From selective substrate analogue factor Xa inhibitors to dual inhibitors of thrombin and factor Xa. Part 3

Article abstract of DOI:10.1016/j.bmcl.2007.03.105

Highly potent and selective substrate analogue factor Xa inhibitors were obtained by incorporation of non-basic or modestly basic P1 residues known from the development of thrombin inhibitors. The modification of the P2 and P3 amino acids strongly influenced the selectivity and provided potent dual factor Xa and thrombin inhibitors without affecting the fibrinolytic enzymes. Several inhibitors demonstrated excellent anticoagulant efficacy in standard clotting assays in human plasma.

Full text of DOI:10.1016/j.bmcl.2007.03.105

Bioorganic & Medicinal Chemistry Letters 17 (2007) 3322–3329
From selective substrate analogue factor Xa inhibitors to
dual inhibitors of thrombin and factor Xa. Part 3
Daniel Do¨nnecke,a Andrea Schweinitz,a Anne Sturzebecher,a Peter Steinmetzer,a
¨
Maj Schuster,b Uta Sturzebecher,c Silke Nicklisch,b Jo¨rg Sturzebecherc and
¨
¨
Torsten Steinmetzera,*
aCuracyte Discovery GmbH, Winzerlaer Str. 2, D-07745 Jena, Germany
bCuracyte Discovery GmbH, Deutscher Platz 5d, D-04103 Leipzig, Germany
cInstitut fu¨r Vaskula¨re Medizin, Klinikum der Universita¨t Jena, Bachstraße 18, D-07740 Jena, Germany
Received 22 February 2007; revised 30 March 2007; accepted 31 March 2007
Available online 6 April 2007
Abstract—Highly potent and selective substrate analogue factor Xa inhibitors were obtained by incorporation of non-basic or mod-
estly basic P1 residues known from the development of thrombin inhibitors. The modification of the P2 and P3 amino acids strongly
influenced the selectivity and provided potent dual factor Xa and thrombin inhibitors without affecting the fibrinolytic enzymes.
Several inhibitors demonstrated excellent anticoagulant efficacy in standard clotting assays in human plasma.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Thrombotic complications are a major reason for mor-
tality and morbidity worldwide. The presently approved
anticoagulants have several limitations, in that most can
be used only parenterally, and the orally active vitamin-
K antagonists require drug monitoring and are influ-
enced by drug and food interactions. Ximelagatran,
the first orally available thrombin inhibitor was recently
withdrawn from the market due to some cases of
hepatotoxicity.
make strong interactions to Tyr228 on the back of the
specificity pocket.4–6 Some non-basic fXa inhibitors
have recently entered clinical trials, and in preliminary
reports it was described that the most advanced com-
pound Bay 59-7939 was well tolerated in healthy male
subjects and showed predictable pharmacodynamics
and pharmacokinetics.7,8
We have recently described several series of substrate
analogue fXa inhibitors containing a 4-amidinobenzyla-
mide as the P1 residue.9 The incorporation of aromatic
P3 amino acids with different side-chain lengths revealed
serendipitously the strongest fXa affinity for inhibitors
containing D-homophenylalanine analogues at this posi-
tion.10 Optimized compounds, such as 1 with a D-homo-
2-pyridylalanine(N-oxide) as P3 residue (Table 1), have
excellent selectivity for fXa over a whole set of tryp-
sin-like serine proteases and possess a strong activity
in the standard clotting assays and for the inhibition
of the prothrombinase complex. However, all pharma-
cokinetic studies with these benzamidine derived
inhibitors, including their hydroxyamidino-, acetylhydr-
oxyamidino- and hexyloxycarbonyl-amidino prodrugs,
revealed a rapid elimination from the circulation of rats
and an insufficient oral bioavailability. To overcome
these pharmacokinetic problems we replaced the
4-amidinobenzylamide in the P1 position with less basic
residues, some of which have been previously used for
Factor Xa (fXa) is another protease of the clotting cas-
cade and has emerged as an alternative target for the
development of new anticoagulants.1 It is the enzymati-
cally active component of the prothrombinase complex,
which is responsible for thrombin generation. During
the last decade several groups have described different
types of non-substrate like fXa inhibitors.2,3 Most of
the initial inhibitors contained a strongly basic P1 group
that interacts with Asp189 at the bottom of the S1 bind-
ing site. During optimization it was possible to replace
this residue with chloro-substituted aromatics, which
Keywords: Factor Xa; Thrombin; Protease inhibitor; Anticoagulant;
Antithrombotic.
*
Corresponding author at present address: Institute of Pharmaceutical
Chemistry, Philipps-University Marburg, Marbacher Weg 6,
D-35032 Marburg, Germany. Tel.: +49 6421 2825900; fax: +49
6421 2825901; e-mail: torsten.steinmetzer@staff.uni-marburg.de
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.03.105

D. Do¨nnecke et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3322–3329
Table 1. Inhibition of fXa and thrombin by inhibitors with a modified P1 residue
3323
N
O
O
O
O
H
N
S
R
N
N
*
H
H
O
Compound
R
Kia (nM)
*
fXa
Thrombin
NH
1
2
3
D
0.32
2700
88,000
NH2
N
D/L
21
NH2
N
D
3900
>1,000,000
4
5
D
3.3
10,000
7600
Cl
Cl
D/L
2.55
Cl
Cl
H2N
H2N
6b
D
0.095
170
7
D/L
32
40,000
8
9
D/L
D/L
53
52
190,000
140,000
Cl
S
S
a The Ki values were measured as described previously.9,14a
b Compound 6 inhibits uPA, plasmin, and trypsin with Ki values of 150, 930, and 6.5 lM, respectively.
the design of substrate analogue thrombin inhibitors.11
The results of these studies are presented in this
communication.
(segments Asp189-Ser195 and Val213-Cys220, respec-
tively, as well as Tyr228 at its back). The only difference
is position 192, which is Gln in case of fXa and Glu in
thrombin. Although no X-ray structures of these inhib-
itors in complex with fXa are available, we assume that
in analogy to the related thrombin inhibitors the P1
chloro atom makes a lipophilic contact with the aro-
matic ring of the fXa residue Tyr228.
Although some of the compounds summarized in Table
1 were obtained as racemates, they contain an otherwise
constant P4–P2 segment and have a more than 1000-
fold selectivity for fXa over thrombin. All inhibitors
with
a meta-chloro-substituted benzylamide group
retained high fXa affinity with inhibition constants in
the low nanomolar range, as previously found in related
thrombin inhibitor series.11 This was unsurprising,
because both sides of the S1 pocket from fXa and
thrombin are formed by nearly identical amino acids
Inhibitor 6, containing a 2-aminomethyl-5-chloro-ben-
zylamide, is an even more potent fXa inhibitor than
the benzamidine 1, despite a moderate decrease in selec-
tivity. This was not expected because in a previously de-
scribed first X-ray structure with a related thrombin

3324
D. Do¨nnecke et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3322–3329
Table 2. Inhibition of fXa and thrombin by inhibitors with modified P4 residue
N
O
H2N
O
H
R
N
N
H
N
*
H
O
Cl
Compound
R
Ki (nM)
*
fXa
Thrombin
1005
10
D/L
0.35
O
O2S
O
O
11
12
13
14
15
D/L
D/L
D/L
D/L
D/L
0.36
3.4
930
2700
530
HO
O2S
O2
S
O
O2
S
0.76
1.6
O
O
O2
S
8300
6500
HO
O
O
4.9
O
O
HO
16
17
18
19
D/L
D/L
D/L
D/L
4.9
18,000
40,000
O
O
O
200
O
O
O
65
16
140,000
125,000
HO
HO
O
inhibitor it was found that the interaction between the
ortho-aminomethyl group of the P1 residue and
Glu192 of thrombin is essential for their high poten-
cy.11d Obviously Gln192 of fXa also contributes to the
affinity of this P1 group. The comparison of the fXa
Ki values found for inhibitor 7 with those of compounds
4 and 6 (we assumed a Ki-value of 16 nM for the pure
D-enantiomer of 7) reveals a ca. 170- and 35-fold
improved potency due to the 5-chloro and 2-amino-
methyl substitutions, respectively. We can only specu-
late that the 2-aminomethyl group might be involved
in additional hydrogen bonds, similarly as shown by
X-ray crystallography for a second thrombin inhibitor
complex containing an identical P1 group (pdb code
1zrb).12 In this complex the aminomethyl group is in
hydrogen bond distance to the carbonyl oxygen of the
P2 inhibitor residue, to the carbonyl oxygen of the
thrombin residue Gly216 and to a neighbouring water
molecule. Interestingly, in this structure the side chain
of the thrombin residue Glu192 is rotated away and
therefore not involved in inhibitor binding.
In contrast, the 5-chlorothiophene-2-methylamide 8,
which was synthesized analogously to known non-sub-
strate type fXa inhibitors,6,13 possessed only moderate
potency. A similar activity was found for the des-chloro
thiophene inhibitor 9; thus both results indicate that the
chlorine of compound 8 does not contribute to the bind-
ing of fXa. It is also worth mentioning that none of the
compounds (except the benzamidine 1) shown in Tables
1–3 significantly inhibited the fibrinolytic enzymes plas-
min and uPA (Ki values >5 lM, data not shown).

D. Do¨nnecke et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3322–3329
3325
Table 3. Inhibition of fXa and thrombin by inhibitors of the general formula
R1
H2N
R
P2
N
N
*
H
H
O
Cl
Compound
R
R1
P2
Ki (nM)
Thrombin
*
fXa
N
20
21
22
23
24
25
26a
27a
28
29
30
31
32
33
D
Ala
Ala
Ser
0.084
24
O2S
O2S
O2S
O2S
O2S
O2S
O2S
O
N
D
0.033
0.25
0.059
0.24
0.52
0.049
0.08
0.87
0.63
1.7
170
250
860
180
20,000
0.56
7.7
N
D/L
O
O
O
O
O
O
O
O
O
O
O
N
N
N
N
N
N
N
N
N
N
N
D
Ser
D
Glu(OMe)
Glu
Pro
Pro
Pro
Pro
Pro
Pro
Pro
Pro
D/L
D
O
O2
S
D
HO
O
O
D/L
19
O
O
HO
D
17.7
4.9
O
O
D
O
O
D
0.92
1.0
7.8
5
O
O
D
9.0
O
HO
D
0.94
17.3
O
(continued on next page)

3326
D. Do¨nnecke et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3322–3329
Table 3 (continued)
Compound
R
R1
P2
Ki (nM)
Thrombin
*
fXa
2.9
O
N
N
34
35
36
37
D
Pro
3.8
3.5
3.6
3.2
O
O
HO
D
D
D
Pro
Pro
Pro
1.5
17
O
HO
HO
O
HO
8.3
O
*The diastereomers were separated by preparative HPLC during synthesis, if possible.17
a The highest inhibitory potency towards trypsin with Ki values of 6.3, 19 and 61 nM was found for the benzamidine derivative 1 and for the newly
synthesized inhibitors 26 and 27, respectively. All other derivatives showed reduced trypsin affinity (Ki > 0.5 lM).
Table 4. Anticoagulant activity of selected inhibitors
The racemic 4-(carboxymethyl)benzylsulfonyl derivative
11 inhibits fXa with a subnanomolar Ki value and retains
strong anticoagulant activity, however, its elimination rate
was not reduced compared to compound 6. As found pre-
viously with the 4-amidinobenzylamide series,9,10 the
replacement of the benzylsulfonyl residue by non-aromatic
P4 groups leads to reduced potency although several ana-
logues still inhibit fXa with Ki values in the low nanomolar
range. The oxalyl derivative 16 showed a slightly prolonged
retention time in the circulation (t1/2 ꢀ 30 min), but also a
weaker anticoagulant activity, which was further reduced
in case of inhibitor 19, containing a melagatran-like15
N-carboxymethyl group (Table 4).
Compound
IC200 (lM)
aPTT
PT
TT
6
11
12
14
16
19
27
29
30
33
34
35
36
37
0.16
0.49
0.45
0.89
1.85
6.0
0.14
0.29
0.6
0.5
1.2
6.5
1.3
1.3
57
15
3.3
>100
0.15
0.30
0.18
0.32
0.23
0.16
0.58
0.47
0.39
0.79
0.47
0.61
0.60
0.35
0.85
0.73
0.11
0.13
0.10
0.17
0.09
0.03
0.23
0.15
In recent years several groups have reported the devel-
opment of dual fXa and thrombin inhibitors and pro-
posed
a stronger anticoagulant activity for such
compounds compared to monoselective inhibitors.16
Using this approach, we have designed inhibitors with
improved thrombin affinity by replacement of the P2
glycine to recover some loss in anticoagulant activity,
which has been observed with some of the compounds
listed in Table 2. The incorporation of proline not only
enhanced thrombin inhibition, but also improved the
affinity towards fXa (Table 3). It is worth mentioning
that the ratio between the fXa and thrombin inhibition
could also be simply adjusted by the choice of the P3
amino acid: compounds 33 and 35 are superior fXa
inhibitors, whereas the D-homotyrosine analogue 37
has higher affinity for thrombin. As expected, these P2
proline derivatives have stronger anticoagulant activity
than their glycine analogues, for example, as seen with
the inhibitor pairs 14/27 or 19/33 (Table 4).
Although compound 6 had excellent anticoagulant activ-
ity (Table 4), it was very rapidly eliminated from the cir-
culation of rats after intravenous injection (t1/2 < 20 min)
and had marginal oral bioavailability (F < 5%, dose
5 mg/kg). Approximately 75% of the given inhibitor dose
was found in the bile and ꢀ15% in the urine, which indi-
cates that the inhibitor is rapidly cleared mainly via the
hepatobiliary route. Therefore, further analogues were
prepared by modification of the N-terminus to overcome
these problems. The replacement of the benzylsulfonyl-
by the smaller propylsulfonyl-residue (inhibitor 12, Ta-
ble 2) did not change the elimination rate, although we
observed a slight shift in the elimination route; that is,
similar amounts of approximately 45% of the inhibitor
dose were found in the bile and also in the urine. In pre-
vious studies on compounds of different structural type
we were able to demonstrate a significantly reduced
clearance rate after incorporation of carboxyl groups
into the inhibitors.14 Therefore, we synthesized addi-
tional inhibitors containing carboxyl groups at the N-ter-
minus while retaining the structure of the rest of the
molecule (Table 2).
The N-carboxymethylated ethyl ester prodrugs 30, 34
and 36 were prepared in analogy to the structurally re-
lated thrombin inhibitor Ximelagatran, a compound
that has sufficient oral bioavailability.15 A major differ-
ence between Ximelagatran and these inhibitors exists
in the P1 residues: at physiological pH the prodrug

D. Do¨nnecke et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3322–3329
3327
Ximelagatran contains a non-charged 4-hydroxyamidi-
no-benzylamide moiety, whereas the inhibitors of our
series posses a weakly basic 2-aminomethyl-5-chloro-
benzylamide.
activity and excellent selectivity towards the fibrinolytic
enzymes uPA and plasmin. The highest inhibitory po-
tency towards trypsin with Ki values of 19 and 61 nM
was found for inhibitors 26 and 27, respectively. All
other derivatives showed reduced trypsin affinity
(Ki > 0.5 lM). However, due to their insufficient phar-
macokinetics, that is, low oral bioavailability and short
circulation half-life, it is necessary to improve these
compounds further.
Several compounds listed in Tables 2 and 3 were initially
evaluated in a permeability assay across a Caco-2 cell
monolayer as a model for intestinal absorption. Com-
pounds that show sufficient oral bioavailability and good
absorption by passive diffusion across the intestinal mem-
brane normally have a Caco-2 cell apparent permeability
(Papp) of P100 nm/s.18 The highest permeability was
found for the ethyl ester of the P3 D-homotyrosine deriv-
ative 36 (Papp = 89 nm/s), which inhibits thrombin more
strongly than fXa. A moderate permeability was also ob-
served for the P3-D-homo-2-pyridylalanine inhibitor 35
(Papp = 50 nm/s). Surprisingly, its ethyl ester 34 was trans-
ported with reduced efficacy (Papp = 24 nm/s). For all
other compounds tested, especially the more hydrophilic
pyridine-N-oxide derivatives, only poor Papp values
(<50 nm/s) were estimated.
Non-commercially available P1 residues used for inhib-
itors 2, 3 and 6–9 were obtained by known procedures.
2-(Boc-amido)-5-aminomethylpyridine for compound 2
was prepared from 2-amino-5-cyanopyridine by Boc
protection20 and subsequent hydrogenation of the nitrile
using Pd/C as catalyst. The 4-aminoethylpyridine for
analogue 3 was prepared from 4-vinylpyridine by treat-
ment with ammonium chloride.21 2-Boc-amidomethyl-
5-Cl-benzylamine for inhibitor 6 was prepared as
described by Nelson et al.,22 the 5-chloromethyl-2-chlo-
rothiophene was converted into the amine used for 8 by
reaction with sodium azide, followed by brief hydroge-
nation in ethyl acetate using Pd/C as catalyst. The
des-chloro P1 residues for inhibitors 7 and 9 were
obtained by extended hydrogenation of the chloro
aromatics. Most sulfonyl- and acyl chlorides, used for
the modification of the P3 amino acid (Table 2), are
commercially available or can be purchased as appropri-
ate esters. Chlorosulfonyl-isopropyl acetate for inhibitor
13 was obtained from the bis-chloride by reaction with 1
equiv isopropanol in diethyl ether.23 The substituted
In spite of the moderate Caco-2 cell permeability, we
determined the oral bioavailability of selected com-
pounds in rats. Unfortunately, none of the investigated
compounds (6, 14, 29, 35, 36) exhibited a sufficient oral
bioavailability at a dose of 5 mg/kg (F < 5%).19
In conclusion, we have developed highly potent sub-
strate analogue dual inhibitors of the clotting proteases
fXa and thrombin, which have strong anticoagulant
O
O2N
O2N
OtBu
O
O
H
N
H
N
S
OH
S
N
O2
O2
a, b
c, d
O
N
N
38
39
NH2
O
O
OEt
H
O
O
N
O2N
N
OH
O
O
O
CH2
N
O
i
33
35
HN
Cl
N
S
e, f, g
O2
h, i
O
N
N
30
40
Scheme 1. Reagents and conditions for the synthesis of inhibitors 30, 33 and 35: (a) H-Pro-OtBu · HCl, PyBOP, 3 equiv DIEA in DMF, 15 min
0 ꢁC, 3 h room temperature; (b) 1.5 equiv mCPBA over 7 h in DCM, separation of the diastereomers by preparative HPLC; (c) 1.1 equiv Cs2CO3 in
DMF, 15 min; 1.5 equiv BrCH2–COOEt, 12 h, rt; (d) 90% TFA, 1.5 h room temperature; (e) 2-(Boc-amidomethyl)-5-chlorobenzylamine, PyBOP, 2
equiv DIEA in DMF, 15 min 0 ꢁC, 3 h room temperature; (f) 2.25 equiv thiophenol, 4.5 equiv K2CO3 in acetonitrile, 5 h 50 ꢁC, 12 h room
temperature; (g) 90% TFA, 1 h room temperature, preparative HPLC provides 30 as a TFA salt; (h) zinc dust in acetic acid/1 N HCl 1:1, 1 h room
temperature; (i) 1 N LiOH in dioxane (1:1), 1 h room temperature, preparative HPLC.

3328
D. Do¨nnecke et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3322–3329
H. G.; Woltmann, R.; Baskin, E. P.; Lynch, J. J.; Lucas,
R.; Shafer, J. A.; Dancheck, K. B.; Chen, I. W.; Mao, S.
S.; Krueger, J. A.; Hare, T. R.; Mulichak, A. M.; Vacca, J.
P. J. Med. Chem. 1997, 40, 3726; (b) Lumma, W. C.;
Witherup, K. M.; Tucker, T. J.; Brady, S. F.; Sisko, J. T.;
Naylor-Olsen, A. M.; Lewis, S. D.; Lucas, B. J.; Vacca, J.
P. J. Med. Chem. 1998, 41, 1011; (c) Tucker, T. J.; Brady,
S. F.; Lumma, W. C.; Lewis, S. D.; Gardell, S. J.; Naylor-
Olsen, A. M.; Yan, Y.; Sisko, J. T.; Stauffer, K. J.; Lucas,
B. J.; Lynch, J. J.; Cook, J. J.; Stranieri, M. T.; Holahan,
M. A.; Lyle, E. A.; Baskin, E. P.; Chen, I. W.; Dancheck,
K. B.; Krueger, J. A.; Cooper, C. M.; Vacca, J. P. J. Med.
Chem. 1998, 41, 3210; (d) Rittle, K. E.; Barrow, J. C.;
Cutrona, K. J.; Glass, K. L.; Krueger, J. A.; Kuo, L. C.;
Lewis, D. S.; Lucas, B. J.; McMasters, D. R.; Morrissette,
M. M.; Nantermet, P. G.; Newton, C. L.; Sanders, W. M.;
Yan, Y.; Vacca, J. P.; Selnick, H. G. Bioorg. Med. Chem.
Lett. 2003, 13, 3477; (e) Morisette, M. M.; Stauffer, K. J.;
Williams, P. D.; Lyle, T. A.; Vacca, J. P.; Krueger, J. A.;
Lewis, S. D.; Lucas, B. J.; Wong, B. K.; White, R. B.;
Miller-Stein, C.; Lyle, E. A.; Wallace, A. A.; Leonard, Y.
M.; Welsh, D. C.; Lynch, J. J.; McMasters, D. R. Bioorg.
Med. Chem. Lett. 2004, 14, 4161.
benzylsulfonylchloride used for 10 and 11 was obtained
from methyl-2-(4-(bromomethyl)phenyl)acetate by
reaction with sodium sulfite, followed by treatment with
phosphorus pentachloride.24 The n-hexyl- and cyclo-
hexyl esters of bromoacetic acid necessary for inhibitors
31 and 32 were prepared from bromoacetyl bromide.25
A representative synthesis of the inhibitors is exemplari-
ly described for compounds 30, 33 and 35 (Scheme 1).
Racemic homo-2-pyridylalanine10 was converted to the
4-nitrobenzylsulfonyl amino acid 38. PyBOP-mediated
coupling of H-Pro-OtBu, followed by oxidation using
mCPBA26 and separation of the diastereomers by
preparative HPLC, provided intermediate 39, which
was alkylated by reaction with ethyl bromoacetate in
the presence of Cs2CO3 in DMF.27 After tert-butyl ester
cleavage and PyBOP-mediated coupling of 2-(Boc-
amidomethyl)-5-Cl-benzylamine,22 the 4-nitrophenyl-
sulfonyl group was removed using thiophenol/K2CO3
in acetonitrile.27 The Boc group was removed and the
ethyl ester 30 was saponified using a mixture of 1 N
LiOH and dioxane.
12. Stauffer, K. J.; Williams, P. D.; Selnick, H. G.; Nantermet,
P. G.; Newton, C. L.; Homnick, C. F.; Zrada, M. M.;
Lewis, S. D.; Lucas, B. J.; Krueger, J. A.; Pietrak, B. L.;
Lyle, E. A.; Singh, R.; Miller-Stein, C.; White, R. B.;
Wong, B.; Wallace, A. A.; Sitko, G. R.; Cook, J. J.;
Holahan, M. A.; Stranieri-Michener, M.; Leonard, Y. M.;
Lynch, J. J., Jr.; McMasters, D. R.; Yan, Y. J. Med.
Chem. 2005, 48, 2282.
Acknowledgments
The authors thank Ute Altmann, Angelika Grau, Clau-
dia Neuwirth, Cathleen Naumann and Martin Korso-
newski for their excellent technical help. We thank
also Robert Etges for correcting the English version of
13. (a) Mederski, W. W. K. R.; Cezanne, B.; van Amsterdam,
C.; Buhring, K.-U.; Dorsch, D.; Gleitz, J.; Ma¨rz, J.;
¨
Tsaklakidis, C. Bioorg. Med. Chem. Lett. 2004, 14, 5817;
(b) Bauer, S. M.; Goldman, E. A.; Huang, W.; Su, T.;
Wang, L.; Woolfrey, J.; Wu, Y.; Zuckett, J. F.; Arfsten,
A.; Huang, B.; Kothule, J.; Lin, J.; May, B.; Sinha, U.;
Wong, P. W.; Hutchaleelaha, A.; Scarborough, R. M.;
Zhu, B.-Y. Bioorg. Med. Chem. Lett. 2004, 14, 4045.
the manuscript. This work was supported by the ‘Thu-
¨
ringer Aufbaubank’.
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