Synthesis and binding properties of guanidinium biscarboxylates

Article abstract of DOI:10.1007/s00706-011-0660-x

The ammonium ion binding site of the enzyme glutaminase HisF inspired us to design guanidinium biscarboxylates as potential self-organized ionophores in molecular recognition. The syntheses of the title compounds based on aliphatic and aromatic building b

Full text of DOI:10.1007/s00706-011-0660-x

Monatsh Chem (2011) 142:1289–1308
DOI 10.1007/s00706-011-0660-x
ORIGINAL PAPER
Synthesis and binding properties of guanidinium biscarboxylates
•
Andreas Spath Janina Gonschor Burkhard Konig
•
¨
¨
Received: 9 May 2010 / Accepted: 11 September 2011 / Published online: 22 October 2011
Ó Springer-Verlag 2011
Abstract The ammonium ion binding site of the enzyme
glutaminase HisF inspired us to design guanidinium
biscarboxylates as potential self-organized ionophores in
molecular recognition. The syntheses of the title com-
pounds based on aliphatic and aromatic building blocks,
along with a general method for the preparation of d-am-
inoethoxyacetic acids, are presented in this work.
Investigation of the binding properties of the title com-
pounds in dimethyl sulfoxide (DMSO) and methanol
solution revealed no ammonium ion affinity, but interaction
of the guanidinium moiety with acetate ions.
[32–38]). Their pre-organization is of key importance for
achieving high binding affinity and selectivity, as supra-
molecular binding enthalpies are typically small and
unfavorable entropic effects of binding should be mini-
mized [43–47]. The classical approach is the use of
macrocycles, such as crown ethers, but clefts [48–53], tri-
pods [54–57], or tweezers [58] can be valuable alternatives.
Tweezers are pincer-like molecules carrying two side-arms
arranged in the form of a forceps, which can complement a
particular guest by specific interactions. Many different
examples such as 1 (for example [59, 60]), 2 [61, 62], 3
[63], 4 [64, 65], or 5 [66] with a rigid core and flexible
side-arms are known (Fig. 1). Such clefts or tweezer-type
molecules have been used for recognition of carboxylate
[67–69], guanidinium [70], and metal ions [71], amino
acids [72–74], and ion pairs [75].
Keywords Guanidinium Á Unnatural amino acid Á
Recognition Á Ligand Á Pyrrole
Introduction
In the bottom region of the enzyme glutaminase subunit
HisF, two glutamic acids and one arginine side-chain
functionalities form hydrogen bonds to the ammonium ion
of a lysine side-chain in the center of a stabilizing ab-eight
barrel [76]. (An ammonium ion tunnel with gate mechanism
is postulated as the mechanism of action of the enzyme:
ammonium ions may replace the lysine residue and are
coordinated by the carboxylate ions. Rebinding of the lysine
residue moves the ammonium ion into the inner part of the
enzyme to be transported to the HisG enzyme subunit.)
Inspired by this, we derived a guanidinium biscarboxylate 6
as a minimal tweezer-type structure for potential ammo-
nium ion binding. Intramolecular hydrogen bonds should
pre-organize the structure and expose the carboxylate
groups in close proximity as a potential cation binding site.
Molecular modeling (DFT, BLYP6-G-31*) confirmed that
the structure is a stable conformer in the gas phase (Fig. 2).
Unnatural amino acids such as 7 are suitable starting
materials for synthesis of 6 (Scheme 1). The parent
Ionophores find applications in many fields of chemistry
(examples for recognition with selective ligands [1–5],
examples for chemosensors [6–12], selective ligands in
guest transport [13–17]), and medical diagnostics (selective
ligands general in analytic applications [18–23], examples
for ion-selective electrodes [24–31], examples for optodes
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-011-0660-x) contains supplementary
material, which is available to authorized users.
¨
¨
A. Spath Á J. Gonschor Á B. Konig (&)
Institut fu
¨r Organische Chemie, Universita¨t Regensburg,
93040 Regensburg, Germany
e-mail: Burkhard.Koenig@chemie.uni-regensburg.de
¨
A. Spath
e-mail: Andreas.Spaeth@chemie.uni-regensburg.de
123

¨
A. Spath et al.
1290
Fig. 1 Examples of
bifunctional synthetic receptors
with tweezers or cleft structures
_
_
COOH
N
COOH
O
_
O
O
O
O
P
O
O
O
O
P
N
H
N
O
N
O
HO
+
N
N
N
O
O
1
2
3
O
_
_
+
NH3
+
H3N
P
O
O
NH2
H2N
O
N
N
N
O
P
O
_
AcO
OAc
O
_
Zn
N
N
N
O
N
4
5
aminoethoxyacetic acid [77, 78] is commercially available,
but substituted derivatives have not been reported. We
describe the preparation of derivatives of 7 and their con-
version into 6. The ammonium and acetate ion binding
ability of 6 and a derivative were investigated by nuclear
magnetic resonance (NMR) and emission titration.
Results and discussion
Synthesis
N-protected 1,2-amino alcohols 8 were prepared by stan-
dard conditions and used as starting materials. Substituted
amino alcohols 8 were prepared by reduction of the corre-
sponding amino acid methyl esters [79] with sodium
boranate in dry tetrahydrofuran (THF) [80]. The indole
moiety of tryptophanol was protected by Cbz chloride in
dichloromethane in the presence of powdered sodium
hydroxide and 10 mol% Bu4NHSO4 under dry conditions to
give the protected indole in 92% yield [81]. The Cbz group
avoids the formation of side-products. The synthesis using
bromoacetic esters 9 proceeds in moderate to good yields,
and the copper-mediated decomposition of azoesters
[82–84] is a feasible alternative route to prepare derivatives
of 7. (NaH as base or the phase-transfer catalytic conver-
sions were most successful. t-BuOK as base gives lower
yields between 20% and 30%. For the copper(I)-mediated
azoester reaction and the substituted amino alcohols, yields
around 30% were observed. NaH is not compatible with
Cbz protection, as hydrogen developed by the base cleaves
the protection group, leading to a reduced yield and a more
tedious workup.) Table 1 summarizes the results.
Fig. 2 Optimized geometry of compound 6 in the presence of a
methylammonium ion (DFT, BYPL6-G-31*)
+
N
H
O
O
H
H
_
_
O
O
O
.....
.....
O
O
H
H
N
O
+
H2N
OH
N
Deprotection by standard conditions gave the corre-
sponding products in excellent yields (Scheme 2).
N
H
H
7
6 + guest
The amino esters were used to prepare 1,3-disubstituted
guanidines in a two-step procedure via Cbz-protected
Scheme 1
123

Synthesis and binding properties of guanidinium biscarboxylates
1291
(11) [91] was used in our case, as the reaction to yield a
thiourea proceeds rapidly and in high yield [92]. Repre-
sentative examples of amino esters 10 which were
converted by 11 into the corresponding Cbz–thioureas in
good to excellent yields are summarized in Table 2.
Pyrrole (entry 8, Table 2) rigidifies the structure of the
target receptor and can provide an additional hydrogen
bond on each side-arm [93–99]. Pyridine derivatives
(entry 9, Table 2) have been used in tweezer structures
[100–104]. The rigid aromatic ring in combination with a
metasubstituent may induce good pre-organization of
binding sites in the final ligand.
Table 1 Reaction partners and yields for preparation of d-amino-
ethoxyacetic acid esters
O
X
R3
O
R2
R2
O
9
R1
R1
O
R3
OH
N
H
N
H
O
8
7
Entry Amino alcohol 8 Acetic ester 9 Conditions Yield of 7/%
R1 R2
R3
X
a
b
c
a
c
a
1
2
3
4
5
6
Boc H
Boc H
Boc H
Cbz H
Cbz H
Boc
tBu
Et
Br
Br
N2
Br
N2
Br
7a
7b
7b
7c
7d
7e
83
59
78
87
61
77
The next conversion step to the symmetric Cbz–guanidine
moiety uses the esters to facilitate the product purification.
The reaction can be performed in DMF, with NEt3 as base,
using either mercury(II) chloride or 1-ethyl-3-(3-dimethyl-
aminopropyl)carbodiimide (EDC) as an activating agent for
thiourea, observing good yields. All conversions and their
correspondingyieldsare given inTable 3. Thiourea 16 cannot
be converted into the corresponding twofold substituted
thiourea, yielding, by intramolecular reaction, the cyclic
guanidine 21 as the only product (Kilburn et al. observed this
with a similar amide compound (see also [105, 106]).
The Cbz protecting group was removed by hydrogena-
tion using 10% Pd on charcoal at 30 bar of hydrogen
pressure. The guanidinium esters (22a/b and 23) are iso-
lated as their hexafluorophosphate salts. Basic or acidic
ester cleavage gives the deprotected tweezers 6 and 24 in
excellent overall yields (Scheme 3).
Et
tBu
Et
tBu
b
a
b
a
7
Boc
Boc
Boc
Boc
Et
Br
Br
Br
Br
7f
52
89
50
63
8
tBu
Et
7g
7h
7i
9
10
tBu
Recognition properties of the tweezer receptors
NCbz
Carboxy guanidinium tweezers exist as zwitterionic struc-
tures over a wide range of pH. The pKA values of their
guanidinium functionality are 12 and 11.5 for 6 and 24,
respectively, in good agreement with the value for the
arginine side-chain (pKA = 12.5) [107]. The carboxylic
acids in 6 show a pKA value of 3.5, the carboxy groups in
24 of 4 (for pKA determination experiments see the sup-
porting information). The large differences in pKA values
between ammonium salt guest molecules and the
a
b
c
Bu4 N?HSO-4 , Dichloromethane(DCM), H2O, NaOH, RT, overnight
NaH, THF (dry), RT, 3 h
Cu(OTf)2, PhHN–NH2, Tetramethylethylenediamine (TMEDA),
DCM (dry, degassed), N2, 0 °C ? RT, 8 h
thioureas. Benzyloxycarbonyl isothiocyanate (Cbz–NCS)
[85–87] and ethoxycarbonyl isothiocyanate [88, 89] have
been widely used for thiourea synthesis [90]. Cbz–NCS
Scheme 2
O
a
R1 = Boc, R2 = H, R3 = t-Bu
+
H3N
_
10a
10b
10c
O
O
O
O
O
O
R2
O
O
O
R1
b
c
R3
+
H3N
R1 = Boc, R2 = H, R3 = Et
O
N
H
O
7
R1 = Cbz, R2 = H, R3 = t-Bu
+
H3N
a) MeOH, THF, or acetone, HCl 1N, RT, 5 h, quant.; b) DCM,
Trifluoroacetic acid (TFA),
or HCl in Et2O, RT, 2-4 h, 93-98%; c) MeOH, HOAc, 10 bar H2, Pd/C, RT, overnight, 94%
123

¨
A. Spath et al.
1292
Table 2 Conversion of amines 10 with 11 into the corresponding Cbz–isothioureas 12–16
O
S
C
N
O
Ph
S
O
+
R
R NH3
N
N
O
Ph
H
H
10
12-16
11
Entry
1
Amine
Conditions
Cbz–thiourea
Yield/%
84
a
O
O
S
_
_
O
+
O
Cbz
Cbz
Cbz
12a
12b
O
NH3
O
N
N
H
H
10a
2
3
b, 2 h
91
89
O
O
O
S
S
O
+
O
O
O
O
NH3
O
O
N
N
H
H
10b
b, 2 h
O
O
+
12c
13a
NH3
N
H
N
H
10c
a
4
81
–
S
S
+
O
O
_
NH3
O
Cbz
Cbz
N
H
N
H
O
O
O
O
10d
10e
5
6
b, 2 h
83
78
+
NH3
O
13b
14a
N
H
N
H
a
O
O
S
H
N
H
_
_
N
Cbz
+
O
NH3
O
N
N
H
H
O
O
10f
7
b, 3 h
84
O
O
S
H
N
H
N
Cbz
+
14b
O
NH3
O
N
H
N
H
O
O
10g
123

Synthesis and binding properties of guanidinium biscarboxylates
1293
Table 2 continued
Entry
8
Amine
Conditions
b, 2 h
Cbz–thiourea
Yield/%
92
O
O
O
S
H
N
H
N
Cbz
15
NH2
N
N
O
H
H
10h
9
b, 2 h
81
O
O
S
N
N
Cbz
O
NH2
O
N
N
16
H
H
10i
a
Dioxane, H2O, NaOH, Cbz–NCS, 12 h, RT
DCM, NEt3, Cbz–NCS, 2–3 h, RT
b
guanidinium carboxylate hosts exclude significant intra-
molecular proton transfer in water or DMSO.
Ester 23 was investigated for comparison by emission
titration with tetrabutylammonium acetate in methanol and
methanol/water (4:1 v/v) and by NMR titration in DMSO.
Binding constants of K = 1.1 9 104 dm3/mol in methanol
and of K = 4.2 9 103 dm3/mol in methanol/water 4:1 were
observed. The stoichiometry of all binding processes is 1:1
as determined by Job plot analysis. NMR titration in DMSO
gave a binding constant of K = 1.3 9 103 dm3/mol, which
is in good agreement with literature values for tetrabutyl-
ammonium acetate binding by alkyl-guanidines [108].
Compounds 15, 20, 23, and 24 show absorption maxima
in methanol at 270 nm and emit upon excitation at 340 nm
with quantum yield of about / = 0.1. [All quantum yields
determined with quinine disulfate in 1 N H2SO4 as the
reference compound (/ = 0.546)]. The absorption and
emission properties are only marginally affected by the
protection groups (Fig. 3).
The binding of compounds 6 and 24 to n-butylammo-
nium chloride and to tetrabutylammonium acetate were
investigated by NMR and emission titrations. Figure 4
shows the chemically induced shift for the NMR titration
of 24 with tetrabutylammonium acetate in DMSO (left) and
the emission titration with a Job plot analysis in methanol
(right). Nonlinear fitting of the titration data gives a bind-
ing constant of K = 1,780 dm3/mol (1,540 dm3/mol for
compound 6) from the NMR data and of K = 2,320 dm3/
mol from the emission data. The addition of n-butylam-
monium chloride does not result in significant changes in
resonance or emission properties under the experimental
conditions. The same applies to the addition of excess
sodium or potassium ions. This indicates that in solution an
interaction with the acetate anion occurs, but no binding of
the ammonium ion as predicted from the gas-phase cal-
culations. The emission titrations were extended in
MeOH:H2O 4:1 to the amino acids glycine, b-alanine, and
gamma-aminobutyric acid (GABA). The acetate ion bind-
ing was determined as K = 1,220 dm3/mol under these
conditions, while the affinity of all amino acids was sig-
nificantly lower at K = 200–250 dm3/mol (see supporting
info for titration data).
Conclusions
Symmetrically substituted biscarboxy guanidinium salts are
accessible from unnatural glycol-d-amino acids. While gas-
phase calculations predicted intramolecular guanidinium–
carboxylate interactions leading to a possible ammonium
ion binding site with similar structure as observed in the
glutaminase subunit HisF, measurements in DMSO and
methanol revealed intermolecular binding between the
guanidinium moiety and added acetate ions. No interaction
with n-butylammonium ions could be detected in these
solvents. Compounds 6 and 24 are, in contrast to the pro-
tein’s ammonium ion binding site, very flexible in their
structure and are not pre-organized. The formation of the
calculated aggregate with an ammonium ion is energetically
favored in the absence of stabilizing solvents due to charge
neutralization. However, in solution the significant loss in
entropy upon aggregate formation may be energetically
prohibitive and can only be overcome in more rigid and
suitably pre-organized receptor structures.
123

¨
A. Spath et al.
1294
Table 3 Preparation of 1,3-symmetrically disubstituted guanidines
+
R
NH3
10
R
S
O
N
O
R
R
N
H
N
H
O
Ph
EDC or Hg2+, NEt3
DMF, CHCl3
RT
N
H
N
H
O
Ph
12-16
17-21
Entry
1
Amine
Cbz–thiourea
Cbz–guanidine
Yield/%
73
12b
O
O
Cbz
O
N
O
O
O
+
O
NH3
17b
O
O
O
EtO
N
N
H
OEt
10b
H
2
3
12c
13b
78
78
Cbz
O
N
O
+
O
NH3
O
tBu
O
N
N
H
OtBu
17a
H
10c
O
O
Cbz
+
NH3
N
O
O
18
2
2
N
N
H
H
10e
O
O
O
4
14b
76
O
Cbz
H
N
O
N
H
N
H
N
+
O
NH3
19
20
O
N
H
N
H
O
O
O
O
10g
5
15
87
O
Cbz
H
N
N
H
N
H
O
O
NH2
O
N
N
H
N
H
O
O
10h
6
16
66
O
O
Cbz
O
HN
N
O
NH2
21
N
N
10i
DMF, CHCl3, NEt3, EDC or HgCl2, RT, overnight
Experimental
with a Bio-Rad FT-IR Excalibur FTS 3000. Ultraviolet
(UV) spectra were recorded on a Cary 50 BIO spectrometer
with temperature control at 25 °C. Electrospray mass
spectra were performed on a Finnigan MAT TSQ 7000 ESI
spectrometer. Other mass spectra were recorded on Varian
CH-5 (EI), Finnigan MAT 95 (CI; FAB and FD). Xenon
Analytical characterization of the synthesized compounds
was done by common methods. Melting points were
determined on Bu¨chi SMP or a Lambda Photometrics
OptiMelt MPA 100. Infrared (IR) spectra were recorded
123

Synthesis and binding properties of guanidinium biscarboxylates
1295
1.) H2, Pd/C
EtOH
+
NH2
Cbz
O
O
O
N
O
R3
O
O
R3
R3
O
O
R3
2.) NH4PF6
EtOH
O
N
H
N
H
O
O
N
N
O
H
H
92 - 95%
22a R3 = t-Bu
22b R3 = Et
17a R3 = t-Bu
17b R3 = Et
HCl(aq), EtOH
quant.
6
R3 = H
1.) H2, Pd/C
MeOH
+
NH2
Cbz
N
H
H
N
H
N
H
N
O
N
O
O
O
2.) NH4PF6
MeOH
N
H
N
H
N
N
H
H
R
R
O
R
R
O
O
O
quant.
20 R = Me
23 R = Me
LiOH(aq), THF
quant.
R = Li
24
Scheme 3
served as the ionization gas for fast-atom bombardment
(FAB).
from special spectroscopic purity purchased from Acros
or Baker or Uvasol from Merck. Millipore water (18 MX,
Milli Q Plus) was used; the 4-(2-hydroxyethyl)-1-piper-
azineethanesulfonic acid (HEPES) buffer was of according
purity, suitable for biochemical optical screenings. If not
otherwise specified, tetraethylammonium hydroxide or
0.1 M hydrochloric acid, both of analytical grade, were used
to adjust the pH in the titrations and screenings.
NMR spectra were recorded on Bruker Avance 600
(1H: 600.1 MHz, 13C: 150.1 MHz), Bruker Avance 400 (1H:
400.1 MHz, 13C: 100.6 MHz), or Bruker Avance 300 (1H:
300.1 MHz,13C:75.5 MHz)at300 K.Characterizationofthe
signals: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet;
bs, broad singlet; dd, double doublet; dt, double triplet; ddd,
double double doublet. Integration was determined as the
relativenumberofatoms, andthe couplingconstantsare given
in Hertz (Hz). The multiplicity of the carbon atoms is given
as (?) = CH3 or CH, (-) = CH2 and (Cquat) for quater-
nary carbon atoms. Structural assignments are based on
distortionless enhanced polarization transfer (DEPT) and
correlation spectroscopy (COSY) experiments where appli-
cable. Error of reported values: chemical shift: 0.01 ppm for
1H NMR, 0.1 ppm for 13C NMR, and 0.1 Hz for coupling
constants. The solvent used is reported for each spectrum.
Analytical thin-layer chromatography (TLC) plates
(silica gel 60 F254) and silica gel 60 (70–230 or 230–400
mesh) were used for chromatographic separations. Visu-
alization of the spots was by UV light and/or staining with
phosphomolybdate or ninhydrine, both in ethanol. DMF,
CH3CN, CHCl3, THF, and Et2O were dried by standard
procedures and stored over molecular sieves. PE refers to
petroleum ether with a boiling range of 70–90 °C; EA
refers to ethyl acetate. All other solvents and chemicals
were of reagent grade and used without further purification.
All test substances were of pro analysis grade, checked by
NMR or HPLC, and used as purchased without further puri-
fication. The solvents for fluorescence measurements were
1-N-Boc-ethanolamine and 1-N-Cbz–ethanolamine are
commercially available. N-Boc-L-leucine methyl ester [79],
N-Boc-L-tryptophane methyl ester [79], N-Boc-L-phenylala-
ninemethylester[79], 3-[2-(N-Boc-amino)ethoxy]propionic
acid ethyl ester [109], glycylglycine methyl ester hydro-
chloride [79], 2,6-pyridinedicarboxylic acid ethyl ester
[79], 5-[(N-Boc-amino)-(4-methylphenylsulfonyl)methyl]-
pyrrole-2-carboxylic acid methyl ester [110–112], 5-[(N-
Boc-amino)methyl] pyrrole-2-carboxylic acid methyl ester
[110–112], and 5-(aminomethyl)pyrrole-2-carboxylic acid
methyl ester [110–112] were prepared according to known
procedures.
General procedure 1 (GP 1): reduction of Boc-amino
acids with NaBH4 [113]
The corresponding N-Boc-protected amino acid methyl
ester (2.00 g) was dissolved in 30.0 cm3 dry THF in
nitrogen atmosphere, and NaBH4 and LiCl were subse-
quently added in small portions. The mixture was stirred
overnight at room temperature under moisture protection.
After having added 20.0 cm3 water slowly, 1.2 cm3 acetic
acid (20.0 mmol) was dropped in. After short stirring, the
123

¨
A. Spath et al.
1296
Fig. 4 Binding studies of 24 with tetrabutylammonium acetate by 1H
NMR (chost = 3.33 9 10-3mol/dm3) in DMSO (left) and fluores-
cence spectroscopy (chost = 3 9 10-5mol/dm3) in methanol (right);
insert: Job’s plot
Fig. 3 Absorption spectra of compounds 15, 20, 23, and 24
(c = 8 9 10-5 mol/dm3) and the emission spectrum of compound
24 (c = 4 9 10-5 mol/dm3) in methanol
2-(tert-Butoxycarbonylamino)-3-phenylpropan-1-ol
(C14H21NO3)
THF was removed under reduced pressure. A 1:1 mixture
of water and diethyl ether (50.0 cm3 each) was added; the
solid was completely dissolved. The organic phase was
separated, and the aqueous layer was extracted with diethyl
ether (3 9 50 cm3). The combined ether phases were dried
over MgSO4, and the solvent was evaporated under
reduced pressure. The residue was purified by column
chromatography (ethyl acetate/petroleum ether 1:2).
N-Boc-L-phenylalanine methyl ester (2.00 g, 6.6 mmol)
was reduced with 0.55 g NaBH4 (14.5 mmol) and 0.62 g
LiCl (14.5 mmol) to give 1.71 g of an off-white solid
1
(6.2 mmol, 94%). M.p.: 88–89 °C; H-NMR (300 MHz,
CDCl3): d = 1.40 (s, 9 H), 2.76 (m, 1 H), 2.88 (d, 2 H,
J = 6.0 Hz), 3.49–3.70 (m, 2 H), 3.87 (bs, 1 H), 4.86 (bs, 1
H), 7.19–7.28 (m, 3 H), 7.28–7.36 (m, 2 H) ppm; MS (ESI–
MS, CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) =
252.3 (100, MH?).
2-(tert-Butoxycarbonylamino)-4-methylpentan-1-ol
(C11H23NO3)
N-Boc-L-leucine methyl ester (2.00 g, 7.72 mmol) was
reacted with 0.65 g NaBH4 (17 mmol) and 0.72 g LiCl
(17 mmol) to obtain 1.66 g colorless oil (7.2 mmol, 87%). 1H-
NMR (300 MHz, CDCl3): d = 0.86 (d, 6 H, J = 6.0 Hz),
1.22 (m, 2 H), 1.40 (s, 9 H), 1.61 (m, 1 H), 3.46 (m, 1 H), 3.68
(bs, 2 H), 4.78 (bs, 2 H) ppm; MS (ESI–MS, CH2Cl2/
MeOH ? 10 mmolNH4OAc): m/z(%) = 218.2(100, MH?).
2-(tert-Butoxycarbonylamino)-3-(1H-indol-3-yl)propan-1-
ol (C16H22N2O3)
N-Boc-L-tryptophan methyl ester (2.00 g, 5.85 mmol) was
reacted with 0.49 g NaBH4 (12.9 mmol) and 0.55 g LiCl
(12.9 mmol) to obtain 1.74 g of a white solid (5.5 mmol,
93%). M.p.: 119–120 °C; 1H-NMR (300 MHz, CDCl3):
123

Synthesis and binding properties of guanidinium biscarboxylates
1297
d = 1.40 (s, 9 H), 2.79 (m, 1 H), 2.99 (d, 2 H, J = 6.0 Hz),
3.51–3.72 (m, 2 H), 4.00 (bs, 1 H), 4.89 (bs, 1 H), 7.01 (m,
1 H), 7.06–7.24 (m, 2 H), 7.34 (app. d, 1 H), 7.68 (app. d, 1
portions in 11.5 cm3 well-stirred SOCl2 under moisture
protection, not allowing the temperature to rise over 0 °C.
After 90 min the solution was allowed to reach room
temperature and the excess of SOCl2 was removed under
reduced pressure without heating. Toluene (20.0 cm3) was
added to the oily residue, and the solution was washed with
cold aqueous 1 M NaHCO3 (2 9 10.0 cm3) and dried
over MgSO4. Evaporation of the solvent afforded 6.5 g
2-(chloromethyl)pyridine-6-carboxylic acid ethyl ester
(32.5 mmol, 83%) as an orange oil.
H), 8.29 (bs,
1 H)ppm; MS (ESI–MS, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 291.2 (100,
MH?).
3-[1-(Benzyloxycarbonyl)-1H-indol-3-yl]-2-(tert-butoxy-
carbonylamino)-propan-1-ol (C24H28N2O5)
Powdered sodium hydroxide (120 mg, 3.0 mmol) was added
to a solution of 290 mg N-Boc-L-tryptophanol (1.0 mmol)
and 290 mg tetrabutylammonium hydrogen sulfate (0.6
mmol) in 5.0 cm3 dried CH2Cl2, and the mixture was stirred
for 2.5 h at room temperature. Benzylchloroformate
(412 mg, 2.4 mmol) was then added, and the mixture was
allowed to stir for 20 h. After dilution with 10.0 cm3 ethyl
acetate and stirring for 0.5 h, it was washed three times with
5.0 cm3 water. After being dried over MgSO4, the solvent
was removed under reduced pressure. The residue was
purified by flash chromatography (silica gel, ethyl acetate/
petroleum ether 1:3) to afford the title compound as a white
solid (380 mg, 0.921 mmol, 92%). M.p.: 76–78 °C; 1H
NMR (300 MHz, CDCl3): d = 1.47 (s, 9 H), 3.03 (m, 2 H),
4.16 (m, 2 H), 4.25 (m, 1 H), 4.70 (m, 1 H), 4.94 (bs, 1 H),
5.20 (s, 2 H), 6.95 (m, 1 H), 7.12 (m, 1 H), 7.21 (m, 1 H),
7.28–7.46 (m, 6 H), 7.65 (m, 1 H) ppm; IR (FT-IR, film):
2-(Chloromethyl)pyridine-6-carboxylic acid ethyl ester
(6.0 g, 30 mmol) in 10.0 cm3 anhydrous DMF was slowly
added to a solution of 6.12 g sodium phthalimide
(36 mmol) in 10.0 cm3 dry DMF. After stirring for 2 h at
room temperature the reaction mixture was centrifuged, the
solvent was removed under reduced pressure, and the
residue was dissolved in 100.0 cm3 CHCl3. The resulting
solution was washed with 0.2 M NaOH (2 9 100.0 cm3),
then with water, and dried. Removal of the solvent yielded
a solid residue, which was dissolved in 500.0 cm3 warm
ethanol. Hydrazine (1.13 g, 35 mmol) was added, and the
mixture was refluxed until disappearance of the starting
material (monitored by TLC). The mixture was cooled to
5 °C and filtered, and the solvent was removed to finally
obtain 4.05 g of yellow syrup (22.5 mmol, 75%).
ꢀ
m = 3,346 (bm), 2,976 (m), 2,886 (m), 1,726 (m), 1,689 (s),
2-[2-(Benzyloxycarbonylamino)ethoxy]acetic acid
tert-butyl ester (7c, C16H23NO5)
1,528 (m), 1,454 (m), 1,397 (m), 1,367 (m), 1,254 (s), 1,162
(s), 1,067 (m), 1,019 (m), 941 (m), 896 (m), 852 (m),
791 (m), 739 (s), 697 (s), 660 (m), 593 (m), 556
(m) cm-1; MS (ESI, CH2Cl2/MeOH ? 10 mmol NH4OAc):
m/z (%) = 425.1 (48, M ? H?), 442.0 (25, M ? NH4?),
849.3 (100, 2M ? H?), 866.4 (11, 2M ? NH?4 ).
To a solution of 2.0 g 2-(benzyloxycarbonylamino)ethanol
(12.5 mmol) in 60 cm3 toluene, 5.73 g tert-butylbromo-
acetate (25.0 mmol) and 2.12 g tetrabutylammonium
hydrogen sulfate (6.25 mmol) were added. The reaction
mixture was vigorously stirred, and 30 cm3 30% NaOH
was slowly added. After 12 h of stirring at room temper-
ature, another portion of tert-butylbromoacetate (1.15 g,
5.0 mmol) was added. Stirring was continued for 6 h, then
the organic phase was separated. It was washed with
20 cm3 5% aqueous acetic acid and with 3 9 20 cm3 of
water. After drying over MgSO4 the solvent was removed
at reduced pressure. The excess of tert-butylbromoacetate
was evaporated in vacuo, and the oily raw material was
purified by column chromatography with ethyl acetate/
petroleum ether 1:4 to yield the benzyloxycarbonylamino
acid ester as clear, colorless oil (3.36 g, 10.86 mmol,
Syntheses and deprotections of the building blocks
6-(Hydroxymethyl)pyridine-2-carboxylic acid
ethyl ester [114]
2,6-Pyridinedicarboxylic acid diethyl ester (27.9 g,
0.125 mol) and 3.8 g NaBH4 (0.1 mol) were dissolved in
250 cm3 dry THF and refluxed for 2 h under moisture
protection. The solvent was removed, and 50.0 cm3 water
was added. After stirring for 10 min, the mixture was
extracted with CHCl3 (3 9 50.0 cm3). The organic phases
were combined and dried over MgSO4, and the solvent was
evaporated under reduced pressure. The residue was
purified by flash chromatography on silica gel with ethyl
acetate/ethanol 6:1 ? 3:1 to give the product as colorless
solid (15.39 g, 85.02 mmol, 85%).
1
87%). H NMR (300 MHz, CDCl3): d = 1.41 (s, 9 H),
3.36 (t, 2 H, J = 5.2 Hz), 3.54 (t, 2 H, J = 5.2 Hz), 3.91
(s, 2 H), 5.07 (s, 2 H), 5.48 (bs, 1 H), 7.26–7.32 (m, 5H)
ppm; 13C NMR (75 MHz, CDCl3): d = 27.1 (?, 3 C), 40.0
(-, 1 C), 64.1 (-, 1 C), 65.6 (-, 1 C), 67.7 (-, 1 C), 69.6
(-, 1 C), 80.7 (Cquat, 1 C), 127.0 (?, 1 C), 127.7 (?, 2 C),
128.5 (?, 2 C), 135.6 (Cquat, 1 C), 155.5 (Cquat, 1 C),
6-(Aminomethyl)pyridine-2-carboxylic acid ethyl
ester [115]
ꢀ
168.7 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,351 (bm),
2,978 (m), 2,937 (m), 2,886 (m), 1,713 (s), 1,518 (m),
Thoroughly dried 6-(hydroxymethyl)pyridine-2-carboxylic
acid ethyl ester (5.8 g, 35 mmol) was dissolved in small
123

¨
A. Spath et al.
1298
1,455 (m), 1,368 (m), 1,228 (s), 1,131 (s), 1,026 (m), 915
(Cquat, 1 C), 166.2 (Cquat, 1 C) ppm; IR (FT-IR, film):
(m), 845 (m), 798 (m), 735 (m), 698 (m), 584 (m) cm-1
;
m = 3,354 (bm), 2,959 (m), 2,934 (m), 2,874 (m), 1,748
ꢀ
MS (CI, NH3): m/z (%) = 219.2 (90, M ? NH?4 –C7H8O),
254.2 (9, M ? H?–C4H8), 271.2 (63, M ? NH?4 –C4H8),
310.2 (9, M ? H?), 327.2 (100, M ? NH?4 ).
(m), 1,710 (s), 1,503 (m), 1,457 (m), 1,387 (m), 1,367 (s),
1,295 (m), 1,228 (s), 1,164 (s), 1,128 (s), 1,049 (m), 941
(m), 846 (m), 778 (m), 751 (m) cm-1; MS (CI, NH3):
m/z
(%) = 219.1
(13,
M ? H?–2C4H8),
237.0
General procedure 2 (GP 2): phase-transfer catalytic
etherification of substituted amino alcohols
(20, M ? NH?4 –2C4H8), 275.2 (48, M ? H?–C4H8),
293.2 (69, M ? NH?4 –C4H8), 332.2 (100, M ? H?),
349.2 (83, M ? NH?4 ).
The N-Boc-amino alcohol was dissolved with 98.8 mg tet-
rabutylammonium hydrogen sulfate (0.4 mmol) in 6.0 cm3
CH2Cl2. Aqueous NaOH (5.0 M, 6.0 cm3) was added,
233 mg bromoacetic acid tert-butyl ester (1.2 mmol) in
1.0 cm3 CH2Cl2 was dropped in, and the reaction mixture
was vigorously stirred for 3 h at room temperature. The same
amount bromoacetic acid tert-butyl ester was added again,
and the heterogeneous solution was stirred overnight. The
mixture was poured on 10 g ice in a separation funnel,
the organic phase was separated, and the aqueous layer
was extracted with dichloromethane (3 9 10.0 cm3). The
organic phases were combined and dried over MgSO4, and
the solvent was removed under reduced pressure. The
crude product was purified by column chromatography
with ethylacetate/petroleum ether 1:4 to obtain the pure
compound.
2-[2-(tert-Butoxycarbonylamino)-3-phenylpropoxy]acetic
acid tert-butyl ester (7g, C20H31NO5)
N-Boc-L-phenylalaninol (250 mg, 1.0 mmol) was employed
to yield 328 mg 7g as a colorless wax (0.89 mmol, 89%). 1H
NMR (300 MHz, CDCl3): d = 1.33 (s, 9 H), 1.39 (s, 9 H),
2.82 (m, 2 H), 3.37 (m, 2 H), 3.65 (m, 1 H), 3.87 (s, 2 H), 5.17
(bs, 1 H), 7.18–7.30 (m, 5 H) ppm; 13C NMR (75 MHz,
CDCl3): d = 28.1 (?, 3 C), 28.4 (?, 3 C), 37.1 (-, 1 C), 51.8
(?, 1 C), 68.5 (-, 1 C), 71.6 (-, 1 C), 79.2 (Cquat, 1 C), 82.9
(Cquat, 1 C), 126.3 (?, 1 C), 128.4 (?, 2 C), 129.5 (?, 2 C),
138.3 (Cquat, 1 C), 155.5 (Cquat, 1 C), 166.3 (Cquat, 1 C)
ꢀ
ppm; IR (FT-IR, film): m = 3,375 (bm), 2,978 (m), 2,932
(m), 2,880 (m), 1,746 (m), 1,710 (s), 1,497 (m), 1,456 (m),
1,389 (m), 1,367 (m), 1,294 (m), 1,230 (m), 1,163 (s), 1,130
(s), 1,058 (m), 943 (m), 917 (m), 846 (m), 742 (m),
701 (m) cm-1; MS (CI, NH3): m/z (%) = 253.1 (14,
M ? H?–2C4H8), 271.0 (21, M ? NH4?–2C4H8), 309.1
(82, M ? H?–C4H8), 327.1 (63, M ? NH4?–C4H8), 366.2
(100, M ? H?), 383.2 (78, M ? NH?4 ); HRMS (EI, 70 eV):
calc. for C20H31NO5*? 365.2202, found 365.2197.
2-[2-(tert-Butoxycarbonylamino)ethoxy]acetic acid
tert-butyl ester (7a, C13H25NO5)
N-Boc-ethanolamine (192 mg, 1.2 mmol) was converted
according to GP 2 to give 7a as colorless oil (273 mg,
0.996 mmol, 83%). 1H NMR (300 MHz, CDCl3): d = 1.37
(s, 9 H), 1.42 (s, 9 H), 3.26 (m, 2 H), 3.53 (t, J = 5.2 Hz, 2
H), 3.90 (s, 2 H), 5.12 (br s, 1 H) ppm; 13C NMR (75 MHz,
CDCl3): d = 28.1 (?, 3 C), 28.4 (?, 3 C), 40.4 (-, 1 C),
68.7 (-, 1 C), 70.7 (-, 1 C), 79.1 (Cquat, 1 C), 81.8
(Cquat, 1 C), 156.0 (Cquat, 1 C), 169.6 (Cquat, 1 C) ppm;
2-[3-[1-(Benzyloxycarbonyl)-1H-indol-3-yl]-2-(tert-but-
oxycarbonyl-amino)-propoxy]acetic acid tert-butyl ester
(7i, C30H38N2O7)
3-[1-(Benzyloxycarbonyl)-1H-indol-3-yl]-2-(tert-butoxy-
carbonylamino)-propan-1-ol (298 mg, 0.7 mmol) was used
1
to give 236 mg 7i as colorless oil (0.441 mmol, 63%). H
ꢀ
IR (FT-IR, film): m = 3,371 (bm), 2,978 (m), 2,932 (m),
1,761 (m), 1,714 (s), 1,509 (m), 1,486 (m), 1,416 (m),
NMR (300 MHz, CDCl3): d = 1.41 (s, 9 H), 1.43 (s, 9 H),
3.06 (m, 2 H), 3.46 (m, 1 H), 3.98 (s, 2 H), 4.11 (m, 2 H),
4.61 (m, 2 H), 5.19 (bs, 1 H), 6.98 (m, 1 H), 7.10 (m, 1 H),
7.19 (m, 1 H), 7.29–7.41 (m, 7 H) ppm; 13C NMR
(75 MHz, CDCl3): d = 28.1 (?, 3 C), 28.4 (?, 3 C), 48.5
(-, 1 C), 51.0 (?, 1 C), 67.7 (-, 1 C), 68.5 (-, 1 C), 68.9
(-, 1 C), 79.1 (Cquat, 1 C), 81.8 (Cquat, 1 C), 108.9 (?, 1
C), 111.9 (Cquat, 1 C), 119.4 (?, 1 C), 121.9 (?, 1 C),
127.4 (?, 1 C), 127.9 (-, 1 C), 128.1 (?, 2 C), 128.4 (?, 2
C), 128.6 (?, 1 C), 136.8 (Cquat, 1 C), 137.4 (Cquat, 1 C),
155.6 (Cquat, 1 C), 167.6 (Cquat, 1 C), 169.0 (Cquat, 1 C),
1,367 (m), 1,250 (s), 1,202 (s), 1,137 (s), 762 (m) cm-1
;
MS (CI, NH3): m/z (%) = 181.1 (43, M ? NH?4 –2C4H8),
219.2 (24, M ? H?–C4H8), 237.2 (100, M ? NH?4 –C4H8),
276.2 (46, M ? H?), 293.3 (60, M ? NH?4 ).
2-[2-(tert-Butoxycarbonylamino)-4-methylpentoxy]acetic
acid tert-butyl ester (7e, C17H33NO5)
N-Boc-L-leucinol (217 mg, 1.0 mmol) was reacted to
obtain 256 mg of a white oily solid (0.77 mmol, 77%).
1H NMR (300 MHz, CDCl3): d = 0.86 (d,
6 H,
J = 7.2 Hz), 1.36 (m, 2 H), 1.38 (s, 9 H), 1.41 (s, 9 H),
1.61 (m, 1 H), 3.43 (m, 2 H), 3.70 (m, 1 H), 3.89 (s, 2 H),
4.83 (bs, 1 H) ppm; 13C NMR (75 MHz, CDCl3): d = 22.4
(?, 1 C), 22.9 (?, 1 C), 24.8 (?, 1 C), 28.1 (?, 3 C), 28.4
(?, 3 C), 41.0 (-, 1 C), 48.5 (?, 1 C), 68.4 (-, 1 C), 73.8
(-, 1 C), 78.9 (Cquat, 1 C), 82.8 (Cquat, 1 C), 155.6
ꢀ
169.6 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,061 (bm),
2,977 (m), 2,933 (m), 1,745 (s), 1,491 (m), 1,468 (m),
1,389 (m), 1,367 (m), 1,299 (m), 1,227 (s), 1,189 (s), 1,128
(s), 1,050 (m), 943 (m), 847 (m), 803 (m), 740 (m), 699
(m), 584 (m) cm-1; MS (CI, NH3): m/z (%) = 539.1 (33,
123

Synthesis and binding properties of guanidinium biscarboxylates
1299
M ? H?), 556.1 (36, M ? NH?4 ), 577.1 (100, M ? K?),
599.1 (64, M ? H? ? HOAc).
(m) cm-1
;
MS (ESI, CH2Cl2/MeOH ? 10 mmol
NH4OAc): m/z (%) = 304.2 (100, M ? H?).
2-[2-(tert-Butoxycarbonylamino)-3-phenylpropoxy]acetic
acid ethyl ester (7h, C18H27NO5)
General procedure 3 (GP 3): NaH-catalyzed ether
synthesis
N-Boc-L-phenylalaninol (1.25 g, 5.0 mmol) was employed
to yield 820 mg of the product as a faintly yellow waxy
solid (2.44 mmol, 49%). 1H NMR (300 MHz, CDCl3):
d = 1.27 (t, 3 H, J = 7.1 Hz), 1.41 (s, 9 H),2.86 (m, 2 H),
3.42 (m, 2 H), 3.81 (s, 2 H), 4.07 (m, 1 H), 4.22 (q, 2 H,
J = 7.1 Hz), 5.17 (bs, 1 H), 7.13–7.31 (m, 5 H) ppm; 13C
NMR (75 MHz, CDCl3): d = 14.2 (?, 1 C), 28.3 (?, 3 C),
37.7 (-, 1 C), 50.8 (?, 1 C), 61.0 (-, 1 C), 66.6 (-, 1 C),
68.5 (-, 1 C), 79.4 (Cquat, 1 C), 126.8 (?, 1 C), 128.4 (?,
2 C), 129.3 (?, 2 C), 138.2 (Cquat, 1 C), 155.5 (Cquat, 1
In a nitrogen-flushed Schlenckflask, N-Boc-amino alcohol
(5.0 mmol) was added to a suspension of 380 mg NaH
(60% susp., 8.0 mmol) and 150 mg KI (0.8 mmol) in
30.0 cm3 dry THF at 0 °C. After dropwise addition of
1.67 g ethyl bromoacetate (10.0 mmol) in 10.0 cm3 dry
THF, the reaction mixture was stirred for 4 h at room
temperature. The NaH suspension and precipitated solids
were settled by a centrifuge, and the THF solution was
decanted off and evaporated to give the crude product. The
residue was purified by column chromatography (petro-
leum ether/ethyl acetate, 9:1 ? 4:1) to give the
corresponding glycol-d-amino acid ester.
ꢀ
C), 167.1 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,366
(bm), 2,978 (m), 2,933 (m), 1,743 (m), 1,703 (s), 1,498 (s),
1,453 (m), 1,388 (m), 1,366 (m), 1,276 (m), 1,247 (m),
1,208 (m), 1,163 (s), 1,132 (s), 1,057 (m), 1,026 (m), 851
(m), 742 (m), 701 (m) cm-1; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 338.1 (100,
M ? H?).
2-[2-(tert-Butoxycarbonylamino)ethoxy]acetic acid ethyl
ester (7b, C11H21NO5)
N-Boc-ethanolamine (800 mg, 5.0 mmol) was reacted
according to GP 3 to give 7b as faintly yellow oil
(727 mg, 2.955 mmol, 59%). 1H NMR (300 MHz, CDCl3):
d = 1.23 (t, 3 H, J = 7.1 Hz), 1.38 (s, 9 H), 3.28 (m, 2 H),
3.54 (t, 2 H, J = 5.2 Hz), 4.02 (s, 2 H), 4.15 (q, 2 H,
J = 7.1 Hz), 5.11 (bs, 1 H) ppm; 13C NMR (75 MHz,
CDCl3): d = 14.2 (?, 1 C), 28.4 (?, 3 C), 40.4 (-, 1 C),
61.0 (-, 1 C), 68.3 (-, 1 C), 70.8 (-, 1 C), 79.2 (Cquat, 1
C), 156.0 (Cquat, 1 C), 170.4 (Cquat, 1 C) ppm; IR (FT-IR,
2-[3-[1-(Benzyloxycarbonyl)-1H-indol-3-yl]-2-(tert-
butoxycarbonylamino)propoxy]acetic acid
ethyl ester (7j, C28H34N2O7)
3-[1-(benzyloxycarbonyl)-1H-indol-3-yl]-2-(tert-butoxy-
carbonyl-amino)-propan-1-ol (298 mg, 0.7 mmol) was
reacted to give 64 mg as colorless waxy solid
(0.126 mmol, 18%). 1H NMR (300 MHz, CDCl3):
d = 1.23 (t, 3 H, J = 7.1 Hz), 1.39 (s, 9 H), 3.08 (m, 2
H), 3.47 (m, 1 H), 3.96 (s, 2 H), 4.11 (m, 2 H), 4.15 (q, 2 H,
J = 7.1 Hz), 4.63 (m, 2 H), 5.17 (bs, 1 H), 6.97 (m, 1 H),
7.12 (m, 1 H), 7.18 (m, 1 H), 7.26–7.43 (m, 7 H) ppm; IR
ꢀ
film): m = 3,374 (bm), 2,978 (m), 2,935 (m), 1,761 (m),
1,706 (s), 1,511 (m), 1,455 (m), 1,367 (m), 1,248 (m),
1,206 (s), 1,136 (s), 1,025 (s), 864 (m), 782 (m), 717 (m),
581 (m) cm-1; MS (CI, NH3): m/z (%) = 192.1 (98,
M ? H?–C4H8), 248.2 (16, M ? H?), 265.2 (40,
M ? NH?4 ); HRMS (PI-LSIMS FAB, glycerol): calc. for
ꢀ
(FT-IR, film): m = 3,060 (bm), 2,981 (m), 2,936 (m), 2,889
(m), 1,743 (s), 1,493 (m), 1,470 (m), 1,386 (m), 1,368 (m),
1,301 (m), 1,225 (s), 1,189 (s), 1,123 (s), 1,054 (m), 944
?
C11H22NO5 248.1498, found 248.1493.
(m), 846 (m), 805 (m), 743 (m), 699 (m) cm-1
.
2-[2-(tert-Butoxycarbonylamino)-4-methylpentoxy]acetic
acid ethyl ester (7f, C15H29NO5)
N-Boc-L-leucinol (1.09 g, 5.0 mmol) was reacted to obtain
Alternative preparation of 7b catalyzed by potassium
tert-butylate
1
790 mg 7f as a white oily solid (2.62 mmol, 52%). H
NMR (300 MHz, CDCl3): d = 0.84 (d, 6 H, J = 7.1 Hz),
1.23 (t, 3 H, J = 7.1 Hz), 1.38 (s, 9 H), 1.39 (m, 2 H), 1.61
(m, 1 H), 3.48 (m, 2 H), 3.72 (m, 1 H), 4.01 (s, 2 H), 4.16
(q, 2 H, J = 7.1 Hz), 4.81 (bs, 1 H) ppm; 13C NMR
(75 MHz, CDCl3): d = 14.1 (?, 1 C), 22.1 (?, 1 C), 22.9
(?, 1 C), 24.7 (?, 1 C), 25.7 (?, 1 C), 28.4 (?, 3 C), 40.6
(-, 1 C), 47.7 (-, 1 C), 61.1 (-, 1 C), 68.2 (-, 1 C), 71.4
(-, 1 C), 79.5 (Cquat, 1 C), 155.4 (Cquat, 1 C), 167.2
A 100-cm3 Schlenckflask was evacuated and then filled
with nitrogen several times. To a solution of 0.80 g N-Boc-
ethanolamine (5.0 mmol) in 10.0 cm3 dry THF were added
1.14 cm3 ethyl bromoacetate (10.0 mmol) and 1.39 g
potassium tert-butylate (12.0 mmol) in small portions. The
mixture changed color to orange. The mixture was stirred
for 3 h at 0 °C. The base suspension was settled by a
centrifuge, the THF solution was decanted off, and the
solvent was evaporated. The crude orange oil was purified
by column chromatography (ethyl acetate/petroleum ether
1:1). The product (Rf = 0.5, EE/PE 1:1) was isolated as a
slightly yellowish oil (0.58 g, 2.34 mmol, 47%).
ꢀ
(Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,354 (bm), 2,958
(m), 2,921 (m), 2,874 (m), 1,739 (m), 1,690 (s), 1,517 (m),
1,454 (m), 1,388 (m), 1,366 (m), 1,277 (s), 1,251 (m),
1,163 (s), 1,115 (m), 1,049 (m), 1,003 (m), 870 (m), 780
123

¨
A. Spath et al.
1300
Synthesis of 7b using azoacetic acid ethyl ester
and copper(I)
Example procedure for hydrogenolytic deprotection:
2-(2-aminoethoxy)acetic acid tert-butyl ester,
hydroacetate (10c, C10H21NO5)
A solution of 1.60 g N-Boc-ethanolamine (10.0 mmol) in
30 cm3 dichloromethane (DCM) was cooled to 5 °C in a
nitrogen atmosphere. Copper(II) triflate (362 mg,
1.0 mmol, 10 mol%) and three drops of phenyl hydrazine
were added, immediately followed by 120 mg tetra-
methylethylenediamine (TMEDA)(1.0 mmol, 10 mol%).
After stirring for 30 min, 8.8 cm3 15% azoacetic acid
ethyl ester in DCM (1.71 g, 15.0 mmol) was slowly
added over a period of 6 h at 5 °C. The mixture was
allowed to warm up slowly to room temperature and was
stirred overnight. The solution was filtered over alumina
N, the filter cake was washed with dichloromethane, and
the solvent was evaporated. The remaining oil was puri-
fied by column chromatography (ethyl acetate/petroleum
ether 2:1 ? 1:1) to yield the product (1.93 g, 7.81 mmol,
78%) as a clear yellow oil (Rf = 0.3, ethyl acetate/
petroleum ether 1:2).
Compound 7c (1.55 g, 5.0 mmol) was dissolved in
20.0 cm3 methanol, three spatula tips of palladium on
charcoal (10% Pd) and 0.3 g acetic acid (5.0 mmol) were
added, and the mixture was stirred for 2 days in hydrogen
atmosphere (20 bar). The solution was filtered over Celite,
the filter cake was washed with small amounts of methanol,
and the solvent was removed under reduced pressure. The
product was isolated as clear, colorless oil (1.14 g,
4.84 mmol, 94%). 1H NMR (300 MHz, CDCl3):
d = 1.45 (s, 9 H), 2.00 (s, 3 H), 3.15 (t, 2 H,
J = 5.2 Hz), 3.77 (t, 2 H, J = 5.2 Hz), 3.99 (s, 2 H),
9.13 (bs, 3 H) ppm; 13C NMR (75 MHz, CDCl3): d = 22.0
(?, 1 C), 28.1 (?, 3 C), 39.5 (-, 1 C), 68.0 (-, 1 C), 68.4
(-, 1 C), 82.6 (Cquat, 1 C), 170.3 (Cquat, 1 C), 176.9
ꢀ
(Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,373 (bm), 2,977
(m), 2,929 (m), 1,737 (m), 1,666 (m), 1,556 (m), 1,398 (m),
1,369 (m), 1,241 (m), 1,128 (s), 1,014 (m), 839 (m), 721
(m), 652 (m) cm-1; MS (CI, NH3): m/z (%) = 120.1 (13,
M ? H?–C4H8), 176.1 (100, M ? H?).
2-[2-[2-(tert-Butoxycarbonylamino)ethoxy]ethoxy]acetic
acid ethyl ester (C13H25NO6)
A solution of 500 mg 2-[2-(tert-butoxycarbonylamino)eth-
oxy]ethanol (2.5 mmol) in 10.0 cm3 DCM was cooled to
5 °C in a nitrogen atmosphere. Copper(II) triflate (90 mg,
0.25 mmol, 10 mol%) and one drop of phenyl hydrazine
were added, followed by 40 mg TMEDA (0.25 mmol,
10 mol%). After stirring for 30 min, 2.20 cm3 15%
azoacetic acid ethyl ester in DCM (0.43 g, 3.8 mmol)
was dropped in over a period of 3 h at 5 °C. The mixture
was allowed to warm up slowly to room temperature and
was stirred overnight. The solution was filtered over
alumina N, the filter cake was washed with DCM, and
the solvent was evaporated. The raw material was purified
by column chromatography (ethyl acetate/petroleum ether
2:1 ? 1:1) to yield a yellow oil (468 mg, 1.69 mmol,
68%). Rf = 0.2 (EE/PE 1:2); 1H NMR (300 MHz, CDCl3):
d = 1.28 (t, 3 H, J = 7.1 Hz), 1.43 (s, 9 H), 3.31 (m, 2 H),
3.54 (t, 2 H, J = 5.2 Hz), 3.65 (m, 2 H), 3.71 (m, 2 H),
4.13 (s, 2 H), 4.21 (q, 2 H, J = 7.1 Hz), 5.03 (bs, 1 H)
ppm; 13C NMR (75 MHz, CDCl3): d = 14.2 (?, 1 C), 28.4
(?, 3 C), 40.4 (-, 1 C), 61.0 (-, 1 C), 61.2 (-, 1 C), 61.3
(-, 1 C), 68.7 (-, 1 C), 70.8 (-, 1 C), 79.1 (Cquat, 1 C),
156.0 (Cquat, 1 C), 170.4 (Cquat, 1 C) ppm; IR (FT-IR,
2-(2-Aminoethoxy)acetic acid ethyl ester, hydrochloride
(10b, C6H14ClNO3)
To a solution of 0.52 g 7b (2.0 mmol) in 10.0 cm3 dried
DCM, 4.0 cm3 diethyl ether saturated with HCl was slowly
added. After 3 h of stirring at room temperature, the
solvent was evaporated. The residue was taken up in a little
DCM, and the solvent was removed under reduced pressure
again. This process was repeated once. The remaining
sticky solid was dried under vacuum overnight. The yield
was 0.38 g of a yellow, deliquescent solid (1.96 mmol,
98%). 1H NMR (300 MHz, CDCl3): d = 1.26 (t, 3 H,
J = 7.1 Hz), 3.32 (m, 2 H), 3.91 (m, 2 H), 4.09–4.21 (q, 2
H, J = 7.1 Hz), 4.18 (s, 2 H), 6.50 (bs, 3 H) ppm; 13C
NMR (75 MHz, CDCl3): d = 14.1 (?, 1 C), 39.4 (-, 1 C),
61.3 (-, 1 C), 67.4 (-, 1 C), 68.2 (-, 1 C), 171.0 (Cquat, 1
ꢀ
C) ppm; IR (FT-IR, film): m = 3,470 (bm), 2,937 (m),
2,916 (m), 1,726 (m), 1,669 (s), 1,516 (m), 1,430 (m),
1,183 (s), 1,127 (s), 1,020 (m), 965 (m), 836 (m), 799 (s),
721 (m), 673 (m), 517 (m) cm-1; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 147.8 (100,
M ? H?), 294.9 (5, 2M ? H?).
The same result is achieved if trifluoroacetic acid (TFA)
is used instead of HCl in diethyl ether. The yields and the
appearance of the product are comparable. Both products
show no difference in reactivity. 10b, trifluoroacetate
ꢀ
film): m = 3,379 (bm), 2,977 (m), 2,953 (m), 1,749 (m),
1,705 (s), 1,515 (m), 1,455 (m), 1,366 (m), 1,248 (m),
1,202 (s), 1,117 (s), 907 (m), 863 (m), 779 (m), 718 (m),
579 (m), 530 (m) cm-1; MS (CI, NH3): m/z (%) = 192.2
(8, M ? H?–CO2–C4H8), 235.2 (100, M ? H?–C4H8),
253.1 (16, M ? NH4?–C4H8), 292.2 (4, M ? H?), 309.2
(6, M ? NH?4 ).
1
(C8H14F3NO5): H NMR (300 MHz, CDCl3): d = 1.28 (t,
3 H, J = 7.1 Hz), 3.32 (m, 2 H), 3.81 (t, 2 H, J = 5.2 Hz),
4.14 (s, 2 H), 4.21 (q, 2 H, J = 7.1 Hz), 7.78 (bs, 3 H)
ꢀ
ppm; IR (FT-IR, film): m = 3,460 (bm), 2,984 (m), 2,937
123

Synthesis and binding properties of guanidinium biscarboxylates
1301
(m), 1,720 (m), 1,671 (s), 1,519 (m), 1,428 (m), 1,378 (m),
1,184 (s), 1,127 (s), 1,021 (m), 966 (m), 836 (m), 799 (s),
721 (m), 677 (m), 596 (m), 517 (m), 495 (m) cm-1; MS
(CI, NH3, ?Q1MS): m/z (%) = 148.1 (100, M ? H?); MS
(CI, NH3, -Q1MS): m/z (%) = 146.0 (100, M - H?).
6-[3-(Benzyloxycarbonyl)-2-thioureido]hexanoic acid
(13a, C15H20N2O4S)
e-Aminohexanoic acid (131 mg, 1.0 mmol) was used to
obtain 261 mg of an oily, colorless solid (0.806 mmol,
81%). 1H NMR (300 MHz, MeOH-d4): d = 1.37 (m, 2 H),
1.62 (m, 4 H), 2.39 (t, 2 H, J = 7.2 Hz), 3.61 (t, 2 H,
J = 7.2 Hz), 5.17 (s, 2 H), 7.21–7.47 (m, 5 H) ppm; 13C
NMR (75 MHz, MeOH-d4): d = 25.6 (-, 1 C), 27.5 (-, 1
C), 29.1 (-, 1 C), 34.9 (-, 1 C), 46.0 (-, 1 C), 68.8 (-, 1
C), 128.9 (?, 1 C), 129.5 (?, 2 C), 129.7 (?, 2 C), 136.9
(Cquat, 1 C), 154.9 (Cquat, 1 C), 177.6 (Cquat, 1 C), 181.2
2-(2-Aminoethoxy)acetic acid (10a, C4H9NO3)
Compound 7a or 7b (1.0 mmol) was dissolved in 0.5 cm3
methanol, 1.0 cm3 1 N aqueous HCl was added, and the
mixture was stirred for 3 h. The solvent was evaporated,
and the residue was lyophilized to yield 10a as colorless
sticky oil (118 mg, 1.0 mmol, quant). 1H NMR (300 MHz,
MeOH-d4): d = 3.19 (t, 2 H, J = 5.2 Hz), 3.81 (t, 2 H,
J = 5.2 Hz), 4.26 (s, 2 H), 4.86 (bs, 3 H), 8.08 (bs, 1 H)
ppm; 13C NMR (75 MHz, MeOH-d4): d = 40.8 (-, 1 C),
68.4 (-, 1 C), 68.8 (-, 1 C), 173.2 (Cquat, 1 C) ppm; MS
(ESI, CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) =
119.8 (100, M ? H?).
ꢀ
(Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,390 (bm), 3,040
(m), 2,989 (m), 2,937 (m), 2,870 (m), 1,770 (m), 1,706 (s),
1,654 (s), 1,462 (m), 1,397 (m), 1,341 (m), 1,215 (m),
1,044 (m), 744 (m), 666 (m), 591 (m) cm-1; MS (ESI,
CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) = 324.9
(100, M ? H?), 649.1 (46, 2M ? H?), 666.1 (46,
2M ? NH?4 ).
General procedure 4 (GP 4): thiourea synthesis
with free acids in dioxane and water
2-[2-[3-(Benzyloxycarbonyl)-2-thioureido]-1-oxoethylami-
no]acetic acid (14a, C13H15N3O5S)
Glycylglycine (132 mg, 1.0 mmol) was employed to give
1
Benzyloxycarbonyl isothiocyanate (250 mg, 1.3 mmol) in
4.0 cm3 dioxane was added to a solution of the amino
compound (1.0 mmol) in 2.2 cm3 aqueous 1 M NaOH. The
mixture was stirred at room temperature for 12 h.
The organic solvent was removed under reduced pressure.
The aqueous phase was acidified with 5% KHSO4 solution
to pH 2. After addition of 10.0 cm3 ethyl acetate, the
phases were separated and the water phase was extracted
twice with 20.0 cm3 ethyl acetate. The combined organic
phases were dried over Na2SO4, and the solvent was
removed under reduced pressure. The residue was sus-
pended in 10.0 cm3 diethyl ether, the ether was decanted
off, and the remaining precipitate was dried. The crude
product was purified by column chromatography on silica
gel (ethyl acetate).
253 mg of an oily product (0.778 mmol, 78%). H NMR
(300 MHz, MeOH-d4): d = 3.95 (s, 2 H), 4.38 (s, 2 H),
5.19 (s, 2 H), 7.22–7.48 (m, 5 H) ppm; 13C NMR (75 MHz,
MeOH-d4): d = 41.8 (-, 1 C), 67.4 (-, 1 C), 68.8 (-, 1
C), 128.9 (?, 1 C), 129.5 (?, 2 C), 129.7 (?, 2 C), 137.0
(Cquat, 1 C), 154.8 (Cquat, 1 C), 170.9 (Cquat, 1 C), 173.0
(Cquat, 1 C), 182.1 (Cquat, 1 C) ppm; IR (FT-IR, film):
ꢀ
m = 3,299 (bm), 3,236 (m), 3,057 (m), 2,900 (m), 2,456
(m), 2,332 (m), 1,711 (s), 1,656 (m), 1,538 (s), 1,398 (m),
1,360 (m), 1,216 (s), 1,136 (m), 1,034 (m), 976 (m), 877
(m), 802 (m), 765 (m), 731 (m), 693 (m), 662 (m), 606
(m) cm-1
;
MS (ESI, CH2Cl2/MeOH ? 10 mmol
NH4OAc): m/z (%) = 326.0 (86, M ? H?), 343.0 (26,
M ? NH?4 ), 651.0 (100, 2M ? H?), 668.0 (39, 2M ?
NH?4 ), 976.1 (17, 3M ? H?), 993.2 (30, 3M ? NH?4 ),
1,301.3 (9, 4M ? H?), 1,318.4 (16, 4M ? NH?4 ).
2-[2-[3-(Benzyloxycarbonyl)-2-thioureido]ethoxy]acetic
acid (12a, C13H16N2O5S)
2-(2-Aminoethoxy)acetic acid (118 mg, 1.00 mmol) was
General procedure 5 (GP 5): preparation
of benzyloxycarbonylthioureas from amino acid esters
1
used to give 262 mg of yellow oil (0.839 mmol, 84%). H
NMR (300 MHz, CDCl3): d = 3.78 (t, 2 H, J = 5.2 Hz),
3.91 (t, 2 H, J = 5.2 Hz), 4.14 (s, 2 H), 5.19 (s, 2 H), 7.28–
7.43 (m, 5 H), 8.10 (bs, 1 H), 9.95 (bs, 1 H) ppm; 13C NMR
(75 MHz, CDCl3): d = 44.5 (-, 1 C), 67.2 (-, 1 C), 67.2
(-, 1 C), 68.0 (-, 1 C), 127.3 (?, 2 C), 127.7 (?, 2 C),
127.8 (?, 1 C), 133.5 (Cquat, 1 C), 151.2 (Cquat, 1 C),
169.8 (Cquat, 1 C), 178.2 (Cquat, 1 C) ppm; IR (FT-IR,
Benzyloxycarbonylisothiocyanate (500 mg, 2.6 mmol) in
10.0 cm3 dichloromethane was added slowly to a solution
of the corresponding amino compound (2.0 mmol) and
300 mg (3.0 mmol) or 500 mg (5.0 mmol, if the amine salt
is employed) triethylamine in 10.0 cm3 dichloromethane at
2–5 °C. The solution was stirred at room temperature until
TLC indicated no further reaction (2–6 h). All volatiles
were removed under reduced pressure. Ethyl acetate
(30.0 cm3) was added, and the organic phase was washed
with 5.0 cm3 saturated ammonium chloride solution and
twice with 10.0 cm3 water. After drying over MgSO4, the
ꢀ
film): m = 3,285 (bm), 2,950 (m), 1,719 (s), 1,512 (s),
1,452 (m), 1,396 (m), 1,315 (m), 1,218 (s), 1,128 (s), 1,082
(m), 1,023 (m), 974 (m), 838 (m), 738 (m), 694 (s), 580
(m), 490 (m) cm-1; MS (EI, 70 eV): m/z (%) = 312.1 (73,
M*?), 221.0 (6, M*?), 91.1 (100, C7H7*?).
123

¨
A. Spath et al.
1302
(300 MHz, MeOH-d4): d = 1.39 (m, 2 H), 1.67 (m, 4
H), 2.40 (t, 2 H, J = 7.2 Hz), 3.62 (t, 2 H, J = 7.2 Hz),
3.65 (s, 3 H), 5.18 (s, 2 H), 7.23–7.45 (m, 5 H) ppm; 13C
NMR (75 MHz, MeOH-d4): d = 25.4 (-, 1 C), 26.9 (-, 1
C), 28.3 (-, 1 C), 34.4 (-, 1 C), 46.1 (-, 1 C), 52.1 (?, 1
C), 68.9 (-, 1 C), 128.8 (?, 1 C), 129.4 (?, 2 C), 129.5 (?,
2 C), 136.6 (Cquat, 1 C), 155.2 (Cquat, 1 C), 175.7 (Cquat,
solvent was evaporated and the residue was suspended in
5.0 cm3 diethyl ether/petroleum ether 1:4. The product was
allowed to settle completely, the ether mixture was dec-
anted off, and after drying the precipitate, it was purified by
column chromatography if necessary (ethyl acetate/petro-
leum ether 1:1 if not stated otherwise).
2-[2-[3-(Benzyloxycarbonyl)-2-thioureido]ethoxy]acetic
acid ethyl ester (12b, C15H20N2O5S)
ꢀ
1 C), 181.1 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,040
(m), 2,986 (m), 2,936 (m), 2,874 (m), 1,772 (m), 1,705 (s),
1,656 (s), 1,464 (m), 1,398 (m), 1,342 (m), 1,216 (m),
1,045 (m), 746 (m), 665 (m), 592 (m) cm-1; MS (ESI,
CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) = 339.2
(100, M ? H?).
The TFA salt of 2-(2-aminoethoxy)acetic acid ethyl ester
(520 mg, 2.0 mmol) was reacted according to GP 5 to give
the thiourea 12b as pale-yellow oil (574 mg, 1.77 mmol,
89%). 1H NMR (300 MHz, CDCl3): d = 1.20 (t, 3 H,
J = 7.2 Hz), 3.70 (t, 2 H, J = 5.2 Hz), 3.83 (t, 2 H,
J = 5.2 Hz), 4.05 (s, 2 H), 4.15 (q, 2 H, J = 7.2 Hz), 5.11
(s, 2 H), 7.22–7.35 (m, 5 H), 8.50 (bs, 1 H), 9.90 (bs, 1 H)
ppm; 13C NMR (75 MHz, CDCl3): d = 14.2 (?, 1 C), 45.4
(-, 1 C), 61.0 (-, 1 C), 68.1 (-, 1 C), 68.4 (-, 1 C), 69.0
(-, 1 C), 128.3 (?, 1 C), 128.7 (?, 2 C), 128.8 (?, 2 C),
134.6 (Cquat, 1 C), 152.4 (Cquat, 1 C), 170.3 (Cquat, 1 C),
2-[2-[3-(Benzyloxycarbonyl)-2-thioureido]-1-
oxoethylamino]acetic acid methyl ester
(14b, C14H17N3O5S)
Glycylglycine methyl ester hydrochloride (364 mg,
2.0 mmol) was converted by GP 5 to the corresponding
thiourea. The product was isolated as a colorless solid
1
ꢀ
(540 mg, 0.839 mmol, 84%). M.p.: 137–138 °C; H NMR
179.4 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,291 (bm),
(300 MHz, acetone-d6): d = 3.68 (s, 3 H), 4.04 (d, 2 H, J
1,1 = 6.0 Hz), 4.40 (d, 2 H, J 1,1 = 6.0 Hz), 5.23 (s, 2 H),
7.29–7.45 (m, 5 H), 7.83 (bs, 1 H), 9.98 (bs, 1 H), 10.32
(bs, 1 H) ppm; 13C NMR (75 MHz, acetone-d6): d = 41.6
(-, 1 C), 48.9 (-, 1 C), 52.3 (?, 1 C), 68.2 (-, 1 C), 129.1
(?, 2 C), 129.2 (?, 2 C), 129.4 (?, 1 C), 136.6 (Cquat, 1
C), 154.1 (Cquat, 1 C), 154.2 (Cquat, 1 C), 168.6 (Cquat, 1
C), 170.6 (Cquat, 1 C), 180.6 (Cquat, 1 C) ppm; IR (FT-IR,
2,981 (m), 2,942 (m), 2,905 (m), 1,721 (s), 1,515 (s), 1,453
(m), 1,375 (m), 1,208 (s), 1,126 (s), 1,022 (s), 911 (m), 839
(m), 796 (m), 740 (m), 696 (m), 580 (m) cm-1; MS (ESI,
CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z(%) = 340.9
(100, M ? H?), 357.9 (8, M ? NH?4 ), 681.1 (22,
2M ? H?), 698.1 (14, 2M ? NH?4 ).
2-[2-[3-(Benzyloxycarbonyl)-2-thioureido]ethoxy]acetic
acid tert-butyl ester (12c, C17H24N2O5S)
ꢀ
film): m = 3,258 (bm), 3,032 (m), 2,955 (m), 1,719 (s),
1,513 (s), 1,437 (m), 1,371 (s), 1,196 (s), 1,131 (m), 1,031
The acetate salt of 2-(2-aminoethoxy)acetic acid tert-butyl
ester (470 mg, 2.0 mmol) was reacted according to GP 5 to
1
yield 12c as a colorless oil (640 mg, 1.82 mmol, 91%). H
(m), 970 (m), 905 (m), 796 (m), 778 (m), 697 (m), 603
(m) cm-1
;
MS (ESI, CH2Cl2/MeOH ? 10 mmol
NH4OAc): m/z (%) = 339.3 (31, M ? H?), 357.0 (46,
M ? H? ? MeCN), 679.2 (69, 2M ? H?), 696.2 (100,
NMR (300 MHz, CDCl3): d = 1.47 (s, 9 H), 3.76 (t, 2 H,
J = 5.2 Hz), 3.90 (t, 2 H, J = 5.2 Hz), 4.00 (s, 2 H), 5.17
(s, 2 H), 7.30–7.41 (m, 5 H), 8.30 (bs, 1 H), 9.93 (bs, 1 H)
ppm; 13C NMR (75 MHz, CDCl3): d = 28.1 (?, 1 C), 45.6
(-, 1 C), 68.1 (-, 1 C), 68.8 (-, 1 C), 68.9 (-, 1 C), 81.9
(Cquat, 1 C), 128.4 (?, 2 C), 128.7 (?, 1 C), 128.8 (?, 2
C), 134.6 (Cquat, 1 C), 152.3 (Cquat, 1 C), 169.4 (Cquat, 1
2M ? NH?4 );
C14H17N3O5S*? 339.0889, found 339.0884.
HRMS
(EI,
70 eV):
calc.
for
5-[3-(Benzyloxycarbonyl)-2-thioureidomethyl]pyrrole-2-
carboxylic acid methyl ester (15, C16H17N3O4S)
The TFA salt of 5-(aminomethyl)pyrrole-2-carboxylic acid
methyl ester(534 mg, 2.0 mmol) was converted following
GP 5 to give the corresponding thiourea as a beige solid
ꢀ
C), 179.3 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,290
(bm), 2,977 (m), 2,937 (m), 2,883 (m), 1,720 (s), 1,515 (s),
1,453 (m), 1,368 (m), 1,319 (m), 1,220 (s), 1,187 (s), 1,123
(s), 1,023 (s), 961 (m), 913 (m), 840 (m), 758 (m), 735 (m),
696 (m), 583 (m) cm-1; MS (ESI, CH2Cl2/MeOH ? 10 m-
mol NH4OAc): m/z (%) = 369.0 (100, M ? H?), 386.0
(14, M ? NH?4 ), 737.1 (19, 2M ? H?), 754.1 (29,
2M ? NH?4 ).
1
(610 mg, 1.84 mmol, 92%). M.p.: 155–156 °C; H NMR
(300 MHz, CDCl3): d = 3.82 (s, 3 H), 4.85 (d, 2 H,
J = 5.8 Hz), 5.16 (s, 2 H), 6.15 (d, 1 H, J = 6.0 Hz), 6.81
(d, 1 H, J = 6.0 Hz), 7.28–7.41 (m, 5 H), 8.30 (s, 1 H),
10.01 (bs, 2 H) ppm; 13C NMR (75 MHz, CDCl3):
d = 41.7 (-, 1 C), 51.6 (?, 1 C), 68.5 (-, 1 C), 110.1
(?, 1 C), 115.4 (?, 1 C), 122.8 (Cquat, 1 C), 128.4 (?, 2
C), 128.8 (?, 2 C), 129.0 (?, 1 C), 132.7 (Cquat, 1 C),
134.3 (Cquat, 1 C), 152.4 (Cquat, 1 C), 161.3 (Cquat, 1 C),
6-[3-(Benzyloxycarbonyl)-2-thioureido]hexanoic acid
methyl ester (13b, C16H22N2O4S)
e-Aminohexanoic acid methyl ester hydrochloride
(170 mg, 1.0 mmol) was used to obtain 281 mg of an
oily, colorless solid (0.831 mmol, 83%). 1H NMR
ꢀ
180.1 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,279 (bm),
2,955 (m), 2,926 (m), 2,855 (m), 1,701 (s), 1,614 (m),
123

Synthesis and binding properties of guanidinium biscarboxylates
1303
twice with 10.0 cm3 water. After drying over MgSO4, the
solvent was evaporated and the oily residue was purified by
column chromatography with ethyl acetate/petroleum ether
4:1 to yield the guanidine.
1,519 (m), 1,492 (s), 1,452 (m), 1,385 (m), 1,315 (m),
1,214 (s), 1,089 (m), 1,037 (m), 1,003 (m), 910 (m), 796
(m), 765 (m), 733 (m), 697 (m), 605 (m) cm-1; MS (ESI,
CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) = 348.0
(44, M ? H?), 389.0 (12, M ? H? ? MeCN), 695.0 (100,
2M ? H?), 712.1 (19, 2M ? NH?4 ); HRMS (EI, 70 eV):
calc. for C16H17N3O4S*? 347.0940, found 347.0946; UV
(MeOH): kmax (e) = 264 (6900), 209 (6100) nm
(mol-1 dm3 cm-1).
Instead of EDC, mercury(II) chloride can be used in this
reaction. DCM has to be replaced by DMF then. For
example: the benzyloxycarbonylthiourea (0.5 mmol) was
added to a solution of the appropriate amino compound
(0.6 mmol) and 130 mg triethylamine (1.3 mmol) in
5.0 cm3 DMF. Mercury(II) chloride (170 mg, 0.62 mmol)
was added, and the reaction mixture was stirred for 15 h at
room temperature. The mixture was filtered over Celite,
and the solvent was removed under reduced pressure. The
crude product was dissolved in H2O and acidified with 5%
KHSO4 to pH = 3. The aqueous layer was extracted three
times with EtOAc. The combined organic phases were
dried over MgSO4 and the solvent removed under reduced
pressure. The crude product was purified by column
chromatography as given.
6-(3-[Benzyloxycarbonyl)-2-thioureidomethyl]pyridine-2-
carboxylic acid ethyl ester (16, C18H19N3O4S)
2-(Aminomethyl)pyridine-6-carboxylic acid ethyl ester
(360 mg, 2.0 mmol) was submitted to GP 5 to give the
corresponding thiourea as an orange oil (604 mg,
1.62 mmol, 81%). 1H NMR (300 MHz, CDCl3):
d = 1.49 (t, 3 H, J = 7.2 Hz), 4.09 (q, 2 H, J = 7.2 Hz),
5.06 (d, 2 H, J = 4.7 Hz), 5.23 (s, 2 H), 7.30–7.46 (m, 5H),
7.52 (d, 1 H, J = 7.7 Hz), 7.85 (dd, 1 H, J = 7.7 Hz), 8.06
(d, 1 H, J = 7.7 Hz), 8.12 (bs, 1 H), 10.81 (bs, 1 H) ppm;
13C NMR (75 MHz, CDCl3): d = 13.2 (?, 1 C), 49.4 (-, 1
C), 61.0 (-, 1 C), 67.2 (-, 1 C), 122.2 (?, 1 C), 123.1 (?,
1 C), 124.1 (?, 1 C), 127.3 (?, 2 C), 127.7 (?, 2 C), 127.9
(?, 1 C), 133.5 (Cquat, 2 C), 136.8 (Cquat, 1 C), 146.7
(Cquat, 1 C), 151.2 (Cquat, 1 C), 154.2 (Cquat, 1 C), 178.0
2-[2-[N-(Benzyloxycarbonyl)-N-[2-(ethoxycarbonylmeth-
oxy)ethyl]guanidinyl]ethoxy]acetic acid ethyl ester
(17b, C21H31N3O8)
Thiourea 12b (161 mg, 0.5 mmol) was reacted with
157 mg of the TFA salt of 2-(2-aminoethoxy)acetic acid
ethyl ester (10b, 0.6 mmol) following GP 6 to give a clear
yellow oil (166 mg, 0.366 mmol, 73%). 1H NMR
(300 MHz, CDCl3): d = 1.25 (t, 6 H, J = 7.1 Hz), 3.56
(m, 4 H), 3.70 (m, 4 H), 4.18 (q, 4 H, J = 7.4 Hz), 4.10 (s,
4 H), 5.12 (s, 2 H), 7.21–7.43 (m, 5 H), 9.00 (bs, 2 H) ppm;
13C NMR (75 MHz, CDCl3): d = 14.2 (?, 2 C), 42.8 (-, 2
C), 47.5 (-, 2 C), 61.2 (-, 2 C), 67.2 (-, 2 C), 68.3 (-, 1
C), 70.8 (-, 2 C), 128.0 (?, 1 C), 128.1 (?, 2 C), 128.4 (?,
2 C), 136.8 (Cquat, 1 C), 160.0 (Cquat, 2 C), 161.5 (Cquat,
1 C), 170.1 (Cquat, 1 C) ppm; IR (FT-IR, film): ꢀm = 3,284
(bm), 2,981 (m), 2,936 (m), 1,740 (s), 1,668 (m), 1,598 (s),
1,518 (m), 1,451 (m), 1,382 (m), 1,275 (m), 1,209 (s),
1,132 (s), 1,051 (m), 1,025 (m), 912 (m), 852 (m), 799 (m),
729 (m), 691 (m), 582 (m) cm-1; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 454.0 (100,
M ? H?).
ꢀ
(Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,425 (bm), 3,348
(bm), 2,960 (m), 2,929 (m), 1,709 (s), 1,591 (m), 1,495
(m), 1,453 (m), 1,394 (m), 1,305 (m), 1,220 (s), 1,163 (s),
1,086 (m), 1,053 (m), 1,024 (m), 910 (m), 861 (m), 761
(m), 735 (m), 697 (m), 581 (m) cm-1; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 373.9 (100,
M ? H?), 747.1 (31, 2M ? H?); HRMS (PI-LSIMS
FAB, glycerol): calc. for C18H20N3O4S? 374.1175, found
374.1165.
Syntheses and deprotections of the receptors
General procedure 6 (GP 6): preparation of symmetric 1,3-
disubstituted benzyloxycarbonylguanidines
The benzyloxycarbonylthiourea (0.5 mmol) was dissolved
together with the appropriate amino compound (0.6 mmol)
and 130 mg triethylamine (1.3 mmol) in 5.0 cm3 DCM.
EDC hydrochloride (110 mg, 0.6 mmol) was added at
2–5 °C in one portion. The mixture was allowed to reach
room temperature in 1 h and was then stirred overnight. In
the case where TLC indicated thiourea still being present at
this point, another portion of triethylamine (30 mg,
0.3 mmol) and EDC hydrochloride (54 mg, 0.3 mmol) was
added and stirring was continued for 4 h. It was diluted
with 25.0 cm3 DCM, and the organic solution was washed
with 5.0 cm3 saturated ammonium chloride solution and
2-[2-[N-(Benzyloxycarbonyl)-N-[2-(ethoxycarbonylmeth-
oxy)ethyl]guanidinyl]ethoxy]acetic acid tert-butyl ester
(17a, C25H39N3O8)
Thiourea 12c (176 mg, 0.5 mmol) was reacted with
158 mg of the TFA salt of 2-(2-aminoethoxy)acetic acid
tert-butyl ester (10c, 0.6 mmol) following GP 6 to yield a
1
clear, slightly yellow oil (198 mg, 0.389 mmol, 78%). H
NMR (300 MHz, CDCl3): d = 1.46 (s, 18 H), 3.52 (m, 4
H), 3.67 (m, 4 H), 3.99 (s, 4 H), 5.11 (s, 2 H), 7.28–7.41
(m, 5 H), 9.23 (bs, 2 H) ppm; 13C NMR (75 MHz, CDCl3):
123

¨
A. Spath et al.
1304
d = 28.1 (?, 6 C), 41.6 (-, 2 C), 48.8 (-, 2 C), 66.4 (-, 2
C), 70.8 (-, 1 C), 82.0 (Cquat, 2 C), 127.5 (?, 1 C), 128.0
(?, 2 C), 128.3 (?, 2 C), 137.8 (Cquat, 1 C), 161.1 (Cquat,
2 C), 164.1 (Cquat, 1 C) ppm(further signals were not
NH4OAc): m/z (%) = 468.0 (100, M ? H?); HRMS
?
(PI-LSIMS FAB, glycerol): calc. for C23H26N5O6
468.1883, found 468.1879; UV (MeOH): kmax (e) = 266
(12,800), 226 (9,800) nm (mol-1 dm3 cm-1).
ꢀ
detectable); IR (FT-IR, film): m = 3,343 (bm), 2,977 (m),
3-(Benzyloxycarbonylamino)imidazo[1,5-a]pyridine-5-
carboxylic acid ethyl ester (21, C18H17N3O4)
2,929 (m), 1,743 (s), 1,633 (s), 1,599 (s), 1,455 (m), 1,386
(m), 1,368 (m), 1,303 (m), 1,228 (m), 1,129 (s), 1,052 (m),
942 (m), 845 (m), 801 (m), 743 (m), 699 (m) cm-1; MS
(ESI, CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) =
510.2 (100, M ? H?); HRMS (LSIMSFAB, glycerol):
Thiourea 16 (187 mg, 0.5 mmol) was submitted to the
conditions of GP 6 to give product 21 as a red glass
(112 mg, 0.328 mmol, 66%). 1H NMR (300 MHz,
CDCl3): d = 1.39 (t, 3 H, J = 7.2 Hz), 4.31 (q, 2 H,
J = 7.2 Hz), 5.18 (s, 2 H), 6.67 (t, 1 H, J = 7.2 Hz),
7.27–7.42 (m, 6 H), 7.58 (d, 2 H, J = 4.6 Hz), 8.61 (bs, 1
H) ppm; 13C NMR (75 MHz, CDCl3): d = 14.1 (?, 1 C),
62.5 (-, 1 C), 67.5 (-, 1 C), 116.4 (?, 1 C), 120.7 (?, 1
C), 121.6 (?, 1 C), 123.6 (?, 1 C), 124.9 (Cquat, 1 C),
128.2 (?, 2 C), 128.3 (?, 2 C), 128.7 (?, 1 C), 130.7
(Cquat, 1 C), 135.9 (Cquat, 1 C), 153.7 (Cquat, 1 C),
?
calc. for C25H40N3O8 510.2815, found 510.2808.
6-[N-(Benzyloxycarbonyl)-N-[5-(methoxycarbonyl)pentyl]-
guanidinyl]hexanoic acid methyl ester
(18, C23H35N3O6)
Thiourea 13b (161 mg, 0.5 mmol) was reacted with
103 mg e-aminohexanoic acid methyl ester hydrochloride
(0.6 mmol) according to GP 6 to give a colorless oil
(175 mg, 0.391 mmol, 78%). 1H NMR (300 MHz, CDCl3):
d = 1.38 (m, 4 H), 1.66 (m, 8 H), 2.42 (m, 4 H), 3.63 (m, 4
H), 3.68 (s, 6 H), 5.17 (s, 2 H), 7.25–7.43 (m, 5 H) ppm;
13C NMR (75 MHz, CDCl3): d = 25.5 (-, 2 C), 27.1 (-, 2
C), 28.2 (-, 2 C), 34.5 (-, 2 C), 46.2 (-, 2 C), 52.0 (?, 2
C), 68.4 (-, 1 C), 128.0 (?, 1 C), 128.2 (?, 2 C), 128.4 (?,
2 C), 136.9 (Cquat, 1 C), 160.1 (Cquat, 2 C), 161.7 (Cquat,
ꢀ
162.8 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,255
(bm), 2,981 (m), 2,908 (m), 1,722 (s), 1,531 (m), 1,454
(m), 1,408 (m), 1,372 (m), 1,266 (s), 1,235 (s), 1,212 (s),
1,171 (m), 1,141 (m), 1,106 (m), 1,036 (m), 913 (m), 872
(m), 810 (m), 743 (m), 698 (m), 595 (m), 495 (m) cm-1
;
MS (ESI, CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) =
340.0 (100, M ? H?), 679.1 (11, 2 M ? H?); UV (MeOH):
kmax (e) = 380 (1,300), 280 (3,200), 230 (13,200) nm
(mol-1 dm3 cm-1).
ꢀ
1 C), 172.1 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,280
(bm), 2,986 (m), 2,938 (m), 1,743 (s), 1,666 (m), 1,597 (s),
1,519 (m), 1,452 (m), 1,384 (m), 1,276 (m), 1,213 (s),
1,134 (s), 1,056 (m), 1,027 (m), 913 (m), 854 (m), 801 (m),
2-[2-[N-(Benzyloxycarbonyl)-N-[2-(methoxy-
carbonylmethylamino)-2-oxoethyl]guanidinyl]-1-
732 (m), 693 (m) cm-1
.
oxoethylamino]acetic acid ethyl ester (19, C19H25N5O8)
Compound 14b (161 mg, 0.5 mmol) was reacted with
110 mg glycylglycine hydrochloride (0.6 mmol) using GP
6 (10% N-methylpyrrolidone (NMP) was added to the
reaction mixture) to give a pale-yellow solid (172 mg,
5-[N-(Benzyloxycarbonyl)-N-[5-(methoxycarbonyl)-1H-
pyrrol-2-ylmethyl]guanidinylmethyl]-1H-pyrrole-2-
carboxylic acid methyl ester (20, C23H25N5O6)
Thiourea 15 (165 mg, 0.5 mmol) was reacted with 160 mg
of the TFA salt of 5-(aminomethyl)pyrrole-2-carboxylic
acid methyl ester (0.6 mmol) according to GP 6 to yield a
yellow solid (203 mg, 0.434 mmol, 87%). M.p.: 122–
1
0.381 mmol, 76%). M.p.: 92–94 °C; H NMR (300 MHz,
DMSO-d6): d = 3.66 (s, 6 H), 3.96 (m, 4 H), 4.08 (m, 4
H), 5.05 (s, 2 H), 7.28–7.42 (m, 5 H), 7.90 (bs, 1 H), 9.05
(bs, 2 H) ppm; 13C NMR (75 MHz, DMSO-d6): d = 41.5
(-, 2 C), 44.9 (-, 2 C), 52.2 (?, 2 C), 66.8 (-, 1 C), 128.3
(?, 1 C), 128.7 (?, 2 C), 129.0 (?, 2 C), 139.2 (Cquat, 1
C), 161.6 (Cquat, 2 C), 170.9 (Cquat, 1 C), 175.7 (Cquat, 2
C) ppm (further signals were not detectable); IR (FT-IR,
1
124 °C; H NMR (300 MHz, DMSO-d6): d = 3.54 (s, 6
H), 4.41 (s, 4 H), 5.14 (s, 2 H), 6.12 (d, 2 H, J = 6.0 Hz),
6.77 (d, 2 H, J = 6.0 Hz), 7.38–7.42 (m, 5 H), 8.16 (s, 2
H), 10.81 (bs, 1 H), 11.05 (bs, 1 H) ppm; 13C NMR
(75 MHz, MeOH-d4): d = 38.9 (-, 2 C), 51.8 (?, 2 C),
67.9 (-, 1 C), 109.9 (?, 2 C), 116.9 (?, 2 C), 122.1 (Cquat,
2 C), 128.8 (?, 1 C), 128.9 (?, 2 C), 129.5 (?, 2 C), 138.9
(Cquat, 1 C), 158.2 (Cquat, 1 C), 161.3 (Cquat, 2 C), 163.1
(Cquat, 2 C), 165.0 (Cquat, 1 C) ppm; IR (FT-IR, film):
ꢀ
film): m = 3,352 (m), 3,290 (bm), 3,069 (m), 2,951 (m),
1,744 (m), 1,639 (s), 1,572 (m), 1,530 (m), 1,434 (m),
1,378 (m), 1,330 (m), 1,283 (m), 1,259 (m), 1,203 (s),
1,171 (s), 1,094 (s), 1,030 (m), 1,009 (m), 983 (m), 941
(m), 794 (m), 719 (m), 672 (m), 603 (m), 539 (m), 456
ꢀ
m = 3,415 (bm), 3,281 (s), 2,949 (m), 1,679 (m), 1,642 (m),
1,584 (m), 1,549 (m), 1,490 (m), 1,443 (m), 1,390 (m),
1,333 (m), 1,277 (m), 1,220 (s), 1,127 (m), 1,065 (m),
1,002 (m), 927 (m), 799 (m), 764 (s), 701 (m), 627 (m), 597
(m), 508 (m) cm-1; MS (ESI, CH2Cl2/MeOH ? 10 mmol
(m) cm-1
;
MS (ESI, CH2Cl2/MeOH ? 10 mmol
NH4OAc): m/z (%) = 452.1 (100, M ? H?), 903.5 (5,
2M ? H?); HRMS (LSI-MSFAB, glycerol): calc. for
?
C19H26N5O8 452.1781, found 452.1782.
123

Synthesis and binding properties of guanidinium biscarboxylates
1305
?
General procedure 7: deprotection
of the benzyloxycarbonylguanidine esters
FAB, glycerol): calc. for C13H26N3O6 320.1822, found
320.1827.
2-[2-[N-[2-(Carboxymethoxy)ethyl]guanidinyl]eth-
oxy]acetic acid hydrochloride (6, C9H18ClN3O6)
Deprotection of the Cbz–guanidine (GP 7a)
The symmetric 1,3-disubstituted benzyloxycarbonylg-
uanidine ester (0.4 mmol) was dissolved, depending on the
ester in the molecule, in 3.0 cm3 ethanol or methanol. Three
spatula tips of palladium on charcoal (10% Pd) were added. It
was stirred in hydrogen atmosphere (30 bar) for 2 days at
room temperature. The reaction mixture was diluted with
10.0 cm3 of the respective alcohol and filtered over Celite.
Thefilter cakewaswashedwithsmallportionsofthisalcohol.
The clear filtrate was evaporated to dryness to give the gua-
nidine free base.
The guanidine free base 22b (181 mg, 0.4 mmol) was
dissolved in 5 cm3 methanol, and 2.0 cm3 aqueous hydro-
chloric acid (2 M, 4.0 mmol) was added. The mixture was
stirred at room temperature until TLC showed complete
consumption of the starting material (4–6 h). The solvent
was evaporated, and the product was lyophilized to give
the product as colorless, sticky oil (97 mg, 0.369 mmol,
1
92%). H NMR (300 MHz, acetone-d6): d = 3.51 (m, 4
H), 3.71 (s, 4 H), 3.78 (m, 4 H), 7.60 (bs, 2 H), 8.71 (bs, 1
H) ppm; 13C NMR (75 MHz, MeOH-d4): d = 43.2 (-, 2
C), 68.8 (-, 2 C), 71.3 (-, 2 C), 158.8 (Cquat, 1 C), 155.9
(Cquat, 1 C), 177.2 (Cquat, 2 C) ppm; IR (FT-IR, film):
Synthesis of the guanidinium hexafluorophosphate salts
(GP 7b)
ꢀ
m = 3,178 (bm), 2,935 (m), 2,250 (m), 1,736 (m), 1,638
(m), 1,435 (m), 1,349 (m), 1,218 (m), 1,134 (s), 1,070 (m),
The guanidine free base (0.2 mmol) was dissolved in
6.0 cm3 methanol or ethanol, according to its ester.
Ammonium hexafluorophosphate (38 mg, 0.24 mmol) was
added, and the mixture was warmed to 40 °C for 3 h. The
solvent was evaporated, and the residue was extracted with
1.0 cm3 DCM thrice. The solvent was distilled off, and the
remaining guanidinium salt was properly dried in vacuo.
The products appear as colorless solids in nearly quanti-
tative yield.
1,010 (s), 875 (m), 824 (m), 762 (m), 682 (m), 669 (m), 623
(m) cm-1
;
MS (ESI, CH2Cl2/MeOH ? 10 mmol
NH4OAc): m/z (%) = 264.1 (100, M ? H?).
5-[N-[5-(Methoxycarbonyl)-1H-pyrrol-2-ylmethyl]guanid-
inylmethyl]-1H-pyrrole-2-carboxylic acid methyl ester
(23, C15H19N5O4)
Compound 20 (187 mg, 0.4 mmol) was reacted according
to GP 7a to yield the free base (C15H19N5O4) as yellow
1
solid (117 mg, 0.384 mmol, 96%). H NMR (300 MHz,
2-[2-[N-[2-(Ethoxycarbonylmethoxy)ethyl]guanidinyl]eth-
oxy]acetic acid ethyl ester (22b, C13H25N3O6)
MeOH-d4): d = 3.81 (s, 6 H), 4.41 (s, 4 H), 6.12 (m, 2 H),
6.79 (m, 2 H) ppm; 13C NMR (75 MHz, MeOH-d4):
d = 39.5 (-, 2 C), 51.8 (?, 2 C), 110.2 (?, 2 C), 117.0 (?,
2 C), 124.1 (Cquat, 2 C), 133.5 (Cquat, 2 C), 157.3 (Cquat,
Compound 17b (181 mg, 0.4 mmol) was deprotected
according to GP 7a to give the free base (C13H25N3O6)
as colorless, sticky oil (97 mg, 0.369 mmol, 92%). 1H
NMR (300 MHz, CDCl3): d = 1.22 (t, 6 H, J = 7.1 Hz),
3.33 (m, 4 H), 3.62 (m, 4 H), 4.06 (s, 4 H), 4.12 (q, 2 H,
J = 7.1 Hz), 7.76 (bs, 1 H), 9.71 (bs, 1 H), 10.43 (bs, 1 H)
ꢀ
1 C), 163.0 (Cquat, 2 C) ppm; IR (FT-IR, film): m = 3,179
(bm), 2,953 (m), 1,660 (s), 1,627 (s), 1,489 (m), 1,439 (m),
1,320 (s), 1,279 (m), 1,218 (s), 1,132 (m), 1,003 (s), 930
(m), 800 (m), 762 (s), 656 (m) cm-1; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 334.0 (100,
M ? H?), 375.0 (38, M ? H? ? MeCN), 713.3 (13,
2M ? H? ? HCOOH); HRMS (EI, 70 eV): calc. for
C15H19N5O4*? 333.1437, found 333.1439.
ꢀ
ppm; IR (FT-IR, film): m = 3,308 (bm), 3,183 (bm), 2,971
(m), 2,927 (m), 2,882 (m), 1,749 (m), 1,630 (s), 1,586 (m),
1,404 (m), 1,317 (m), 1,208 (s), 1,134 (s), 1,049 (m), 881
(m), 704 (m), 587 (m) cm-1
.
The free base was reacted following GP 7b in ethanol to
give the hexafluorophosphate salt (C13H26F6N3O6P) as
1
colorless, sticky oil (97 mg, 0.369 mmol, 92%). H NMR
The free base was reacted following GP 7b in methanol
to yield the hexafluorophosphate salt as yellow solid
(300 MHz, acetone-d6): d = 1.24 (t, 6 H, J = 7.1 Hz),
3.59 (m, 4 H), 3.79 (m, 4 H), 4.19 (q, 4 H, J = 7.1 Hz),
4.23 (s, 4 H), 5.16 (s, 2 H), 7.30 (m, 5 H) ppm; 19F NMR
(300 MHz, acetone-d6): d = -70.2 (?), -72.7 (?) ppm;
13C NMR (75 MHz, MeOH-d4): d = 14.6 (?, 2 C), 43.2
(-, 2 C), 62.2 (-, 2 C), 69.1 (-, 2 C), 71.3 (-, 2 C), 158.8
(Cquat, 1 C), 172.3 (Cquat, 2 C), 171.0 (Cquat, 1 C)
ppm; MS (ESI, CH2Cl2/MeOH ? 10 mmol NH4OAc):
m/z (%) = 320.0 (100, M ? H?); HRMS (PI-LSIMS
1
(117 mg, 0.384 mmol, 96%). M.p.: 141–144 °C; H NMR
(300 MHz, acetone-d6): d = 3.78 (s, 6 H), 4.67 (s, 4 H),
6.23 (d, 2 H, J = 3.6 Hz), 6.75 (d, 2 H, J = 3.6 Hz), 7.76
(bs, 1 H), 11.09 (bs, 1 H) ppm; 13C NMR (75 MHz,
MeOH-d4): d = 51.9 (?, 2 C), 110.3 (?, 2 C), 117.0 (?, 2
C), 124.1 (Cquat, 2 C), 133.4 (Cquat, 2 C), 157.2 (Cquat, 1
C), 163.0 (Cquat, 2 C) ppm; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 334.0 (100,
123

¨
A. Spath et al.
1306
M ? H?); UV (MeOH): kmax (e) = 263 (11,600) nm
chloroform was added dropwise, and the mixture was
stirred for 2 days at room temperature. The solvent was
evaporated and all volatiles were removed under reduced
pressure. The residue was dissolved in 10.0 cm3 ethanol
and cooled to 0 °C for 3 h. It was filtered with suction, and
the crystals were washed with a little ice-cold ethanol. The
filtrate was reduced to about 2.0 cm3, and the thiourea was
precipitated by slow addition of diethyl ether. The solvent
was decanted off, the residue was suspended in diethyl
ether, and after the solid had settled completely, the ether
was decanted off again. The product was dried in vacuum.
If necessary, the material can be purified by column
chromatography with ethyl acetate/petroleum ether 1:2.
(mol-1 dm3 cm-1).
5-[N-(5-Carboxy-1H-pyrrol-2-ylmethyl)guanidinylmethyl]-
1H-pyrrole-2-carboxylic acid (24, C13H15N5O4)
Compound 23 (187 mg, 0.4 mmol) was dissolved in 5 cm3
methanol, and 0.8 cm3 aqueous LiOH (1 M, 0.8 mmol)
was added. The mixture was stirred at room temperature
until TLC showed complete conversion (6–8 h). The
solvent was evaporated, and the product was lyophilized
to yield the guanidine bisacid lithium salt as a pale-beige
powder (117 mg, 0.384 mmol, 96%). M.p.: 161–165 °C
(decomp.); 1H NMR (300 MHz, MeOH-d4): d = 4.31 (s, 4
H), 6.02 (m, 2 H), 6.41 (m, 2 H) ppm; 13C NMR (75 MHz,
MeOH-d4): d = 39.9 (-, 2 C), 108.6 (?, 2 C), 113.5 (?, 2
C), 130.6 (Cquat, 2 C), 132.0 (Cquat, 2 C), 137.3 (Cquat, 1
C), 170.1 (Cquat, 2 C) ppm; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 306.0 (100,
M ? H?); UV (MeOH): kmax (e) = 264 (11,200) nm
(mol-1 dm3 cm-1).
2-[2-[3-[2-(Ethoxycarbonylmethoxy)ethyl]thioureido]eth-
oxy]acetic acid ethyl ester [116] (E-4, C13H24N2O6S)
2-(2-Aminoethoxy)acetic acid ethyl ester hydrochloride
(640 mg, 3.5 mmol) was submitted to GP 8. The clean
product was obtained as a clear, yellow sticky oil (417 mg,
1
1.24 mmol, 73%). H NMR (300 MHz, CDCl3): d = 1.26
(t, 6 H, J = 7.1 Hz), 3.81 (m, 4 H), 3.98–4.13 (m, 8 H),
4.14–4.29 (q, 4 H, J = 7.1 Hz), 6.61 (bs, 2 H) ppm; 13C
NMR (75 MHz, CDCl3): d = 14.2 (?, 2 C), 40.8 (-, 2 C),
46.9 (-, 2 C), 54.2 (-, 2 C), 60.9 (-, 2 C), 169.0 (Cquat, 2
An alternative route to the receptors via the thiourea
using trifluoroacetamide as reagent for guanidination
3-(2-Aminoethoxy)propionic acid ethyl ester hydrochloride
(C7H16ClNO3)
ꢀ
C), 181.7 (Cquat, 1 C) ppm; IR (FT-IR, film): m = 2,979
(m), 2,944 (m), 2,870 (m), 1,740 (s), 1,496 (m), 1,418 (m),
1,350 (m), 1,261 (m), 1,206 (s), 1,123 (s), 1,026 (m), 861
(m), 805 (m), 661 (m), 579 (m) cm-1; MS (CI, NH3):
m/z (%) = 337.2 (100, M ? H?), 354.2 (19, M ? NH4?).
To a solution of 0.52 g 3-[2-(tert-butoxycarbonylami-
no)ethoxy]propionic acid ethyl ester (2.0 mmol) in
10.0 cm3 DCM was slowly added 4.0 cm3 diethyl ether
saturated with HCl. After 3 h of stirring at room temper-
ature, the solvent was evaporated. The residue was taken
up in a little DCM, and the solvent was removed under
reduced pressure again. The remaining sticky solid was
dried under vacuum overnight. The yield was 0.38 g of a
yellow, hygroscopic solid (1.96 mmol, 98%). 1H NMR
(300 MHz, CDCl3): d = 1.94 (t, 3 H, J = 7.1 Hz), 2.56 (t,
2 H, J = 6.8 Hz), 3.21 (m, 2 H), 3.68–3.81 (m, 4 H), 4.08
(q, 2 H, J = 7.1 Hz), 8.11 (bs, 3 H) ppm; 13C NMR
(75 MHz, CDCl3): d = 14.2 (?, 1 C), 34.7 (-, 1 C), 39.5
(-, 1 C), 60.9 (-, 1 C), 65.6 (-, 1 C), 66.2 (-, 1 C), 172.1
3-[2-[3-[2-(2-Ethoxycarbonylethoxy)ethyl]thioureido]eth-
oxy]propionic acid ethyl ester (E-5a, C15H28N2O6S)
3-(2-Aminoethoxy)propionic acid ethyl ester hydrochloride
(690 mg, 3.5 mmol) was reacted after GP 8 to yield the
thiourea as a clear, yellow oil (439 mg, 1.21 mmol, 69%).
1H NMR (300 MHz, CDCl3): d = 1.20 (t,
6 H,
J = 7.1 Hz), 2.51 (t, 4 H, J = 6.0 Hz), 3.51–3.70 (m, 8
H), 3.67 (t, 4 H, J = 6.3 Hz), 4.08 (q, 4 H, J = 7.1 Hz),
6.61 (bs, 2 H) ppm; 13C NMR (75 MHz, CDCl3): d = 14.2
(?, 2 C), 34.9 (-, 2 C), 44.3 (-, 2 C), 60.7 (-, 2 C), 66.2
(-, 2 C), 69.5 (-, 2 C), 171.8 (Cquat, 2 C), 182.8 (Cquat, 1
ꢀ
(Cquat, 1 C) ppm; IR (FT-IR, film): m = 3,468 (bm), 2,976
ꢀ
C) ppm; IR (FT-IR, film): m = 2,978 (m), 2,942 (m), 2,874
(m), 2,934 (m), 1,716 (m), 1,672 (s), 1,521 (m), 1,425 (m),
1,379 (m), 1,183 (s), 1,123 (s), 1,024 (m), 968 (m), 834
(m), 799 (s), 722 (m), 678 (m), 598 (m) cm-1; MS (ESI,
CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) = 162.1
(100, M ? H?).
(m), 1,728 (s), 1,546 (m), 146 (m), 1,373 (m), 1,261 (m),
1,189 (s), 1,124 (s), 1,064 (m), 860 (m), 802 (m), 659 (m),
603 (m) cm-1; MS (CI, NH3): m/z (%) = 364.9 (100,
M ? H?), 381.9 (4, M ? NH?4 ).
2-[2-[N-[2-(Ethoxycarbonylmethoxy)ethyl]-N-(2,2,2-
trifluoroacetyl)guanidinyl]ethoxy]acetic acid ethyl
ester (E-6)
General procedure 8 (GP 8): preparation of symmetric
thioureas from glycol-amino acid esters
To a solution of 364 mg compound E-5a (1.0 mmol) in
20.0 cm3 acetone, 0.2 cm3 methyl iodide (2.0 mmol) was
added, and the reaction mixture was stirred at room
temperature overnight under light protection. All volatiles
To an ice-cold solution of the amino compound (3.5 mmol)
in 20.0 cm3 chloroform containing 727 mg triethylamine
(7.2 mmol), 0.13 cm3 thiophosgene (1.7 mmol) in 5.0 cm3
123

Synthesis and binding properties of guanidinium biscarboxylates
1307
were removed under reduced pressure without heating. The
residue was dissolved in 30.0 cm3 of methanol and
dichloromethane (1:1). Ammonium hexafluorophosphate
(424 mg, 2.6 mmol) was added, and the solution was
stirred overnight at room temperature. The solvents were
distilled off, and the remaining yellow oil was taken up in
100 cm3 of dichloromethane and washed with two 20 cm3
portions of water. After drying over MgSO4, the solvent
was evaporated to give the according hexafluorophosphate
in nearly quantitative yield.
References
1. Evans DA, Murry JA, von Matt P, Norcross RD, Miller SJ
(1995) Angew Chem Int Ed 34:798
2. Evans DA, Johnson JS (1997) J Org Chem 62:786
3. Hechavarrıa Fonseca M, Konig B (2003) Adv Syn Cat 345:1173
4. Evans DA, Kozlowski MC, Burgey CS, MacMillan DWC
(1997) J Am Chem Soc 119:7893
5. Sohtome Y, Hashimoto Y, Nagasawa K (2005) Adv Synth Catal
347:1643
6. Davis M, Bonnat M, Guillier F, Kilburn JD, Bradley M (1998) J
Org Chem 63:8696
´
¨
It was redissolved in a mixture of 20.0 cm3 toluene and
5.0 cm3 chloroform, and 0.45 cm3 1,8-diazabicyclo[5.4.0]
undec-7-ene (DBU) (3.0 mmol) and 540 mg trifluoroaceta-
mide (4.8 mmol) were added. The mixture was refluxed
overnight under vigorous stirring. After cooling to room
temperature the solvents were distilled off at reduced pressure.
The oily raw material was purified by column chromatography
with ethyl acetate/petroleum ether 1:2 to yield the guanidine as
clear, yellow oil (279 mg, 0.773 mmol, 77%). 1H NMR
(300 MHz, CDCl3): d = 1.25 (t, 6 H, J = 7.1 Hz), 2.61 (m, 4
H),3.61(m, 4H),3.72(m, 4H),4.16(q,4H, J = 7.1 Hz), 6.59
(bs, 2 H), 9.67 (bs, 1 H) ppm; MS (ESI, CH2Cl2/
MeOH ? 10 mmol NH4OAc): m/z (%) = 444.0 (100,
M ? H?), 909.3 (6, 2M ? Na?).
7. Hunter CA, Purvis DH (1992) Angew Chem Int Ed 31:792
8. Ragusa A, Rossi S, Hayes JM, Steinand M, Kilburn JD (2005)
Chem Eur J 5:5674
9. Rensing S, Schrader T (2002) Org Lett 4:2161
10. Chang S-Y, Kim HS, Chang K-J, Jeong K-S (2004) Org Lett
6:181
¨
11. Kovbasyuk L, Kramer R (2004) Chem Rev 104:3161
12. Chin J, Walsdorff C, Stranix B, Oh J, Chung HJ, Park SM, Kim
K (1999) Angew Chem Int Ed 38:2756
13. Bell TW, Hext NM (2004) Chem Soc Rev 33:589
14. Raker J, Glass TE (2002) J Org Chem 67:6113
´
15. Holy P, Morf WE, Seller K, Simon W, Vigneron J-P (2004)
Helv Chim Acta 73:1171
16. Krause C, Werner T, Huber C, Wolfbeis OS (1999) Anal Chem
71:1544
17. Shih Y, Huang HJ (1999) Anal Chim Acta 392:143
18. Lee MH, Yoo CL, Lee JS, Cho I-S, Kim BH, Cha GS, Nam H
(2002) Anal Chem 74:2603
19. Riggs JA, Litchfield RK, Smith BD (1996) J Org Chem 61:1148
20. Tsukube H, Shinoda S, Uenishi J, Kanatani T, Itoh H, Shiode M,
Iwachido T, Yonemitsu O (1998) Inorg Chem 37:1585
21. Boudouche S, Jacquet L, Lobo-Recio MA, Marzin C, Tarrago G
(1993) J Inclusion Phenom Macrocyclic Chem 16:81
22. Metzger A, Gloe K, Stephan H, Schmidtchen FP (1996) J Org
Chem 61:2051
¨
23. Fricke T, Hamann J, Bahadir M, Konig B (2002) Anal Bioanal
Chem 374:148
24. Stefan R-I, Baiulescu GE, Aboul-enien HY (1997) Crit Rev
Analyt Chem 27:307
Deprotection to the free guanidine
The above material (270 mg, 0.75 mmol) was dissolved in
2.0 cm3 methanol, and a solution of 138 mg potassium
carbonate (1.0 mmol) in 0.5 cm3 water was added. After
warming the well-stirred mixture to 50 °C for 4 h, the sol-
vents were evaporated. The residue was dissolved in a 1:1
mixture of DCM and water (10.0 cm3). The phases were
separated, and the aqueous layer was extracted with 5.0 cm3
DCM twice. Thecombined organic phases were washed with
brine, dried over MgSO4, and evaporated to dryness. The
product E-7 was obtained as a yellow oil (232 mg,
0.68 mmol, 90%). 1H NMR (300 MHz, MeOH-d4):
d = 1.25 (t, 6 H, J = 7.1 Hz), 2.62 (t, 4 H, J = 6.0 Hz), 3.11
(t, 4 H, J = 4.9 Hz), 3.67 (t, 4 H, J = 4.9 Hz), 3.76 (t, 4 H,
J = 6.0 Hz), 4.16 (q, 4 H, J = 7.1 Hz) ppm; 13C NMR
(75 MHz, MeOH-d4): d = 14.5 (?, 2 C), 35.5 (-, 2 C), 40.5
(-, 2 C), 61.8 (-, 2 C), 67.5 (-, 2 C), 67.6 (-, 2 C), 160.4
(Cquat, 1 C), 173.7 (Cquat, 2 C) ppm; IR (FT-IR, film):
25. Fry CH, Hall SK, Blatter LA, McGuigan JAS (1990) Exp
Physiol 75:187
26. Smith JW, Le Calvez H, Parra-Gessert L, Preece NE, Jia X,
Assa-Munt N (2002) J Biol Chem 277:10298
27. Singh AK, Singh R, Saxena P (2004) Sensors 4:187
28. Peczuh MW, Hamilton AD (2000) Chem Rev 100:2479
29. Abbastabar-Ahangar H, Shirzadmehr A, Marjani K, Khoshsafar
H, Chaloosi M, Mohammadi L (2009) J Inclusion Phenom
Macrocyclic Chem 63:287
30. Mazurov A, Hauser T, Miller CH (2006) Curr Med Chem
13:1567
31. Alexandratos SD, Zhu X (2007) Inorg Chem 46:2139
32. Bu¨lhman P, Prestch E, Bakker E (1998) Chem Rev 98:1593
´
33. Ehala S, Kasicka V, Makrlık E (2008) Electrophoresis 29:652
34. Bakker E (2004) Anal Chem 76:3285
ꢀ
m = 3,419 (bm), 2,986 (m), 2,934 (m), 2,891 (m), 1,670 (s),
1,438 (m), 1,379 (m), 1,276 (m), 1,189 (s), 1,123 (s), 1,029
(m), 840 (m), 800 (m), 721 (m), 606 (m) cm-1; MS (ESI,
CH2Cl2/MeOH ? 10 mmol NH4OAc): m/z (%) = 348.0
(100, M ? H?).
35. Bakker E, Qin Y (2006) Anal Chem 78:3965
36. Jiang P, Guo Z (2004) Coord Chem Rev 248:205
37. Valeur B, Leray I (2000) Coord Chem Rev 202:40
38. de Silva AP, Gunaratne HQN, Gunnlaugsson T, Huxley AJM,
McCoy CP, Radermacher JT, Rice TE (1997) Chem Rev
97:1515
Acknowledgments We thank the Deutsche Forschungsgemeins-
chaft (GRK 640) and the University of Regensburg for support of this
work. We thank Dr. R. Vasold and K. Lehner for HPLC analyses.
39. Spichiger UE, Freiner D, Bakker E, Rosatzin T, Simon W
(1993) Sens Actuators B 11:263
40. Lu¨bbers DW (1995) Acta Anaesthesiol Scand 39:37
123

¨
A. Spath et al.
1308
41. Buck SM, Lee Koo Y-E, Park E, Xu H, Philbert MA, Brasuel
77. Chandrakumar NS, Stapelfeld A, Beardsley PM, Lopez OT,
Drury B, Anthony E, Savage MA, Williamson LN, Reichman M
(1992) J Med Chem 35:2928
MA, Kopelman R (2004) Curr Opin Chem Biol 8:540
42. Davidsohn I, Henry JB (1978) Clinical diagnosis by laboratory
methods. WB Saunders, Philadelphia
43. Spichiger-Keller UE (1999) Analyt Chim Acta 400:65
44. Lehn JM (1995) Supramolecular chemistry—concepts and per-
spectives. VCH, New York
45. Kirkovits GJ, Shriver JA, Gale PA, Sessler JL (2001) J Inclusion
Phenom Macrocyclic Chem 41:69
46. Izatt RM, Pawlak K, Bradshaw JS (1995) Chem Rev 95:2529
47. Zhang XX, Bradshaw JS, Izatt RM (1997) Chem Rev 97:3313
48. Grady T, Harris SJ, Smyth MR, Diamond D, Hailey P (1996)
Anal Chem 68:3775
78. Korosec E, Poljsak D, Urleb U (1992) Arch Pharm 325:251
79. Garner P, Park JM (1998) Org Synth Coll 9:300
80. Meyers AI, McKennon MJ (1993) J Org Chem 58:3568
81. Depew KM, Marsden SP, Zatorska D, Zatorski A, Bornmann
WG, Danishefsky SJ (1999) J Am Chem Soc 121:11953
82. Zhu Z, Espenson JH (1996) J Am Chem Soc 118:9901
83. Lottes AC, Landgrebe JA, Larsen K (1989) Tetrahedron Lett
30:4089
´
84. Fructos MR, Belderrain TR, de Fremont P, Scott NM, Nolan SP,
Mar Dıaz-Requejo M, Perez PJ (2005) Angew Chem Int Ed
´
´
49. Grady T, Joyce T, Smyth MR, Harris SJ, Diamond D (1998)
Anal Commun 35:123
50. Sakaki T, Harada T, Deng G, Kawabata H, Kawahara Y, Shinkai
S (1992) J Inclusion Phenom Macrocyclic Chem 14:285
51. Kim H-J, Asif R, Chunga DS, Hong J-I (2003) Tetrahedron Lett
44:4335
44:5284
85. Groziak MP, Townsend LB (1986) Org Chem 51:1277
86. Martin NI, Woodward JJ, Marletta MA (2006) Org Lett 8:4035
87. Martin NI, Beeson WT, Woodward JJ, Marletta MA (2008) J
Med Chem 51:924
88. Lewellyn ME, Wang SS, Strydom PJ (1990) J Org Chem
55:5230
52. Imai H, Misawa K, Munakata H, Uemori Y (2001) Chem Lett
688
¨
89. Suhs T, Konig B (2006) Chem Eur J 12:8150
53. Wright AT, Anslyn EV (2004) Org Lett 6:1341
54. Ahn KH, Kim S-G, Jung J, Kim K-H, Kim J, Chin J, Kim K
(2000) Chem Lett 170
55. Tomas S, Prohens R, Vega M, Rotger MC, Deya PM, Ballester
P, Costa A (1996) J Org Chem 61:9394
56. Jon SY, Kim J, Kim M, Park S-H, Jeon WS, Heo J, Kim K
(2001) Angew Chem Int Ed 40:2116
57. Chin J, Walsdorff C, Stranix B, Oh J, Chung HJ, Park S-M, Kim
K (1999) Angew Chem Int Ed 38:2756
58. Schmidtchen FP, Berger M (1997) Chem Rev 97:1609
59. Schrader T (1996) Angew Chem Int Ed 35:2649
60. Schrader T (1998) J Org Chem 63:264
61. Bell TW, Khasanov AB, Drew MGB (2002) J Am Chem Soc
124:14092
62. Bell TW, Hext NM, Khasanov AB (1998) Pure Appl Chem
70:2371
63. Rebek J Jr, Nemeth D (1985) J Am Chem Soc 107:6738
90. Katritzky AR, Kirichenko N, Rogovoy BV, Kister J, Tao H
(2004) Synthesis 1799 and literature therein
91. Linton BR, Carr AJ, Orner BP, Hamilton AD (2000) J Org
Chem 65:1566
92. Esmail R, Kurzer F (1975) Synthesis 301
93. Schmuck C, Lex J (1999) Org Lett 1:1779
94. SchmuckC (1999) Eur J Org Chem 2397
95. Schmuck C, Dudaczek J (2005) Tetrahedron Lett 46:7101
96. Schmuck C (2006) Coord Chem Rev 250:3053
97. Schmuck C, Bickert V (2003) Org Lett 5:4579
98. Schmuck C (2000) Chem Eur J 6:709
99. Schmuck C (1999) Chem Commun 843
100. Kyne GM, Light ME, Hursthouse MB, de Mendoza J, Kilburn
JD (2001) J Chem Soc Perkin Trans I 1258
101. Hunter CA, Purvis DH (1992) Angew Chem Int Ed 31:792
102. Ragusa A, Rossi S, Hayes JM, Stein M, Kilburn JD (2005)
Chem Eur J 5:5674
¨
64. Fokkens M, Schrader T, Klarner F-G (2005) J Am Chem Soc
127:14415
65. Talbiersky P, Bastkowski F, Klarner FG, Schrader T (2008) J
Am Chem Soc 130:9824
66. Aıt-Haddou H, Wiskur SL, Lynch VM, Anslyn EV (2001) J Am
Chem Soc 123:11296
67. Zheng Y-S, Zhang C (2004) Org Lett 6:1189
68. Schmuck C, Bickert V (2007) J Org Chem 72:6832
¨
69. Rehm T, Stepanenko V, Zhang X, Wu¨rthner F, Grohn F, Klein
K, Schmuck C (2008) Org Lett 10:1469
103. Chang S-Y, Kim HS, Chang K-J, Jeong K-S (2004) Org Lett
6:181
104. Schmuck C, Machon U (2005) Chem Eur J 11:1109
105. Fitzmaurice RJ, Gaggini F, Srinivasan N, Kilburn JD (2007) Org
Biomol Chem 5:1706
106. Bourdais J, Omar A, Mohsen ME (1980) J Heterocycl Chem
17:555
107. Schwesinger R (1985) Chimica 39:269
108. Linton B, Hamilton AD (1999) Tetrahedron 55:6027 and liter-
ature therein
¨
70. Calnan BJ, Tidor B, Biancalana S, Hudson D, Frankel AD
(1991) Science 252:1167
71. Hancock RD, Melton DL, Harrington JM, McDonald FC,
Gephart RT, Boone LL, Jones SB, Dean NE, Whitehead JR,
Cockrell GM (2007) Coord Chem Rev 251:1678
109. Srinivasa Reddy D, Vander Velde D, Aube J (2004) J Org Chem
69:1716
110. Bonauer C (2004) PhD thesis, University of Regensburg
¨
111. Bonauer C, Konig B (2005) Synthesis 2367
112. Bonauer C, Zabel M, Konig B (2004) Org Lett 6:1349
¨
¨
72. Kruppa M, Mandl C, Miltschitzky S, Konig B (2005) J Am
Chem Soc 127:3362
73. Escuder B, Rowan AE, Feiters MC, Nolte RJM (2004) Tetra-
hedron 60:291
74. Suzuki I, Obata K, Anzai J-I, Ikeda H, Ueno A (2000) J Chem
Soc Perkin Trans II 1705
75. Gale PA (2003) Coord Chem Rev 240:191
76. Douangamath A, Walker M, Beismann-Driemeyer S, Vega-
Fernandez MC, Sterner R, Wilmanns M (2002) Structure 10:185
113. Evans DA, Peterson GS, Johnson JS, Barnes DM, Campos KR,
Woerpel KA (1998) J Org Chem 63:4541 workup changed
114. Fife TH, Przystas TJ (1982) J Am Chem Soc 104:2251
115. Fornasier R, Milani D, Scrimin P, Tonellato U (1986) J Chem
Soc Perkins Trans II 233
116. Mansuy D, Fontecave M, Bartoli JF (2003) J Am Chem Soc,
Chem Commun 68(24):9419
123