Bicycle synthesis through peptide macrocyclization using aziridine aldehydes followed by late stage disulfide bond installation

Article abstract of DOI:10.1039/c3md00054k

We present a method that can be applied to generate medium-sized peptidomimetic macrocycles equipped with disulfide bonds. The reaction hinges on amphoteric aziridine aldehydes and their ability to bridge the ends of linear peptides in the presence of isocyanides. Aziridine aldehyde dimers enable the initial cyclization, which is followed by site-specific aziridine ring-opening with sodium azide. Subsequent to that, thallium-induced oxidative disulfide bond formation furnishes the final product. The NMR characterization of the molecules obtained using this method indicates that conformationally well-behaved systems are readily accessible. The site-specific introduction of azide functionality should open the doors to subsequent functionalization using well-established protocols.

Full text of DOI:10.1039/c3md00054k

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Bicycle synthesis through peptide macrocyclization
using aziridine aldehydes followed by late stage
disulfide bond installation†
Cite this: Med. Chem. Commun., 2013,
4, 1124
Benjamin K. W. Chung, Jennifer L. Hickey, Conor C. G. Scully, Serge Zaretsky
*
and Andrei K. Yudin
We present a method that can be applied to generate medium-sized peptidomimetic macrocycles
equipped with disulfide bonds. The reaction hinges on amphoteric aziridine aldehydes and their ability
to bridge the ends of linear peptides in the presence of isocyanides. Aziridine aldehyde dimers enable
the initial cyclization, which is followed by site-specific aziridine ring-opening with sodium azide.
Subsequent to that, thallium-induced oxidative disulfide bond formation furnishes the final product.
The NMR characterization of the molecules obtained using this method indicates that conformationally
well-behaved systems are readily accessible. The site-specific introduction of azide functionality should
open the doors to subsequent functionalization using well-established protocols.
Received 15th February 2013
Accepted 26th May 2013
DOI: 10.1039/c3md00054k
www.rsc.org/medchemcomm
cyclization constraints. Modications such as N-methylation and
intramolecular covalent bridge formation have been previously
Introduction
Macrocycles are a therapeutically relevant class of molecules that
oen show increased binding affinity and selectivity to various
biological targets,1 as well as improved stability compared to the
corresponding linear congeners.2 Notable naturally occurring
macrocycles include erythromycin, a broad-spectrum macrolide
antibiotic, amphotericin B, a polyene antifungal, and cyclo-
sporine A, a cyclic peptide immunosuppressant.3 Cyclic peptides,
as a subset of macrocycles, are particularly noteworthy for their
ability to inhibit protein–protein interactions. Recent successes
include the development of cyclic peptide inhibitors of the p53/
MDM2 interaction,4 avb3 integrin antagonists,1a,5 and hydro-
carbon stapled peptides for the disruption of the BID-Bcl-2
complex,1b among others.6
reported to improve potency and bioavailability of peptide mac-
rocycles.9 Conformationally restricted peptide macrocycles can
also act as models for epitopes of folded proteins by conferring
structural stability to peptides that would otherwise exist as
random coils in solution.10 Previous examples include the alkyl-
ation of cysteine residues within a peptide using tris-(bromo-
methyl)benzene, promoting internal hydrogen-bond formation
within the structure and ultimately stabilizing the g- and b-turns
within the macrocycle.11
Inspired by the ubiquity of disuldes in biology, we sought to
prepare synthetic peptidomimetic macrocycles containing a
disulde bridge. We also hoped to explore the compatibility of
While there has been much development in contemporary
strategies for the synthesis of cyclic peptides, peptide macro-
cyclization still proves to be challenging, especially for small- to
medium-sized rings.7 As increasing numbers of protein–protein
interactions are validated as drug targets, there has been
demand for new methods of peptide cyclization. Recently, we
reported a robust process for head-to-tail peptide macro-
cyclization using aziridine aldehydes and isocyanides, enabling
the synthesis of peptidomimetic macrocycles of various ring
sizes and sequences (Scheme 1).8
As part of a program aimed at delineating drug-like properties
of macrocycles, we became interested in creating additional post-
Department of Chemistry, University of Toronto, 80 St George Street, Toronto, Canada.
Tel: +1 4169465042
Scheme 1 Mechanism of the macrocyclization of N-terminal proline peptides
using (S)-aziridine-2-carboxaldehyde dimer and tert-butyl isocyanide. TFE ¼ 2,2,2-
trifluoroethanol.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3md00054k
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aziridine aldehyde-derived peptide macrocyclization with
disulde bridge formation. Ultimately, the intramolecular
disulde could serve as a structurally stabilizing element to
further rigidify a macrocyclic scaffold. In this report, we outline
a strategy for disulde-bond formation in the synthesis of
challenging medium-sized peptidomimetic macrocycles
derived from the aziridine aldehyde-driven cyclization.
Synthesis
We envisioned the synthesis of disulde-bridged macrocycles
through the macrocyclization of peptides containing two
cysteine residues. This approach can be realized by two
different pathways – the rst requiring formation of the disul-
de bond followed by cyclization, while the second involving
cyclization and subsequent disulde bridge formation. For the
rst approach in which the disulde is formed before cycliza-
tion (Scheme 2), the linear peptide Fmoc-PC(PG)LFC(PG)-OH
(PG ¼ Acm or Trt) was prepared on 2-chlorotrityl resin. The solid
supported peptide was then subjected to oxidative deprotection
using either iodine/base or thallium triuoroacetate, Tl(TFA)3.
Oxidation was monitored by performing microcleavages of the
resin-bound peptide followed by liquid chromatography-mass
spectrometry (LC-MS) analysis. Out of four reactions, oxidative
deprotection of the Acm-protected peptide by Tl(TFA)3 gave the
cleanest outcome. The disulde-containing peptide then
underwent Fmoc-deprotection and resin cleavage using
1,1,1,3,3,3-hexauoroisopropan-2-ol/dichloromethane (HFIP/
DCM, 1 : 3), followed by macrocyclization. The macro-
cyclization was monitored by LC-MS, which revealed a small
amount of the desired product. However, the mass of an imi-
dazolidinone product was observed to be the major component
of the reaction mixture.
Scheme 3 Mechanism for formation of the imidazolidinone side product.
The mechanism of imidazolidinone formation (Scheme 3) rst
involves reversible dissociation of the aziridine aldehyde to the
reactive open dimer form.12 Next, condensation of the peptide N-
terminal nitrogen with the open dimer form generates the imi-
nium intermediate. Finally, nucleophilic attack of the (n + 1)
amide nitrogen on the resulting iminium gives the ve-membered
imidazolidinone. This process is hypothesized to occur because
the disulde forces the peptide to exist in a conformation where
the C-terminal carboxylate is oriented away from the intermediate
iminium and unfavourable for cyclization to proceed.
Since the presence of the disulde appears to prohibit head-
to-tail peptide macrocyclization and increases the possibility of
imidazolidinone by-product formation, we wanted to see if
macrocyclization before disulde formation could form the
desired disulde-bridged peptidomimetic macrocycle. In the
alternative approach, linear peptide PC(PG)LFC(PG) (PG ¼ Acm
or Trt) underwent cyclization rst, followed by oxidation
(Scheme 4). As head-to-tail macrocyclization of peptides using
Scheme 2 Disulfide formation prior to peptide macrocyclization. (a) I2 (8 eq.),
DIPEA (8 eq.), DCM/MeOH/H2O (15 : 6 : 1, 18 mL gꢀ1 resin), RT, 45 min. OR Tl(TFA)3
(1.5 eq.), DMF (0.06 M), RT, 2 h; (b) 20% (%v/v) piperdine, DMF (10 mL gꢀ1 resin), RT,
30 min; (c) HFIP/DCM (1 : 3, 10 mL gꢀ1 resin), RT, 1 h; (d) (S)-aziridine-2-carbox-
aldehyde dimer (1 eq.), tert-butyl isocyanide (1 eq.), TFE (0.1 M), RT, 6h.
Scheme 4 PCLFC cyclization followed by disulfide formation. (a) (S)-aziridine-2-
carboxaldehyde dimer (0.51 eq.), tert-butyl isocyanide (1.01 eq.), TFE (0.1 M), RT, 6
h; (b) NaN3 (10 eq.), MeCN/DMF (0.04 M), 60 ꢁC, 4 h; (c) Tl(TFA)3 (1.5 eq.), anisole
(2 eq.), TFA (2 mM), RT, 5 h.
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Table 1 List of 1H NMR chemical shifts for 2 (methanol-d4). Chemical shifts are
reported in ppm
Residue position
a
b
g
d
P1
C2
G3
L4
G5
F6
C7
3.63
4.37
4.09, 3.67
4.57
3.78, 3.41
4.60
4.73
3.66 (CH)
4.28 (CH)
2.19, 1.93
3.69, 3.05
—
1.57
—
3.44, 2.85
3.57, 3.35
1.40 (tBu)
3.33 (CH2)
1.84, 1.67
—
—
1.48
—
3.30, 2.93
—
—
0.92
—
—
—
—
—
Exocyclic amide
N3
Fig. 1 Structures of disulfide-bridged peptidomimetic macrocycles derived from
PC(Acm)GLGFC(Acm) (2) and PC(Acm)LGLGFC(Acm) (3).
aziridine aldehydes requires free N- and C-termini, the side-
This spectrum indicates that the scaffolds are either struc-
chain protected peptides must rst be cleaved from resin aer turally homogeneous and exist as one major conformation in
solution, or have multiple conformations that interconvert
preparation on solid phase.
rapidly in the NMR time scale. Given the size (from ve to eight
Cyclization of the Acm-protected peptide in solution pro-
ceeded cleanly, but the reaction with the Trt-protected peptide residues, medium length) of the peptidomimetic macrocycles
was slow and did not result in formation of the desired product. as well as the presence of an intramolecular disulde, the rapid
This can be attributed to steric bulk at the Trt-protected interconversion is most likely but future work involves further
C-terminal cysteine residue which prohibited macrocyclization. experimentation in order to clarify this.
The Acm-protected macrocycle was treated with sodium azide to
afford the aziridine ring-opened product. Finally, oxidative Acm-
deprotection with Tl(TFA)3 yielded the nal disulde-bridged
macrocycle 1 in 8% isolated yield.13 Assay yields of the three step
Summary
In summary, a method for the preparation of synthetically
sequence were determined by LC-MS for the synthesis of 1 using
caffeine as an external standard (details in ESI†). The rst two
steps (macrocyclization and azide functionalization) gave an
assay yield of 92% while the nal step (oxidative Acm-depro-
tection) gave an assay yield of 87%.
challenging medium-sized disulde-bridged peptidomimetic
macrocycles has been developed. The disulde-containing
compounds that were prepared illustrate an enabling approach
to generate novel scaffolds whose bicyclic connectivities repre-
sent an underexplored area of structural space. In addition, the
use of an azide nucleophile for late-stage aziridine ring-opening
creates opportunities for downstream application of macro-
cycles. Given the tremendous interest in the development of
macrocyclic scaffolds, the ability to generate disulde-bridged
macrocycles in a systematic fashion should facilitate the
discovery of novel biological probes.
This method was used to generate two additional disulde-
bridged peptidomimetic macrocycles 2 and 3 derived from linear
peptides PC(Acm)GLGFC(Acm) and PC(Acm)LGLGFC(Acm)
(Fig. 1). 1- and 2-D NMR studies of the disulde-bridged macro-
1
cycles demonstrate sharp peaks. A representative H NMR spec-
trum and a list of 1H resonances is shown for 2 (Fig. 2 and Table 1).
Fig. 2 1H NMR spectrum of 2. Sample was prepared in methanol-d4 and obtained on an Agilent DD2 500 MHz spectrometer with an Agilent HC 5-mm XSens
cryogenically-cooled probe.
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(m, 4H), 3.27–3.13 (m, 5H), 2.17–2.06 (m, 1H), 1.95 (m, 1H), 1.85
(m, 1H), 1.68 (m, 1H), 1.36 (s, 12H), 0.87 (d, J ¼ 6.1 Hz, 1H), 0.81
(d, J ¼ 6.2 Hz, 1H). 13C NMR (126 MHz, methanol-d4) d 176.9,
174.4, 174.0, 172.2, 172.1, 171.9, 130.4, 129.5, 127.9, 115.8, 66.6,
66.3, 56.8, 54.33, 52.7, 52.6, 52.2, 51.6, 49.5, 49.3, 49.1, 49.0,
40.9, 40.4, 31.0, 28.8, 28.7, 25.7, 25.4, 22.9, 22.5. HRMS (ESI)
calculated for C38H56N12O8S2 (M + H+): 759.3429; found:
759.3423. Retention time (analytical HPLC/MS): 6.99 min.
Disulde 2. 1H NMR (500 MHz, methanol-d4) d 7.31–7.15
(m, 5H), 4.73 (dd, J ¼ 12.2, 4.9 Hz, 1H), 4.60 (dd, J ¼ 11.2, 3.7 Hz,
1H), 4.57 (dd, 1H), 4.37 (dd, J ¼ 12.0, 4.2 Hz, 1H), 4.28 (td, J ¼
7.0, 3.8 Hz, 1H), 4.10 (d, J ¼ 16.9 Hz, 1H), 3.80 (d, J ¼ 15.0 Hz,
1H), 3.68 (d, J ¼ 10.0 Hz, 2H), 3.66 (d, J ¼ 3.2 Hz, 1H), 3.65–3.61
(m, 1H), 3.58 (dd, J ¼ 12.1, 4.9 Hz, 1H), 3.46–3.32 (m, 6H), 3.05
(dd, J ¼ 15.1, 4.2 Hz, 1H), 2.93 (td, J ¼ 9.7, 5.6 Hz, 1H), 2.85 (dd,
J ¼ 14.4, 11.1 Hz, 1H), 2.18 (tdd, J ¼ 12.4, 9.9, 6.8 Hz, 1H), 1.96–
1.89 (m, 1H), 1.88–1.80 (m, 1H), 1.72–1.61 (m, 1H), 1.61–1.53
(m, 2H), 1.49 (q, J ¼ 8.4, 7.5 Hz, 1H), 1.40 (s, 9H), 0.93 (dd, J ¼
6.1, 4.3 Hz, 6H). 13C NMR (126 MHz, methanol-d4) d 179.1,
174.4, 173.8, 172.9, 172.1, 171.9, 171.1, 170.9, 139.0, 130.0,
129.5, 127.7, 71.5, 68.2, 60.5, 56.2, 54.4, 53.5, 53.1, 52.9, 52.6,
44.9, 44.5, 44.0, 39.2, 37.3, 34.3, 32.3, 28.9, 28.8, 26.1, 25.6, 23.3,
23.0. HRMS (ESI) calculated for C38H56N12O8S2 (M + H+):
Experimental
2-Chlorotrityl chloride resin, coupling reagents, and all Fmoc-
protected amino acids were purchased from AAPPTec Inc.
(Louisville, KY, U.S.A.). 2,2,2-Triuoroethanol was purchased
from Oakwood Products (West Columbia, SC, U.S.A.). All other
reagents and solvents were purchased from commercial sources
and used as received, unless otherwise noted. All reactions
involving chemical transformations on peptides were analyzed
using analytical high-performance liquid chromatography-
mass spectrometry (HPLC/MS) with an Agilent Technologies
(Mississauga, ON, Canada) 1200 Series liquid chromatography
system. Purication of nal peptide products was performed
with semi-preparative HPLC/MS using an Agilent 1260 Innity
Series liquid chromatography system. 1H, 13C, gCOSY, zTOCSY,
and gc2HSQCse nuclear magnetic resonance (NMR) spectra
were recorded on an Agilent DD2 500 MHz spectrometer. 1H
and 13C NMR spectra chemical shis (d) are reported in parts
per million (ppm) referenced to residual solvent peaks (MeOD, d
¼ 3.31 for 1H, d ¼ 49.00 for 13C).
General procedure for synthesis of compounds 1–3
Side-chain protected linear peptide (1 eq.) and (S)-aziridine-2- 873.3858; found: 873.3831. Retention time (analytical HPLC/
carboxaldehyde dimer (0.51 eq.) were weighed in a 20 mL glass MS): 7.22 min.
vial. 2,2,2-Triuoroethanol (0.1 M) was added, and the resultant
Disulde 3. 1H NMR (500 MHz, methanol-d4) d 7.33–7.11 (m,
solution/suspension stirred vigorously. tert-Butyl isocyanide 5H), 4.71 (t, J ¼ 7.5 Hz, 1H), 4.66 (dd, J ¼ 10.0, 4.9 Hz, 1H), 4.41
(1.01 eq.) was added, and the reaction was stirred at room (td, J ¼ 7.2, 4.4 Hz, 1H), 4.27 (dd, J ¼ 9.5, 5.2 Hz, 1H), 4.20 (s,
temperature for 3–5 hours until complete consumption of linear 2H), 4.14 (d, J ¼ 17.2 Hz, 1H), 3.88–3.81 (m, 1H), 3.74–3.70 (m,
peptide as demonstrated by LC-MS analysis. The solvent was 1H), 3.67 (dd, J ¼ 3.9, 1.5 Hz, 1H), 3.65–3.60 (m, 1H), 3.57 (dd,
removed under reduced pressure and triturated once with tert- J ¼ 16.7, 1.6 Hz, 1H), 3.53–3.32 (m, 6H), 3.28–3.24 (m, 2H), 3.17
butyl methyl ether (TBME). The triturated aziridine-containing (dd, J ¼ 13.9, 10.3 Hz, 1H), 3.01 (q, J ¼ 7.8, 7.1 Hz, 1H), 2.23–2.10
macrocycle was then dissolved in a mixture of acetonitrile : N,N- (m, 1H), 1.95 (dt, J ¼ 12.2, 4.4 Hz, 1H), 1.82 (ddd, J ¼ 13.4, 6.8,
dimethylformamide (MeCN : DMF 1 : 1, 0.04 M) followed by the 3.8 Hz, 2H), 1.80–1.60 (m, 5H), 1.59–1.50 (m, 1H), 1.40–1.36
addition of sodium azide (10 eq.). The reactions were heated to 65 (m, 9H), 0.97–0.90 (m, 12H). 13C NMR (126 MHz, methanol-d4) d
ꢁC with stirring for 4 hours. The solvent was partially removed 177.4, 174.7, 172.9, 171.6, 171.1, 171.0, 170.7, 170.4, 169.7,
under reduced pressure. The remaining mixture in DMF was 137.9, 128.8, 128.1, 126.2, 68.8, 66.3, 55.8, 54.3, 53.2, 53.0, 52.6,
diluted with ten volumes of water, and extracted ve times with 52.4, 51.9, 51.4, 51.3, 43.3, 42.3, 39.5, 38.5, 34.7, 30.6, 27.5, 24.7,
ethyl acetate (EtOAc). The organic extracts were combined, dried 24.6, 24.5, 22.3, 21.9, 20.7, 20.5. HRMS (ESI) calculated for
with anhydrous sodium sulfate, ltered, and concentrated under
C
38H56N12O8S2 (M + H+): 986.4699 found: 986.4685. Retention
reduced pressure. The residue containing azide ring-opened time (analytical HPLC/MS): 7.39 min.
macrocycle was dissolved in triuoroacetic acid (0.002 M), fol-
lowed by addition of anisole (2 eq.) and thallium(III) tri-
uoroacetate (1.5 eq.). The reactions were stirred at room
Acknowledgements
temperature for 4 hours. Next, the solvent was removed under
The authors would like to acknowledge CQDM and OGI (SPARK
reduced pressure and the macrocycles were triturated once with
TBME. The crude material was then subjected to silica gel
chromatography using a CombiFlashꢀ Rf system equipped with
a RediSep Rf Goldꢀ Normal-Phase Silica 4 g column and a
gradient of 0–50% methanol in EtOAc as eluent. Aer concen-
tration under reduced pressure, the roughly puried product was
subjected to semi-preparative HPLC/MS purication. The frac-
tions containing the desired mass as determined by ESI-MS were
combined and lyophilized overnight.
program) for funding this project.
Notes and references
1 (a) R. Haubner, R. Gratias, B. Diefenbach, S. L. Goodman,
A. Jonczyk and H. Kessler, J. Am. Chem. Soc., 1996, 118,
7461; (b) L. D. Walensky, A. L. Kung, I. Escher, T. J. Malia,
S. Barbuto, R. D. Wright, G. Wagner, G. L. Verdine and
S. J. Korsmeyer, Science, 2004, 305, 1466.
Disulde 1. 1H NMR (500 MHz, methanol-d4) d 7.30–7.18
(m, 5H), 4.22 (ddd, J ¼ 7.0, 5.4, 4.0 Hz, 1H), 3.95 (t, J ¼ 7.5 Hz,
1H), 3.75 (dd, J ¼ 15.2, 5.3 Hz, 1H), 3.62–3.54 (m, 2H), 3.51–3.32
2 J. D. Tyndall, T. Nall and D. P. Fairlie, Chem. Rev., 2005, 105,
973.
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3 E. M. Driggers, S. P. Hale, J. Lee and N. K. Terrett, Nat. Rev.
Drug Discovery, 2008, 7, 608.
A. Friedler, Y. Ben-Neriah and C. Gilon, Biopolymers, 2009,
91, 157; (i) L. Rizzi, S. Sundararaman, K. Cendic, N. Vaiana,
R. Korde, D. Sinha, A. Mohmmed, P. Malhotra and
S. Romeo, Eur. J. Med. Chem., 2011, 46, 2083; (j) V. Dewan,
T. Liu, K. M. Chen, Z. Qian, Y. Xiao, L. Kleiman,
K. V. Mahasenan, C. Li, H. Matsuo, D. Pei and K. Musier-
Forsyth, ACS Chem. Biol., 2012, 7, 761.
7 C. J. White and A. K. Yudin, Nat. Chem., 2011, 3, 509.
8 R. Hili, V. Rai and A. K. Yudin, J. Am. Chem. Soc., 2010, 132,
10986.
4 (a) R. Fasan, R. L. A. Dias, K. Moehle, O. Zerbe,
J. W. Vrijbloed, D. Obrecht and J. A. Robinson, Angew.
Chem., Int. Ed., 2004, 43, 2109; (b) M. M. Madden,
A. Muppidi, Z. Li, X. Li, J. Chen and Q. Lin, Bioorg. Med.
Chem. Lett., 2011, 21, 1472; (c) S. Baek, P. S. Kutchukian,
G. L. Verdine, R. Huber, T. A. Holak, K. W. Lee and
G. M. Popowicz, J. Am. Chem. Soc., 2012, 134, 103.
¨
¨
5 (a) M. Pfaff, K. Tangemann, B. Muller, M. Gurrath, G. Muller,
H. Kessler, R. Timpl and J. Engel, J. Biol. Chem., 1994, 269,
9 (a) D. J. Craik and D. J. Adams, ACS Chem. Biol., 2007, 2, 457;
(b) T. R. White, C. M. Renzelman, A. C. Rand, T. Rezai,
C. M. McEwen, V. M. Gelev, R. A. Turner, R. G. Linington,
S. S. F. Leung, A. S. Kalgutkar, J. N. Bauman, Y. Zhang,
S. Liras, D. A. Price, A. M. Mathiowetz, M. P. Jacobson and
R. S. Lokey, Nat. Chem. Biol., 2011, 7, 810.
´
20233; (b) A. Roxin, J. Chen, C. C. G. Scully, B. H. Rotstein,
A. K. Yudin and G. Zheng, Bioconjugate Chem., 2012, 23, 1387.
6 (a) A.-M. Leduc, J. O. Trent, J. L. Wittliff, K. S. Bramlett,
S. L. Briggs, N. Y. Chirgadze, Y. Wang, T. P. Burris and
A. F. Spatola, Proc. Natl. Acad. Sci. U. S. A., 2003, 100,
11273; (b) S. Fletcher and A. D. Hamliton, J. R. Soc., 10 A. C. Gibbs, L. H. Kondejewski, W. Gronwald, A. M. Nip,
Interface, 2006, 3, 215; (c) C. J. Porter, J. M. Matthews,
J. P. Mackay, S. E. Pursglove, J. W. Schmidberger,
R. S. Hodges, B. D. Sykes and D. S. Wishart, Nat. Struct.
Biol., 1998, 5, 284.
P. J. Leedman, S. C. Pero, D. N. Krag, M. C. J. Wilce and 11 (a) D. S. Kemp and P. E. McNamara, J. Org. Chem., 1985, 50,
J. A. Wilce, BMC Struct. Biol., 2007, 7, 58; (d) J. Liu, C. Li,
S. Ke and S. D. Satyanarayanajois, J. Med. Chem., 2007, 50,
4038; (e) A. Tavassoli, Q. Lu, J. Gam, H. Pan, S. J. Benkovic
and S. N. Cohen, ACS Chem. Biol., 2008, 3, 757; (f)
5834; (b) P. Timmerman, J. Beld, W. C. Puijk and
R. H. Meloen, ChemBioChem, 2005, 6, 821; (c) C. Heinis,
T. Rutherford, S. Freund and G. Winter, Nat. Chem. Biol.,
2009, 5, 502.
Y. Zhang, S. Zhou, A. S. Wavreille, J. DeWille and D. Pei, J. 12 N. Assem, R. Hili, Z. He, T. Kasahara, B. L. Inman, S. Decker
Comb. Chem., 2008, 10, 247; (g) T. Liu, S. H. Joo, and A. K. Yudin, J. Org. Chem., 2012, 134, 9926.
J. L. Voorhees, C. L. Brooks and D. Pei, Bioorg. Med. Chem., 13 Low isolated yields are due to product loss during semi-
2009, 17, 1026; (h) N. Qvit, A. Hatzubai, D. E. Shalev, preparative HPLC-MS purication.
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