Capillary gas chromatographic separation of enantiomers with stable dipentyl-α, β- and...

Analytrca Chrmrca Acta, 234 (1990) 365-380

Elsewer Science Pubhshers B V . Amsterdam - Pnnted m The Netherlands

365

Capillary gas chromatographic separation of enantiomers with stable dipentyl-a-, /?- and y-cyclodextrin-derivatized

stationary phases

DANIEL W ARMSTRONG *, WEIYONG LI and APRYLL M STALCUP

Department of Chemrstry, Unrversrfy of Musourr-Rolla, Rolla, MO 65401 (U S A )

HENRY V SECOR, RICHARD R. IZAC and JEFFREY I SEEMAN *

Phrlrp Morns Research Center, P 0 Box 26583, Richmond, VA 23261 (U S A )

(Received 13th November 1989)

ABSTRACT

Llquld and amorphous sohd pentyl derwatlves of a-, p- and y-cyclodextnn were evaluated as gas chromatographlc

(GC) chral stationary phase coatmgs on fused-slhca caplllanes The factors controllmg the physlcal state of the

pentyl-substituted cyclodextnn denvatlves were consldered Sixty-five racemlc solutes were resolved, mcludmg alcohols,

ammes, lactones, epoxldes, ammo alcohols, esters, ethers, ammo acids, haloalkanes, sugars and alkaloids The

pentyl-substituted a-. p- and y-cyclodextnn stationary phases showed pronounced selectwty differences. apparently

based on the size, shape and functlonahty of the analytes Inclusion complexatlon may more readdy occur at high

temperatures m gas than m hquld chromatography (LC) Enantlomenc separations can be accomphshed by this GC

method that cannot be done by related cyclodextnn LC techniques

Keywords Enantlomers, Cyclodextrms, Choral stationary phase

In 1966, Gil-AV et al. [l] reported the first chiral stationary phase (CSP) for the separation of volatile amino acid derivatives by gas chromatography (GC). Over the next decade, several mvesti- gators applied a variety of amino acid derivatives and peptides as GC CSPs [2-61. Derivatives of other naturally occurring chtral molecules, such as tartanc, malic, mandehc and chrysanthemic acid, also showed some promise as GC CSPs [7,8]. Subsequently, chtral metal complexes were used as GC stationary phases [9]. The only commercially successful GC CSP was made by coupling L-

vahne-tert-butylarmde to a siloxane copolymer, i.e., Clnrasil-valme (Fig. 1) [lO,ll]. In the 1980s a tremendous expansion occurred in the liquid chro- matograpmc (LC) separation of enantiomers and

0003-2670/90/$03 50 0 1990 - Elsevler Science Pubhshers B V

a large number of new techniques were developed [12-191. These LC methods were so successful that the importance of “chiral GC” seemed to be ignored. There were several reasons for this. First, many of the chiral GC phases had limited apphca- bihty Most of them were based on one or more ammo acids and they tended to be useful prim- arily for resolving other racemtc ammo acid derivatives and analogous solutes. The analyte had to be volattle for analysts by GC and this limited the utility of the method for a variety of choral compounds including many of those of pharmaco- logical importance. The higher column temperatures needed for GC often resulted in racemiza- tion, decompositton and bleeding of the CSP. Fur- thermore, enantioselecttvittes were significantly

D W ARMSTRONG ET AL

7 “1 cH3 I I

-S1---&-cH3

I l I cm; CH3

-I -7n

AH I

1 /(7H3; CH-CH

l \cH31 I c=G

I r$I

I 1 ; H3C-C-CH3 I

I 1 cH3 I

I- ‘n _I

Fig 1 Structure of Chlrastl-vahne

decreased at higher temperatures. Finally, most preparative-scale separations were not feasible with GC.

Recently, there have been reports on new types of GC CSPs that will probably revitalize the separation of stereoisomers, including enantiomers by capillary GC [20-241. All of these CSPs are based

on derivatized cyclodextrins (CDs) (Fig. 2). The versatility, stability and utility of the newest gen-

1, a-CD n=6

2. P-CD n=7

3, y-CD n=8

4, P-a-CD ~6. R=~-CSHII

5, P-P-CD n=7. R=n-CSHII

6, P-~-CD n=R. R=n-CSHII

Fig 2 Structures of (a) cyclodextnns and (b) denvatmed cyclodextnns

eration of CD derivatives may be surprising because several investigators [25-311 have attempted to use native CDs and simple derivatives m GC for years with much less success (at least for enantiomeric separations). Most of the initial work involving (Y- and P-CDs (1 and 2, respectively) and then polymers was done by Smolkova- Keulemansova and co-workers [25-281. They found that the CD stationary phases gave unusual selectivities (particularly for structural isomers)

that could be accounted for only by mclusion complex formation. The selectivity of these stationary phases was affected by the solvent used m the coating process. Also, it was noted that the

efficiency of these columns was not as high as desirable. Sybilska and co-workers [29,30] also worked with packed GC columns containing an (Y-CD stationary phase. Again, interesting selectivities and poor efficiencies were found. However, they did report one of the few successful enantt- omeric GC separations (for /?-pinene) done with a

native CD stationary phase [29,30]. It was evident from extensive LC work and

from the aforementioned GC work that CDs had potential as GC stationary phases. However, the fact that native CDs and most of their simpler derivatives (e.g., dimethyl, permethyl, peracetyl) are crystalline solids with very high melting or decomposition points makes them difficult to use as GC stationary phase coatings. Even when dissolved in various higher boilmg solvents, poor efficiencies were obtained and high temperatures

had to be avoided [25-301. Better results were obtained when permethylated /?-CD was dissolved m a typical GC stationary phase liquid, such as

OV-1701, and coated onto glass capillary columns [31]. Reasonably efficient enantiomeric separations of a few cyclic alkanes, ethers and alcohols were demonstrated [31,32]. Recently, KGmg and co-workers [20-231 indicated that certain derivatives of CX- and P-CD (i.e., perpentyl and 3-0- acetyl-2,6-di-0-pentyl analogs) were liquids and could be used as GC stationary phases. They were able to effect a number of enantiomenc separations using 40-m glass capillaries coated with

these compounds.

In thts work, three di-0-pentylcyclodextrms were exammed, namely 2,6-dt-0-pentyl-a-, p- and

CAPILLARY GC OF ENANTIOMERS 367

y-cyclodextrin (4, 5 and 6, respecttvely) for then physical properties and their performance as GC choral stattonary phases. The posstbihty that pur- ity controls the state (I.e., solid or hqutd) of the vartous CD dertvattves IS considered. The first compartson of analogous (r-, /3- and y-CD dertva-

ttves as GC choral stattonary phases 1s made. The use of CD dertvattves that can be effecttvely coated on fused-sthca captllaries 1s emphasized.

EXPERIMENTAL

Deactivated and untreated fused-sthca capillary tubing (0.25 mm i.d.) was obtained from Alltech and Supelco. Dtpentylcyclodextrms were prepared followmg a literature procedure [33]. The pentylated CDs were purified by LC usmg a Whatman Magnum 20 Parttstl 10 column with 15% ethyl acetate-hexane as eluent at a flow-rate of 9 ml min-‘.

Heptaku(2,6-dl-0-pentyI)-P-cyclodextrln (5) A

solution of P-cyclodextrin hydrate (1.0 g, 0.87 mmol) u-t dimethyl sulfoxtde (20 ml) was treated with finely ground NaOH (2.0 g, 50.0 mmol) and l-bromopentane (5.97 g, 39.5 mmol). The mixture was cooled slightly, stirred for 48 h and then quenched by the addttton of chloroform (60 ml) and water (60 ml). The organic phase was separated and treated with MgSO, and Filter Cel, stirred for 20 h and filtered. The clear, pale yellow solutton was concentrated under vacuum (0.15 mmHg, 45” C) to give 1.37 g (74.6% yield) of partially crystalhne 5: ‘H NMR (CDCI,), S 0.87-

0.93 (m, 42H), 1.26-1.33 (m, 56H), 1.56-1.71 (m, 28H), 3.35 (dd, 7H, .I = 9.6, 3.6 Hz), 3.40-3.60 (m, 21H), 3.59-3.75 (m, 28H), 3.89-5.10 (m, 14H),

4.90 (d, 7H, J = 3.6 Hz); 13C NMR (CDCI,), 6 14.08, 14.13,22.50,22.61, 27.85, 27.93,28 32,28.41,

29.38, 29.42,29.51,69.03, 70.38,71.51,72.96,73.41, 80.34, 83.47, 101.78. Analysis. calculated for (C,,H,,O,),, C 63.54, H 10.00; found C 63.93, H 10.08%.

Hexaku(2,6-dr-0-pentyl)-cr-cyclodextnn (4). The above procedure was used. cY-Cyclodextrm hydrate (1.0 g, 1.01 mmol) yielded 2.02 g (nominal 110%) of crude 4 as a colorless VISCOUS 011: 13C NMR (CDCI,), S 14.14, 14.16, 22.51, 22.64, 27.96,

28.32, 28.44,29.41,29.54, 69.35, 70.42, 71.80, 72.70, 73.77, 79.85, 83.49, 101.41. LC assay of thts material showed two large, broadly elutmg fractions. The larger of the two fractions was collected to yield 700 mg (38.2%) of partially crystalline 4 having a 13C NMR spectrum identical with that of the crude material with the sole exception of the

disappearance of the peak at 27.96 ppm. Analysts: calculated for (C,,H,,O,),, C 63.54, H 10.00; found, C 64.23, H 10.13%.

Octaku(2,6-dl-0-pentyl)-y-cyclodextrm (6). The procedure described above was used. y-cyclodextrin (1 0 g, 0.77 mmol) yielded 2.12 g (nominal 114%) of crude, partially crystallme 6. 13C NMR (CDCI,), 6 14.13, 14.18, 22.55, 22.66, 28.02, 28.39, 28.46,29.48,29.56,69.17, 70.46, 71.54, 73.11, 73.40, 80.74, 83.22, 101.77. LC assay of thts material showed two overlapping broad peaks. The larger peak collected to yield 900 mg (48.3%) of partially crystallme 6 having a 13C NMR spectrum stmtlar to that of the crude material wtth the exception

that some of the closely spaced resonances in the ahphattc region (22-30 ppm) had altered relative

intenstttes. Analysts: calculated for (C,,H,,O,),: C 63.54, H 10.00; found, C 64.29, H 10.12%.

The fused-silica capillaries were coated by the static method usmg a solution of 0.2% (w/v) of the stationary phase material m dtethyl ether. Wtth the dipentyl-cu-cyclodextrm dertvattve 4, two dtf- ferent coating mixtures were compared; one contamed 4 and the other both 4 and polyethylene glycol 400.

All racemrc or enanttomertc solutes were obtained from Sigma or Aldrich, except for the tobacco alkaloids and nicotine analogs, reported

prevtously [34-421. Ammes and alcohols were de- rtvattzed with trifluoroacetrc anhydride (TFAA), acetic anhydrtde, monochloroacetic anhydrtde or trtchloroacetic anhydrtde (Aldrich). In each case,

ca. 1.0 mg of the analyte was dtssolved m 0.5 ml of methylene chloride and 200 ~1 of the desired anhydrtde were added. Ammes reacted qutckly (a few seconds to minutes) and alcohols more slowly (20-30 min). When the reaction was complete, dry

nitrogen was bubbled through the solutton to re- move the excess of anhydrtde reagent. Chiral carboxylic acids were converted to stlyl esters by reaction with N-methyl-N-trimethylsilyltrtfluoro-

368 D W ARMSTRONG ET AL

acetannde at 60” C for 30 min. The solvent type and volumes were identical with those m the aforementioned procedure. Sugars were trifluro- acetylated by a procedure sunilar to that for amines and alcohols except that tetrahydrofuran was used as the solvent. Also, because the time for complete reaction was longer and TFAA is vola- tale, additional aliquots of TFAA were added every 5-10 min.

Typical capillary columns had efficiencies of 2300 plates m ‘. Unless indicated otherwise, the separations were done on a relatively short 10-m column. All of the reported racemates could be baseline resolved on columns < 40 m long and most were resolved on the 10-m column. The columns can be operated at temperatures between 30 and 300 a C, although oxygen should be ex- cluded from the carrier gas at higher temperatures. There do not seem to be problems when operating at higher temperatures (> 150” C) for extended period of time. However, on returmng to low-temperature separations, a decrease in column efficiency was sometimes noticed. Consequently, we recommend that a column be dedicated to high- temperature or to lower temperature (< 150 o C) separations. These columns have been used con- tinuously for several months. Another interesting feature of these GC phases is that the peaks do not tail as extensively as has been reported for LC CSPs. Consequently, relatively better resolutions are obtained for compounds with small (Y values.

Either a Hewlett-Packard Mode1 5710A or a Varian Mode1 3700 gas chromatograph was used for all separations. Split mlectton and flame ioni- zation detection were applied. The injection port temperature was 200°C. Both dned nitrogen and hydrogen were used as carrier gases.

RESULTS AND DISCUSSION

One of the important properties of certain CD derivatives that allow them to be utilized success- fully as GC stationary phase coatings IS their liquidity. In addition, they have very low volatili- ties, are stable at hrgh temperatures m the absence of oxygen, and have many “chual centers”. Al- kylation of (Y-, p- or y-CD with sufficiently large

groups can produce viscious liquid products. However, extensive purification of 4-6 yields semi-crystallme materials (see Experimental). For example, with 5, the exhaustively purified liquid yielded a solid of m.p. 90-92°C. For the P-a-CD 4 and the P-y-CD 6, LC analysis mdicated that a mixture of compounds IS present. Interestmgly, the 13C NMR spectra of the crude products 4-6 are nearly identical. In these spectra, the ap- pearance of more than eight ahphatic carbons m the region 22-30 ppm also mdicates the presence of a mixture of alkylated CDs. Apparently, extensive derivatization of CDs produces a series of closely related species with shghtly different ex- tents of substitution or substituted in different posittons [33,43]. Because these species are not the same, they do not crystalhze. This is not surpris- mg, as it has been noted that this same phenomenon 1s responsible for greatly increasing the water solubihty of a variety of P-CD derivatives [44]. The heterogeneity of the derivatized product is not the only effect that leads to hquification of certain derivatized cyclodextrms. There are numerous examples of solid but heterogeneous cyclodextrm derivatives [44].

In addition to bemg heterogeneous, it is benefi- cial if most of the polar primary and secondary hydroxyl groups on the CD are functionalized with non-polar, non-hydrogen-bonding groups. Finally, functional groups with some steric bulk seem to inhibit crystallization [45]. However, very large or bulky functional groups can sometimes be counter-productive, because larger substituents on the cyclodextrin mmbit the reaction of adlacent hydroxyls for steric reasons. Also, tf the molecular weight of the derivatized CD is too high, it tends to exist as an amorphous solid. This seems to be true for exhaustively hexadecylated P-CD [45].

The question arises as to whether the purified solid CD derivative behaves differently as a GC stationary phase than the closely related but more heterogeneous liquid. Fused-sihca capillaries (20 m) were coated with the solid and liquid “dipentyl#CD” derivatives. At lower temperatures (20-8O”C), there were obvious differences due to the state of the stationary phase. The capillaries coated with the heterogeneous liquid gave good, efficient enantiomeric separations whereas those


TABLE 1

Influence of polyethylene glycol 400 on low-temperature GC separations wth an amorphous sohd pentyl-a-CD stationary phase

System a Compound k; k; a RS T(OC)

I Tetrahydro-2-(2-propynyloxy)-2H-pyran 12 89 14.16 1.10 1 30 70 2-(Bromomethyl)tetrahydro-2H-pyran 19.0 20.29 1.07 1 30 70 2-(Chloromethyl)tetrahydro-2H-pyran 7.71 8 14 1.06 1 25 70 3,4-Dlhydro-2-ethoxy-2H-pyran 8.68 9 17 1.06 11 50 ( +)-exe-2-Chloronorbomane 9.75 10 31 1.06 08 50

II Tetrahydro-2-(2-propynyloxy)-2H-pyran 18.35 19 86 1.08 1 25 70 2-(BromomethyQtetrahydro-2H-pyran 26.10 27 82 107 1 25 70 2-(Chloromethyl)tetrahydro-2H-pyran 10.94 1142 1.04 1 25 70 3,4-Dlhydro-2-ethoxy-2 H-pyran 8 89 943 106 1.25 40 ( f )-exe-2-Chloronorbomane 22 23 23 19 104 1.0 40

a I, P-a-CD, coating solution 0.2’S, PEG 0.07%, column length 9 m, II, P-a-CD, coatmg solution 0 2%, PEG 0.128, column length 9

m.

coated with the pure material did not. Micro- scopic examination of the latter capillaries showed that they were poorly coated with the stationary phase. In these cases, the stationary phase seemed to have agglomerated into clumps or sohd drop- lets. However, at higher temperatures (> 90” C), the two stationary phases were remarkably similar in their chromatographic properties, m particular their enantioselectivity. Consequently, from a practical standpoint, there seems to be little rea- son to use the highly purified CD derivative. How- ever, for theoretical and mechanistic studies, the purified substrate may be essential. Interestingly, many of the solid or semi-solid CD derivatives can be used at low temperatures if diluted slightly with polyethylene glycol (PEG) 400. Small amounts of the PEG do not seem to interfere with enantioselectivity but can have a significant effect on retention time, as shown in Table 1.

There had been no comprehensive comparisons of analogous LX-, /3- and y-CD denvatized GC stationary phases. The dipentylcyclodextrins 4-6 seem to have somewhat different selectivlties to those reported by K&rig and co-workers [20-231. Also, they are easily coated on deactivated fused- silica capillaries.

A series of 65 racemic compounds were individ- ually injected onto capillary columns coated with 4-6. Very different enantioselectivities were observed. Table 2 lists the compounds and pertinent separation data generated with the dipentyl-a-CD (4) capillary column. Tables 3 and 4 give analo-

gous data for the derivatized P-CD (5) and y-CD (6) columns, respectively. Only the data for racemates that were effectively resolved are given in each table. It is apparent that a variety of different types of enantiomeric compounds can be resolved by capillary GC, including alcohols, amines, ammo alcohols, amino acids, epoxides, carboxylic acids, esters, lactones, ethers, sugars and haloalkanes. The largest number of compounds were separated on the derivatized P-CD column, followed by the a-CD and then the y-CD column. Interestingly, this trend seems to parallel that found for the analogous CD LC bonded phase columns [46]. However, both the dipentyl-y-CD and particularly the dipentyl-a-CD are clearly superior at resolving certain types of racemates. For example, the a-CD column effectively resolves a variety of epoxldes and other cyclic ethers that are difficult to resolve on the larger CD stationary phases (Table 2). Particularly illustrative examples of the selectivity differences between (Y-, /3- and y-CD phases are shown in Figs. 3 and 4. 3-Ammomethyl-3,5,5-u+ methylcyclohexanol has two “chiral centers”, i.e., it can exist as four stereoisomers, two pairs of enantiomers (Table 2). Only on the a-CD stationary phase are both pairs of enantiomers easily and quickly resolved (Fig. 3). Conversely, the larger compound 1-(1-naphthyl)ethylamine is best resolved on the y-CD column (Fig. 4). The y-CD column also effectively resolved a number of truns-cyclic diols (Table 4).

The pentylated P-CD column seemed to have a


TABLE 2

Enantlomenc separation of racenuc compounds by capdlary CC wth a dlpentyl-a-CD stationary phase

Compound Structure Column Column R a

temperature length (m)

(“0

Alcohols (R = H)

1,2,3,4-Tetrahydro-1-naphthol

OR

cl3 0 160 20 COCH, 1 07

OR

1-Phenylethanol

I-Phenylpropan-2-01

60

60

10 a

10

H 102

H 1 02 \ s OR

Terpmen-4-01

exo-2-Hydroxymethylnorbomane

40

40

10 a

10 a

H 105

H 1 07

L&f CH20R

70 10 = COCF, 1 30

NHR

d

Amrnes (R = H)

I-Ammomdan 90 10 COCF, 1 20

NHR

cd 0

1,2,3,4-Tetrahydro-1-naphthylamme

US- and rruns-3’-Methylnormcotme

N-[1-(3-Pyndyl)ethyl]-ethylanune

170

140

20

20

COCH ,Cl 1.05

COCF, 106, 107 h c? 0 N R

N

160 20 COCF, 107

CY 0 NJ R

N

TABLE 2 (contmued)

Compound Structure

371

Column Column R a


(“C)

Amrnes (R = H)

2-Ammopentane

1-Methoxypropylamme

1-Phenylethylamme

1-Cyclohexylethylamme

endo-2-Ammonorbornane

3-Ammomethyl-3,5,5-tnmethylcyclohexanol

Epoxrdes

1,2-Epoxyhexane

1,2-Epoxyoct-7-ene

Glycldyl isopropyl ether

terr-Butyl glycldyl ether

Ally1 glycldyl ether

Ltmonene oxide

Styrene oxide

Other ethers

2-(Bromomethyl)tetrahydro-2 H-pyran

NHR

/-O r HR NHR

d NHR

NHR OR

-Q- CH2NHR

0

2 0

CT

30

40

50

45

30

90

40

40

30

30

30

30

30

30

10 d COCF, 108

10 d COCF, 103

10 = COCF, 103

10 = COCF, 108

10 = COCF,

10 COCF,

10 d _

10 = _

10 ‘% _

10 * _

10 d _

10 d _

10 a _

10 a _

108

120.1 19 b

1 03

1 05

1 06

1 02

104

1 20, 1 06 b

105

115

(Contmued on p 372)

312

TABLE 2 (contmued)

Compound Structure

D W ARMSTRONG ET AL

Column Column R a


(“C)

Other ethers

Tetrahydro-2-(2_propynyloxy)-2 H-pyran

2-(Chloromethyl)tetrahydro-2 H-pyran

2-Ethoxy-3,4-dlhydro-2H-pyran

2-Ethoxytetrahydrofuran

Other compoundr

endo- and exe-2-benzylnorbornane

endo- and exe-2-acetylnorbornane

exe-2-Bromonorbomane

exo-2-Chloronorbornane

0-Acetylmandehc acid

c), 40 10 a _ 1 25

0 0CH2CzCH

0, 0 40 10 a

H2C1

114

Q 0 30 10 a

C2H5

1 20

I 0 OCZH5 30 10 d

GB”

CH2C~ll~ 70 10 d

0

& E-CH3 40 10 a

1 10

102,103 h

105.1 02 h

Br

&

30 10 = 1 05

a9

Cl

30 10 a _ 1 08

OCOCH3

OR 120 10 VCH,), 127

* This low-temperature CSP also contamed 0 1% PEG 400 h Two pans of enantlomers (a total of four stereolsomers) exist for this

material

htgh enanttoselectivtty for ammes (including het- used with these stationary phases, as bleeding

erocyclic compounds) and sugars (Table 2). Also, seems to be minimal. Figure 5 shows a typical

tt effectively resolved a number of other classes of isothermal separation of three related, racemtc

compounds such as esters and amino alcohols. amines and Fig. 6 shows a gradtent separation of

Both isothermal and gradtent separations can be the methyl ester of racemtc tyrosme.

CAPILLARY CC OF ENANTIOMERS 373

TABLE 3

Enantlomenc separation of racenuc compounds by capdlary GC wth a dlpentyl-P-CD stationary phase

Compound Structure Column Column R (1


(“C)

Alcohols (R = H)

1,2,3,4-Tetrahydro-1-naphthol

2-Methoxy-2-phenylethanol

Amrnes

1-Ammomdan

1,2,3,4-Tetrahydro-1-naphthylamme

I-(1-Naphthyl)ethylamme

endo-2-Ammonorbornane

2-Ammo-1-methoxypropane

QMethylnormcotme

2-(4-Pyndyl)pyrrohdme

Anabasme

OH

a!l 0

0CH3

100 10 _ 105

100 10 COCF, 1.02

80 10 COCF, 107

NHR

dJ 0

NHR

NHR

A/ \

140

130

150

100

115

170 10 COCF, 105

170 10 COCF, 1.03

180 10 COCF, 1 03

20 COCF, 1 05

20 COCF, 1 01

10 COCF, 1 06

10 COCF, 103

30 COCCl 3 1.02

(Contmued on p 374)


TABLE 3 (contmued)



(“C)

Amrnes

2-Phenylpyrrohdme

2-(2-Pyndyl)pyrrohdme

2-Phenethylpyrrohdme

l-(3-Pyndyl)ethylarnme

I-Phenylethylamme

2-(3-Pyndyl)-1-azacycloheptane

N ‘-benzylnomxotme

N ‘-(2,2,2-TnfluoroethyQnomxotme

N ‘-(2,2-Dlfluoroethyl)normcotme

Hexobarbltal

Mephobarbltal

140 10 COCF,

140 10 COCF,

140 20 COCF,

NHR 140 20 COCF,

NHR 120 20 COCF,

140-195 a 20 COCF,

103

102

101

102

103

1.01

160-200 a 20

140 20

_ 101

101

160 30 _

O= 180 10

o= 180 10

102

1 02

102

CAPILLARY GC OF ENANTIOMERS 37s

TABLE 3 (contmued)



(“C)

Ammo alcohols

2-Ammopropan-2-01 -Y-OR 140-180 d 30 COCH,CI

12s 30 COCH,

101

102

1 06, 1 01 h

1 04

NHR

NHR

OR

CHZNHR

2-Ammopentan-1-01

3-Ammomethyl-3,5,5-

-tnmethylcyclohexanol 150 30 COCF,

120 20 COCF,

OR CH2NHR

OR

q-( f )Ephedrme

HR

Octopamme 140-200 A 30 COCF, 1 01

COCF, 102 2-Ammo-3-phenylpropanol

Ammo nclds

a-Methyl-p-tyrosmemethyl ester 0CH3 170-200 .a 30 COCF, 1 02

RO

RoMCH3 170-200 = 30 COCF, 1 02 Tyrosme methyl ester

Carboxylrr acids and esters

0-Acetylmandehc acid

OCOCH.

o+ 0 5

OR 130 30

30

WCH,), 1 03

_ 1 04 a-(Tnchloromethyl)benzyl acetate

OH OCH3

100 30 Mandehc acid methylester 1 02

(Contmued on p 376)


TABLE 3 (contmued)



(“C)

Ammo acrdr

Mandehc acid ethylester

LAxtones

Pantolactone

N-Acetylhomocystemetholactone

Sugars CH20R

DL-Glucose

DL-Mannose

DL-Galactose

DL-kbose

Dt_-Arabmose

DL-XylOSe

DL-LyXOSe

DL-Erythrose

OR

oCH2CH2

-tT OH

0 ‘0

NHR

CH OR

0 42 R H(OR) RO

OR (D)

@)

CHO

100 30

110 30

145 30

100 10 COCF, 1 21

100 10 COCF, 1 23

90 10 COCF,

80 10 COCF,

70 10 COCF,

70 10 COCF,

80 10 COCF,

80 10 COCF,

104

107

1.21

1.04

105

107

COCF,

_

1.02

1 05

102


TABLE 3 (contmued)

Compound Structure Column Column R (I


(“C)

DL-Sorbose 100 10 COCF, 104

1-0-Methyl-P-DL-arabmopyranostde OR CD)

90 10 COCF, 111

A Thts separation used a temperature gradrent of lo C mu-’ ’ Two pa us of enantromers (a total of four stereotsomers) extst for thus

maternal

TABLE 4

Enanttomertc separatton of racemrc compounds by captllary GC wrth a dtpentyl-y-CD statronary phase

Compound Structure Column Column R o


(“C)

Alcohols

trans-Cyclohexane-1,2-dlol

rrans-Cycloheptane-1,2-dtol

(runs-Cyclooctane-1.2-dtol

A~IIWS NHR

I-(1-Naphthyl)ethylamme

1,2,3,4-Tetrahydro-l-naphthylamme

1-Ammomdan

2-(3-Pyndyl)-1-azacycloheptane

Anatabme

NHR

NHR

90 10 H

90 10 COCH,

150 10 COCH,-Cl

100 10 H

90 10 COCH,

150 10 COCH ,Cl

120

150 10 COCF, 1 08

123 10 COCF, 1 03

10 H

123 10 COCF, 1 05

173 10 COCF, 106

140 10 COCF, 102

102

103

1 05

1 04

1 05

1 05

1 05

D W ARMSTRONG ET AL

I I I I I I 0 12 24 36 4s GO

TIME, MIN

Fig 3 GC separation of the four lsomenc forms of bls- tnfluoroacetylated 3-ammomethyl-3,5,5-tnmethylcyclohexanol

on the dlpentyl-y-cyclodextrm column (P-y-CD), the dlpentyl- fi-cyclodextnn column (P-B-CD) and the dlpentyl-ol-cyclodextnn column (P-a-CD). Only on the smallest CD phase (P-a-CD)

are both pars of enantlomers resolved. All separations were done on 10-m fused-slhca capdlary columns at 90 o C

There seems to be a certain size and shape selectivity between the derivatized OL-, fl- and y-CD stationary phases, which indicates that mclusion complexation probably is occurring despite the elevated temperatures. It is well known that the association constant of a CD-substrate complex in aqueous solution decreases with increasing temperature. A similar phenomenon seems to occur in the gas-liquid system. An increase m column temperature causes enantiomeric peaks to elute sooner and with smaller (Y values. In theory, at sufficiently high temperatures, no enantioselectivity will be observed even if there is appreciable retention. The temperature at which enantioselectivity is lost is different for every racemate. In aqueous or aqueous-organic solutions, most CD-substrate complexes can be broken at temperatures between 60 and 90°C. Of course, even at high temperatures, there is a fmite residence time during wluch there is complexation-decomplexation.

I 1 I I I 0 10 20 30 40

TIK, MIN

Fig 4 GC separation of enantlomers of tnfluoroacetylated l-(1-naphthyl)ethylamme (R = -COCF,) on the dlpentyl-y- cyclodextnn column (P-y-CD), the dlpentyl-p-cyclodextnn col-

umn (P-P-CD) and the dlpentyl-a-cyclodextnn column (P-y- CD) Basehne separation was achieved on the largest CD phase

(P-y-CD) All separations were done on 10-m columns at 150 o c.

With these GC phases, enantioselectivity has been observed at temperatures > 200 o C. The fact that there are no competmg solvent molecules m

-

I 1

0 16 32 ils

T I M E, M I N

Fig. 5. Isothermal enantlomerlc separation of tnfluoroacetylated 1-ammomdan, 1,2,3,4-tetrahydro-1-naphthylamme

and 1-(1-naphthyl)ethylamme on a 30-m fused-shca capillary

coated wth P-P-CD (column temperature 160 o C)

CAPILLARY GC OF ENANTIOMERS

I I 1 , , 1 1 I

0 8 16 2'1 32 40 '18 55

TIME, MIN

Fig. 6 Gradlent GC resolution of racermc tnfluoroacetylated

tyrosme methyl ester on a 30-m P-P-CD column (temperature

range. 170-200°C m 15 mm).

GC may allow complexation to occur at htgher temperatures. In addition, the functional groups on the derivatized CDs greatly alter their enantioselectivity. Consequently, compounds can be resolved on these GC stationary phases that cannot be separated on the corresponding LC columns. Good examples are the linear substituted alkanes and the small saturated cyclic compounds (Tables 2-4).

All of the recently reported derivatized “liquid” CD stationary phases used in GC have been hy- drophobic. Currently, several liquid CD derivatives are being developed that have hydrophilic groups and it will be interesting to compare their applicability and selectivity with those of the pentyl-substituted CD phases described here. This will be discussed in a subsequent publication.

Support of this work by the Smokeless Tobacco Research Council (to D.W.A.) (0154-01) is grate- fully acknowledged.

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Capillary gas chromatographic separation of enantiomers with stable dipentyl-α, β- and...

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