Synthesis and conformational studies of peptidomimetics containing a carbocyclic 1,3-diacid

Article abstract of DOI:10.1016/S0040-4020(01)00921-8

A rigid carbocyclic scaffold comprising of an all-cis 4,5-dihydroxy-1,3-cyclopentanedicarboxylic acid is developed. Attachment of peptide strands to the carboxylic groups of this novel template led to the peptidomimetics 2 and 3. Conformational analysis by circular dichroism and NMR studies revealed that these molecules adopt a unique folded structure in nonpolar solvent involving intramolecular hydrogen bonding between PheNH of one strand and LeuC=O (in 2) or GlyC=O (in 3) of the other strand. This structure is very different from the structures observed earlier in their sugar counterparts (1). The paper describes in detail the synthesis and structural studies of compounds 2 and 3.

Full text of DOI:10.1016/S0040-4020(01)00921-8

TETRAHEDRON
Pergamon
Tetrahedron 57 42001) 9169±9175
Synthesis and conformational studies of peptidomimetics
containing a carbocyclic 1,3-diacid
Tushar K. Chakraborty,a,p Animesh Ghosh,a R. Nagaraj,b A. Ravi Sankara and Ajit C. Kunwara,p
aIndian Institute of Chemical Technology, Hyderabad 500 007, India
bCentre for Cellular and Molecular Biology, Hyderabad 500 007, India
Received 4 June 2001; revised 15 August 2001; accepted 6 September 2001
AbstractÐA rigid carbocyclic scaffold comprising of an all-cis 4,5-dihydroxy-1,3-cyclopentanedicarboxylic acid is developed. Attachment
of peptide strands to the carboxylic groups of this novel template led to the peptidomimetics 2 and 3. Conformational analysis by circular
dichroism and NMR studies revealed that these molecules adopt a unique folded structure in nonpolar solvent involving intramolecular
hydrogen bonding between PheNH of one strand and LeuCvO 4in 2) or GlyCvO 4in 3) of the other strand. This structure is very different
from the structures observed earlier in their sugar counterparts 41). The paper describes in detail the synthesis and structural studies of
compounds 2 and 3. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
of the tetrahydrofuran ring, as the core foundation on which
various peptide chains could be anchored. A new molecular
entity of an all-cis 4,5-dihydroxy-1,3-cyclopentanedi-
carboxylic acid moiety,6 carrying the essential `cis-b-
hydroxycarboxyl' motif on both sides, was conceived as
an ideal template that could nucleate topologically simple
intramolecular folding. In this paper, we describe the
synthesis and detail structural studies of the peptido-
mimetics 2 and 3 prepared from this novel carbocycle-
based template.
Reverse-turn peptidomimetics constitute one of the most
important areas of research in structure±activity relation-
ship studies of peptides.1 Several conformationally rigid,
non-peptidic scaffolds have been designed with different
functional groups anchored on a single molecular frame-
work and used for crafting many interesting reverse-turn
mimetics.2 Many of these templates, especially the ones
with identical functional groups, have also been used
extensively to nucleate parallel b-sheet structures in
peptides.3 Recently, we have developed a structurally
rigid, novel molecular framework of a hexose-derived
2,5-anhydrosugar diacid.4 Bi-directional elongation of its
diacid moieties with identical peptide strands led to the
formation of architecturally beautiful novel C2-symmetric
peptidomimetics 1, which displayed very ordered structures
consisting of identical intramolecular H-bonds at two ends
between AANH!sugar±OH, as shown schematically in
structure A.4 The presence of two `cis-b-hydroxycarboxyl'
moieties, the core structural motif believed to be responsible
for such intramolecular H-bonds,5 on two sides of the
tetrahydrofuran ring nucleated identical b-turn-like
structures in 1 at both ends. To examine whether similar
secondary structure could also be present in other ®ve-
membered ring systems, we decided to construct a struc-
turally similar scaffold with a cyclopentane ring, in place
Keywords: peptide mimetics; hydrogen bonding; conformation; NMR;
Diels±Alder reactions.
p
Corresponding authors. Tel.: 191-40-717-2976; fax: 191-40-717-3757/
3387; e-mail: chakraborty@iict.ap.nic.in
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020401)00921-8

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T. K. Chakraborty et al. / Tetrahedron 57 "2001) 9169±9175
involving a Diels±Alder reaction in the ®rst step between
cyclopentadiene and vinylene carbonate, followed by
alkaline hydrolysis of the carbonate ring. The diol 4, thus
obtained, was protected as an acetonide 5 in 95% yield,
which was subjected to an oxidative cleavage reaction
using a catalytic amount of RuCl3´3H2O in the presence of
NaIO4 to give the diacid 6 in 85% yield. Compound 6 was
then treated with an excess of amino-terminal free peptide
molecule H2N±Phe±Leu±OMe under standard solution
phase methods using 1-ethyl-3-43-4dimethylamino)propyl)-
carbodiimide hydrochloride 4EDCI) and 1-hydroxybenzo-
triazole 4HOBt) as coupling agents and amine-free dry
DMF and/or dry CH2Cl2 as solvent. The puri®ed product
7 4yield 80%) was deprotected under acidic conditions to
furnish the ®nal product 2 in 55% yield. The same protocol
was followed for the synthesis of the other molecule 3, by
attaching H2N±Phe±Gly±OMe to the diacid 7 under the
same conditions described in Scheme 1 for the synthesis
of 2.
2.2. Conformational analysis. Circular dichroism studies
The conformational analysis of the peptidomimetics 2 and 3
were carried out by studying their circular dichroism 4CD)
spectra in tri¯uoroethanol 4TFE). A strong positive band
with very high ellipticity at 222 nm exhibited by 2 in its
CD spectrum 4Fig. 1) suggested the possibility of the
existence of a turn structure in the molecule.8 Compound
3 also showed a positive band at 216 nm indicating the
likely occurrence of a similar structure in solution.
Scheme 1. Synthesis of the peptidomimetic 2.
2. Results and discussion
2.3. Conformational analysis. NMR studies
The dimethyl ester of 6 showed a two-fold molecular
symmetry in its proton NMR spectrum. In the energy-mini-
mized structure of the molecule, that was supported by the
1H NMR coupling constants, the C1±C5±C4±C3 dihedral
angle was zero making it a perfect meso isomer. However,
addition of the identical dipeptide residues on both sides
resulted in the loss of two-fold molecular symmetry in the
peptidomimetics 2 and 3. The difference of the behavior of
the left and right halves of the molecules resulted from their
diastereomeric relationship to each other. The protons of the
left side peptide chain 4peptide I), attached to the C3 end of
the cyclopentane ring, with 4L)LRR con®guration 4LLRR
for 3, LRR for 2, amino acids were all L) showed different
chemical shifts than those in the right side peptide 4peptide
I0) linked to the C1 end of the ring with 4L)LSS con®gu-
ration.9 This was also largely due to the different confor-
mational behavior of the two peptide chains discussed
below in detail.
2.1. Synthesis of peptidomimetics 2 and 3
Scheme 1 outlines the synthesis of 2. The starting material,
endo-cis-bicyclo[2.2.1]hept-5-ene-2,3-diol 44), was pre-
pared according to the reported procedure7 in two steps
The absence of molecular symmetry leads to the spectral
complexity due to the appearances of the resonances close
to each other, having very similar chemical shifts. It was,
therefore, not possible to derive all the spectral parameters,
especially the couplings involved in the Leucine side-
chains. The sequential aH±NH, H3±Phe4I)NH and H1±
Phe4I0)NH cross-peaks in the ROESY spectra were used
to make assignments of the resonances in the peptide chains
I and I0. The spectral parameters for 2 and 3 are given in
Tables 1 and 2, respectively.
Figure 1. CD spectra 4in TFE) of 2 4a) and 3 4b).

T. K. Chakraborty et al. / Tetrahedron 57 "2001) 9169±9175
9171
Table 1. 1H chemical shifts 4d) in ppm, Coupling constants 4J) in Hz, and temperature coef®cients ꢀDd=DT; in ppb/K) of the amide protons of 2 4500 MHz,
CDCl3)
Protons
Phe 4I)
Leu 4I)
Phe 4I0)
Leu 4I0)
NH
7.60 4d, JNH±aHˆ7.6 Hz)
4.63 4ddd, Jˆ7.9, 7.6, 6.0 Hz)
3.17 4dd, Ja±bˆ6.0 Hz)
6.67 4d, JNH±aHˆ8.6 Hz)
4.58 4ddd, Ja±bˆ9, 5 Hz)a
{1.55±1.43 4m)
6.37 4d, JNH±aHˆ7.7 Hz)
4.73 4ddd, Jˆ7.7, 6.5, 6.2 Hz)
3.26 4dd, Ja±bˆ6.2 Hz)
6.69 4d, JNH±aHˆ8.6 Hz)
4.59 4ddd, Ja±bˆ9, 5 Hz)a
{1.55±1.43 4m)
CaH
CbH
CbH0
0
0
0
0
3.09 4dd, Ja±b ˆ7.9 Hz, Jb±b
ˆ
3.08 4dd, Ja±b ˆ6.5 Hz, Jb±b
ˆ
13.9 Hz)
14.0 Hz)
CgH
CdH
0.86 4d, CH3, Jg±dˆ6.3 Hz)
0.87 4d, CH3, Jg±dˆ6.3 Hz)
3.69 4s, CO2CH3)b
6.5
0.89 4d, CH3, Jg±dˆ6.3 Hz)
0.90 4d, CH3, Jg±dˆ6.3 Hz)
3.71 4s, CO2CH3)b
5.2
Others
2Dd=DT 4ppb/K) for NH
7.20±7.36 4m, ArH)
9.1
7.20±7.36 4m, ArH)
10.7
Cyclopentane ring: 4.47 4d, 4-OH, JH4±OHˆ7.4 Hz), 4.28 4d, 5-OH, JH5±OHˆ6.1 Hz), 4.21 4m, H4), 4.19 4m, H5), 2.81 4m, H1), 2.79 4m, H3), 2.26 4dt, H2,
J2±2 ˆ14:3 Hz, J1±2ˆJ3±2ˆ7:2 Hz), 2.06 4dt, H20, J2±2 ˆ14:3 Hz, J1±2 ˆJ3±2 ˆ9:3 Hz)
0
0
0
0
a
Errors in these couplings are ^1 Hz.
These assignments are interchangeable.
b
Table 2. 1H Chemical shifts 4d) in ppm, coupling constants 4J) in Hz, and temperature coef®cients ꢀDd=DT; in ppb/K) of the amide protons of 3 4500 MHz,
CDCl3)
Protons
Phe 4I)
Gly 4I)
7.30 4bs)
Phe 4I0)
Gly 4I)
7.2 4bs)
NH
CaH
8.06 4d, JNH±aHˆ8.5 Hz)
4.61 4dt, Jˆ8.5, 5.5 Hz)
6.74 4d, JNH±aHˆ8.0 Hz)
4.86 4dt, Jˆ8.0, 5.8 Hz)
aˆ4.08 4dd,
aˆ4.04 4dd,
JNH±aHˆ6.0 Hz),
a0ˆ3.86 4dd,
JNH±aHˆ 5.6 Hz),
a0ˆ3.88 4dd,
0
0
JNH±a Hˆ5.0 Hz)
JNH±a Hˆ5.0 Hz)
0
0
4Ja±a ˆ18.0 Hz)
4Ja±a ˆ18.3 Hz)
CbH
CbH0
3.24 4dd, Ja±bˆ5.5 Hz)
3.23 4dd, Ja±bˆ5.8 Hz)
0
0
3.04 4dd, Ja±b ˆ8.5 Hz,
3.08 4dd, Ja±b ˆ8.0 Hz,
0
0
Jb±b ˆ13.9 Hz)
Jb±b ˆ14.2 Hz)
Others
2Dd=DT 4ppb/K) for NH
7.18±7.32 4m, ArH)
14.1
3.68 4s, CO2CH3)a
9.1
7.18±7.32 4m, ArH)
2.45
3.69 4s, CO2CH3)a
9.3
0
Cyclopentane ring: 4.56 4bs, OH), 4.42 4bs, OH), 4.21 4m, H4), 4.22 4m, H5), 2.82 4m, H1), 2.83 4m, H3), 2.32 4dt, H2, J2±2 ˆ14.5 Hz, J1±2ˆJ3±2ˆ5.2 Hz), 1.9
4dt, H20, J1±2 ˆJ3±2 ˆ9.3 Hz)
0
0
a
These assignments are interchangeable.
In noncompetitive solvents like CD2Cl2, CDCl3, intra-
molecularly hydrogen bonded amide protons resonate
down®xeld and typically exhibit a relatively large tempera-
ture dependence of the chemical shifts compared to the free
amide protons.3c The temperature coef®cients of the amide
proton chemical shifts ꢀDd=DT† are listed in Tables 1 and 2.
The Phe4I)NH signals in both 2 and 3 show larger tempera-
ture coef®cients than the other amide protons indicating
their possible involvements in intramolecular H-bonding.
The variations in the chemical shifts of the amide protons in
nonpolar 4CDCl3)-polar solvent 4DMSO-d6) titration also
provide information on their participation in the intra-
molecular hydrogen bondings.10 Addition of strongly hydro-
gen bonding solvents like DMSO-d6 to CDCl3 solution of
the peptides perturbs all the exposed amide protons, which
start forming H-bonds with the added polar solvent
molecules causing substantial down®eld shifts in their
NMR signals. The intramolecularly hydrogen bonded
amide proton chemical shifts, on the other hand, show rela-
tively smaller solvent dependence. The solvent induced
shifts of the amide proton resonances of 2 and 3 are
shown in Figs. 2 and 3, respectively. Only Phe4I)NH showed
small shifts in both the compounds indicating its partici-
pation in intramolecular H-bonds.
2.4. Conformational analysis of 2
The 3JNH±aH were about 8.6 Hz for the Leucines in 2, imply-
ing the predominance of w values of approximately 21208,
whereas for its Phe residues, the 3JNH±aH were about 7.5 Hz,
which may arise due to either the propensity of a structure
Figure 2. Solvent titration plots for the amide protons of 2.

9172
T. K. Chakraborty et al. / Tetrahedron 57 "2001) 9169±9175
Figure 5. Stereoview of the 20 superimposed energy-minimized structures
sampled during 20 ps MD simulations of 2.
membered intramolecularly hydrogen bonded ring involv-
ing Phe4I)NH and Leu4I0)CO, as depicted schematically in
Fig. 4.
The cross-peak intensities in the ROESY spectra were used
for obtaining the restraints in the MD calculations. The
two-spin approximation was used to obtain the inter-
molecular distances. The lower and upper bounds of these
distance restraints were ®xed as 15% less and 15% more,
respectively, than the derived distances. Several long-range
distance constraints, torsional restraints and an H-bonding
restraint were used in the energy calculations. The results
derived from the 20 ps dynamics run for compound 2 are
shown in Fig. 5. The structures are consistent with NMR
restraints and have an H-bond between Phe4I)NH and the
Leu4I0)-carbonyl. The cyclopentane ring showed a C2-endo
structure, which is in agreement with the NMR data.
Interestingly, the dimethyl ester of 6 also showed such a
cyclopentane ring geometry.
Figure 3. Solvent titration plots for the amide protons of 3.
with w<21208 or the average of several conformations.
The presence of a strong sequential aH±NH connectivity
is consistent with the w ±c values in the b-region of
the Ramachandran plots. Yet, the presence of NH±NH
sequential connectivity indicates that some fraction of the
molecules have w ±c values in the a-region. The 3JaH±bH for
the Phe residues were about 7 Hz. This is a result of more
than one signi®cant rotamers about Ca±Cb. For Leu, the
two JaH±bH are 5 and 9 Hz 4due to spectral overlap these
values are accurate to about 1 Hz). Inability to make stereo-
speci®c assignments for the b-protons in these residues
precluded obtaining information on the rotamer populations
about Ca±Cb.
2.5. Conformational analysis of 3
The solution of structure of 3 as determined by NMR studies
was found to be very similar to that of 2. The H NMR
1
data of 3 is shown in Table 2. Its Phe4I)NH also showed a
down®eld chemical shift 48.06 ppm) that had a very large
temperature coef®cient 4214.1 ppb/K) and was less solvent-
dependent compared to the other amide proton chemical
shifts as can be seen from its solvent titration plots 4Fig.
3). The structure of 3 as determined from the various
coupling constants, ROESY cross-peaks and subsequent
MD calculations was found to be very similar to that of 2,
involving an intramolecular H-bond between Phe4I)NH and
Gly4I0)-carbonyl. In fact, the Phe4I)NH in 3 had a more
down®eld chemical shift, a much larger temperature co-
ef®cient and also lesser solvent dependence compared to
those exhibited by the Phe4I)NH in 2, suggesting a stronger
H-bonded ring structure in the former.
Phe4I)NH of 2 appeared at very low ®eld 47.6 ppm)
compared to Phe4I0)NH indicating its participation in an
intramolecular hydrogen bond.11,12 This was con®rmed by
the temperature coef®cient of Phe4I)NH 429.1 ppb/K) that
was larger than those of other amide protons. The solvent-
titration also provided further support for intramolecular
H-bonding. Addition of DMSO-d6 in the CDCl3 solution
of 2 4Fig. 2) resulted in a very small shift of its Phe4I)NH
signal compared to the other amide proton chemical shifts,
which exhibited large solvent dependence.
The rOe cross-peaks suggested the existence of a 14-
The interstrand H-bonded turn structure observed here for
these cabocycle-based peptidomimetics 2 and 3 is quite
different from the structure observed in the corresponding
sugar analogue Idac-4Phe±Leu±OMe)2 41) that displayed a
very symmetrical structure 4A) consisting of identical intra-
molecular H-bonds at two ends between LeuNH!sugar±
OH and had a different sugar pucker consisting of a twist
conformation.4 A close look at the structures obtained from
the MD simulations of Idac-4Phe±Leu±OMe)2 showed that
in this molecule the distance between the Phe amide-proton
Ê
and the sugar ring oxygen is ,2.2A and the resulting
electrostatic interaction was possibly responsible for the
Figure 4. Schematic representation of the 14-membered hydrogen bonded
ring structure of 2 with the long range ROEs seen in its NMR spectrum.

T. K. Chakraborty et al. / Tetrahedron 57 "2001) 9169±9175
9173
induction of the observed pseudo b-turn in the molecule.4a
Replacement of the ring oxygen with carbon resulted in the
loss of similar electrostatic interactions in 2 and 3 forcing
the peptide chains to adopt different orientations. However,
the `1,3-cis' relationship between the peptide chains
anchored on the carbocyclic ring in these molecules could
still bring them into close proximity that probably induced
the observed interstrand H-bond. These three factors,
namely, the `1,3-cis' relationship of the peptide chains,
the absence of the oxygen atom in the core ®ve-membered
ring and a different puckering of the cyclopentane ring
probably worked synergistically to instill this unique struc-
ture in these molecules which is distinctly different from
their sugar-based counterparts.
were carried out in the phase sensitive mode using the
procedure of States et al.14 The spectra were acquired with
2£256 or 2£192 free induction decays 4FID) containing
8±16 transients with relaxation delays of 1.5±2.0 s. The
ROESY experiments were performed with mixing time of
0.3 s. For ROESY experiments a spin-locking ®eld of about
2 kHz and pulsed ®eld locking with 308 pulses were used.
The TOCSY experiments were performed with the spin
locking ®elds of about 10 kHz and a mixing time of
0.08 s. The two-dimensional data were processed with
gaussian apodization in both the dimensions. The chemical
shifts, coupling constants and temperature coef®cients
ꢀDd=DT† of amide proton chemical shifts of 2 and 3 are
given in Tables 1 and 2, respectively. To obtain the tempera-
ture coef®cients of NH-chemical shifts, the spectra were
recorded between 30 and 508C.
The rigid scaffold of the cyclopentane ring described here
can serve as a novel template to design interesting peptido-
mimetics that may lead to the development of de novo
molecular entities with well-de®ned three dimensional
structures and useful properties.
3.3. Molecular dynamics
All molecular mechanics/dynamics calculations were
carried out using Sybyl 6.7 program on a Silicon Graphics
O2 workstation. The Tripos force ®eld with default
parameters was used throughout the simulations. Minimi-
zations were done ®rst with steepest decent, followed
by conjugate gradient methods for a maximum of 2000
iterations each or RMS deviation of 0.05 kcal mol21, which-
ever was earlier. The energy-minimized structures were
then subjected to MD simulations. A number of inter-atomic
distance and torsional angle constraints15 obtained from
NMR data were used as restraints in the minimization as
well as MD runs. For MD runs a temperature of 300 K was
used. The energy-minimized molecules were initially
equilibrated for 1 ps and subsequently subjected to a 20 ps
dynamics with a step size of 1 fs, sampling the trajectory at
equal intervals of 1 ps. The samples were minimized using
the above mentioned energy minimization protocol. The
twenty energy-minimized structures were compared and
superimposed as shown in Fig. 4.
3. Experimental
3.1. General procedures
All reactions were carried out in oven or ¯ame-dried glass-
ware with magnetic stirring under nitrogen atmosphere
using dry, freshly distilled solvents, unless otherwise
noted. Reactions were monitored by thin layer chroma-
tography 4TLC) carried out on 0.25 mm silica gel plates
with UV light, I2, 7% ethanolic phosphomolybdic acid-
heat and 2.5% ethanolic anisaldehyde 4with 1% AcOH
and 3.3% conc. H2SO4)-heat as developing agents. Silica
gel ®ner than 200 mesh was used for ¯ash column chroma-
tography. Yields refer to chromatographically and spectro-
scopically homogeneous materials unless otherwise stated.
Melting points are uncorrected.
IR spectra were recorded as neat liquids or KBr pellets.
Mass spectra were obtained under electron impact 4EI)
and liquid secondary ion mass spectrometric 4LSIMS)
techniques, respectively. For LSIMS m-nitrobenzyl alcohol
was used as a matrix.
3.3.1. The acetonide 5. To a solution of 4 41.5 g, 11.9
mmol) in dry CH2Cl2 45 mL) was added 2,2-dimethoxy-
propane 42.92 mL, 23.8 mmol) followed by the addition of
camphorsulphonic acid 428 mg, 0.12 mmol) at 08C. The
reaction mixture was stirred at room temperature for 4 h.
It was then diluted with CHCl3, washed with saturated
NaHCO3 and brine, dried 4Na2SO4), ®ltered and concen-
trated in vacuo. Puri®cation by column chromatography
4SiO2, 5% EtOAc in petroleum ether eluant) afforded 5
41.877 g, 95%) as a syrupy liquid. Rfˆ0.5 4silica gel, 25%
EtOAc in petroleum ether); IR 4neat): nmax 3425, 3000,
The CD spectra were recorded in quartz cell of 1 mm path
length at 258C using a peptide concentration of 0.7 mM in
TFE.
3.2. NMR spectroscopy
1
2950, 1500, 1050 cm21; H NMR 4CDCl3, 200 MHz): d
NMR spectra were recorded on 200 4218C) and 500 MHz
4308C) spectrometers with 7±10 mM solutions in appro-
priate solvents using tetramethylsilane as internal standard
or the solvent signals as secondary standards and the
chemical shifts are shown in d scales. 13C NMR spectra
were recorded with complete proton decoupling. The
assignments were carried out with the help of two-dimen-
sional total correlation spectroscopy 4TOCSY).13 For some
cases the rotating frame nuclear Overhauser effect spectro-
scopy 4ROESY) experiments,13 which provide the infor-
mation on the proximity of protons, were additionally
used to con®rm the assignments made. All the experiments
6.1 4s, 2H, ole®nicH), 4.65 4s, 2H, CH±O), 2.93 4s, 2H,
tertiary C±H), 1.65 and 1.43 4two d, Jˆ7.5 Hz, 2H, CH2),
1.3 and 1.2 4two s, 6H, acetonide methyls); 13C NMR
4CDCl3, 125 MHz): d 134.03, 114.5, 81.27, 46.21, 45.58,
25.68, 25.22; MS 4EI): m/z 4%): 151 410) [M12CH3];
HRMS 4EI): calcd for C9H11O2 [M12CH3]: 151.0759,
found: 151.0758.
3.3.2. The diacid 6. To a solution of 5 41.5 g, 9.036 mmol)
in CCl4/CH3CN/H2O 41:1:1.5, 28 mL) was added NaIO4
48.697 g, 40.66 mmol) at 08C. To this biphasic solution,
RuCl3´3H2O 452.3 mg, 0.2 mmol) was added at the same

9174
T. K. Chakraborty et al. / Tetrahedron 57 "2001) 9169±9175
temperature and the reaction mixture was stirred vigorously
for 12 h at room temperature. It was then diluted with
acetone 4100 mL), ®ltered through a short pad of Celitew
and the ®lter cake was washed with acetone. The ®ltrate
and washings were combined and concentrated in vacuo.
The residue was again dissolved in acetone 430 mL), ®ltered
and the ®ltrate was concentrated in vacuo to get the diacid 6
41.76 g, 85%) as a white solid. Rfˆ0.5 4silica gel, EtOAc);
mp 238±2408C; IR 4neat): nmax 3450, 3275, 2975, 2940,
42£50 mL). The organic extracts were combined, washed
with brine, dried 4Na2SO4) and concentrated in vacuo.
Puri®cation by column chromatography 4SiO2, 5% MeOH
in CHCl3 eluant) afforded 2 40.202 g, 55%) as a white solid.
Rfˆ0.41 4silica, CHCl3/MeOH/AcOH, 80:8:2); [a]D
22
ˆ
236.3 4c 0.31, CHCl3); mp 193±1948C; IR 4neat): nmax
1
3300, 2950, 1735, 1650, 1530 cm21; H NMR 4CDCl3,
500 MHz): see Table 1; 13C NMR 4CDCl3, 125 MHz): d
173.40, 173.27, 172.85, 170.77, 170.25, 136.80, 136.28,
129.42, 129.35, 128.81, 128.56, 127.25, 126.89, 74.75,
74.50, 55.03, 54.40, 52.37, 52.26, 50.89, 50.73, 47.59,
46.83, 41.37, 41.28, 37.80, 37.46, 28.34, 24.70, 22.75,
22.71, 21.86, 21.79; MS 4LSIMS): m/z 4%): 739 424)
[M11H], 761 416) [M11Na]; HRMS 4LSIMS): calcd for
C39H55N4O11 [M11H]: 739.3918, found: 739.3917.
1
2890, 1375, 1040 cm21; H NMR 4DMSO-d6, 500 MHz):
d 12.06 4br s, 2H, COOH), 4.72 4m, 2H, CH±O), 2.70 4m,
2H, CHCO2H), 2.08 4dt, Jˆ13.1, 12.6 Hz, 1 H, methyl-
eneCH), 1.64 4dt, Jˆ13.1, 6.1 Hz, 1H, methyleneCH0),
1.26 and 1.19 4two s, 6H, acetonide methyls); 13C NMR
4DMSO-d6, 125 MHz): d 171.60, 109.38, 80.17, 46.98,
25.62, 25.57, 24.11; MS 4LSIMS): m/z 4%): 231 442)
[M11H]; HRMS 4LSIMS): calcd for C10H15O6 [M11H]:
231.0868, found: 231.0867.
3.3.5. Peptidomimetic 3. This compound was synthesized
following the same method as described above for the
synthesis of 2. Rfˆ0.31 4silica, CHCl3/MeOH/AcOH,
22
80:8:2); [a]D ˆ221.3 4c 0.11, MeOH); mp 206±2088C;
3.3.3. The acetonide protected peptidomimetic 7. To a
solution of BocNH±Phe±Leu±OMe 41.078 g, 2.75 mmol)
in dry CH2Cl2 44 mL) was added tri¯uoroacetic acid 42 mL)
at 08C and stirred under nitrogen for 1 h. The reaction
mixture was then concentrated in vacuo. In another round-
bottom ¯ask, the diacid 6 40.287 g, 1.25 mmol) dissolved in
dry DMF 42 mL) and dry CH2Cl2 44 mL) was sequentially
treated with HOBt 40.337 g, 2.5 mmol) and EDCI 40.479 g,
2.5 mmol) at 08C. After 15 min, TFA´H2N±Phe±Leu±OMe
prepared above and dissolved in dry DMF 42 mL) was
added to the reaction mixture followed by the addition of
DIPEA 40.97 mL, 5.5 mmol). After stirring for 12 h at room
temperature, the reaction mixture was diluted with EtOAc,
washed with saturated NH4Cl and brine, dried 4Na2SO4) and
concentrated in vacuo. Puri®cation by column chroma-
tography 4SiO2, 2.5% MeOH in CHCl3 eluant) afforded 7
40.78 g, 80%) as a white solid. Rfˆ0.43 4silica, CHCl3/
1
IR 4neat): nmax 3300, 2925, 1735, 1650, 1525 cm21; H
NMR 4CDCl3, 500 MHz): see Table 2; 13C NMR 4DMSO-
d6, 125 MHz): d 172.76, 172.70, 171.48, 169.98, 138.00,
137.59, 129.15, 128.96, 128.00, 127.96, 126.15, 74.34,
74.11, 53.84, 53.79, 51.64, 51.61, 46.72, 45.88, 40.64,
40.57, 37.33, 36.93, 27.05; MS 4LSIMS): m/z 4%): 649
458) [M11Na]; HRMS 4LSIMS): calcd for C31H39N4O10
[M11H]: 627.2666, found: 627.2660.
Acknowledgements
We thank Drs M. Vairamani and R. Srinivas for mass
spectrometric assistance, CSIR, New Delhi for research
fellowships 4A. G. and A. R. S.) and DST, New Delhi for
®nancial support.
22
MeOH/AcOH, 80:8:2); [a]D ˆ233.9 4c 0.32, CHCl3);
mp 266±2688C; IR 4neat): nmax 3425, 3275, 1720, 1635
1
cm21; H NMR 4CDCl3, 500 MHz): d 7.31±7.20 410H,
References
ArH), 6.58 and 6.57 4two d, Jˆ7.9 and 7.7 Hz, 2H,
PheNH), 6.34 and 6.32 4two d, Jˆ8.4 and 8.2 Hz, 2H,
LeuNH), 4.74 4m, 2H, C4±H and C5±H), 4.71 4m, 2H,
PheaH), 4.55 4m, 2H, LeuaH), 3.70 and 3.69 4two s, 6H,
CO2CH3), 3.21 and 3.17 4two dd, Jˆ14.0, 6.4 Hz, 2H,
PhebH), 3.07 and 3.06 4two dd, Jˆ14, 6.4 Hz, 2H, PhebH0),
2.49 4m, 2H, carbocyclic C1±H and C3±H), 2.09 4dt,
Jˆ13.1, 6.1 Hz, 1H, C2±H), 2.01 4dt, Jˆ13.1, 12.6 Hz,
1H, C2±H0), 1.62±1.40 4m, 6H, LeubH and LeugH), 1.27
and 1.25 4two s, 6H, acetonide methyls), 0.90 and 0.89 4four
d, Jˆ6.6 Hz, 12H, Leu methyls); 13C NMR 4CDCl3, 125
MHz): d 172.75, 170.37, 169.62, 169.48, 136.58, 136.40,
129.46, 129.42, 128.68, 128.60, 127.08, 126.99, 111.38,
79.88, 79.79, 54.10, 54.00, 52.23, 50.88, 50.81, 49.27,
49.10, 41.54, 41.37, 37.51, 28.94, 25.33, 24.75, 24.68,
23.89, 22.72, 21.94; MS 4LSIMS): m/z 4%): 779 4100)
[M11H], 801 472) [M11Na]; HRMS 4LSIMS): calcd for
C42H59N4O10 [M11H]: 779.4231, found: 779.4225.
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6. For some other 1,3-cyclopentanedicarboxylic acids see: