Design, synthesis and evaluation of a PLG tripeptidomimetic based on a pyridine scaffold

Article abstract of DOI:10.1021/jm049484q

A 2,3,4-substituted pyridine derivative has been identified as a potential tripeptidomimetic scaffold. The design of the scaffold was based on conformational and electrostatic comparisons with a natural tripeptide. The scaffold has been used in the synthe

Full text of DOI:10.1021/jm049484q

J. Med. Chem. 2004, 47, 6595-6602
6595
Design, Synthesis and Evaluation of a PLG Tripeptidomimetic Based on a
Pyridine Scaffold
Stina Saitton,†,‡ Andria L. Del Tredici,§ Nina Mohell,§,| Roeland C. Vollinga, Dan Bostro¨m,‡ Jan Kihlberg,‡,# and
Kristina Luthman*,†
Department of Chemistry, Medicinal Chemistry, Go¨teborg University, SE-412 96 Go¨teborg, Sweden, Department of Chemistry,
Organic Chemistry, Umeå University, SE-901 87 Umeå, Sweden, and ACADIA Pharmaceuticals Inc.,
3911 Sorrento Valley Boulevard, San Diego, California 92121
Received June 29, 2004
A 2,3,4-substituted pyridine derivative has been identified as a potential tripeptidomimetic
scaffold. The design of the scaffold was based on conformational and electrostatic comparisons
with a natural tripeptide. The scaffold has been used in the synthesis of a Pro-Leu-Gly-NH2
(PLG) mimetic. The different substituents in the 2-, 3-, and 4-positions of the pyridine ring
were introduced via an aromatic nucleophilic substitution reaction, a “halogen-dancing” reaction,
and a Grignard coupling of a Boc-protected amino aldehyde, respectively. The synthetic route
involves eight steps and provides the mimetic in 20% overall yield. The pyridine based PLG-
mimetic was evaluated for its ability to enhance the maximum response of the dopamine agonist
N-propylapomorphine (NPA) at human D2 receptors using a cell based assay (the R-SAT assay).
The dose-response curve of the mimetic was found to exhibit a down-turn phase, similar to
that of PLG. In addition, the mimetic was more potent than PLG to enhance the NPA response;
the maximum response was found to be 146% at 10 nM concentration, as compared to 115%
for PLG at the same concentration. Interestingly, conformational analysis by molecular modeling
showed that the pyridine mimetic cannot adopt a type II â-turn conformation that previously
has been suggested to be the bioactive conformation of PLG.
Introduction
development of a general tripeptidomimetic scaffold
useful for mimicry and for investigation of conforma-
tional properties of various tripeptides and tripeptide
segments. In this study the design of a general tripep-
tidomimetic based on a 2,3,4-substituted pyridine scaf-
fold and its application to the tripeptide L-Pro-L-Leu-
Gly-NH2 (PLG) by the synthesis and biological evaluation
of mimetic 1 is presented.
Several short peptides have been shown to be involved
in vital physiological processes. Therefore, such peptides
are interesting as lead compounds for drug develop-
ment.1 In general, the therapeutic potential of natural
peptides is limited by their physicochemical properties.
The instability of amide bonds toward enzymatic deg-
radation prevents oral administration, peptides are
usually poorly transported through cell membranes and
they are rapidly excreted through liver and kidneys, and
the conformational flexibility of short peptides often
results in low receptor selectivity. To circumvent these
limitations, nonpeptidic compounds that mimic the
physiological effects of endogenous peptides are desired.
These compounds are commonly referred to as pepti-
domimetics.2,3 One approach toward the development
of peptidomimetics is to use a molecular template or
scaffold to which important pharmacophoric groups,
supposedly amino acid side chains, can be attached.4-7
Such compounds have the potential of being orally
active, selective, and metabolically stable. Conforma-
tionally more rigid structures can also provide impor-
tant information about the bioactive conformation of
peptides. We have for some time been interested in the
PLG is an endogenous tripeptide found in the central
nervous system. Pharmacological studies have shown
that PLG exerts its biological effects by interaction with
the dopaminergic pathways.8 In particular, PLG en-
hances the effect of dopamine agonists. This modulation
of the dopamine receptors emanates from an enhanced
binding of agonists to the D2 receptor in the presence
of PLG and a shift in the ratio between receptors in the
low-affinity state versus the high-affinity state.9 Nu-
merous peptide analogues10-17 and peptidomimetics18-27
of PLG have been synthesized and evaluated in binding
assays and in vivo models in the search for compounds
with enhanced activity and stability in vivo. In addition,
to provide structure-activity information of the interac-
tion of PLG with the dopamine D2 receptors, such
compounds have potential use in the treatment of
dopamine-related disorders such as Parkinson’s disease
and tardive dyskinesia.
* To whom correspondence should be addressed. Telephone: +46-
31-772 2894. Fax: +46-31-772 3840. E-mail: luthman@chem.gu.se.
† Go¨teborg University.
‡ Umeå University.
§ ACADIA Pharmaceuticals Inc.
| Present address: R&D, Biovitrum AB, SE-112 76 Stockholm,
Sweden.
Present address: DSM Anti-Infectives B.V., P.O. Box 425, 2600
AK Delft, The Netherlands.
# Present address: AstraZeneca R&D Mo¨lndal, SE-431 83 Mo¨lndal,
Sweden.
10.1021/jm049484q CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/18/2004

6596 Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26
Scheme 1. Design Strategy
Saitton et al.
Figure 1. A geometrical comparison of two selected low-
energy conformations of the model tripeptide and the corre-
sponding mimetic (X ) NH).
Scheme 2. Retrosynthetic Analysis of the PLG Mimetic
Table 1. Semiempirical Calculations of Atomic Charges for the
Ac-Ala-Ala-Ala-NHMe Tripeptide and the Corresponding
Tripeptidomimeticsa
compound
atom 1
atom 2
atom 3
peptide
-0.56
-0.23
-0.68
-0.70
0.75
0.31
0.92
0.82
-0.65
-0.58
-0.74
-0.42
benzene mimetic
pyridine mimetic X ) NH
pyridine mimetic X ) O
a See Scheme 1 for structures and numbering of the atoms.
PLG mimetic 1 is shown in Scheme 2. The key inter-
mediate is the functionalized scaffold 2, in which the
leucine side chain is attached to the 3-position of the
pyridine ring and the neighboring positions are func-
tionalized by fluorine and iodine, respectively. This
substitution pattern allows for selective introduction of
suitable groups in the 2- and 4-positions. The N-
terminal amino acid (proline) could be attached to the
scaffold via a metal-mediated coupling reaction, and the
C-terminal amino acid equivalent (glycolamide) could
be introduced via a nucleophilic aromatic substitution
reaction. For synthetic reasons, the connecting atom X
in the proposed tripeptidomimetic, was chosen to be an
oxygen atom.31
The functionalized scaffold 4 was synthesized in two
steps starting from 2-fluoropyridine as described earlier
(Scheme 3).31 In the first step, treatment of 2-fluoropy-
ridine with LDA followed by addition of iodine afforded
2-fluoro-3-iodopyridine (3) in 78% yield.32 To introduce
the substituent in the 3-position, a phenomenon referred
to as “halogen-dancing” was used.33 Metalation of 3 with
LDA initially forms an anion in the 4-position. This
anion is rearranged to the 3-position while iodine
migrates to the 4-position, presumably due to the
additional stabilization of the anion by fluorine. Sub-
sequent addition of an electrophile results in attachment
of the desired side chain equivalent in the 3-position
and provides a 3-alkyl-2-fluoro-4-iodopyridine. Attempts
failed to introduce an isobutyl group directly by using
isobutyl bromide or isobutyl iodide as electrophiles.
Instead, an unsaturated side-chain equivalent was
employed; reaction with metallyl bromide produced the
isobutylene derivative 4 in 93% yield.
Results and Discussion
Design of a Tripeptidomimetic Scaffold. Initially
a trisubstituted aromatic ring was investigated as a
peptidomimetic scaffold (Scheme 1). Such system seemed
attractive from a synthetic point of view due to the
thoroughly explored chemistry of benzene derivatives
and the lack of stereocenters in the ring. The idea was
to replace the second amino acid of the tripeptide with
the aromatic ring containing the amino acid side chain
moiety and then let the peptide chain continue from the
neighboring (ortho) positions.
To improve the mimicking ability of the six-mem-
bered, aromatic scaffold, different heterocyclic ring
systems were examined by computer-based methods.28
Molecular mechanics calculations29 were used to exam-
ine conformational preferences, and AM1 semiempirical
methods30 were used to calculate electrostatic proper-
ties. An Ac-Ala-Ala-Ala-NHMe tripeptide was used as
a model compound together with various suggestions
of appropriate peptidomimetic structures. By comparing
the predicted atomic charges with those of the model
tripeptide, it was found that the pyridine ring system
showed interesting properties. The nitrogen atom in the
pyridine ring provides a good electrostatic mimic of the
carbonyl oxygen in an amide bond (Table 1). The
connecting atom (X) in the proposed tripeptidomimetic
could be either a nitrogen or an oxygen atom. Figure 1
shows a geometrical comparison of the natural tripep-
tide and the proposed mimetic (X ) NH). As can be seen,
the geometric fit between the peptide and the mimetic
is good overall, except for the direction of the second
amino acid side chain, which is constrained by the
aromatic ring.
The metal-mediated coupling of the proline residue
to the 4-position proved troublesome at first.31 Several
attempts to accomplish the coupling via a Stille reaction
or a lithiated scaffold failed. Instead, the coupling was
accomplished using a Grignard reaction. The Grignard
reagent was formed by treatment of the pyridine scaffold
with iPrMgCl in THF at room temperature.34 Subse-
To explore the synthetic feasibility of the designed
scaffold, PLG was chosen as the first tripeptide target.
Synthesis. A strategy for the synthesis of the 2,3,4-
trisubstituted pyridine system, where the introduction
of the different substituents was investigated, has
recently been reported.31 A retrosynthetic analysis of

A Novel Pyridine-Based PLG Mimetic
Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26 6597
Scheme 3. Synthesis of the PLG Mimetica
a Reaction conditions: (i) LDA, I2, THF, -78 °C, 5 h; (ii) LDA, metallyl bromide, THF, -78 °C, 3.5 h; (iii) iPrMgCl, Boc-Pro-CHO, THF,
rt, overnight; (iv) BnBr, NaH, cat. Bu4NI, THF, rt, 28 h; (v) Glycolamide, KH, DMSO, 55 °C, 60 h; (vi) H2, cat. Pd/C, MeOH, rt, 1.5 h; (vii)
Dess-Martin periodinane, CH2Cl2, rt, overnight; (viii) TFA, CH2Cl2, rt, 5 min.
glycolamide to silyl-protected37 5a gave rapid deprotec-
tion and a mixture of 5a and the oxazolidinone (see
above) was observed. Instead, treatment of 5a with
benzyl bromide in the presence of NaH and tetrabutyl-
ammonium iodide afforded the benzyl ether 6 in 86%
yield. The nucleophilic substitution reaction could then
be accomplished by reaction of 6 with glycolamide and
KH in DMSO at 55 °C to give amide 7 in 74% yield.
Attempts to use mixtures of DMF and THF as solvents
or NaH as base were considerably less successful. When
the reaction was run at a higher temperature (80 °C), a
byproduct (10) resulting from a nucleophilic attack of
the amide functionality of glycolamide was detected in
∼10% yield.
Figure 2. X-ray crystal structure of 5b, showing 50% prob-
ability displacement ellipsoids.51
quent addition of Boc-protected proline aldehyde af-
forded the expected alcohols (5a and 5b) in a combined
yield of 88% (approximately 85:15 diastereomeric ratio)
and the deiodinated pyridine derivative corresponding
to 4 (5c, 8% yield). The stereoisomers were separated
in order to study the diastereoselectivity in this step and
thereafter the reaction sequence was continued only
with the major isomer. The minor isomer was crystal-
lized and analyzed by X-ray crystallography.35 It was
found to be the R-isomer (Figure 2), which indicates that
Simultaneous cleavage of the benzyl ether and reduc-
tion of the double bond in the isobutylene group were
achieved by catalytic hydrogenation using Pd/C to afford
8 in 92% yield. In some instances this reaction stopped
before completion and the rearranged product 11 was
formed. Fortunately, the conjugated double bond in 11
was easy to reduce by the same method using fresh
catalyst. It should also be mentioned that in one
instance methanolysis of the terminal amide was ob-
served. Change of solvent to THF eliminated this side
reaction. Oxidation of the secondary alcohol with Dess-
Martin periodinane afforded ketone 9 in 84% yield. For
similar substrates, this method was superior to the use
of PDC or Jones’ reagents. The optical purity of 9 was
confirmed by chiral HPLC analysis. The chromatogram
showed one single peak, as compared to the two peaks
for the racemate (see above).
the reaction proceeds according to Cram’s chelation
model for carbonyl addition with preferential formation
of the S-isomer.36
A first attempt to directly oxidize alcohol 5a to the
corresponding ketone (Dess-Martin periodinane), fol-
lowed by nucleophilic substitution in the 2-position
(glycolamide, KH), gave the desired product 9. However,
this product showed no optical activity, which indicated
that a racemization occurred during the strongly basic
conditions used in the substitution reaction. Analysis
by chiral HPLC using a Pirkle Covalent (S,S)-Whelk-O
1 10/100 Krom Fec column confirmed the conclusion,
as the chromatogram showed two peaks with a 1:1 ratio.
Attempts to introduce the 2-substituent on alcohol 5a
by a nucleophilic substitution reaction (KH, glycola-
mide) gave no desired product. The main product was
instead identified by mass spectroscopy as a bicyclic
oxazolidinone, resulting from attack by the hydroxy
group on the carbonyl in the Boc group. Obviously, there
was a need to protect the secondary alcohol of 5a. First
the use of various silyl protecting groups was explored.
It was found that addition of a mixture of KH and
Finally, cleavage of the Boc group was achieved by
TFA/CH2Cl2 (1:3). By shortening the reaction time from
30 to 5 min an unwanted amide hydrolysis38 to acid 12
was avoided and the final product 1 was isolated as a
TFA salt in 65% yield after purification by HPLC.

6598 Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26
Saitton et al.
Table 2. Druglike Properties of Mimetic 1 and PLG
entry
propertya
PLG
284
5
7
-1.62
7
mimetic 1
1
MW (g/mol)
305
3
6
2
H-bond donors
H-bond acceptors
C log P
3b
4c
5
0.77
7
7
rotable bonds
H-bond total
6
10
122
7d
polar surface area (Å2)
99
Figure 3. Stimulation of NPA response at the D2 receptor
by PLG and 1. Data represent the percentage of maximum
NPA response over the media-alone control in the cell-based
functional assay R-SAT. The indicated concentrations of PLG
or 1 were added with NPA. Shown are the average and
standard deviations of three to five separate experiments
carried out in triplicate. Paired t-test indicates significant
differences from media-alone values: *, p < 0.05; **, p < 0.01.
a Druglike properties are limited to MW e 500 g/mol, H-bond
donors e 5, H-bond acceptors e 10, C log P e 5, rotable bonds e
10, H-bond total e 12, polar surface area e 140 Å2 (for definition
of the parameters, see ref 42 for entries 1-4 and ref 43 for entries
5-7). b The pyrrolidine nitrogen was not counted as a hydrogen-
bond acceptor since it is charged at physiological pH. c C log P was
calculated using Spartan v. 5.1.3 ab initio calculation of log P
(Ghose-Crippen). d Polar surface area was calculated using MAR-
EA version 2.4.50
Pharmacological Testing. The PLG mimetic 1 was
evaluated in the R-SAT assay, a pharmacologically
predictive cell-based functional assay based on ligand-
dependent transformation of NIH/3T3 cells as previ-
ously described.39,40 Compound 1 and native PLG were
tested for their ability to enhance the maximum re-
sponse of the dopamine agonist N-propylapomorphine
(NPA) at the human D2 receptor at three different
concentrations (10, 50, and 100 nM). As described by
Dolbeare et al.,27 an enhancement in NPA maximum
response was observed at 10 nM PLG (115 ( 6%, n)5)
but not at higher concentrations (50 nM, 104 ( 13%,
n)3; 100 nM, 98 ( 6%, n)3). However, for compound
1, an enhancement in NPA response was observed at
both 10 nM (146 ( 28%, n)4) and 50 nM (135 ( 14%,
n)3) concentration of 1 (Figure 3). No enhancement of
NPA response was however observed at 100 nM of 1
(117 ( 39%, n)3). In addition, no enhancement in NPA
response was observed with the unrelated peptide,
somatostatin-14 (10 or 50 nM). The down-turn phase
exhibited by both PLG and mimetic 1 in this assay is
in accordance with previous findings where PLG has
shown a bell-shaped dose-response curve.41 The mech-
anism of action for this behavior is however not pres-
ently known.
Druglike Properties. In the design of new drug
candidates, it is of great importance to evaluate the
physicochemical properties of the candidates in order
to qualitatively estimate oral bioavailability and mem-
brane permeability. The properties originally proposed
by Lipinski et al.42 (MW, H-bond donors, H-bond accep-
tors, log P) together with the properties more recently
suggested by Veber et al.43 [rotable bonds, sum of
H-bond donors and acceptors, polar surface area (PSA)]
have been calculated for mimetic 1 and PLG (Table 2).
Both PLG and mimetic 1 satisfy all the parameters
mentioned above and are hence expected to show
acceptable absorption properties. However, the positive
log P value and the lower number of H-bond donors and
acceptors of mimetic 1 as compared to PLG are reflected
in more favorable PSA predictions (99 vs 122 Å2). In
fact, it has been estimated that a polar surface area over
120 Å2 only permits approximately 20% permeability
in vivo.44 Thus, even though both compounds can be
considered to be within the “druglike limits”, mimetic
1 is predicted to show more favorable membrane perme-
ability properties than PLG.
Figure 4. (a) PLG in a â-turn conformation and mimetic 1
with the æ and ψ dihedral angles shown. (b) Overlap picture
between a selected turnlike low-energy conformation of mi-
metic 1 and PLG in a type II â-turn conformation.
Structure-Activity Relationships. On the basis
of results from previous studies, a type II â-turn has
been proposed as the bioactive conformation of
PLG.17,21,22,45 The conformational properties of peptido-
mimetic 1 have been examined by molecular modeling.
In particular, the probability of 1 to adopt a type II
â-turn was investigated. As expected, it was found that
mimetic 1 cannot adopt a type II â-turn due to the
aromaticity of the pyridine ring, which restricts the φ2
and æ2 torsion angles to approximately -180° and
+180°, respectively (Figure 4a). In order for the mimetic
to adopt any turnlike conformation, the pyrrolidine ring
of the proline residue must be at the same face of the
pyridine ring (which lies perpendicular to the plane of
the turn) as the glycol amide tail. When doing so, the
carbonyl group is forced to point in the direction out of

A Novel Pyridine-Based PLG Mimetic
Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26 6599
with syringe to a solution of the iodo derivative 4 (1.65 g, 5.95
mmol) in freshly distilled THF (8 mL), kept under nitrogen
atmosphere at room temperature. The reaction was exothermic
and became warm, cloudy, and somewhat discolored. After 1
h, Boc-Pro-CHO (1.19 g, 5.95 mmol) was added as a solution
in THF (6 mL), whereby the reaction mixture again became
warm but clear and kept the yellow-brown color. The reaction
was stirred overnight before it was quenched by the addition
of H2O (20 mL) and extracted to diethyl ether. The combined
organic phases were washed with brine, dried, and concen-
trated to yield 2.2 g of a crude oil (dr 85:15 according to crude
NMR analysis). Purification by flash chromatography (EtOAc/
heptane 1:6 then 1:2) afforded the diastereomers 5a and 5b
(1.84 g, 88% combined yield) and the deiodinated byproduct
5c (68 mg, 8%):
5a: A colorless oil; [R]D +9.2 (c 1.3, CHCl3); 1H NMR (CDCl3)
δ 8.03 (d, 1H, J ) 5.3 Hz); 7.27 (d, 1H, J ) 5.3 Hz), 6.08 (br s,
1H), 4.76 (s, 1H), 4.73 (d, 1H, J ) 8.7 Hz), 4.37 (s, 1H), 4.13
(m, 1H), 3.44-3.30 (m, 4H), 1.75 (s, 3H), 1.72-1.59 (m, 3H),
1.44 (s, 9H), 1.34-1.22 (m, 1H); 13C NMR (CDCl3) δ 162.27 (d,
J ) 239 Hz), 158.22, 155.12 (d, J ) 4 Hz), 145.24 (d, J ) 15
Hz), 142.54, 120.20 (d, J ) 3 Hz), 119.14 (d, J ) 31 Hz), 111.69,
81.01, 73.57, 63.13, 47.54, 32.55, 28.22 (4 C:s), 23.96, 22.73;
IR (neat) 3270, 2968, 1679, 1374 cm-1; HRMS (FAB) calcd for
C19H28FN2O3 [M + H]+ 351.2084, found 351.2076. Anal.
(C19H27FN2O3) C, H, N.
the turn, which makes a hydrogen bond between this
functionality and the terminal amide impossible (Figure
4b). The H‚‚‚O distance between the (CdO)i and (HN)i+3
residues is approximately 6 Å, as compared to 2 Å for
the peptide in the suggested bioactive conformation.
Although a type II â-turn as a bioactive conformation
is supported by test results of several highly constrained
PLG mimetics,20 the biological activity of 1 cannot
emanate from its ability to adopt such a conformation.
It has, however, been suggested that there might be two
different ways in which mimetics can interact with the
PLG modulatory site.15 Our findings support this sug-
gestion but today the properties of this second bioactive
conformation are unknown.
Conclusion
A novel pyridine-based tripeptidomimetic scaffold has
been designed, synthesized, and tested as a mimetic of
PLG. The synthetic sequence, which involves eight
steps, afforded the desired mimetic in 20% total yield.
Compound 1 was found to efficiently mimic the dopam-
ine agonist enhancing response of PLG. This effect
makes 1 interesting as a modulator of dopaminergic
activity, especially in dopamine dependent degenerative
diseases such as Parkinson’s disease and tardive dys-
kinesia. To investigate the structure-activity relation-
ship for this type of compound, synthesis and evaluation
of analogues of 1 as dopamine modulators are in
progress.
5b: A colorless oil; [R]D -34.7 (c 1.2, CHCl3); 1H NMR
(CDCl3) δ 8.04 (d, 1H, J ) 4.9 Hz), 7.40 (br s, 1H), 5.39 (s,
1H), 4.74 (s, 1H), 4.30 (br s, 1H), 4.00 (br s, 1H), 3.75-3.15
(m, 5H), 2.00-1.60 (m, 4H), 1.81 (s, 3H), 1.46 (s, 9H); 13C NMR
(CDCl3) δ 162.38 (d, J ) 239 Hz), 155.69, 154.73, 144.88 (d, J
) 15 Hz), 143.15, 119.55, 118.42 (d, J ) 31 Hz), 110.85, 79.95,
69.70, 61.67, 47.95, 32.43, 28.44 (3 C:s), 24.97, 24.33, 23.09;
IR (neat) 3355, 2974, 1681, 1395 cm-1; HRMS (FAB) calcd for
C19H28FN2O3 [M + H]+ 351.2084, found 351.2070. Anal.
(C19H27FN2O3) C, H, N.
Experimental Section
1
5c: A colorless oil; H NMR (CDCl3) δ 8.04 (m, 1H,), 7.57
Chemistry. General Data. All chemical reagents were
commercially available. THF was distilled from potassium/
sodium. TLC analysis was performed on silica gel F254 (Merck)
and detection was carried out by examination under UV light
and staining with phosphomolybdic acid. After workup, all
organic phases were dried with MgSO4. Flash column chro-
matography was performed on silica gel with solvent of HPLC
grade or analytical grade. Analytical reversed-phase HPLC
was performed on a Beckman System Gold HPLC equipped
with a Kromasil C-8 column (250 × 4.6 mm) using acetonitrile
(0.1% TFA) in H2O (0.1% TFA), 0-100% linear gradient over
60 min. A flow rate of 1.5 mL/min was used and detection was
at 254 nm. Preparative reversed-phase HPLC was performed
on a Kromasil C-8 column (250 × 20 mm) using the same
eluent, a flow rate of 11 mL/min, and detection at 254 nm.
Chiral HPLC chromatography was performed on a Pirkle
Covalent (S,S)-Whelk-O 1 10/100 Krom Fec column using CH2-
Cl2/iPrOH/heptane (48:4:48) as eluent, a flow rate of 1.0 mL/
min, and detection at 254 nm. Melting points were determined
on a Bu¨chi melting point apparatus and are uncorrected.
Specific rotations were measured with a Perking-Elmer model
343 polarimeter. 1H and 13C NMR spectra were recorded on a
Bruker DRX-400 spectrometer at 400 and 100 MHz, respec-
tively, and calibrated using the residual peak of solvent as
internal reference [CDCl3 (CHCl3 δH 7.26, CDCl3 δC 77.0) or
CD3OD (CD2HOD δH 3.34, CD3OD δC 49.0)]. Infrared spectra
were recorded using an ATI Mattson Infinity Series FTIR
spectrometer. High-resolution mass spectral analysis was
performed at Instrumentstationen, Lund University, Sweden.
Elemental analyses were performed by MikroKemi AB, Upp-
sala, Sweden. Compounds 332 and 431 were synthesized as
reported earlier.
(m, 1H), 7.10 (m, 1H), 4.83 (s, 1H), 4.65 (s, 1H), 3.29 (s, 2H),
1.69 (s, 3H); 13C NMR (CDCl3) δ 162.06 (d, J ) 239 Hz), 145.38
(d, J ) 15 Hz), 142.39, 141.16 (d, J ) 6 Hz), 121.44 (d, J ) 32
Hz), 121.30 (d, J ) 4 Hz), 112.80, 36.62, 22.05; IR (neat) 3077,
2971, 2917, 1604, 1577, 1436 cm-1; HRMS (EI) calcd for C9H10-
FN [M]+ 151.0797, found 151.0786.
4-[(1S)-Benzyloxy-1-[(2S)-1-tert-butoxycarbonylpyrro-
lidin-2-yl]methyl]-2-fluoro-3-(2-methylallyl)pyridine (6).
Alcohol 5a (1.13 g, 3.22 mmol) was dissolved in freshly distilled
THF and stirred while NaH (0.19 g, 4.84 mmol, 60%) was
added carefully. Benzyl bromide (0.58 mL, 4.84 mmol) and
tetrabutylammonium iodide (60 mg, 0.16 mmol) were added.
The reaction was stirred at room temperature for 28 h before
it was quenched by the addition of NH4Cl (aq, sat.) and
extracted into Et2O. The combined organic phases were
washed with brine, dried, and concentrated to a yellow oil. The
crude product was purified by flash chromatography (EtOAc/
heptane 1:10) to yield 6 (1.22 g, 86%) as a colorless oil: [R]D
1
-40.4 (c 0.94, CHCl3); H NMR (CDCl3) (rotamers) δ 8.07 (br
s, 1H), 7.42 (br s, 1H), 7.36-7.23 (m, 5H), 5.03 (br s, 0.5H),
4.84-4.70 (m, 1.5H), 4.45-4.29 (m, 3H), 4.27-4.12 (m, 1H),
3.55-3.10 (m, 3.5H), 3.05-2.91 (m, 0.5H), 2.07-1.10 (m, 4H),
1.76 (s, 3H), 1.44 (s, 5H), 1.23 (s, 4H); 13C NMR (CDCl3) [δ for
minor rotamer when observed and assigned (above 27 ppm)]
δ 162.28 (d, J ) 238 Hz), 154.92 [154.35], 152.88 (d, J ) 4
Hz), 144.58 [144.93] (d, J ) 16 Hz), 142.44 [142.10], 137.64
[137.37], 128.09 (2 C:s), 127.40, 127.30 (2 C:s), 120.16 [119.58]
(d, J ) 31 Hz), 120.05, 111.36 [111.55], 79.28, 76.68 [78.04],
71.31 [71.20], 60.46, 46.60 [46.47], 31.95 [32.10], 28.19 [27.89]
(3 C:s), 25.80, 23.78, 23.11, 22.58; IR (neat) 3066, 2975, 2885,
1691, 1390 cm-1; HRMS (FAB) calcd for C26H34FN2O3 [M +
H]+ 441.2553, found 441.2548.
(S)-[(2S)-1-tert-Butoxycarbonylpyrrolidin-2-yl)[2-fluoro-
3-(2-methylallyl)pyridin-4-yl]methanol (5a), (R)-[(2S)-1-
tert-Butoxycarbonylpyrrolidin-2-yl)[2-fluoro-3-(2-meth-
ylallyl)pyridin-4-yl]methanol (5b), and 2-Fluoro-3-(2-
methylallyl)pyridine (5c). A 2.0 M solution of isopropyl-
magnesium chloride in THF (3.00 mL, 5.95 mmol) was added
2-{4-[(1S)-Benzyloxy-1-((2S)-1-tert-butoxycarbonylpyr-
rolidin-2-yl)methyl]-3-(2-methylallyl)pyridin-2-yloxy}-
acetamide (7). Glycolamide (63 mg, 0.84 mmol) was dissolved
in dry DMSO (1 mL) and KH was added carefully. After a few
minutes, the gas evolution stopped and the pyridine derivative

6600 Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26
Saitton et al.
6 (74 mg, 0.17 mmol) was added as a solution in DMSO (1
mL). The reaction was heated at 55 °C for 60 h or until no
starting material could be detected by TLC. The reaction was
quenched by the addition of H2O, and aq HCl (5%) was added
until neutral pH was achieved. The product was extracted to
CH2Cl2, and the combined organic phases were washed with
brine, dried, and concentrated to a crude yellow oil. Purifica-
tion by flash chromatography (EtOAc/heptane 2:1) afforded 7
Hz), 5.34-5.30 (m, 1H), 4.94-4.84 (m, 2H), 3.50-3.43 (m, 2H),
2.97 (dd, 1H, J ) 12.9 Hz, 7.4 Hz), 2.70 (dd, 1H, J ) 12.9 Hz,
7.2 Hz), 2.47-2.40 (m, 1H), 2.19-2.01 (m, 3H), 1.97-1.88 (m,
1H), 0.97-0.93 (m, 6H); 13C NMR (CD3OD) δ 197.79, 173.94,
163.46, 146.07, 143.75, 125.98, 117.29, 66.66, 65.50, 47.25,
35.51, 30.48, 29.39, 24.89, 22.95, 22.85; IR (neat) 1665 cm-1
;
HRMS (FAB) calcd for C16H24N3O3 [M + H]+ 306.1818, found
306.1832. Anal. (C18H26F3N3O6) [M + TFA + H2O]: C, H, N.
1
(62 mg, 74%) as a white foam: [R]D -30.9 (c 1.1, CHCl3); H
X-ray Crystallography of 5b. The X-ray crystal structure
of 5b was determined at the Department of Inorganic Chem-
istry, Umeå University. Single colorless crystals of 5b were
obtained as prisms from heptane (dimensions: 0.45 mm × 0.15
mm × 0.10 mm) and are, at 150 K, orthorombic, space group
P212121 (No. 19) with a ) 9.9070(4) Å, b ) 10.0770(3) Å, c )
19.6780(4) Å, V ) 1964.5 (1) Å3, and Z ) 4 {dcalcd ) 1.185
g‚cm-3; µa(Mo KR) ) 0.086 mm-1}. A full hemisphere of
diffracted intensities (æ and ω scans) was measured, using
graphite-monochromated Mo KR radiation on a Nonius Kap-
paCCD single-crystal diffraction system. Lattice constants
were determined with the HKL Scalepack software package
using 10 590 reflections.46 A total of 19 281 integrated reflec-
tion intensities having 2θ(Mo KR) < 60.08° were produced
using the Nonius program Collect.47 Of these, 5726 (4101 >
2σ) reflections were unique and gave Rint ) 0.079. Nonius
SIR9748 and SHELXL-97 software49 were used to solve the
structure using direct methods techniques. Weighted full-
NMR (CDCl3) (rotamers) δ 8.05 (br s, 1H), 7.36-7.15 (m, 6H),
6.48-6.33 (m, 1H), 6.27-5.97 (m, 1H), 5.92-5.78 (m, 0.5H),
5.15 (m, 0.5H), 5.03-4.10 (m, 7H), 3.54-2.90 (m, 3H), 2.09-
1.06 (m, 17H); 13C NMR (CDCl3) (rotamers) [δ for minor
rotamer when assigned (above 27 ppm)] δ 172.14 [171.91],
160.16 [159.10], 154.75 (br), 150.33 [150.00], 144.99 [144.24],
138.96 [138.38], 137.96 (br), 128.27 (2 C:s), 127.54 (3 C:s),
120.82 (br), 116.50 [116.96], 110.07 (br), 79.16 [79.48], 77.99
(br), 71.08 [71.47], 64.81 [64.13], 60.85 [60.35], 46.56 (br), 32.64
(br), 28.34 (br, 3 C:s), 26.62, 25.96, 25.41, 23.95, 23.32, 19.89;
IR (neat) 1685, 1388 cm-1; HRMS (FAB) calcd for C28H38N3O5
[M + H]+ 496.2811, found 496.2814. Anal. (C28H37N3O5) C, H,
N.
2-{4-[1-[(2S)-1-tert-Butoxycarbonylpyrrolidin-2-yl]-(1S)-
hydroxymethyl]-3-isobutylpyridin-2-yloxy}acetamide (8).
Compound 7 (60 mg, 0.12 mmol) was dissolved in MeOH (1
mL), and Pd/C (10%, 22 mg) was added. The reaction was
stirred vigorously under H2 at room temperature for 1.5 h. The
catalyst was filtered off using a plug of Celite and the filtrate
was concentrated to afford 8 (45 mg, 92%) as a colorless oil/
2
matrix least-squares refinements were conducted using Fo
data with the SHELXL-97 software package. Final agreement
factors are R1 ) 0.0423 (unweighted, based on F, I > 2σ); R1
1
foam: [R]D -35.3 (c 1.0, CHCl3); H NMR (CDCl3) δ 8.02 (d,
) 0.0668 (unweighted, based on F, all reflections); wR2
)
0.0862 (weighted, based on F2, I > 2σ) and wR2 ) 0.0939
1H, J ) 5.3 Hz), 7.14 (d, 1H, J ) 4.7 Hz), 6.28 (br s, 1H), 6.06
(br s, 1H), 5.79 (br s, 1H), 4.91-4.75 (m, 3H), 4.19-4.10 (m,
1H), 3.53-3.38 (m, 2H), 2.70 (dd, 1H, J ) 13.5, 6.8 Hz), 2.45
(dd, 1H, J ) 13.5, 7.7 Hz), 1.99-1.86 (m, 1H), 1.85-1.75 (m,
2H), 1.72-1.61 (m, 1H), 1.51 (s, 9H), 1.37-1.27 (m, 1H), 0.96
(app t, 6H, J ) 6.3 Hz); 13C NMR (CDCl3) δ 172.00, 160.19,
158.42, 152.21, 144.30, 121.63, 116.66, 81.15, 73.33, 64.92,
63.79, 47.73, 34.22, 29.48, 28.38 (3 C:s), 28.26, 24.21, 22.79,
22.35; IR (neat) 3315, 2957, 1665, 1395 cm-1; HRMS (FAB)
calcd for C21H34N3O5 [M + H]+ 408.2498, found 408.2484.
(weighted, based on F2, all reflections)
The structural model incorporated anisotropic thermal
parameters for all nonhydrogen atoms and isotropic thermal
parameters for all hydrogen atoms. The hydrogen atoms were
located from difference Fourier synthesis and refined using a
riding model.
Biological Activity. Functional Assays. The R-SAT
(receptor selection and amplification technology) assay was
performed essentially as previously described.39 Briefly, NIH/
3T3 cells at 70-80% confluency were transfected using Poly-
fect (Qiagen, Los Angeles, CA) as described in the manufac-
turer’s protocols with DNA encoding the human D2 receptor
at 5 ng/well and DNA encoding â-galactosidase at 20 ng/well
of a 96-well plate. After 24 h incubation, saturating amounts
of N-propylapomorphine (0.75-1 µM) were added with or
without PLG or 1 (NPA, pEC50 ) 7.6 ( 0.4, n ) 7). After a
5-day incubation, the â-galactosidase activity was measured
by the addition of o-nitrophenyl â-D-galactopyranoside (in
phosphate-buffered saline with 5% Nonidet P-40 detergent).
The resulting colorimetric reaction was measured with a
spectrophotometric plate reader (Titertek, Huntsville, AL) at
420 nM.
2-{4-[(2S)-1-tert-Butoxycarbonylpyrrolidine-2-carbonyl]-
3-isobutylpyridin-2-yloxy}acetamide (9). Alcohol 8 (105
mg, 0.26 mmol) was dissolved in dry CH2Cl2 and Dess-Martin
periodinane (15% in CH2Cl2, 0.71 mL) was added. After
stirring at room temperature for 4 h, another 0.15 mL of the
oxidizing agent was added and the reaction was stirred
overnight. The reaction was quenched by the addition of
Na2S2O5 (350 mg, 1.8 mmol) in NaHCO3 (aq, sat.) and
extracted into CH2Cl2. The combined organic layers were
washed with brine, dried, and concentrated to a crude oil.
Purification by flash chromatography (EtOAc:heptane 2:1)
afforded 9 (88 mg, 84%) as a white foam: [R]D +9.1 (c 1.0,
CHCl3); 1H NMR (CDCl3) major [minor] rotamer integral ratio
0.55 [0.45] δ 8.09 [8.13] (d, 1H, J ) 5.2 Hz), 7.24 [7.10] (d, 1H,
J ) 5.2 Hz), 6.26 (br s, 1H), 5.91 (br s, 1H), 4.97-4.87 (m,
1H), 4.87-4.78 (m, 2H), 3.71-3.56 (m, 1H), 3.54-3.39 (m, 1H),
2.83-2.74 (m, 1H), 2.50-2.39 (m, 1H), 2.19-1.75 (m, 5H), 1.45
[1.39] (s, 9H), 0.95-0.86 (m, 6H); 13C NMR (CDCl3) rotamers
[δ for minor rotamer when assigned (above 30 ppm)] δ 203.03
[201.17], 171.46 [171.29], 161.21 [160.95], 154.41 [153.63],
147.48 [146.44], 144.36 [144.50], 122.42 [122.96], 116.32
[116.04], 79.95 [80.25], 65.03 [65.13], 64.06 [63.95], 46.84
[46.62], 35.03 [35.12], 29.50, 29.45, 29.29, 28.83, 28.37 [28.32]
(3 C:s), 24.21, 23.07, 22.74, 22.60, 22.50; HRMS (FAB) calcd
for C21H32N3O5 [M + H]+ 406.2342, found 406.2346.
Statistical Analysis. Statistical analysis was performed
using a paired two-tailed Student’s t-test, to assess whether
the NPA response in the presence of 1 or PLG was significantly
different from the NPA response in the media alone as a
control.
Acknowledgment. We thank the Knut and Alice
Wallenberg Foundation, the Norwegian Research Coun-
cil and the Swedish Research Council for providing
financial support for this project. We also thank Umeå
University for providing laboratory facilities. In addi-
tion, we thank Uli Hacksell for advice during the
development of the functional assay.
2-{3-Isobutyl-4-[(2S)-pyrrolidine-2-carbonyl]pyridin-2-
yloxy}acetamide TFA Salt (1). Trifluoroacetic acid (0.5 mL)
was added to a solution of 9 (20 mg, 0.049 mmol) in freshly
distilled CH2Cl2 (1.5 mL) at room temperature. The reaction
was allowed to stir for 5 min before it was concentrated under
reduced pressure without heating to afford a yellowish solid.
Purification by reversed-phase preparative HPLC afforded 1
(13 mg, 65%) as a yellow solid: [R]D -25.4 (c 1.2, H2O); 1H
NMR (CD3OD) δ 8.23 (d, 1H, J ) 5.3 Hz), 7.34 (d, 1H, J ) 5.3
Supporting Information Available: Experimental pro-
cedure and tables with structural information regarding the
X-ray crystallographic analysis of 5b in CIF format and
elemental analysis results. This material is available free of
charge via the Internet at http://pubs.acs.org.

A Novel Pyridine-Based PLG Mimetic
Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26 6601
(22) Sreenivasan, U.; Mishra, R. K.; Johnson, R. L. Synthesis and
Dopamine Receptor Modulating Activity of Lactam Conforma-
tionally Constrained Analogues of Pro-Leu-Gly-NH2. J. Med.
Chem. 1993, 36, 256-263.
(23) Khalil, E. M.; Ojala, W. H.; Pradhan, A.; Nair, V. D.; Gleason,
W. B.; Mishra, R. K.; Johnson, R. L. Design, synthesis, and
dopamine receptor modulating activity of spiro bicyclic pepti-
domimetics of L-prolyl-L-leucyl-glycinamide. J. Med. Chem. 1999,
42, 628-637.
(24) Khalil, E. M.; Pradhan, A.; Ojala, W. H.; Gleason, W. B.; Mishra,
R. K.; Johnson, R. L. Synthesis and dopamine receptor modulat-
ing activity of substituted bicyclic thiazolidine lactam peptido-
mimetics of L-prolyl-L-leucyl-glycinamide. J. Med. Chem. 1999,
42, 2977-2987.
(25) Weber, K.; Ohnmacht, U.; Gmeiner, P. Enantiopure 4-and
5-aminopiperidin-2-ones: Regiocontrolled synthesis and confor-
mational characterization as bioactive beta-turn mimetics. J.
Org. Chem. 2000, 65, 7406-7416.
(26) Dolbeare, K.; Pontoriero, G. F.; Gupta, S. K.; Mishra, R. K.;
Johnson, R. L. iso-lactam and reduced amide analogues of the
peptidomimetic dopamine receptor modulator 3(R)-(2(S)pyrro-
lidinylcarbonyl)amino-2-oxo-1-pyrrolidineacetamide. Bioorg. Med.
Chem. 2003, 11, 4103-4112.
(27) Dolbeare, K.; Pontoriero, G. F.; Gupta, S. K.; Mishra, R. K.;
Johnson, R. L. Synthesis and dopamine receptor modulating
activity of 3-substituted gamma-lactam peptidomimetics of
L-prolyl-L-leucyl-glycinamide. J. Med. Chem. 2003, 46, 727-733.
(28) Borg, S.; Vollinga, R. C.; Labarre, M.; Payza, K.; Terenius, L.;
Luthman, K. Design, synthesis, and evaluation of Phe-Gly
mimetics: Heterocyclic building blocks for pseudopeptides. J.
Med. Chem. 1999, 42, 4331-4342.
(29) The conformational analysis was performed in Macromodel (v.
5.5) using the AMBER force-field in vacuo. Conformations within
a 3 kcal/mol range from the lowest conformation found were
compared. See ref 28.
(30) The semiempirical calculations were performed in the Spartan
program (v. 5.1.3) using AM1 as approximate Hamiltonian. The
atomic charges were calculated by electrostatic fitting of the
single point energies of the minimized structures.
(31) Saitton, S.; Kihlberg, J.; Luthman, K. A synthetic approach to
2,3,4-substituted pyridines useful as scaffolds for tripeptidomi-
metics. Tetrahedron 2004, 60, 6113-6120.
(32) Rocca, P.; Marsais, F.; Godard, A.; Queguiner, G. Connection
between metalation and cross-coupling strategies. A new con-
vergent route to azacarbazoles. Tetrahedron 1993, 49, 49-
64.
(33) Rocca, P.; Cochennec, C.; Marsais, F.; Thomas-dit-Dumont, L.;
Mallet, M.; Godard, A.; Queguiner, G. 1st Metalation of Aryl
IodidessDirected Ortho-Lithiation of Iodopyridines, Halogen-
Dance, and Application to Synthesis. J. Org. Chem. 1993, 58,
7832-7838.
(34) Trecourt, F.; Breton, G.; Bonnet, V.; Mongin, F.; Marsais, F.;
Queguiner, G. Pyridylmagnesium chlorides from bromo and
dibromopyridines by bromine-magnesium exchange: A conve-
nient access to functionalized pyridines. Tetrahedron Lett. 1999,
40, 4339-4342.
(35) Depostition number CCDC 243122 at the Cambridge Crystal-
lographic Data Centre; web site (http://www.ccdc.cam.ac.uk).
(36) Cram, D. J.; Elhafez, F. A. A. Studies in stereochemistry. X. The
rule of “Steric control of asymmetric induction” in the synthesis
of acyclic systems. J. Am. Chem. Soc. 1952, 74, 5828-5835.
(37) Silyl-protecting groups used were TES (triethylsilyl), TBDMS
(tertbutyldimethylsilyl), and TIPS (triisopropylsilyl).
(38) Compare the ease of hydrolysis of the amide with the ease of
forming the methyl ester in methanol. Presumably this is due
to the intramolecular assistance of the pyridine nitrogen.
(39) Weiner, D. M.; Burstein, E. S.; Nash, N.; Croston, G. E.; Currier,
E. A.; Vanover, K. E.; Harvey, S. C.; Donohue, E.; Hansen, H.
C.; Andersson, C. M.; Spalding, T. A.; Gibson, D. F. C.; Krebs-
Thomson, K.; Powell, S. B.; Geyer, M. A.; Hacksell, U.; Brann,
M. R. 5-hydroxytryptamine(2A) receptor inverse agonists as
antipsychotics. J. Pharmacol. Exp. Ther. 2001, 299, 268-
276.
(40) Burstein, E. S.; BraunerOsborne, H.; Spalding, T. A.; Conklin,
B. R.; Brann, M. R. Interactions of muscarinic receptors with
the heterotrimeric G proteins G(q) and G(12): Transduction of
proliferative signals. J. Neurochem. 1997, 68, 525-533.
(41) Mishra, R. K.; Marcotte, E. R.; Chugh, A.; Barlas, C.; Whan, D.;
Johnson, R. L. Modulation of dopamine receptor agonist-induced
rotational behavior in 6-OHDA-lesioned rats by a peptidomi-
metic analogue of Pro-Leu-Gly-NH2 (PLG). Peptides 1997, 18,
1209-1215.
References
(1) Hokfelt, T.; Broberger, C.; Xu, Z. Q. D.; Sergeyev, V.; Ubink, R.;
Diez, M. NeuropeptidessAn overview. Neuropharmacology 2000,
39, 1337-1356.
(2) Giannis, A.; Rubsam, F. Peptidomimetics in Drug Design.
Advances in Drug Research; Academic Press: London, 1997; pp
1-78.
(3) Ripka, A. S.; Rich, D. H. Peptidomimetic design. Curr. Opin.
Chem. Biol. 1998, 2, 441-452.
(4) Hanessian, S.; McNaughtonSmith, G.; Lombart, H. G.; Lubell,
W. D. Design and synthesis of conformationally constrained
amino acids as versatile scaffolds and peptide mimetics. Tetra-
hedron 1997, 53, 12789-12854.
(5) Souers, A. J.; Ellman, J. A. Beta-Turn mimetic library synthe-
sis: Scaffolds and applications. Tetrahedron 2001, 57, 7431-
7448.
(6) Eguchi, M.; Shen, R. Y. W.; Shea, J. P.; Lee, M. S.; Kahn, M.
Design, synthesis, and evaluation of opioid analogues with
nonpeptidic beta-turn scaffold: Enkephalin and endomorphin
mimetics. J. Med. Chem. 2002, 45, 1395-1398.
(7) Le, G. T.; Abbenante, G.; Becker, B.; Grathwohl, M.; Halliday,
J.; Tometzki, G.; Zuegg, J.; Meutermans, W. Molecular diversity
through sugar scaffolds. Drug Discovery Today 2003, 8, 701-
709.
(8) Mishra, R. K.; Chiu, P.; Mishra, C. P. Pharmacology of L-Prolyl-
L-Leucyl-Glycinamide (PLG): A Review. Methodol. Find. Exp.
Clin. 1983, 5, 203-233.
(9) Srivastava, L. K.; Bajwa, S. B.; Johnson, R. L.; Mishra, R. K.
Interaction of L-Prolyl-L-Leucyl Glycinamide with Dopamine-
D2 ReceptorsEvidence for Modulation of Agonist Affinity States
in Bovine Striatal Membranes. J. Neurochem. 1988, 50, 960-
968.
(10) Rajakumar, G.; Naas, F.; Johnson, R. L.; Chiu, S.; Yu, K. L.;
Mishra, R. K. Down-Regulation of Halperidol-Induced Striatal
Dopamine Receptor Supersensitivity by Active Analogues of
L-Prolyl-L-Leucyl-Glycinamide (Plg). Peptides 1987, 8, 855-861.
(11) Johnson, R. L.; Rajakumar, G.; Yu, K. L.; Mishra, R. K. Synthesis
of Pro-Leu-Gly-NH2 Analogues Modified at the Prolyl Residue
and Evaluation of Their Effects on the Receptor-Binding Activity
of the Central Dopamine Receptor Agonist, ADTN. J. Med.
Chem. 1986, 29, 2104-2107.
(12) Johnson, R. L.; Rajakumar, G.; Mishra, R. K. Dopamine Receptor
Modulation by Pro-Leu-Gly-NH2 Analogues Possessing Cyclic
Amino-Acid-Residues at the C-Terminal Position. J. Med. Chem.
1986, 29, 2100-2104.
(13) Johnson, R. L.; Bontems, R. J.; Yang, K. E.; Mishra, R. K.
Synthesis and Biological Evaluation of Analogues of Pro-Leu-
Gly-NH2 Modified at the Leucyl Residue. J. Med. Chem. 1990,
33, 1828-1832.
(14) Baures, P. W.; Pradhan, A.; Ojala, W. H.; Gleason, W. B.; Mishra,
R. K.; Johnson, R. L. Synthesis and dopamine receptor modulat-
ing activity of unsubstituted and substituted triproline analogues
of L-prolyl-L-leucyl-glycinamide (PLG). Bioorg. Med. Chem. Lett.
1999, 9, 2349-2352.
(15) Evans, M. C.; Pradhan, A.; Venkatraman, S.; Ojala, W. H.;
Gleason, W. B.; Mishra, R. K.; Johnson, R. L. Synthesis and
dopamine receptor modulating activity of novel peptidomimetics
of L-prolyl-L-leucyl-glycinamide featuring alpha,alpha-disubsti-
tuted amino acids. J. Med. Chem. 1999, 42, 1441-1447.
(16) Thomas, C.; Ohnmacht, U.; Niger, M.; Gmeiner, P. beta-
analogues of PLG (L-prolyl-L-leucyl-glycinamide): Ex-chiral pool
syntheses and dopamine D-2 receptor modulating effects. Bioorg.
Med. Chem. Lett. 1998, 8, 2885-2890.
(17) Palomo, C.; Aizpurua, J. M.; Benito, A.; Miranda, J. I.; Fratila,
R. M.; Matute, C.; Domercq, M.; Gago, F.; Martin-Santamaria,
S.; Linden, A. Development of a new family of conformationally
restricted peptides as potent nucleators of beta-turns. Design,
synthesis, structure, and biological evaluation of beta-lactam
peptide analogue of melanostatin. J. Am. Chem. Soc. 2003, 125,
16243-16260.
(18) Yu, K. L.; Rajakumar, G.; Srivastava, L. K.; Mishra, R. K.;
Johnson, R. L. Dopamine Receptor Modulation by Conforma-
tionally Constrained Analogues of Pro-Leu-Gly-NH2. J. Med.
Chem. 1988, 31, 1430-1436.
(19) Baures, P. W.; Ojala, W. H.; Costain, W. J.; Ott, M. C.; Pradhan,
A.; Gleason, W. B.; Mishra, R. K.; Johnson, R. L. Design,
synthesis, and dopamine receptor modulating activity of dike-
topiperazine peptidomimetics of L-prolyl-L-leucylglycinamide. J.
Med. Chem. 1997, 40, 3594-3600.
(20) Genin, M. J.; Mishra, R. K.; Johnson, R. L. Dopamine-Receptor
Modulation by a Highly Rigid Spiro Bicyclic Peptidomimetic of
Pro-Leu-Gly-NH2. J. Med. Chem. 1993, 36, 3481-3483.
(21) Subasinghe, N. L.; Bontems, R. J.; McIntee, E.; Mishra, R. K.;
Johnson, R. L. Bicyclic Thiazolidine Lactam Peptidomimetics of
the Dopamine-Receptor Modulating Peptide Pro-Leu-Gly-NH2.
J. Med. Chem. 1993, 36, 2356-2361.
(42) Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J.
Experimental and computational approaches to estimate solubil-
ity and permeability in drug discovery and development settings.
Adv. Drug Deliv. Rev. 1997, 23, 3-25.

6602 Journal of Medicinal Chemistry, 2004, Vol. 47, No. 26
Saitton et al.
(47) Nonius B. V., “Collect” data collection software, 1999.
(43) Veber, D. F.; Johnson, S. R.; Cheng, H. Y.; Smith, B. R.; Ward,
K. W.; Kopple, K. D. Molecular properties that influence the oral
bioavailability of drug candidates. J. Med. Chem. 2002, 45,
2615-2623.
(48) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. SIR97: A new tool for crystal structure determina-
tion and refinement. J. Appl. Crystallogr. 1999, 32, 115-119.
(49) Sheldrick, G. M. 1997. SHELXL-97. Program for the Refinement
of Crystal Structures. University of Go¨ttingen, Germany.
(50) The program MAREA is available from J. Gråsjo¨, Department
of Pharmaceutics, Uppsala University, Uppsala, Sweden (e-
mail: johan.grasjo@farmaci.uu.se).
(44) Palm, K.; Stenberg, P.; Luthman, K.; Artursson, P. Polar
molecular surface properties predict the intestinal absorption
of drugs in humans. Pharm. Res. 1997, 14, 568-571.
(45) Baures, P. W.; Ojala, W. H.; Gleason, W. B.; Mishra, R. K.;
Johnson, R. L. Design, Synthesis, X.-Ray-Analysis, and Dopam-
ine Receptor-Modulating Activity of Mimics of the C5 Hydrogen-
Bonded Conformation in the Peptidomimetic 2-Oxo-3(R)-(2(S)-
pyrrolidinylcarbonyl)amino-1-pyrrolidineacetamide. J. Med. Chem.
1994, 37, 3677-3683.
(51) Dowty, E. 2000. ATOMS for Windows and Macintosh. Version
5.1. Shape Software, 521 Hidden Valley Road, Kingsport, TN
37663.
(46) Otwinowski, Z.; Minor, W. Methods in Enzymology. In Macro-
molecular Crystallography; Carter, C. W., Jr., Sweet, R. M., Eds.;
Academic Press: New York, 1997; Part A, pp 307-326.
JM049484Q