantiomer separations, the derivatized polysaccharide, silica-based Chiralcel and Chiralpak chiral stationary phases (CSPs) have been most used, with many applications, particularly in pharmaceutical analysis, readily found in the recent literature (reviewed in Refs. 1 and 2). To a lesser extent, applications employing Pirkle brush-type, cyclodextrin and antibiotic CSPs have also been described. In addi-... 

Most chiral HPLC analyses are performed on CSPs. General classification of CSPs and rules for which columns may be most appropriate for a given separation, based on solute structure, have been described in detail elsewhere. Nominally, CSPs fall into four primary categories (there are additional lesser used approaches) donor-acceptor (Pirkle) type, polymer-based carbohydrates, inclusion complexation type, and protein based. Examples of each CSP type, along with the proposed chiral recognition mechanism, analyte requirement(s), and mode of operation, are given in Table 3. Normal-phase operation indicates that solute elution is promoted by the addition of polar solvent, whereas in reversed-phase operation elution is promoted by a decrease in mobile-phase polarity. 

Enantiomeric purity of binaphthol is determined using chiral stationary phase HPLC Pirkle Type 1-A column (Regis Chemical Company) eluted with 20 1 hexane/2-propanol 1 or poly(triphenylmethyl)methacrylate on silica gel (Chiralpak OT, Daicel Chemical Industries, LTD) eluted with methanol. To determine enantiomeric purities >99% ee an HPLC trace of the unknown is compared to the HPLC trace of unknown containing 0.2% deliberately-added racemic material. 

Figure 10.3 Representative bonded chiral phases for HPLC. (I) Pirkle phase (II) N-(3, 5-dinitrobenzoyl) phenylglycine ionically bonded to 3-asinopropylsilanized silica (III) N-n-valeryl-L-valyl-3-aminopropylsilanized silica (IV) R-(-)-2-(2-4-5-7-tetranitro-9-fluorenylideneaminooxy) propionamidepropyl silanized silica.

Terfloth, G.J. Pirkle, W.H. Lynam, K.G. Nicolas, E.C. Broadly applicable polysUoxane-based chiral stationary phase for high-performance liquid chromatography and supercritical fluid chromatography. J.Chromatogr.A, 1995, 705, 185-194 [chiral HPLC SFC also carprofen, dcloprofen, etodolac, fenoprofen, flurbiprofen, naproxen, pirprofen, warfarin]... 

Fenvalerate Fenvalerate has four isomers (2 = 4). A CHIRALCEL OD colunm separated the enantiomer pair I (isomers A + D) from II (isomers B + C) (Li et al. 2006) (Table C7, Appendix C). Of the four individual isomers, the first and secrnid peaks were only partially resolved (Li et al. 2009). Huang et al. (1991) resolved fenvalerate into four well-separated peaks on a Pirkle-type 1-A chiral HPLC colunm using 99.9 0.1 hexane/isopropanol (v/v) as the mobile phase. The respective elution order was B, C, D, and A. The enantiomer pair II eluted before pair I. 

Pirkle and coworkers [59] compared retention and selectivity factors between HPLC and SFC using Poly Whelk-O chiral stationary phases and a-naphthyl-1-ethylamine carbamates. The results indicate that both retention and selectivity factors in SFC were higher than those in HPLC. This can be mainly attributed to the weaker solvating power of the carbon dioxide supercritical fluid as compared to a liquid such as methanol or hexane. 

Chiral separations result from the formation of transient diastereomeric complexes between stationary phases, analytes, and mobile phases. Therefore, a column is the heart of chiral chromatography as in other forms of chromatography. Most chiral stationary phases designed for normal phase HPLC are also suitable for packed column SFC with the exception of protein-based chiral stationary phases. It was estimated that over 200 chiral stationary phases are commercially available [72]. Typical chiral stationary phases used in SFC include Pirkle-type, polysaccharide-based, inclusion-type, and cross-linked polymer-based phases. 

Macaudiere P, Lienne M, Tambute A, Caude M, Pirkle type and related chiral stationary phases for enantiomeric resolution, in Chiral Separations by HPLC, Krstulovic AM (Ed.), Ellis Horwood, New Y>rk (1989). 

Chiral separations can be considered as a special subset of HPLC. The FDA suggests that for drugs developed as a single enantiomer, the stereoisomeric composition should be evaluated in terms of identity and purity [6]. The undesired enantiomer should be treated as a structurally related impurity, and its level should be assessed by an enantioselective means. The interpretation is that methods should be in place that resolve the drug substance from its enantiomer and should have the ability to quantitate the enantiomer at the 0.1% level. Chiral separations can be performed in reversed phase, normal phase, and polar organic phase modes. Chiral stationary phases (CSP) range from small bonded synthetic selectors to large biopolymers. The classes of CSP that are most commonly utilized in the pharmaceutical industry include Pirkle type, crown ether, protein, polysaccharide, and antibiotic phases [7]. 

The enantiomeric excess of this product was determined to be >99.5X using a chiral stationary phase HPLC (preparative Regis Pirkle Type 1-A, 10 x 250 tm I.D., 7.5 mt/min flow rate, 1000 psi pressure, lOX 2-propanol in hexane, detector at 284 nm). The R-(-)-enant1omer Is eluted first and the peaks are well separated. Another batch of phosphate ([ 3p -507.7 C, THF, e 1.17) was shown to have 96.5% ee using the same conditions. 

Recently, optically active polythiophenes, incorporating as ring substituents chiral selectors such as (R)-(-)- and CS )-(+)-/V-(3,5-dinitrobcnzoyl)-a-phcnylglycinc used in Pirkle-type stationary phases, have been synthesized.167 These may have potential in enantioselective analysis of chiral chemicals using high performance liquid chromatography (HPLC). 

Cleveland, T. Pirkle-concept chiral stationary phases for the HPLC separation of pharmaceutical race-mates. J.Liq.Chromatogr, 1995, 18, 649—671... 

---