Substituted imidazo[1,2- a ]pyridines as β-strand peptidomimetics

Article abstract of DOI:10.1021/ol302850n

New conformationally extended dipeptide surrogates based on an imidazo[1,2-a]pyridine scaffold are described. Efficient synthesis and incorporation into host peptides affords structures with native side-chain functionality and hydrogen bonding elements on

Full text of DOI:10.1021/ol302850n

ORGANIC
LETTERS
2012
Vol. 14, No. 24
6162–6165
Substituted Imidazo[1,2‑a]pyridines
as β‑Strand Peptidomimetics
Chang Won Kang, Yongmao Sun, and Juan R. Del Valle*
Drug Discovery Department, H. Lee Moffitt Cancer Center and Research Institute,
Tampa, Florida 33612, United States
juan.delvalle@moffitt.org
Received October 16, 2012
ABSTRACT
New conformationally extended dipeptide surrogates based on an imidazo[1,2-a]pyridine scaffold are described. Efficient synthesis and
incorporation into host peptides affords structures with native side-chain functionality and hydrogen bonding elements on one face of the
backbone. Structural analysis by NMR suggests that model peptidomimetics adopt a β-strand-like conformation in solution.
β-Strands are key structural motifs in a number of
proteinꢀprotein interfaces relevant to human disease.1
For example, the Akt-GSK3β,2 Ras-Raf1,3 and KRas-
FTase4 interactions are all implicated in oncogenesis
and involve the recognition of an extended or β-strand
domain. Isolated β-strand peptides are also substrates for
various proteolytic enzymes5 and major histocompatibility
complex (MHC) proteins.6 Conformational mimicry of
β-strands has thus gained increasing attention as a strategy
toward new chemical probes and therapeutics.7
rigidity.7 In these cases, enhanced stability may require
large scaffolding elements or complementary strands that
are auxiliary toa sequenceof interest. The incorporation of
constrainedresiduesorbackboneisostereswithinextended
host peptides represents a more “minimalist” approach
toward peptidomimetic drug candidates. The utility of
hybrid peptides featuring β-strand prosthetics such as
the Hao subunit,8 @-tide residues,9 and other dipeptide
surrogates10 have been demonstrated in various applica-
tions. The development of artificial β-strands comprised
entirely of nonpeptide subunits has also emerged as an
approach toward novel proteomimetic foldamers.11
Synthetic templated β-strands/sheets often feature
β-hairpin or macrocyclic motifs to impart conformational
Our interest in peptidomimetics targeting the Akt-
GSK3β interaction12 led us to explore scaffolds that could
be easily prepared and incorporated into host sequences,
ꢀ
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(1) (a) Dou, Y.; Baisnee, P.-F.; Pollastri, G.; Pecout, Y.; Nowick, J.;
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r
10.1021/ol302850n
Published on Web 12/04/2012
2012 American Chemical Society

enforce an extended backbone conformation, and impart
more “drug-like” character onto short peptides. Here, we
describe the design and synthesis of peptidomimetics
incorporating a novel imidazo[1,2-a]pyridine scaffold.
Structuralanalysisofmodel compounds byNMR suggests
that hybrid imidazo[1,2-a]pyridine-based peptides adopt
an extended conformation in solution. These studies lay
the groundwork for the development of imidazo[1,2-
a]pyridines as new core motifs for peptido- and proteomi-
metic drug design.
In our search for constrained templates with favorable
pharmacokinetic properties, we were particularly attracted
to the imidazo[1,2-a]pyridine core structure due to its
presence in therapeutic agents such as the hypnotics
zolpidem (Ambien) and alpidem,13 and the vasodilator
olprinone.14 We envisioned that imidazo[1,2-a]pyridines
appropriately substituted with terminal amine and car-
boxy groups, and with side-chain diversity at the putative
β carbon, could serve as “drug-like” extended dipeptide
surrogates. Figure 1 depicts the design of our scaffold as
well as an overlay of a model tetrapeptide mimic with an
idealized strand from an antiparallel β-sheet. Although the
planarity of the aromatic core results in some deviation in
main chain and side chain geometry, the imidazole nitro-
gen (H-bond acceptor) overlays reasonably well with the
carbonyl oxygen in the native peptide strand. We thus
pursued this scaffold as a potential backbone replacement
for highly extended dipeptides. Despite a wealth of litera-
tureonthe chemistry and pharmacologyof imidazo[1,2-a]-
pyridines,15 to the best of our knowledge, these scaffolds
have not previously been synthesized or studied in the
context of peptide mimicry.
Figure 1. Design of IP-based β-strand mimics.
Scheme 1. Synthesis of Orthogonally Protected IP-based
Extended Dipeptide Surrogates
Scheme 1 depicts the synthesis of imidazo[1,2-a]pyridine
(IP) dipeptide surrogates via monobromination of aceto-
acetate derivatives and condensation with 2,3-diaminopyri-
dine. Although various methods exist for the R-bromination
of β-ketoesters, we found reaction of 1 with 1.1 equiv of
N-bromosuccinimide in PEG-400 to be the most convenient.
Optimal yields of IP scaffolds 2 were obtained by directly
heating the crude R-bromo-β-ketoesters with 1.0 equiv 2,3-
diaminopyridine in the presence of NaHCO3. We found that
the yields of imidazopyridine formation could be increased
by using mono-Cbz-protected 2,3-diaminopyridine16 in
the condensation reaction. To show that a wider array of
substituents could be similarly incorporated, we also pre-
pared orthogonally protected dipeptide surrogates featuring
butenylglycine (3c), homoserine (3d), tyrosine (3e), ornithine
(3f), and arginine (3g) side chains.
We next studied the incorporation of our IP building
blocks into short peptides using conventional coupling
reactions. Condensation of 2a with Cbz-Leu-OH proved
challenging, presumably due to the low nucleophilicity
of the IP aryl amine. As shown in Table 1, a variety of
standard conditions afforded poor yields of the desired
amide (entries 1ꢀ3). The use ofEDC withcatalytic DMAP
(entries 4 and 5) led to an improvement in yield, but was
attended by significant racemization of the Leu residue.17
Surprisingly, we found that omission of an auxiliary base
resulted in higher conversions. Optimal results were ob-
tained by using 2.0 eq. of both EDC and Cbz-Leu-OH
without any addedbasetogive4in91%yieldand >98:2 er
(entry 7). Other coupling reagents such as PyBOP, COMU,
DEPBT, and DCC consistently gave inferior results
(entries 8ꢀ11).
(11) (a) Smith, A. B.; Guzman, M. C.; Sprengeler, P. A.; Keenan,
T. P.; Holcomb, R. C.; Wood, J. L.; Carroll, P. J.; Hirschmann, R. J. Am.
Chem. Soc. 1994, 116, 9947. (b) Angelo, N. G.; Arora, P. S. J. Am. Chem.
Soc. 2005, 127, 17134. (c) Wyrembak, P. N.; Hamilton, A. D. J. Am.
Chem. Soc. 2009, 131, 4566. (d) Jamieson, A. G.; Russell, D.; Hamilton,
A. D. Chem. Commun. 2012, 48, 3709. (e) Raghuraman, A.; Ko, E.;
Perez, L. M.; Ioerger, T. R.; Burgess, K. J. Am. Chem. Soc. 2011, 133,
12350. (f) Ko, E.; Liu, J.; Perez, L. M.; Lu, G.; Schaefer, A.; Burgess, K.
J. Am. Chem. Soc. 2010, 133, 462. (g) Chandrasekhar, S.; Babu, B. N.;
Prabhakar, A.; Sudhakar, A.; Reddy, M. S.; Kiran, M. U.; Jagadeesh, B.
Chem. Commun. 2006, 1548.
(12) (a) Ranatunga, S.; Del Valle, J. R. Bioorg. Med. Chem. Lett.
2011, 21, 7166. (b) Ranatunga, S.; Liyanage, W.; Del Valle, J. R. J. Org.
Chem. 2010, 75, 5113.
(13) Langtry, H. D.; Benfield, P. Drugs 1990, 40, 291.
(14) Mizushige, K.; Ueda, T.; Yukiiri, K.; Suzuki, H. Cardiol. Drug
Rev. 2002, 20, 163.
(15) (a) Enguehard-Gueiffier, C.; Gueiffier, A. Mini Rev. Med. Chem.
2007, 7, 888. See also: (b) Shao, N.; Pang, G.-X.; Yan, C.-X.; Shi, G.-F.;
Cheng, Y. J. Org. Chem. 2011, 76, 7458 and references therein.
(16) Choi, J. Y.; Plummer, M. S.; Starr, J.; Desbonnet, C. R.; Soutter,
H.; Chang, J.; Miller, J. R.; Dillman, K.; Miller, A. A.; Roush, W. R.
J. Med. Chem. 2012, 55, 852.
(17) Hydrogenolysis of 4 (from entries 4 and 5) and HBTU/HOBt-
mediated coupling to Cbz-Phe-OH afforded a ∼83:17 mixture of
diastereomers by HPLC.
Org. Lett., Vol. 14, No. 24, 2012
6163

Table 1. N-Terminal Coupling of IP Scaffold 2a
Scheme 2. Synthesis and Selected ROESY Correlations (in red)
of Tetrapeptide Mimic 6
entry
1
conditionsa
yieldb
1.2 equiv Z-Leu-OH, 1.2 equiv HBTU, HOBt, trace
NEt3, MeCN
2
3
1.2 equiv Z-Leu-OH, 1.2 equiv PyBOP, NEt3, DMF trace
1.2 equiv Z-Leu-OH, 1.2 equiv EDC, HOBt, NEt3, trace
DCM
4
5
1.2 equiv Z-Leu-OH, 1.2 equiv EDC, 0.2 eq. DMAP, 33c
DCM
2.0 equiv Z-Leu-OH, 2.0 equiv EDC, 0.2 eq. DMAP, 58c
DCM
in an unstructured peptide.18 This is consistent with the
chemical shift of β-strand/sheet CHR protons relative to
those in random coil peptides. Most important, the 2D
ROESY spectrum of 6 exhibited a correlation between the
IPMe and PheNH protons, as well as between LeuCHR and
IPNH (Scheme 2). These correlations strongly support trans
amide bond configurations across tetrapeptide mimic 6.
We then synthesized heptapeptide mimic 8 featuring two
IP scaffolds embedded within a larger strand (Scheme 3).
6
1.2 equiv Z-Leu-OH, 1.2 equiv EDC, DCM
2.0 equiv Z-Leu-OH, 2.0 equiv EDC, DCM
2.0 equiv Z-Leu-OH, 2.0 equiv PyBOP, DCM
2.0 equiv Z-Leu-OH, 2.0 equiv COMU, DCM
2.0 equiv Z-Leu-OH, 2.0 equiv DEPBT, DCM
2.0 equiv Z-Leu-OH, 2.0 equiv DCC, DCM
59
91
33
44
26
77
7
8
9
10
11
a All reactions carried out for 24 h at rt. b Isolated yields. c HPLC
analysis of a phenylalanyl derivitive showed significant epimerization of
the Leu chiral center.
1
Compound 8 again exhibited a well-resolved H NMR
Hydrolysis of the ester in 4 required extended reaction
times in the presence of hydroxide. Prolonged exposure to
aqueous LiOH also resulted in an unacceptable degree of
amide bond scission. We found that this issue could be
circumvented by using Cbz-protected building block 3a
for peptide assembly in the CꢀN direction (Scheme 2).
The urethane bond in 3a proved stable to hydrolysis with
LiOH. Condensation of the resulting crude acid with
H-Phe-OMe was followed by hydrogenolysis and coupling
to Cbz-Leu-OH to give tetrapeptide mimic 6 in 45% yield
over 4 steps.
The observed lack of aryl amine reactivity in the pres-
ence of various standard amidation reagents (Table 1,
entries 1ꢀ3) also led us to explore the direct condensation
of a fully unprotected IP derivative. We found that hydro-
lysis of 2a followed by neutralization afforded the crude
amino acid zwitterion, which could be used directly in the
subsequent coupling. Reaction with H-Phe-OMe in the
presence of HBTU, HOBt, and NEt3 gave the desired
amide exclusively, without any detectable homocoupling
of the IP scaffold. The phenylalanyl intermediate was then
reacted with Cbz-Leu-OH and EDC to provide 6 in an
improved 59% overall yield from 2a.
spectrum that could be readily assigned using 2D COSY.
The absence of NMR signals corresponding to minor
rotational isomers was particularly notable due to the
presence of 1 urethane and 4 amide bonds. Each of the
amino acid CHR signals in 8 showed pronounced down-
field shifts in CDCl3 relative to their corresponding
random-coil values.18 In addition, the NHꢀCHR coupling
constants obtained for the Ile and Leu residues were in
the 8ꢀ9 Hz range, consistent with the values typical of a
β-strand conformation. These coupling constants were
used to calculate possible φ dihedral angles of the Ile,
Leu, and Phe residues in 8 using the Pardi modification
tothe Karplus equation.19 Amino acidresidues inidealized
parallel and antiparallel β-sheets exhibit φ values of ap-
proximately ꢀ119° and ꢀ139°, respectively (see Scheme 3).7
The corresponding torsions in 8 are thus in good agreement
with a β-strand-like conformation.
The ROESY data obtained earlier for 6 suggested that
the IPMe 1H NMR signal could serve as a useful reporter
of local conformation. As with 6, we observed ROESY
correlations between the IP1Me-LeuNH and IP2Me-PheNH
protons in heptapeptide mimic 8, indicating the presence
of trans amide bonds between these residues (Figure 2).
Additional correlations between IleCHRꢀIP1NH and
LeuCHRꢀIP2NH further supported an extended structure
for 8 in solution. Dilution studies with 8 indicated a
negligible chemical shift dependence on concentration,
With a short model peptide in hand, we used NMR to
determine whether 6 adopts an extended conformation in
1
solution. The H NMR of 6 in CDCl3 was well resolved
and showed no evidence of rotational isomers. The CHR
proton resonances of both the Leu and Phe residues also
appeared 0.3ꢀ0.5 ppm downfield of their expected values
(19) Two possible torsions for each J value result from the inherent
ambiguity of the Karplus relationship. Values were calculated according
to the parametrization described in: Pardi, A.; Billeter, M.; Wuthrich, K.
J. Mol. Biol. 1984, 180, 741.
(18) Wishart, D. S.; Sykes, B. D.; Richards, F. M. Biochemistry 1992,
31, 1647.
6164
Org. Lett., Vol. 14, No. 24, 2012

Scheme 3. Synthesis of Heptapeptide Mimic 8 and Conforma-
tional Analysis by 1H NMR
Figure 2. ROESY spectrum of 8 (500 MHz in CDCl3) high-
lighting key Overhauser correlations.
ruling out the presence of dimeric species that may give rise
to intermolecular correlations (see Supporting Information).
Conformers with IP scaffold φ0 torsions at or near 0°,
exemplified by A in Figure 3, would be expected to exhibit
a strong Overhauser correlation between IP1NH and Ha.
The absence of (nonvicinal) ROESY cross-peaks involving
0
0
either protons Ha/a or Hc/c suggests that stable turn
conformations are not significantly populated on the
NMR time-scale. While 2D NMR does not rule out the
posibility of acute Ile, Leu, or Phe φ angles, the JHNCH
coupling constants observed for 8 (see Scheme 3) are not
consistent with structures such as B (Figure 3). Moreover,
the position of the Ile, Leu, and Phe NH resonances in the
1H NMR spectrum of 8 (<7.0 ppm) argue against their
participation in intramolecular H-bonds resulting from
tight turns.20 We also carried out Δδ/ΔT experiments in
DMSO-d6 and found that each of the 5 NH protons in 8
exhibited a high (>4.0 ppb/K) temperature dependence
on chemical shift consistent with solvent accessibility (see
Scheme 3 and Supporting Information).21 Taken together,
these data provide compelling evidence that heptapeptide
mimic 8 adopts an extended conformation in CDCl3.
In summary, we have described the design and synthesis
of novel extended dipeptide surrogates based on sub-
stituted imidazo[1,2-a]pyridine scaffolds. These building
blocks can be readily prepared from β-ketoesters and
Figure 3. Structures of possible turn conformations of 8.
incorporated into host peptides in either protected or
unprotected forms. The efficient synthesis of model pepti-
domimetics harboring these scaffolds affords structures
that adopt β-strand-like conformations in solution. We
anticipate that the imidazo[1,2-a]pyridine subunit will
find utility as a probe of peptide conformation, and
as a dipeptide surrogate with enhanced drug-like char-
acter. Efforts toward IP-based β-strand mimics capable
of disrupting proteinꢀprotein interactions are currently
underway in our laboratory and will be reported in due
course.
Acknowledgment. We gratefully acknowledge financial
support from NIH (CA167215) and Dr. Edwin Rivera
(USF) for assistance with 2D NMR acquisitions.
Supporting Information Available. Experimental pro-
cedures and structural characterization of compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Nowick, J. S.; Smith, E. M.; Pairish, M. Chem. Soc. Rev. 1996,
25, 401.
(21) (a) Kessler, H. Angew. Chem., Int. Ed. 1982, 21, 512. (b) Llinas,
M.; Klein, M. P. J. Am. Chem. Soc. 1975, 97, 4731.
The authors declare no competing financial interest.
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