Conclusions

Chiral TLC plays a significant role both in economical routine analyses and in determination of optical purity of individual antipodes. Detection limits of *0.1% d- or l-isomer can be currently achieved.

Table 8 Retention and resolution data for derivatized and free acidic drugs by chiral TLC

Drug

hRf1a

hRF2

«b

Eluentc

Plates and remarks

DNAn-ibuprofen

28 (S)

45 (R)

2.10

A

Precoated 10 cm x 10 cm HPTLC-NH2 F254s plates

DNAn-naproxen

15 (S)

24 (R)

1.79

A

(Altech, Deerfield, IL, USA), derivatized with (R)-(—)-1-

DNAn-fenoprofen

23

33

1.65

A

(1-naphthyl)ethyl isocyanate.

DNAn-flurbiprofen

23

33

1.65

A

Visualization: UV254 and UV360.

DNAn-benoxaprofen

20

30

1.71

A

Flurbiprofen

18

24

1.44

B

MCTA plates.

Carprofen

36

41

1.23

C

Visualization: UV.

Indoprofen

58

63

1.22

D

5 cmx 20 cm chemically-bonded diphenyl-F plates.

cEluents: A = n-hexane-isopropanol-acetonitrile 20 : 8 : 1 (v/v/v); B = ethanol-water 40 : 60 (v/v); C = isopropanol-water 60 : 40 (v/v); D = acetonitrile-0.6mol L~1 NaCl-1% triethylammonium acetate buffer (pH 4.1) containing vancomycin.

Table 9 The structure of racemic flavanones

Re y Rs

R3

R5

R6

R7

r2

Ry

R4'

Name

H

H

H

H

H

H

H

Flavanone

H

och3

H

H

H

H

H

5-Methoxyflavanone

H

H

OH

H

H

H

H

6-Hydroxyflavanone

H

H

och3

H

H

H

H

6-Methoxyflavanone

H

H

H

OH

H

H

H

7-Hydroxyflavanone

H

H

H

H

OH

H

H

2'-Hydroxyflavanone

H

H

H

H

H

H

OH

4'-Hydroxyflavanone

H

H

H

H

H

H

och3

4'-Methoxyflavanone

H

OH

H

OH

H

H

H

Pinocembrin

H

OH

H

och3

H

H

H

Pinocembrin-7-methylether

H

OH

H

OH

H

H

OH

Naringenin

H

OH

H

OH

H

H

och3

Isosakuranetin

H

OH

H

och3

H

H

OH

Sakuranetin

H

OH

H

Gla

H

H

OH

Naringenin-7-glucoside

H

OH

H

Rh-Glb

H

H

OH

Naringin

H

OH

H

OH

H

OH

OH

Eriodictyol

H

OH

H

OH

H

och3

OH

Homoeriodictyol

H

OH

H

OH

H

OH

och3

Hesperetin

OH

OH

H

OH

H

OH

OH

Taxifolin

aGl = Glucoside.

bRh-Gl = Rhamnosidoglucoside.

Table 10 Retention and resolution data for racemic flavanones by chiral TLC

Racemate

hR1

hRF2

Rsc

Plate

Eluentd

Flavanone (F)

16

20

1.31

1.6

SIL C,8-50/UV254

A

22

24

1.12

0.4

MCTA

B

6-Hydroxy-F

36

39

1.14

0.8

MCTA

B

6-Methoxy-F

24

27

1.17

0.8

MCTA

B

2'-Hydroxy-F

10

16

1.71

2.0

SIL C,8-50/UV254

C

19

24

1.35

1.6

SIL C,8-50/UV254

A

4'-Hydroxy-F

38

42

1.18

1.2

SIL C,8-50/UV254

A

4'-Methoxy-F

13

19

1.57

2.0

SIL C,8-50/UV254

A

5,7-Dihydroxy-F

54

60

1.27

1.8

MCTA

D

4',5,7-Trihydroxy-F

23

28

1.30

1.6

MCTA

E

5,7-Dihydroxy-4'-methoxy-F

18

21

1.21

1.3

MCTA

E

4',5-Dihydroxy-7-methoxy-F

43

48

1.22

1.2

MCTA

D

3',4',5,7-Tetrahydroxy-F

26

30

1.21

1.5

MCTA

E

4',5,7-Trihydroxy-3'-methoxy-F

23

26

1.17

0.8

MCTA

E

3',5,7-Trihydroxy-4'-methoxy-F

23

27

1.24

1.5

MCTA

E

3,3',4',5,7-Pentahydroxy-F

44

48

1.17

1.3

MCTA

cRs = 2x (distance between the centres of two adjacent spots)/(sum of the width of the two spots in the direction of development). dEluents: A = 0.15 mol L~1 p-CD aqueous solution with urea (32%) and NaCl (2%)-acetonitrile 80 : 20 (v/v), migration distance 8.5 cm; B = ethanol-water 80:20 (v/v), migration distance 12 cm; C = 0.05 mol L~1 sodium bicarbonate # 0.05 mol L~1 sodium carbonate solution containing 6% BSA and 12% isopropanol, migration distance 8 cm; D = ethanol-water 70 : 30 (v/v), migration distance 14 cm; E = methanol-water 80 : 20 (v/v), migration distance 16 cm.

Figure 3 Remission-location curves recorded on 10cmx 10cm HPTLC-CHIR plates. (A) D,L-Lactic acid (ARF = 0.05); (B) D,L-2-hydroxybutanoic acid (ARF = 0.10); (C) D,L-2-hydroxyoctanoic acid (ARF = 0.14).

Figure 4 Remission-location curves recorded on 20 cm x 20 cm Chiralplates. (A) D-Phe spiked with 0.1 % L-Phe; (B) 0.1% L-Phe.

Figure 4 Remission-location curves recorded on 20 cm x 20 cm Chiralplates. (A) D-Phe spiked with 0.1 % L-Phe; (B) 0.1% L-Phe.

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 |xg per spot

Figure 5 Calibration line for L-phenylalanine. IE, integration units; y = - 463 # 16 349x; r = 0.9992; Sxo = 0.0038 pg per spot; 1 = 540 nm.

J 100:1

Figure 6 Densitograms of (R) and (S) - 1,1'-binaphthyl-2,2'-diamine mixtures in the ratios 50 : 1,100 : 1 and 200 : 1 on MCTA layers, eluted with ethanol-water80 : 20 (v/v). Migration distance 17 cm. (A) (R) = 10 pg, (S)=0.2 pg; (B) (R) = 20 ^g; (S) = 0.2 ^g; (C) (R) = 40 ^g; (S) = 0.2 pg.

Migration distance (cm)

Figure 6 Densitograms of (R) and (S) - 1,1'-binaphthyl-2,2'-diamine mixtures in the ratios 50 : 1,100 : 1 and 200 : 1 on MCTA layers, eluted with ethanol-water80 : 20 (v/v). Migration distance 17 cm. (A) (R) = 10 pg, (S)=0.2 pg; (B) (R) = 20 ^g; (S) = 0.2 ^g; (C) (R) = 40 ^g; (S) = 0.2 pg.

Less work is being carried out on chiral TLC than on column chromatography, even though the two techniques may give complementary results and TLC has advantages such as low cost and easy evaluation of the tests.

Future possibilities of chiral TLC include:

1. the synthesis of enantiomeric derivatives that are easier to resolve and more sensitively detected than those so far investigated;

2. the availability of layers prepared from new cellulose derivatives and, in addition, the availability of more versatile MCTA plates using highly crystalline and homogeneously sized material;

3. more extensive application of normal-phase chromatography with a chiral mobile phase additive (DIOL plates are particularly advisable);

4. the use of eluents containing new chiral selectors in reversed-phase systems, which is the technique most widely used for enantioseparations.

See also: N/Chromatography: Thin-Layer (Planar):

Densitometry and Image Analysis; Layers; Spray Reagents. III/Amino Acids and Derivatives: Chiral Separations: Chiral Separations: Capillary Electrophoresis; Cellulose and Cellulose Derived Phases; Chiral Derivatization; Cyclodextrins and Other Inclusion Complexation Approaches; Ion-Pair Chromatography; Ligand Exchange Chromatography; Liquid Chromatography; Molecular Imprints as Stationary Phases; Protein Stationary Phases; Supercritical Fluid Chromatography; Synthetic Multiple Interaction ('Pirkle') Stationary Phases.

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