A CD exciton chirality method for determination of the absolute configuration of threo-β-aryl-β-hydroxy-α-amino acid derivatives

Article abstract of DOI:10.1021/jo016070l

The absolute configuration of threo-β-aryl-β-hydroxy-α-amino acids was studied by CD exciton chirality method using 7-diethylaminocoumarin-3-carboxylate as a red-shifted chromophore. Bischromophoric derivatives for a series of threo-β-aryl-β-hydroxy-α-amino acids (3a-h) were prepared and their CD spectra measured in CH₂Cl₂. By combining the data of CD and NMR coupling constants, we are able to correlate their preferred conformer (B) and the positive CD to the corresponding (2S,3R)-absolute configuration. These results are consistent with those obtained from serine and threonine derivatives, which represent the simplest form of β-hydroxy-α-amino acids. This CD method could thus become a general method for determining the absolute configuration of threo-β-aryl-β-hydroxy-α-amino acids.

Full text of DOI:10.1021/jo016070l

1368
J . Org. Chem. 2002, 67, 1368-1371
Notes
type of target. â-Aryl-â-hydroxy-R-amino acids, a subclass
A CD Exciton Ch ir a lity Meth od for
Deter m in a tion of th e Absolu te
Con figu r a tion of
of â-hydroxy-R-amino acids carrying an aromatic sub-
stituent at the â-position, are of special interest as they
are usually a key constituent of peptide antibiotics, such
as chloramphenicol and vancomycin.5 Currently, estab-
lishment of the absolute configuration of these com-
pounds relies mostly on comparison of their optical
rotations with those of authentic samples, an approach
which is only applicable to the known compounds.
Another indirect approach would derive the results from
the stereospecificity of enzymes used for the synthesis.
Therefore, it is necessary to develop a spectroscopic
method for the determination of the absolute configura-
tion of this class of compounds. In our continuing efforts
in studying the stereochemical properties of â-hydroxy-
R-amino acids, we have developed a convenient CD
exciton chirality method using a red-shifted chromophore
on model compounds 1 and 2 (Figure 1).6 In this report,
we further extend this CD method to the series of threo-
â-aryl-â-hydroxy-R-amino acids (3).
th r eo-â-Ar yl-â-h yd r oxy-r-a m in o Acid
Der iva tives
Lee-Chiang Lo,* Chun-Tzu Yang, and
Charng-Sheng Tsai
Department of Chemistry, National Taiwan University,
Taipei 106, Taiwan
lclo@ccms.ntu.edu.tw
Received August 29, 2001
Abstr a ct: The absolute configuration of threo-â-aryl-â-
hydroxy-R-amino acids was studied by CD exciton chirality
method using 7-diethylaminocoumarin-3-carboxylate as a
red-shifted chromophore. Bischromophoric derivatives for a
series of threo-â-aryl-â-hydroxy-R-amino acids (3a -h ) were
prepared and their CD spectra measured in CH2Cl2. By
combining the data of CD and NMR coupling constants, we
are able to correlate their preferred conformer (B) and the
positive CD to the corresponding (2S,3R)-absolute configu-
ration. These results are consistent with those obtained from
serine and threonine derivatives, which represent the
simplest form of â-hydroxy-R-amino acids. This CD method
could thus become a general method for determining the
absolute configuration of threo-â-aryl-â-hydroxy-R-amino
acids.
The CD exciton chirality method is a nonempirical
means and has been widely used for determining the
absolute configuration of many natural products.7 The
electric transition moments of two chromophores interact
through space to give an exciton coupled CD spectrum.
The sign and intensity of the couplets depend on the
handedness of the two chromophores and the distance
between them. 7-Diethylaminocoumarin-3-carboxylate (4)
is a red-shifted chromophore (λmax 420 nm in CH2Cl2) with
high ꢀ value (an average ꢀ value of 80 000 was used for
the bischromophoric derivatives).6,8 It gives extremely
strong CD couplets on rigid systems such as (1R,2R)-1,2-
diaminocyclohexane (A ) -203) and (1R,2R)-1,2-cyclo-
hexanediol (A ) -161).8 These results suggest that the
direction of the electric transition moment corresponding
to 420 nm is along the long axis of the chromophore and
is parallel to the C-N and C-O bonds on the parent
molecules, as indicated by the arrow in Figure 1. This
highly sensitive property sets the basis for its applica-
tions in microscale structural determination. Previous
results on simple â-hydroxy-R-amino acids (1 and 2)
showed that the polar carboxylate moiety of compounds
1 and 2 played an important role in determining the
predominant conformer and therefore made them behave
differently from other acyclic vic-amino alcohols.6 To
study the substituent effects of the aromatic ring at the
â-position on the resulting predominant conformer and
CD, and to correlate the results to their absolute config-
â-Hydroxy-R-amino acids belong to a class of com-
pounds of medicinal importance. They have been found
in many natural products possessing a wide range of
biological properties.1 Besides, they could serve as useful
precursors and chiral auxiliaries in organic synthesis.2
Therefore, a large number of synthetic efforts, utilizing
chemical and enzymatic methods,3,4 have focused on this
(1) Solenberg, P. J .; Matsushima, P.; Stack, D. K.; Wilkie, S. C.;
Thomson, R. C.; Baltz, R. H. Chem. Biol. 1997, 4, 195-202. Saeed, A.;
Young, D. W. Tetrahedron 1992, 48, 2507-2514.
(2) Evans, D. A.; Dinsmore, C. J .; Ratz, A. M.; Evrard, D. A.; Barrow,
J . C. J . Am. Chem. Soc. 1997, 119, 3417-3418. Shibata, K.; Shingu,
K.; Vassilev, V. P.; Nishide, K.; Fujita, T.; Node, M.; Kajimoto, T.;
Wong, C.-H. Tetrahedron Lett. 1996, 37, 2791-2794. Drueckhammer,
D. G.; Barbas, C. F., III; Nozaki, K.; Wong, C.-H. J . Org. Chem. 1988,
53, 1607-1611. Coppola, G. M.; Schuster, H. F. Asymmetric Synthe-
sis: Construction of Chiral Molecules Using Amino Acids; J ohn Wiley
& Sons: Toronto, 1987. Millar, M. J . Acc. Chem. Res. 1986, 19, 49-
56.
(3) Kuwano, R.; Okuda, S.; Ito, Y. J . Org. Chem. 1998, 63, 3499-
3503. Li, G.; Chang, H.-T.; Sharpless, K. B. Angew. Chem., Int. Ed.
Engl. 1996, 35, 451-454. Kolb, H. C.; VanNieuwenhze, M. S.; Sharp-
less, K. B. Chem. Rev. 1994, 94, 2483-2547. Seebach, D.; J uaristi, E.;
Miller, D. D.; Schickli, C.; Weber, T. Helv. Chim. Acta 1987, 70, 237-
261. Evans, D. A.; Weber, A. E. J . Am. Chem. Soc. 1986, 108, 6757-
6761.
(4) Kimura, T.; Vassilev, V. P.; Shen, G. J .; Wong, C. H. J . Am.
Chem. Soc. 1997, 119, 11734-11742. Vassilev, V. P.; Uchiyama, T.;
Kajimoto, T.; Wong, C.-H. Tetrahedron Lett. 1995, 36, 4081-4084.
Herbert, R. B.; Wilkinson, B.; Ellames, G. J .; Kunec, E. K. J . Chem.
Soc., Chem. Commun. 1993, 205-206. Lotz, B. T.; Gasparski, C. M.;
Peterson, K.; Miller, M. J . J . Chem. Soc., Chem. Commun. 1990, 1107-
1109.
(5) Rao, A. V. R.; Gurjar, M. K.; Reddy, K. L.; Rao, A. S. Chem. Rev.
1995, 95, 2135-2167. Fles, D.; Balenovic, B. J . Am. Chem. Soc. 1956,
78, 3072-3074.
(6) Lo, L.-C.; Chen, J .-Y.; Yang, C.-T.; Gu, D.-S. Chirality 2001, 13,
266-271.
(7) Berova, N.; Nakanishi, K. In Circular Dichroism-Principles and
Applications; Berova, N.; Nakanishi, K.; Woody, R. W., Eds.; Wiley-
VCH: New York, 2000; pp 337-382. Harada, N.; Nakanishi, K.
Circular Dichroic Spectroscopy-Exciton Coupling in Organic Stereo-
chemistry; University Science Books: Mill Valley, CA, 1983.
(8) Lo, L.-C.; Liao, Y.-C.; Kuo, C.-H.; Chen, C.-T. Org. Lett. 2000, 2,
683-685.
10.1021/jo016070l CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/26/2002

Notes
J . Org. Chem., Vol. 67, No. 4, 2002 1369
F igu r e 1. Structures of the bischromophoric derivatives 1-3 and the chromophore 4. The major electric transition moment of
chromophore 4 is indicated by the arrow.
Sch em e 1. P r ep a r a tion of th e Op tica lly Active th r eo-â-Ar yl-â-h yd r oxy-r-a m in o Acid Der iva tives 3a -h for
NMR, UV/Vis, a n d CD Mea su r em en ts
uration, we prepared a series of optically active threo-â-
aryl-â-hydroxy-R-amino acid derivatives (3a -h ) and
measured their CD (Scheme 1).
scheme was intended to apply to a microscale reaction,
the intermediates 7 and 8 were not purified. The bis-
chromophoric derivatization of (2S,3R)-8 was achieved
by a single coupling reaction using DCC/DMAP method.
Bischromophoric derivatives 3a -h thus obtained were
further HPLC-purified before subjected to UV/vis and CD
measurements. Enantiomeric excess values of these
bischromophoric derivatives were found to be in the range
Derivatives 3a -h cover a wide variety of aromatic
substituents (Figure 1), including a simple phenyl ring
(3a ), electron-withdrawing groups (4-cyano, 3b; 3-nitro,
3c; 2-nitro, 3d ; 3-chloro, 3e), and electron-releasing
groups (2-methoxy, 3f; 2,5-dimethoxy, 3g; 2,4-dimethyl,
3h ). Among these, derivatives 3d , 3f, 3g, and 3h are
expected to be more sterically hindered, as they all have
a substituent at ortho-position. Scheme 1 depicts the
preparation of optically active bischromophoric deriva-
tives 3a -h for the CD study. Methyl esters of racemic
threo-â-aryl-â-hydroxy-R-amino acids (2S*,3R*)-5, pre-
pared from methyl glycinate and corresponding aromatic
aldehydes according to literature procedures,9 were used
as the starting materials. They were first N-Boc-protected
to give (2S*,3R*)-6, followed by enzymatic resolution10
using either R-chymotrypsin or alcalase to offer optically
active acids (2S,3R)-7. The Boc group was chosen as the
amino protecting group, because it could concomitantly
be removed when the acids 7 were treated with SOCl2/
MeOH, offering the methyl esters (2S,3R)-8. The kinetic
resolution of methyl esters 6a -g was carried out with
R-chymotrypsin. In the case of ester 6h , a poor substrate
of R-chymotrypsin, alcalase, which has been shown to
display a broader substrate specificity,11 was used in-
stead. The reaction conditions were not optimized, and
they were stopped at 15-20% conversions to ensure high
optical purity of the acid products. Since this reaction
1
of 80-96% by H NMR analysis in the presence of Eu-
(hfc)3. Well-resolved signals were obtained for the methyl
ester protons.12
By taking advantage of the red-shifted property of the
chromophore as well as the good solubility of the bis-
chromophoric derivatives in CH2Cl2, we were able to
perform all the following UV/vis and CD measurements
in this solvent. Furthermore, the results would better
1
correlate to the H NMR conformational analyses, which
were conducted in CDCl3. We started with compound 3a ,
which carries only a phenyl group at the â-position and
represents the simplest member in the family. Its λmax is
around 422 nm, which is typical for derivatives of the
red-shifted chromophore 4. The CD of (2S,3R)-3a exhibits
an exciton couplet with a positive first Cotton effect (CE)
at 432 nm (∆ꢀ +60) and a negative second CE at 398 nm
(∆ꢀ -32), leading to an A value of +92 (Table 1). This
positive handedness of the two chromophores could be
explained from the conformational analysis. There are
three staggered conformers for (2S,3R)-3a (Figure 2). The
two chromophores are in gauche positions for conformers
A and B, and in anti positions for conformer C. Since
(11) Hwang, B. K.; Gu, Q.-M.; Sih, C. J . J . Am. Chem. Soc. 1993,
115, 7912-7913. Wong, C.-H.; Chen, S.-T.; Hennen, W. J .; Bibbs, J .
A.; Wang, Y.-F.; Liu, J . L.-C.; Pantoliano, M. W.; Whitlow, M.; Bryan,
P. N. J . Am. Chem. Soc. 1990, 112, 945-953. Bones, J . B.; Beck, J . F.
In Applications of Biochemical Systems in Organic Chemistry; Part I,
J ones, J . B.; Sih, C. J .; Perlman, D., Eds.; Wiley: New York, 1976; pp
107-401.
(12) Cheˆnevert, R.; Le´tourneau, M.; Thiboutot, S. Can. J . Chem.
1990, 68, 960-963. Cheˆnevert, R.; Le´tourneau, M. Chem. Lett. 1986,
1151-1154.
(9) Belokon, Y. N.; Bulychev, A. G.; Vitt. S. V.; Struchkov, Y. T.;
Batsanov, A. S.; Timofeeva, T. V.; Tsyryapkin, V. A.; Ryzhov, M. G.;
Lysova, L. A.; Bakhmutov, V. I.; Belikov, V. M. J . Am. Chem. Soc. 1985,
107, 4252-4259. Pines, S. H.; Kozlowski, M. A. J . Org. Chem. 1972,
37, 292-297. Ehrhart, G.; Siedel, W.; Nahm, H. Chem. Ber. 1957, 90,
2088-2094.
(10) Wong, C.-H.; Whitesides, G. M. In Enzymes in Synthetic Organic
Chemistry; Baldwin, J . E.; Magnus, P. D., Eds.; Elsevier: Oxford, 1994;
pp 41-130.

1370 J . Org. Chem., Vol. 67, No. 4, 2002
Notes
Ta ble 1. CD a n d NMR Cou p lin g Con sta n ts of
Bisch r om op h or ic Der iva tives 1, 2, a n d 3a -h
to an A value of +82. Coupling constant between HR and
Hâ was found to be 2.0 Hz in CDCl3 (Table 1). As a matter
of fact, the results for all the (2S,3R)-family members are
very consistent. The coupling constant between HR and
Hâ for all the members falls in the range of 1.7-2.3 Hz,
and their CD A values range from +79 to +129 (Table
1). The high A values together with the small coupling
constants between HR and Hâ help rule out the possibility
of (2R,3S)-enantiomers adopting conformers A (minor)
and C (major). In this case the A values would be
expected to be much smaller, as the contribution from
the less populated conformer A is insignificant. It is
therefore a general trend for the threo-â-aryl-â-hydroxy-
R-amino acid derivatives to adopt the preferred conformer
B. The substituents on the phenyl ring, regardless of
their electronic and steric properties, seemed not affect
the predominant conformer and the resultant CD. It is
noteworthy that the absolute configuration determined
from this CD exciton chirality method is in good agree-
ment with the outcome of the enzymatic stereospecific-
ity12 used in this report to prepare the (2S,3R)-threo-â-
aryl-â-hydroxy-R-amino acids.
coupling constanta
compound CD (CH2Cl2): λext (∆ꢀ) A value
J
HR,Hâ (Hz)
(2S,3R)-2
432 (+54), 396 (-24)
+78
+92
2.0
2.3
2.2
1.7
2.2
2.2
1.9
2.0
2.0
(2S,3R)-3a 432 (+60), 398 (-32)
(2S,3R)-3b 431 (+51), 397 (-30)
(2S,3R)-3c 435 (+61), 403 (-34)
(2S,3R)-3d 431 (+84), 400 (-45)
(2S,3R)-3e 432 (+50), 399 (-29)
(2S,3R)-3f
(2S,3R)-3g 431 (+58), 398 (-34)
(2S,3R)-3h 431 (+52), 398 (-29)
+81
+95
+129
+79
431 (+79), 397 (-45)
+124
+92
+82
a
Spectra were taken in CDCl3.
In conclusion, we applied the CD exciton chirality
method to the threo-â-aryl-â-hydroxy-R-amino acids using
a red-shifted 7-diethylaminocoumarin-3-carboxylate (4)
chromophore. By taking advantage of its red-shifted
property, we were able to use CH2Cl2 as the solvent for
CD and UV/vis measurements in this study. Data from
NMR coupling constants as well as the sign and the
intensities of CD reveal the preferred conformer B for
this series of compounds. Therefore, the aromatic rings
at the â-position do not make them behave differently
from that of threonine derivative, which carries only a
methyl group. A positive CD will correspond to (2S,3R)-
absolute configuration. This is a convenient and reliable
method, and it could generally be applicable to all threo-
â-aryl-â-hydroxy-R-amino acids. Our ongoing study is to
extend this CD method to the series of erythro-â-aryl-â-
hydroxy-R-amino acids. The results will be reported in
due course.
F igu r e 2. Three staggered conformers (A-C) of the bischro-
mophoric derivatives 3a -h .
F igu r e 3. CD and UV/vis spectra of bischromophoric deriva-
tive 3h in CH2Cl2.
Exp er im en ta l Section
Gen er a l Meth od s. Melting points are uncorrected. IR spec-
tra were taken with Nicolet Magna-IR 550 Series II. 1H and 13C
NMR spectra were recorded at 400 and 100 MHz in CDCl3,
respectively. High-resolution mass spectra were recorded on a
J EOL-102A mass spectrometer. Analytical TLC (silica gel, 60F-
54, Merck) and spots were visualized under UV light and/or
phosphomolybdic acid-ethanol. Flash column chromatography
was performed with silica gel 60 (70-230 mesh, Merck). Spec-
trophotometric grade of CH2Cl2 and a J asco J -720 spectropola-
rimeter were used for obtaining CD spectra. The ∆ꢀ values were
normalized to 100% optical purity of the corresponding samples.
Gen er a l P r oced u r e for th e P r ep a r a tion of Boc-P r otect-
ed Der iva tives 6a -h . To an ice-cooled solution of the corre-
sponding methyl ester 5 (1.5 mmol) in 25 mL of CH2Cl2 was
added (Boc)2O (1.65 mmol). The mixture was gradually allowed
to return to room temperature and stirred for 20 h. A solution
of 5% NaHCO3 (20 mL) was added. The organic layer was
separated and dried over anhydrous Na2SO4. It was filtered and
concentrated. The desired product 6 was purified with silica gel
column chromatography eluted with CH2Cl2/MeOH (95/5 f 90/
10).
the dihedral angle between the two chromophores in
conformer C is close to 180°, it is not expected to have a
significant CD. Conformer A will give a negative CD, and
1
conformer B will lead to a positive CD. H NMR study
provides useful information regarding which conformer
will be predominant. The coupling constant between HR
and Hâ was found to be 2.3 Hz in CDCl3 (Table 1). It
supports that the HR proton is gauche to the Hâ proton,
therefore, ruling out the possibility of conformer A. Since
the observed CD has a strong A value of +92, we could
conclude that (2S,3R)-3a prefers conformer B. This
observation of preferred conformer B, leading to positive
CD for (2S,3R)-derivatives, is the same as in the case of
(2S,3R)-threonine derivative 2, which has a methyl group
instead of a phenyl ring at the â-position.
We then studied the substituent effect of the ring by
examining the other members of the family (3b-h ). As
a typical example, the CD and UV/vis spectra of com-
pound 3h , the only member derived from alcalase-
catalyzed resolution, were shown in Figure 3. Its λmax is
around 423 nm, and its CD exhibits an exciton couplet
with a positive first Cotton effect (CE) at 431 nm (∆ꢀ +53)
and a negative second CE at 398 nm (∆ꢀ -29), leading
Gen er a l P r oced u r e for th e P r otea se-Ca ta lyzed Resolu -
tion of (2S*,3R*)-6. R-Chymotrypsin was used for compounds
6a -g, whereas alcalase was used for compound 6h . Individual
substrate 6 (1.0 mmol) was suspended in 25 mL of phosphate
buffer (0.3 M, pH 7.0) containing 10% DMF. R-Chymotrypsin
(25 mg) was added, and the reaction was shaken at 37 °C for

Notes
J . Org. Chem., Vol. 67, No. 4, 2002 1371
12-24 h. The mixture was then extracted with EtOAc (25 mL
×6) to remove the remaining ester. The aqueous layer was
acidified with citric acid and extracted with EtOAc (30 mL ×3).
The combined organic layers were washed with brine and dried
over anhydrous Na2SO4. After filtration and concentration, crude
acid (2S,3R)-7 was obtained (yield 15-20%). In the case of
compound 6h , alcalase (2.2 mL) instead of R-chymotrypsin was
used. The rest part of the procedure was the same. The acid
(2S,3R)-7 was used without further purification.
Gen er a l P r oced u r e for th e Meth yla tion a n d Dep r otec-
tion of th e Boc Gr ou p To Give Com p ou n d s 8a -h . To a
cooled solution of crude acid 7 (0.07 mmol) in anhydrous MeOH
(5 mL) was added 0.2 mL of SOCl2. It was stirred at room
temperature for 20 h. The mixture was then concentrated to give
crude (2S,3R)-8 (yield 60-80%). It was converted to free amine
by adding 10% NaHCO3 (5 mL) and extracted with EtOAc (5
mL). The EtOAc extract was concentrated and subjected to the
bischromophoric derivatization without further purification.
Gen er a l P r oced u r e for th e P r ep a r a tion of Bisch r o-
m op h or ic Der iva tives. To an ice-cooled solution of free amine
from individual (2S,3R)-8 (0.05 mmol), triethylamine (0.1 mmol),
DMAP (0.05 mmol), and 7-diethylaminocoumarin-3-carboxylic
acid (0.11 mmol) in CH2Cl2 (3 mL) was slowly added a solution
of DCC (0.14 mmol) in CH2Cl2 (2 mL). The mixture was
gradually allowed to return to room temperature and stirred
overnight. It was quenched by adding a few drops of 5% citric
acid and stirred for another 30 min. The CH2Cl2 was removed,
and EtOAc was added. The DCU was first filtered off, and the
EtOAc filtrate was washed with 10% NaHCO3 (×3), H2O (×2),
and finally with brine. The organic layer was dried over
anhydrous Na2SO4. It was filtered, concentrated, and subjected
to silica gel column chromatography for purification. The desired
product 3a -h was eluted with CHCl3/EtOAc (9/1). The optical
purity of the bischromophoric derivatives 3a -h was determined
by 1H NMR in the presence of 0.3 equiv of Eu(hfc)3.
Ack n ow led gm en t. This work was supported by the
National Science Council (NSC-89-CPC-7-002-012 to L.-
C.L.).
Su p p or tin g In for m a tion Ava ila ble: Data for compounds
3a -h , 6a -h as well as the 1H and 13C NMR spectra of
compounds 3a -h . This material is available free of charge via
the Internet at http://pubs.acs.org.
J O016070L