Evaluation of the enantiomeric resolution of 7,8-dihydroxy-7,8-dihydrobenzo[a]-pyrene and its 6-fluoro and 6-bromo derivatives on polysaccharide-derived stationary phases

Article abstract of DOI:10.1021/jo020607t

The enantiomeric resolution and the elution order of (±)-trans-7,8-dihydrodiols of benzo[a]pyrene and its 6-fluoro and 6-bromo derivatives were analyzed on three polysaccharide-based columns: Daicel Chiralcel CA-I (cellulose triacetate), OF, and OG [cellu

Full text of DOI:10.1021/jo020607t

been achieved on a column packed with (R)-N-(3,5-dinitro-
benzoyl)phenylglycine, ionically bonded to R-aminopro-
pylsilanized silica (Regis Pirkle I-A).7,8 On the basis of
this result, the separation of BaP dihydrodiol, as well as
the 6-fluoro analogue was attempted on the Regis Pirkle
I-A column. Although the BaP dihydrodiol enantiomers
separated as reported, the 6-fluoro BaP dihydrodiol
enantiomers did not.
Eva lu a tion of th e En a n tiom er ic Resolu tion
of 7,8-Dih yd r oxy-7,8-d ih yd r oben zo[a ]-
p yr en e a n d Its 6-F lu or o a n d 6-Br om o
Der iva tives on P olysa cch a r id e-Der ived
Sta tion a r y P h a ses
Barbara Zajc,*,†,‡ Rok Grahek,§ Andrej Kocijan,§
Mahesh K. Lakshman,‡ J anez Kosˇmrlj,† and J ure Lah†
This led us to investigate other commercially available
columns for the separation of the 6-fluoro BaP dihy-
drodiol. Although other amino acid-based columns are
commercially available, we were surprised to find that
no studies on the separation of dihydrodiol or other PAH
metabolites have been reported using polysaccharide-
based columns, despite their various successful applica-
tions.14-20 Excellent separation of the (()-trans-7,8-dihy-
drodiol of 6-fluoro-BaP on a Daicel Chiralcel OG column4
initiated our present investigation.
Among the commercially available cellulose-based Da-
icel columns, three were chosen for the study: Chiralcel
CA-I (microcrystalline cellulose triacetate), Chiralcel OG
and OF (phenylcarbamate derivatives of cellulose coated
on silica gel). As substrates, 6-H-BaP-DHD, 6-F-BaP-
DHD and 6-Br-BaP-DHD (Figure 1) were chosen. These
close structural analogues offer conformational diversity,
which can be valuable in understanding chiral recogni-
tion. In the C-6 protio analogue, the hydroxyls are
predominantly quasi-diequatorial, while in the C-6 halo
analogues, these are predominantly quasi-diaxial. The
syntheses of BaP dihydrodiol21 and the C-6 fluoro ana-
logue4 have been reported, while the C-6 bromo analogue
was synthesized from 6-bromo-9,10-dihydrobenzo[a]py-
rene4 (see Supporting Information). (()-6-Br-BaP-DHD
Faculty of Chemistry and Chemical Technology, University
of Ljubljana, 1000 Ljubljana, Slovenia, Department of
Chemistry, The City College of CUNY, 138th Street at
Convent Avenue, New York, New York 10031, and
Lek Pharmaceutical Company, Research and
Development, 1526 Ljubljana, Slovenia
barbaraz@sci.ccny.cuny.edu
Received September 16, 2002
Abstr a ct: The enantiomeric resolution and the elution
order of (()-trans-7,8-dihydrodiols of benzo[a]pyrene and its
6-fluoro and 6-bromo derivatives were analyzed on three
polysaccharide-based columns: Daicel Chiralcel CA-I (cel-
lulose triacetate), OF, and OG [cellulose tris(4-chloro- and
4-methylphenylcarbamate)]. For comparison, the separation
of (()-1,1′-bi-2-naphthol was evaluated on the OG and OF
columns. Possibly similar interactions of (S)-1,1′-bi-2-naph-
thol and (7S,8S)-isomers of 6-halo-7,8-dihydroxy-7,8-dihy-
drobenzo[a]pyrene with the chiral sorbent are suggested.
Benzo[a]pyrene (BaP) is a known environmental pol-
lutant1 that is metabolized to carcinogenic bay-region diol
epoxides via dihydrodiols. Introduction of a halo sub-
stituent into position 6 of the BaP system dramatically
affects its tumorigenic properties, resulting in diminished
activity.2 The peri C-6 substituent also influences the
conformation of the dihydro and the tetrahydro ring in
the dihydrodiol and diol epoxide metabolites. Thus,
among the dihydrodiols, a preference for the quasi-diaxial
orientation of vicinal hydroxyl groups is observed in the
6-bromo3 and 6-fluoro4 7,8-dihydrodiols, whereas in the
6-H analogue, these are quasi-diequatorial. Also, among
the diol epoxides, the (7R,8S)-diol (9S,10R)-epoxide of
BaP is a potent tumorigen, whereas a lack of tumorigenic
activity has been reported for the 6-fluoro analogue.5 This
has been attributed to the conformational differences.
Recently, the synthesis of the (()-trans-7,8-dihydrodiol
of 6-fluoro-benzo[a]pyrene was reported.4 In light of the
small quantities needed, direct resolution by chiral HPLC
was preferred, rather than via diastereomer separation.6
Separation of dihydro and tetrahydrodiol derivatives of
several polycyclic aromatic hydrocarbons (PAHs) has
been extensively studied on amino acid-based enantiose-
lective columns.7-13 Resolution of BaP dihydrodiol has
(5) J erina, D. M.; Sayer, J . M.; Agarwal, S. K.; Yagi, H.; Levin, W.;
Wood, A. W.; Conney, A. H.; Pruess-Schwartz, D.; Baird, W. M.; Pigott,
M. A.; Dipple, A. Biological Reactive Intermediates III; Kocsis, J . J .,
J ollow, D. J ., Witmer, C. M., Nelson, J . O., Snyder, R., Eds.; Plenum
Press: New York, 1986; pp 11-30.
(6) Lakshman, M. K.; Chaturvedi, S.; Kole, P. L.; Windels, J . H.;
Myers, M. B.; Brown, M. A. Tetrahedron: Asymmetry 1997, 8, 3375-
3378 and references therein.
(7) Weems, H. B.; Yang, S. K. Anal. Biochem. 1982, 125, 156-161.
(8) Yang, S. K.; Weems, H. B.; Mushtaq, M.; Fu, P. P. J . Chromatogr.
1984, 316, 569-584.
(9) Yang, S. K.; Mushtaq, M.; Fu, P. P. J . Chromatogr. 1986, 371,
195-209.
(10) Weems, H. B.; Mushtaq, M.; Fu, P. P.; Yang, S. K. J . Chro-
matogr. 1986, 371, 211-225.
(11) Yang, S. K.; Mushtaq, M.; Weems, H. B.; Fu, P. P. J . Liq.
Chromatogr. 1986, 9, 473-492.
(12) Weems, H. B.; Fu, P. P.; Yang, S. K. Carcinogenesis 1986, 7,
1221-1230.
(13) Weems, H. B.; Yang, S. K. J . Chromatogr. 1990, 535, 239-253.
(14) Ichida, A.; Shibata, T.; Okamoto, I.; Yuki, Y.; Namikoshi, H.;
Toga, Y. Chromatographia 1984, 19, 280-284.
(15) Fukui, Y.; Ichida, A.; Shibata, T.; Mori, K. J . Chromatogr. 1990,
515, 85-90.
(16) Oguni, K.; Oda, H.; Ichida, A. J . Chromatogr. A 1995, 694, 91-
* Corresponding author. Phone: (212) 650-8926. Fax: (212) 650-6107.
† University of Ljubljana.
100.
(17) (a) Yashima, E.; Okamoto, Y. Bull. Chem. Soc. J pn. 1995, 68,
3289-3307. (b) The effect of the substituents on the adsorption
properties of 4-substituted phenylcarbamate CSPs was quantitatively
evaluated by plotting the retention times of acetone and of the first-
eluted isomer of 2,2,2-trifluoro-1-(9-anthryl)ethanol against the Ham-
mett σp substituent values.
‡ The City College of CUNY.
§ Lek Pharmaceutical Company.
(1) Harvey, R. G. Polycyclic Aromatic Hydrocarbons: Chemistry and
Carcinogenicity; Cambridge University Press: Cambridge, 1991.
(2) Cavalieri, E.; Rogan, E.; Cremonesi, P.; Higginbotham, S.;
Salmasi, S. J . Cancer Res. Clin. Oncol. 1988, 114, 10-15.
(3) Fu, P. P.; Yang, S. K. Biochem. Biophys. Res. Commun. 1982,
109, 927-934.
(18) Yashima, E.; Yamamoto, C.; Okamoto, Y. Synlett 1998, 344-360.
(19) Okamoto, Y.; Yashima, E. Angew. Chem., Int. Ed. 1998, 37,
1020-1043.
(4) Zajc, B. J . Org. Chem. 1999, 64, 1902-1907.
(20) Okamoto, Y.; Kaida, Y. J . Chromatogr. A 1994, 666, 403-419.
10.1021/jo020607t CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/20/2003
J . Org. Chem. 2003, 68, 3291-3294
3291

study a Daicel Chiralcel OF column (a 4-chloro substi-
tuted phenylcarbamate) and a Daicel Chiralcel OG
column (a 4-methyl-substituted phenylcarbamate).
Ch r om a togr a p h y on a Da icel Ch ir a lcel OF Col-
u m n . HPLC of the three racemates was performed at
30 °C and under identical conditions (1:1 2-propanol-n-
hexane). Only 6-H and 6-F-BaP-DHD were resolved,
while 6-Br was not (Table 1).
As evident from Table 1, the elution times of the
enantiomers of 6-H-BaP-DHD were shorter than those
of the 6-F, as well as that of the unresolved 6-Br-BaP-
DHD. This could be explained on the basis of the
conformation of the cyclohexadienyl moiety in the dihy-
drodiols. Such an effect on the HPLC elution orders of
isomeric PAH dihydrodiols was first suggested by Grover
et al.,27 who studied their chromatographic behavior on
an achiral silica column. Within a series of dihydrodiols
derived from a hydrocarbon, trans isomers with quasi-
diequatorial hydroxyl groups eluted much earlier, pos-
sibly due to an intramolecular hydrogen bond.27 Such a
phenomenon may be responsible for the observed reten-
tion times in the chiral resolution as well. As a conse-
quence, there are possibly fewer hydrogen-bonding in-
teractions between the solute and the CSP in the case of
6-H-BaP-DHD enantiomers, which is apparently not the
case with the 6-F and 6-Br derivatives. For both resolved
substrates the (7S,8S)-isomer eluted early.
F IGURE 1. Structures of BaP dihydrodiol enantiomers as
well as the 6-halo analogues.
TABLE 1. En a n tiom er ic Resolu tion of 6-Br , 6-F , a n d
6-H-Ba P -DHD on Ch ir a lcel CA-I, OF , a n d OG
Chiralcel CA-Ia
Rb Rc k1′d
Br 1.45 0.6e 1.62
(7R,8R)
1.47 0.7 2.20
(7R,8R)
1.74 1.5 3.70
(7R,8R)
Chiralcel OFf,g
Rb Rh k1′d
Chiralcel OGf,j
Rb Rh k1′d
1.36 1.4 3.97
(7R,8R)
1.41 1.8 2.56
(7R,8R)
1.33 1.3 1.51
(7S,8S)
X
ui
ui 4.12i
(()
F
1.23 0.9 3.21
(7S,8S)
1.27 0.8 1.31
(7S,8S)
H
a
Analytical column, 0.46 × 25 cm; solvent system, 15% H2O in
MeOH; flow rate, 0.5 mL/min. R ) k2′/ k1′. c Resolution factor,
b
as defined by Cai and Wu,23 was used. k1′: retention factor, early-
d
eluted isomer. e Value ) 0.8 when 20% H2O in MeOH was used.
f Analytical column, 0.46 cm × 25 cm with a 0.46 cm × 5 cm
precolumn; column temperature, 30 °C; solvent, 1:1 n-hexane-i-
PrOH; to was measured by the use of 1,3,5-tri-tert-butylbenzene.26
h
g Flow rate: 0.8 mL/min. Resolution factor: R ) 1.18 (tR(2) - tR(1))/
(w1/2(1) + w1/2(2)), where w1/2 is the peak width on half-height.
i
j
Unresolved. Flow rate: 0.9 mL/min.
Ch r om a togr a p h y on a Da icel Ch ir a lcel OG Col-
u m n . The Daicel Chiralcel OG column was found to be
the best for the resolution of the dihydrodiols (Table 1,
30°C, 1:1 2-propanol-n-hexane, also see Figure B in
Supporting Information). Comparison of the retention
times of the enantiomers of the three substrates shows
that the 6-H-BaP-DHD enantiomers had the shortest
elution times, followed by the 6-F derivative. The reten-
tion times of the 6-Br-BaP-DHD enantiomers were ap-
proximately twice as long as those of 6-H-BaP-DHD. This
could again be explained by lesser solute-CSP hydrogen
bond interactions in the case of 6-H-BaP-DHD, due to
intramolecular hydrogen bonding.27 The elution order of
the 6-H-BaP-DHD enantiomers was the same as on the
Chiralcel OF column, with the (7S,8S)-enantiomer elut-
ing first. On the other hand, in the C-6 halo analogues,
the (7R,8R)-isomer eluted early.
was resolved by chiral HPLC (Daicel Chiralcel OG) and
chirality of the enantiomers was established by compari-
son of their CD spectra to that of trans-(7R,8R)-6-bromo-
7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene3 (Figure A in
Supporting Information).
En a n tiom er ic Sep a r a tion by Ch ir a lcel CA-I. Cell-
ulose triacetate (CTA-I), was first reported by Hesse and
Hagel22 to possess chiral resolving properties, and its
microcrystalline structure was postulated to be crucial
for enantioselection. Table 1 shows the resolution of all
three substrates on Chiralcel CA-I, a microcrystalline
cellulose triacetate.
Comparison of k′ values under identical chromato-
graphic conditions (15% water in methanol) revealed
that, in each set of early- and late-eluting dihydrodiols,
the bromo is the fastest eluting, followed by the fluoro
and then by the protio analogue. This order is in
agreement with the presumption that the flat molecules
fit better into chiral cavities of CTA-I.24 In solution, 6-H-
BaP-DHD is closer to planarity, possibly due to an
intramolecular hydrogen bond between the two equato-
rial hydroxyls,25 which contrasts to the 6-F- and 6-Br-
BaP-DHD, where the hydroxyl groups are predominantly
diaxial. Better separation of the 6-Br derivative was
obtained when 20% water in methanol was used as the
eluent. In each case the (7R,8R)-isomer eluted early.
En a n tiom er ic Sep a r a tion by Cellu lose P h en yl-
ca r ba m a tes. Among polysaccharide derivatives coated
on silica gel, cellulose and amylose phenylbenzoates as
well as phenylcarbamates have proven to possess very
good resolving qualities.16-20 A wide range of cellulose
tris(phenylcarbamate) derivatives have thus been studied
as HPLC chiral stationary phases (CSPs).26 Introduction
of a methyl or halo substituent at the meta and/or para
position of the phenylcarbamate increases the enanti-
oseparating power of the CSP. We therefore chose for our
Eva lu a tion of CSP -P AH Solu te In ter a ction s on
OF a n d OG Colu m n s a n d Com p a r ison to CSP -
Bin ol In ter a ction s. Attractive interactions between the
solute and the polar carbamate groups of the CSP have
been suggested as the primary mechanism for chiral
recognition by phenylcarbamate derivatives of cellu-
(21) McCaustland, D. J .; Engel, J . P. Tetrahedron Lett. 1975, 2549-
2552. Lehr, R. E.; Schaefer-Ridder, M.; J erina, D. M. J . Org. Chem.
1977, 42, 736-744. Harvey, R. G. in Polycyclic Hydrocarbons and
Carcinogenesis; Harvey, R. G., Ed.; ACS Symposium Series 283;
American Chemical Society: Washington, DC, 1985; pp 35-62.
(22) Hesse, G.; Hagel, R. Chromatographia 1973, 6, 277-280. Hesse,
G.; Hagel, R. J ustus Liebigs Ann. Chem. 1976, 996-1008.
(23) Cai, C. P.; Wu, N. S. Chromatographia 1990, 30, 400-404.
(24) Lipkowitz, K. B. J . Chromatogr. A 1995, 694, 15-37.
(25) Neidle, S.; Subbiah, A.; Osborne, M. R. Carcinogenesis 1981,
2, 533-536.
(26) Okamoto, Y.; Kawashima, M.; Hatada, K. J . Chromatogr. 1986,
363, 173-186. Koller, H.; Rimbo¨ck, K.-H.; Mannschreck, A. J . Chro-
matogr. 1983, 282, 89-94.
(27) Tierney, B.; Burden, P.; Hewer, A.; Ribeiro, O.; Walsh, C.;
Rattle, H.; Grover, P. L.; Sims, P. J . Chromatogr. 1979, 176, 329-335.
3292 J . Org. Chem., Vol. 68, No. 8, 2003

F IGURE 2. Geometrically optimized (HyperChem) structures of (A) (S)-Binol, (7S,8S)-6-halo dihydrodiol, and the manually
superimposed structures and (B) (S)-Binol, (7R,8R)-6-halo dihydrodiol, and the manually superimposed structures.
TABLE 2. Com p a r ison of th e k′ of (7S,8S)- a n d (7R,8R)-Isom er s of th e Ba P Dih yd r od iols on Ch ir a lcel OG a n d OF a
Chiralcel OG
Chiralcel OF
substituent Xa
H
k′: (7S,8S)
k′: (7R,8R)
k′: (7S,8S)
k′: (7R,8R)
1.51
2.01
1.31
1.66
∆k′: (k′F - k′H) ) 2.1
∆k′: (k′F - k′H) ) 0.5
∆k′: (k′F - k′H) ) 1.9
∆k′: (k′F - k′H) ) 2.3
F
3.61
2.56
3.21
3.95
∆k′: (k′Br - k′H) ) 3.9
5.40
∆k′: (k′Br - k′H) ) 2.0
3.97
Br
4.12b
4.12b
a
b
Substituent at position 6 of the 7,8-dihydrodiol of BaP. Unresolved.
lose.16-20 These include H-bonding with the NH and
CdO groups, dipole-dipole stacking,26,28 and π-π inter-
actions.15,29 With solutes containing hydroxyl groups,
hydrogen bonding between the -OH and the carbamate
residues is mainly implicated for chiral recognition.19,20,30,31
For a better understanding of PAH dihydrodiol inter-
with the dihydrodiols, we decided to analyze the data
based on the conformational differences. Since separa-
tions of these closely related substrates were performed
under identical conditions, a comparison of the k′ values
within the series of (7S,8S)- and (7R,8R)-isomers was
possible with each column. The k′ values are presented
in Table 2.
actions with phenylcarbamate-derived cellulose, we evalu-
ated the literature for reports on the resolution and chiral
recognition studies of dihydroxylated substrates. Oka-
moto et al. have performed a thorough study of solute-
CSP interactions on 1,1′-bi-2-naphthol (Binol) and the
chloroform-soluble cellulose tris(5-fluoro-2-methylphen-
ylcarbamate).30 The cellulose polymer used in the study
was suggested to have a left-handed 3/2 helical struc-
ture30 such as the crystalline cellulose tris(phenylcar-
bamate)32 and tris(4-chlorophenylcarbamate). The latter
is the OF CSP. In these studies, a model showing a CSP:
(S)-Binol complex with two simultaneous hydrogen bonds
between the two hydroxyls of bi-2-naphthol and CO
groups of the carbamate moieties of two neighboring glu-
cose units was proposed. On the other hand, in the case
of (R)-Binol, hydrogen bonding between only one hydroxyl
group of the solute and CO of the CSP was suggested.
We became interested in assessing whether there were
similarities in the CSP-solute interactions observed in
Binol to those of dihydrodiols with diaxially oriented
hydroxyls. We therefore evaluated the intramolecular
distance of the OH groups in Binol as well as 6-Br- and
6-F-BaP-DHD through the use of the HyperChem suite
of computational programs (see Supporting Information).
The (7S,8S)-isomers of 6-Br- and 6-F-BaP-DHD were geo-
metrically optimized and manually superimposed with
that of (S)-Binol. Nearly equivalent oxygen-oxygen dis-
tances were found in both, 6-Br- and 6-F-BaP-DHD and
(S)-Binol (Figure 2). In (7S,8S)-isomers of 6-Br- and 6-F-
BaP-DHD, the rest of the polyaromatic moiety is extended
in such a fashion that, according to the model reported
for (S)-Binol,30 it could fit into the CSP groove (which is
not the case with the (7R,8R)-isomers, Figure 2).
Com p a r ison of k′ Va lu es for (7S,8S)-Isom er s: On
th e OG CSP . Inspection of k′ of 6-H and 6-F derivatives
for the (7S,8S)-isomers on the OG column reveals a ∆k′
of 2. This difference in k′ may be a consequence of the
differences in hydrogen bonding interactions of the sub-
strates, allowing for an intramolecular hydrogen bond in
the protio analogue.27 This is not possible in 6-F-BaP-
DHD, leaving both hydroxyl groups available for interac-
tions with the CSP. Comparison of the protio analogue
to the 6-Br revealed a ∆k′ of ∼4. This could indicate that
the (7S,8S)-isomer of the 6-bromo derivative forms a
CSP-solute complex with more concomitant interactions
than the 6-fluoro derivative, possibly including a bromine
atom.
On th e OF CSP . The same trend is observed in the
series of (7S,8S)-isomers on the OF CSP as well, though
the values of k′ are lower as compared to those on the
OG CSP, indicating weaker CSP-solute complexes. An
approximately equal difference of 2 is observed between
the k′ of the protio and the fluoro analogue, with the lat-
ter being more retained. This again points to the possible
solute-CSP complex with two simultaneous interactions
in the case of 6-F-BaP-DHD as described above. (()-6-
Br-BaP-DHD, however, was not resolved on this column.
(28) Okamoto, Y.; Kawashima, M.; Hatada, K. J . Am. Chem. Soc.
1984, 106, 5357-5359.
(29) Hopf, H.; Grahn, W.; Barrett, D. G.; Gerdes, A.; Hilmer, J .;
Hucker, J .; Okamoto, Y.; Kaida, Y. Chem. Ber. 1990, 123, 841-845.
(30) Yashima, E.; Yamamoto, C.; Okamoto, Y. J . Am. Chem. Soc.
1996, 118, 4036-4048.
(31) O’Brien, T.; Crocker, L.; Thompson, R.; Thompson, K.; Toma,
P. H.; Conlon, D. A.; Feibush, B.; Moeder, C.; Bicker, G.; Grinberg, N.
Anal. Chem. 1997, 69, 1999-2007.
(32) Vogt, U.; Zugenmaier, P. Ber. Bunsen-Ges. Phys. Chem. 1985,
89, 1217-1224. Steinmeier, H.; Zugenmaier, P. Carbohydr. Res. 1987,
164, 97-105.
To evaluate whether any qualitative indication of the
similarity of the CSP-PAH complex to the model pro-
posed for Binol could be derived from the data obtained
J . Org. Chem, Vol. 68, No. 8, 2003 3293

Com p a r ison of k′ Va lu es for (7R,8R)-Isom er s: On
th e OG CSP . Interestingly, within the series of (7R,8R)-
isomers, there is a small difference between the k′ of the
6-H and 6-F analogues on the OG CSP, indicating a
similar number of attractive interactions in the CSP-
solute complex, despite different conformational prefer-
ences, while a difference of ∼2 between the k′ of the 6-H
and 6-Br derivatives is observed. Although there is an
increase in ∆k′ (∼2) between the protio and the bromo
derivative, it is only half of what was observed for the
corresponding (7S,8S)-isomer on the OG CSP. Thus, the
(7R,8R)-isomers of both 6-halo derivatives show fewer
simultaneous attractive interactions to the OG CSP than
the corresponding (7S,8S)-isomers.
interactions. Since the (7S,8S)-enantiomers of the 6-H,
6-F, and 6-Br dihydrodiols as well as the (7R,8R)-isomer
of the 6-H dihydrodiol show a greater retention on the
OG CSP compared to the OF, it is possible that the
solute-CSP interactions include those where the solute
is a hydrogen bond donor via the hydroxyl protons. On
the other hand, the 6-F (7R,8R)-dihydrodiol enantiomer
is more retained on the OF CSP, and in this case, the
solute-CSP interactions could involve the CSP as the
hydrogen bond donor (through the carbamate NH) to the
solute as well. It is difficult to make any predictions about
the 6-Br dihydrodiols because these were not resolved
on the OF CSP; however, the (7S,8S)-enantiomer is more
retained on the more basic OG column compared to the
racemate on the OF CSP.
On th e OF CSP . On the OF CSP, 6-Br-BaP-DHD was
not resolved. There is a difference of ∼2 between the k′
of the 6-H and 6-F derivatives, with the latter more re-
tained. Here again, the ∆k′ could suggest two simulta-
neous interactions between the solute and the CSP in
the case of 6-F and not in the protio analogue, as de-
scribed earlier. This is in contrast to the observation ob-
tained on the OG CSP for (7R,8R)-isomers of the 6-H and
6-F derivatives, mentioned above, where a similar num-
ber of simultaneous solute-CSP interactions was sug-
gested for both.
Since the basis of our model for the enantiomeric
separation of the 6-halo dihydrodiols was derived by
analogy to (S)-Binol, we reasoned that if effects observed
in the dihydrodiol separation translated to the separation
of Binol, further support for the proposed model could
be obtained. For this, we compared the separation of
(()-6-F-BaP-DHD on the OG and OF CSPs to that of
(()-Binol. As indicated earlier, (7R,8R)-6-F-BaP-DHD
eluted early on the OG column and, on the OF CSP, this
enantiomer eluted late. With (()-Binol, (R)-(+)-Binol was
early eluting on the OG CSP, whereas this elution order
was reversed on the OF CSP, as was observed with the
enantiomers of 6-F-BaP-DHD. Whereas only a small peak
separation between Binol enantiomers was obtained
under the HPLC conditions used to resolve the dihy-
drodiol enantiomers, co-injection of the racemic Binol
with an enantiopure isomer enabled a clear distinction
between the isomers. These results provide additional
support to our proposed solute-CSP interactions in the
enantioresolution of axially constrained non-bay-region
dihydrodiols of PAHs.
The polarity of the carbamate moiety is modulated by
the substituents on the phenyl ring, which consequently
influences the separating properties of the CSP.26 The
effect of the substituents on the adsorption properties of
4-substituted phenylcarbamate CSPs has been quanti-
tatively evaluated by plotting the retention times of the
first-eluted isomer of substrates containing a carbonyl
group or a hydroxy group, against the Hammett σp
substituent values.17a,b With increasing electron-with-
drawing power of substituents on the phenyl ring, the
substrate containing the carbonyl group was retained
more. This was explained by increased acidity of the
carbamate NH moiety leading to stronger hydrogen
bonds to the basic site of the substrate. On the other
hand, increasing electron-withdrawing power of the
substituents diminished the basicity of the carbamate
carbonyl, resulting in lower retention of the hydroxyl
group containing substrate. This was explained by weaker
hydrogen bonds between the carbonyl of the CSP and the
hydroxyl of the substrate.17a,b
In conclusion, various polysaccharide-based enantiose-
lective columns can be successfully used for the resolution
of chiral metabolites of PAHs. Although separation of all
three substrates was attained on both the CA-I and the
Chiralcel OG CSPs, significantly better resolution was
observed on the latter (also partly due to peak broadening
on the CA-I). A large set of close structural analogues
such as a set of PAH dihydrodiols offers a good pool of
substrates for further studies of solute-CSP interactions
on chiral sorbents.
To gain an idea about the nature of the CSP-solute
attractive forces operative in the resolution, the k′ values
of (7S,8S)- and (7R,8R)-isomers were compared on the
two columns, in the light of the relative basicity/acidity
of the sorbents resulting from the electronic effects of the
4-substituent on the phenylcarbamate.17
Ack n ow led gm en t. We are very grateful to Prof.
J ozˇe Koller (University of Ljubljana) for his help with
the computation. Special thanks are due to Drs. Dusˇan
Zˇigon and Bogdan Kralj (J ozˇef Stefan Institute) for mass
spectral measurements. Financial support by the Min-
istry of Education, Science and Sport of Slovenia (Grant
PO-0501-103 to B.Z.) and by the National Institutes of
Health (1 R15 CA094224-01 to M.K.L.) is acknowledged.
The OG CSP with the 4-methyl subsitutent (Hammett
σp substituent value of -0.14) is a more basic sorbent
and therefore a better hydrogen bond acceptor via the
basic carbonyl moiety of the carbamate compared to the
OF CSP. On the other hand, the OF CSP with the
4-chloro substituent (Hammett σp substituent value
+0.24) contains more acidic NH groups and is likely a
better proton donor than the OG CSP in hydrogen
bonding interactions. Therefore, we reasoned that a
comparison of the k′ values (Table 2) of any given
enantiomer on these two CSP systems could provide
insight into the types of hydrogen bond donor-acceptor
Su p p or tin g In for m a tion Ava ila ble: Scheme and de-
scription of synthesis of 6-Br-BaP-DHD, enantiomeric resolu-
tion of 6-Br-BaP-DHD, computational details, CD spectra of
(7R,8R)- and (7S,8S)-6-Br-BaP-DHD, and HPLC traces for
resolution of (()-6-Br-BaP-DHD, (()-6-F-BaP-DHD, and (()-
6-H-BaP-DHD on a Chiralcel OG column. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O020607T
3294 J . Org. Chem., Vol. 68, No. 8, 2003