Inhibition of hepatitis C virus NS5A by fluoro-olefin based γ-turn mimetics

Article abstract of DOI:10.1016/j.bmcl.2012.02.051

The HCV non-structural protein NS5A has been established as a viable target for the development of direct acting antiviral therapy. From computational modeling studies strong intra-molecular hydrogen bonds were found to be a common structural moiety withi

Full text of DOI:10.1016/j.bmcl.2012.02.051

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2938–2942
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Inhibition of hepatitis C virus NS5A by fluoro-olefin based c-turn mimetics
⇑
Wonsuk Chang , Ralph T. Mosley, Shalini Bansal, Meg Keilman, Angela M. Lam, Phillip A. Furman,
Michael J. Otto, Michael J. Sofia
Pharmasset1, Inc. 303A College Road East, Princeton, NJ 08540, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The HCV non-structural protein NS5A has been established as a viable target for the development of
direct acting antiviral therapy. From computational modeling studies strong intra-molecular hydrogen
bonds were found to be a common structural moiety within known NS5A inhibitors that have low
Received 6 January 2012
Revised 14 February 2012
Accepted 16 February 2012
Available online 28 February 2012
pico-molar replicon potency. Efforts to reproduce these
c-turn-like substructures provided a novel
NS5A inhibitor based on a fluoro-olefin isostere. This fluoro-olefin containing inhibitor exhibited picom-
olar activity (EC50 = 79 pM) against HCV genotype 1b replicon without measurable cytotoxicity. This level
of activity is comparable to the natural peptide-based inhibitors currently under clinic evaluation, and
demonstrates that a peptidomimetic approach can serve as a useful strategy to produce potent and struc-
turally unique inhibitors of HCV NS5A.
Keywords:
Antiviral
HCV
Hepatitis C
NS5A
Ó 2012 Elsevier Ltd. All rights reserved.
Peptidomimetics
c
-Turn
Chronic hepatitis C virus (HCV) infection afflicts approximately
the efforts described in recent patent applications concentrate on
simple modifications of the central linker, and the two pyrrolidine
(B,B0) units (Fig. 1).7
160 million people worldwide and is the leading cause of liver cir-
rhosis and hepatocellular carcinoma in the US.1 Among first gener-
ation direct acting antivirals (DAAs), two NS3/4a protease
inhibitors, telaprevir and boceprevir, in combination with pegylat-
ed interferon (PEG-IFN) and ribavirin (RBV) have been approved for
the treatment of HCV infection in mid-2011.2 Both of these combi-
nation treatments have shown improved sustained virological
response (SVR) and reduced treatment duration compared to
PEG-IFN/RBV. However, they are limited to treating only genotype
1 patients, still require PEG-IFN/RBV adjunct therapy and bring
with them additional side effects which require further medical
attention.3 Therefore, the search continues for novel agents with
complementary mechanisms of action that can be combined with
existing or future DAA therapies and that have improved safety
profiles over existing therapy.4
The NS5A inhibitor, BMS-790052 (daclatasvir, 1) has recently
been reported to produce promising clinical outcomes among
HCV infected patients.5 Consequently remarkable interest within
the pharmaceutical community has been focused on related
NS5A inhibitors.6 Currently known development candidates in this
series include BMS-790052, & BMS-824393, GS-5885, PPI-461, PPI-
437, PPI-668, & PPI-833, ABT-267, EDP-239, ACH-2928 & ACH-
3102, GSK-2336805, MK-4882, and IDX-719. Although the exact
structures of many of these agents are undisclosed, a majority of
A general structural feature found in the known NS5A inhibitors
having low pico-molar replicon potency is a peptide substructure
containing multiple functional groups which can act as hydrogen
bond donors or acceptors. In order to better understand if these
chemical features conferred an energetically favored three dimen-
sional shape which could be correlated to potency, we applied the
following molecular modeling approach to all of the compounds.
Conformers were generated using torsional sampling (MCMM)8
followed by energy minimization to a convergence gradient of
0.001 using MMFFs with a distance dependent dielectric of 2r.9
An RMSD of 0.25 and a 12 or 20 kcal/mol energy window were
used as criterion to limit the number of conformers output with
an upper limit of 1000 being possible. Of the 297 conformers gen-
erated for compound 1 within the range of 88–108 kcal/mol
(Fig. 2), the 12 lowest energy conformers were observed to make
internal hydrogen bonds between the valine C@O and imidazole
N–H (distance = 2.22 Å) on both sides of the dimer. The observed
lowest energy conformations were similar to
c-turns typically
found in polypeptide structures: the -turn is a three amino acid
c
residue turn which contains a seven-membered ring constructed
by the hydrogen bond between the carbonyl C@O of the ith residue
and the amide N–H of the i + 2 residue.10
We envisaged that the intra-molecular hydrogen bonds within
the NS5A inhibitor structure might play a crucial role in holding
the molecule in an optimized binding conformation. As an effort
to both verify our hypothesis and to discover a novel inhibitor
⇑
Corresponding author. Tel.: +1 609 613 4100; fax: +1 609 613 4150.
E-mail address: wochang@gmail.com (W. Chang).
Pharmasset was acquired by Gilead Sciences on January 17, 2012.
1
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.02.051

W. Chang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2938–2942
2939
O
O
O
O
O
O
O
O
N
N
HN
NH
O
O
H
H
Fast-follow
N
B
N
B'
O
O
N
N
A
A'
Linker
H
H
N
N
A, A' = Hydrogen bond donors, such as (fused) imidazoles, amides
B, B '= (Un)substituted 3- to 6-membered (fused) heterocycles
N
N
Linker = Rigid and hydrophobic functionalities, such as combinations
1, BMS-790052
of aryl, alkynyl, cycloalkyl, heteroaryl, & fused-aryl groups
Figure 1. Current HCV NS5A inhibitors.6a
Figure 2. Energy-minimized structures of HCV NS5A inhibitors.
class, a number of structural motifs that could potentially repro-
duce the -turn were evaluated computationally. Among them,
the fluoro-olefin based proline analogue 2 was predicted to effec-
tively mimic the -turn. Analog 2 was shown to generate low en-
removed under acidic conditions. Introduction of the methyl car-
bamate functionality to amine 11 provided a crystalline solid, ure-
thane 12, of which regio- and absolute stereochemistries were
unambiguously verified by a single crystal X-ray structure (Fig. 3,
CCDC 866508). Jones oxidation of the primary alcohol 12 yielded
carboxylic acid 13,14a which was then reacted with symmetric
1,10-([1,10-biphenyl]-4,40-diyl)bis(2-bromoethanone) to give di-
meric ester 14. Finally, the two ketoester groups of 14 were cy-
clized upon microwave irradiation for 30 min at 155 °C in the
presence of excess ammonium acetate, thus providing the bis-
imidazolic fluoro-olefin 2.
c
c
ergy conformers that are internally hydrogen-bonded between
the fluoro-olefin @C–F and imidazole N–H (distance = 2.29 Å) on
both sides of the dimer. This fluoroalkene isostere maintain steric
and electronic character similar to a peptide bond,11 and has been
previously employed in thermolysin inhibitor,12 cyclophilin A
inhibitors,13 dipeptidyl peptidase IV inhibitors,14 neuropeptide
substance P analogs,15 and human parathyroid hormone analogs.16
If the peptidomimetic substructure is able to maintains the desired
inhibitor conformation and demonstrate potent inhibition of HCV
replication, a new approach to the development of novel NS5A
inhibitors with a distinctive structural feature is possible. There-
fore, we decided to prepare the fluoro-olefin based structures
and study their anti-HCV activities.
Separately, fluoro-olefin 23 which does not include the neigh-
boring cyclopentane rings (Leu-NMe-trans-Ala mimic) was also
prepared. Unlike proline analog 2, N-methylalanine analog 23 con-
tains a freely rotatable
fore, is less likely to induce the
analog 2. By examining the biological activities of the two fluoro-
olefin analogs 2 and 23, we hoped to determine whether the -turn
r
-bond next to the fluoro-olefin and there-
c-turn sub-structure anticipated for
The synthesis of Leu-trans-Pro mimetic 2 was initiated with
stereoselective organocatalytic aldol reaction of cyclopentanone
where it was treated with L-threonine and aqueous formaldehyde
c
predicted for 2 is actually an important NS5A inhibitory conforma-
tional element. The synthesis of flouro-olefin 22 started from com-
mercially available (S)-methyl 3-hydroxy-2-methylpropanoate 15
(Scheme 2). First, the alcohol was protected with a TBDPS group
to give silyl ether 16 and a Weinreb amide was introduce to the es-
ter moiety. Methyl Grignard reagent was subsequently reacted
with the Weinreb amide 17 at ꢀ10 oC to provide ketone 18 in
excellent yield. Unlike the previous example of cyclopentanone 6,
the Horner-Emmons reaction of methyl ketone 18 with ethyl 2-
(diethoxyphosphoryl)-2-fluoroacetate gave (Z)-olefin 19 as a minor
regioisomer in a 1:1.5 ratio. The remainder of the synthesis mir-
rored that used to prepare compound 2, and gave alanine mimetic
23 in 6.5% overall yield.
(Scheme 1).17 After silyl protection of the hydroxyl group, ketone
6 was obtained in 55% yield and its enantiomeric excess (ee) was
determined to be 60% by chiral SFC. Then, the Horner-Emmons
reaction of the cyclopentanone and ethyl 2-(diethoxyphospho-
ryl)-2-fluoroacetate provided (Z)-fluoro-olefin 7, which was easily
separated from the (E)-isomer by regular column chromatogra-
phy.18 A two-step sequence which included DIBALH reduction
and a Dess–Martin oxidation afforded
a-fluoro-a,b-unsaturated
aldehyde 8. An Ellman’s sulfinyl imine was subsequently con-
structed using (S)-tert-butanesulfinamide and titanium(IV)
ethoxide.19 Diastereoselective addition of isopropyl lithium to b-
fluoroimine 9 (dr 3:1) established the
L
-Leu moiety in a similar
Anti-HCV activity of fluoro-olefins 2 and 23 was measured in a
clone A replicon whole cell assay (Table 1). Inhibitor 23 based on
fashion as the previously reported example.20 The sulfinyl group
of compound 10 and the silyl protecting group were readily
(Z)-Leu-w[CF@C(Me)]-Ala, which displayed a less favorable

2940
W. Chang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2938–2942
O
O
O
O
F
O
EtO
c
b
a
+
OH
OTBDPS
H
H
OTBDPS
4
3
6
5
7
O
S
H
H
O
S
F
F
tBu
N
O
F
g
d,e
f
tBu
N
H
OTBDPS
OTBDPS
OTBDPS
9
8
10
O
O
F
F
F
h
i
j
k
H2N
O
N
H
O
N
H
O
OH
OH
OH
11
13
12
O
O
O
O
O
HN
NH
O
N
H
F
l
F
F
O
O
O
H
H
N
N
N
N
2
14
2
Scheme 1. Reagents and conditions: (a) 0.3 equiv L-threonine, dioxane, rt, 33%, 60% ee; (b) TBDPSCl, imidazole, DCM, rt, 55%; (c) ethyl 2-(diethoxyphosphoryl)-2-
fluoroacetate, NaH, ether, 36%, (E:Z = 1:1.2); (d) DIBALH, ether, 0 °C, 62%; (e) Dess–Martin periodinane, DCM, rt, 85%; (f) (S)-2-tert-butanesulfinamide, Ti(OEt)4, DCM, rt, 63% in
two steps; (g) iPrLi, toluene, ꢀ78 °C, 58% (dr 3:1); (h) 2 N HCl, dioxane/EtOH; (i) methylchloroformate, Et3N, DCM/EtOH, 63% in two steps; (j) CrO3, aq H2SO4, acetone, 0 °C,
61%; (k) 1,10-([1,10-biphenyl]-4,40-diyl)bis(2-bromoethanone), iPrNEt2, CH3CN, 92%; (l) NH4OAc, p-xylene, Microwave, 155 °C, 32%.
substructure as an important conformational motif in the binding
interaction with the NS5A protein that ultimately results in the
inhibition of HCV replication by this class of molecules. In addition,
the (Z)-Leu-
w
[CF@C]-Pro based inhibitor 2 did not exhibit any
M)
cytotoxicity up to the highest concentration tested (CC50 >10
l
rendering a large selectivity index. It was also observed that 2 pro-
duced significantly reduced potency against genotype 1a replicon
relative to genotype 1b. This is in contrast to the result observed
for 1 and consistent with the genotype activities observed for pre-
viously reported NS5A inhibitors.6,21 Consequently, in order to
achieve comparable potency across multiple genotypes further
work is needed.
In 2010, Urban and co-workers, reported ab initio calculation of
simple fluoro-olefins as peptide mimics, where the structure of
(R,Z)-4-fluoro-N,2-dimethylpent-3-enamide (NFA) was predicted
to closely resemble the structure of its natural peptide congener,
(S)-2-acetamido-N-methylpropanamide (ADA).22 However, the in-
ter-atomic distance between the fluorine in the @C–F and the
hydrogen in the N–H in NFA was calculated to be 2.407 Å, which
was significantly longer than the calculated length of the
c-turn
hydrogen bond (2.026 Å) in ADA. This calculated result showed
that the hydrogen-bond accepting ability of the fluorine atom in
a fluoro-olefin analog is less strong than that of the oxygen in a car-
bonyl group.23 Comparing the fluoro-olefins 2 and 23 described
Figure 3. ORTEP drawing of carbamate 12.
here, the (Z)-Me-w[CF@CH]-Ala model (NFA) imparts significantly
tendency to induce a c-turn in computational modeling, did show
nano-molar in vitro activity (EC50 = 27.3 nM) in a genotype 1b rep-
licon assay. However, the proline mimetic 2, which was calculated
less conformational restriction. Consequently, our results demon-
strate that by imposing appropriate conformational constraint
around the putative internal hydrogen bond, even a weak hydro-
to induce a more stable c-turn as is seen for other NS5A inhibitors,
exhibited pico-molar anti-HCV activity (EC50 = 79 pM) in a geno-
type 1b replicon assay. Thus 2 was shown to be >300-fold more po-
gen-bond-acceptor such as fluorine can stabilize a c-turn substruc-
ture and produce effective HCV NS5A inhibitor. Further studies
toward more potent NS5A inhibitors that incorporate strong inter-
nal hydrogen bonds are currently in progress.
tent than the acyclic analog 23 providing support for the
c-turn

W. Chang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2938–2942
2941
O
O
O
O
F
b
c
a
MeO
N
OTBDPS
O
OH
O
OTBDPS
OTBDPS
18
15
16
17
O
O
S
F
F
O
g,h
d
EtO
e,f
tBu
N
H
OTBDPS
OTBDPS
OTBDPS
20
19
21
O
O
O
O
O
HN
NH
k,l,m
i,j
F
O
N
F
F
H
H
H
OH
N
N
22
N
N
23
Scheme 2. Reagents and conditions: (a) TBDPSCl, imidazole, DCM, rt, 61%; (b) N,O-dimethylhydroxylamine hydrochloride, iPrMgCl, THF, ꢀ20 to ꢀ5 °C, 99%; (c) MeMgBr, THF,
ꢀ10 °C, 99%; (d) ethyl 2-(diethoxyphosphoryl)-2-fluoroacetate, NaH, ether, 33% (E:Z = 1.5:1); (e) DIBALH, ether, ꢀ78 °C; (f) Dess–Martin periodinane, DCM, rt, 60%; (g) (S)-2-
tert-butanesulfinamide, Ti(OEt)4, DCM, rt; (h) iPrLi, toluene, ꢀ78 °C, 50% (dr 7:3); (i) 2 N HCl, dioxane/EtOH; (j) methylchloroformate, Et3N, DCM/EtOH, 54%; (k) CrO3, aq
H2SO4, acetone, 0 °C; (l) 1,10-([1,10-biphenyl]-4,40-diyl)bis(2-bromoethanone), iPrNEt2, CH3CN; (m) NH4OAc, o-xylene, microwave, 140 °C, 40%.
Kraft, W. K.; Charlton, M. R.; Lopez-Talavera, J. C.; Grasela, D. Abstract of Papers,
59th Annual Meeting of the American Association for the Study of Liver
Diseases, San Francisco, CA; John Wiley & Sons Inc.: Malden, MA, 2008, Abstract
LB12.; (b) Gao, M.; Nettles, R. E.; Belema, M.; Snyder, L. B.; Nguyen, V. N.;
Fridell, R. A.; Serrano-Wu, M. H.; Langley, D. R.; Sun, J. H.; O’Boyle, D. R., II;
Lemm, J. A.; Wang, C.; Knipe, J. O.; Chien, C.; Colonno, R. J.; Grasela, D. M.;
Meanwell, N. A.; Hamann, L. G. Nature 2010, 465, 96; (c) Nettles, R. E.; Gao, M.;
Bifano, M.; Chung, E.; Persson, A.; Marbury, T. C.; Goldwater, R.; DeMicco, M. P.;
Rodriguez-Torres, M.; Vutikullird, A.; Fuentes, E.; Lawitz, E.; Lopez-Talavera, J.
C.; Grasela, D. M. Hepatology 2011, 54, 1956.
Table 1
HCV replicon activity and cytotoxicity
Compd
EC50 (nM)
CC50 (nM)
GT 1B
GT 1A
GT 2A
23
2
1
27.3
0.079
0.009
659
189
0.050
397
167
0.063
1,400
>10,000
2,800
6. (a) Meanwell, N. A.; Belema, M. Annu. Rep. Med. Chem. 2011, 46, 263; (b)
Najarro, P.; Mathews, N.; Cockerill, S. In Hepatitis C; Tan, S.-L., He, Y., Eds.;
Caister Academic Press: Norfolk, UK, 2011; pp 271–292; (c) Cordek, D. G.;
Bechtel, J. T.; Maynard, A. T.; Kazmierski, W. M.; Cameron, C. E. Drugs Future
2011, 36, 691.
In conclusion, a series of novel HCV NS5A inhibitors were de-
signed and prepared where a fluoro-olefin moiety replaced the
intramolecular hydrogen bonded amide group in a known class
of HCV NS5A inhibitors (1). The fluoro-olefin based proline mimic
2 demonstrated pico-molar in vitro activity supporting the hypoth-
7. Some recent patent applications on related NS5A inhibitors: (a) Bachand, C.;
Belema, M.; Deon, D. H.; Good, A. C.; Goodrich, J.; James, C. A.; Lavoie, R.; Lopez,
O. D.; Martel, A.; Meanwell, N. A.; Nguyen, V. N.; Romine, J. L.; Ruediger, E. H.;
Snyder, L. B.; St. Laurent, D. R.; Yang, F.; Langley, D. R.; Wang, G.; Hamann, L. G.
PCT Int. Appl. WO 2008021927, 2008; (b) Guo, H.; Kato, D.; Kirschberg, T. A.;
Liu, H.; Link, J. O.; Mitchell, M. L.; Parrish, J. P.; Squires, N.; Sun, J.; Taylor, J.;
Bacon, E. M.; Canales, E.; Cho, A.; Cottell, J. J.; Desai, M. C.; Halcomb, R. L.;
Krygowski, E. S.; Lazerwith, S. E.; Liu, Q.; Mackman, R.; Pyun, H.-J.; Saugier, J. H.;
Trenkle, J. D.; Tse, W. C.; Vivian, R. W.; Schroeder, S. D.; Watkins, W. J.; Xu, L.;
Yang, Z.-Y.; Kellar, T.; Sheng, X.; Clarke, M. O. H.; Chou, C.-H.; Graupe, M.; Jin,
H.; Mcfadden, R.; Mish, M. R.; Metobo, S. E.; Phillips, B. W.; Venkataramani, C.
PCT Int. Appl. WO 2010132601, 2010; (c) Li, L.; Zhong, M. PCT Int. Appl. WO
2010065668, 2010; (d) Degoey, D. A.; Kati, W. M.; Hutchins, C. W.; Donner, P.
L.; Krueger, A. C.; Randolph, J. T.; Motter, C. E.; Nelson, L. T.; Patel, S. V.;
Matulenko, M. A.; Keddy, R. G.; Jinkerson, T. K.; Soltwedel, T. N.; Hutchinson, D.
K.; Flentge, C. A.; Wagner, R.; Maring, C. J.; Tufano, M. D.; Betebenner, D. A.;
Rockway, T. W.; Liu, D.; Pratt, J. K.; Lavin, M. J.; Sarris, K.; Woller, K. R.; Wagaw,
S. H.; Califano, J. C.; Li, W. PCT Int. Appl. WO 2010144646, 2010; (e) Or, Y. S.;
Peng, X.; Ying, L.; Ce, W.; Tang, D.; Qiu, Y.-L. PCT Int. Appl. WO 2010096462,
2010; (f) Schmitz, F. U.; Rai, R.; Roberts, C. D.; Kazmierski, W.; Grimes, R. PCT
Int. Appl. WO 2010062821, 2010; (g) Rosenblum, S. B.; Chen, K. X.; Kozlowski, J.
A.; Njoroge, F. G.; Coburn, C. A. PCT Int. Appl. WO 2010132538, 2010; (h)
Dousson, C. B.; Dukhan, D.; Parsy, C. C.; Pierra, C.; Alexandre, F.-R.; Brandt, G.;
Da Costa, D.; Rahali, H.; Paparin, J.-L.; Derock, M.; Convard, T.; Surleraux, D. PCT
Int. Appl. WO 2011075615, 2011.
esis that the c-turn substructure is an active conformation impor-
tant for the inhibition of HCV by NS5A inhibitors. Therefore,
structurally distinct NS5A inhibitors with strong anti-HCV potency
can be obtained via the peptidomimetic approach where the opti-
mal molecular conformer for inhibition is reproduced.
Acknowledgements
We thank Dr. Patrick Carroll, University of Pennsylvania for X-
ray analysis and WuXi AppTec for their contributions to the work
described in this article.
Supplementary data
Supplementary data (experimentals and spectral data for com-
pound 2, 5–14, and 23) associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2012.02.051.
8. Monte Carlo Multiple Minimum: (a) Chang, G.; Guida, W. C.; Still, W. C. J. Am.
Chem. Soc. 1989, 111, 4379; (b) Saunders, M.; Houk, K. N.; Wu, Y. D.; Still, C. W.;
Lipton, M.; Chang, G.; Guida, W. C. J. Am. Chem. Soc. 1990, 112, 1419.
9. (a) Halgren, T. A. J. Comput. Chem. 1999, 20, 720; (b) Halgren, T. A. J. Comput.
Chem. 1999, 20, 730.
References and notes
10. Etzkorn, F. A.; Travins, J. M. In Advances in Amino Acid Mimetics and
Peptidomimetics; Abell, A., Ed.; Jai Press Inc.: Stamford, CT, 1999; Vol. 2, pp
125–163.
11. (a) Couve-Bonnaire, S.; Cahard, D.; Pannecoucke, X. Org. Biomol. Chem. 2007, 5,
1151; (b) Welch, J. T. In Fluorine and Health; Tressaud, A., Haufe, G., Eds.;
Elsevier, 2008; pp 699–735.
1. Negro, F.; Alberti, A. Liver Int. 2011, 31, 1.
2. Pockros, P. J. Expert Opin. Biol. Ther. 2011, 11, 1611.
3. Garber, K. Nat. Biotechnol. 2011, 29, 963.
4. Sofia, M. J.; Chang, W.; Furman, P. A.; Mosley, R. T.; Ross, B. S. J. Med. Chem.
2012. doi:10.1021/jm201384j.
5. (a) Nettles, R.; Chien, C.; Chung, E.; Persson, A.; Gao, M.; Belema, M.; Meanwell,
N. A.; DeMicco, M. P.; Marbury, T. C.; Goldwater, R.; Northup, P.; Coumbis, J.;
12. Bartlett, P. A.; Otake, A. J. Org. Chem. 1995, 60, 3107.

2942
W. Chang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2938–2942
13. Welch, J. T.; Lin, J.; Boros, L. G.; DeCorte, B.; Bergmann, K.; Gimi, R. In Biomedical
Frontiers of Fluorine Chemistry; Ojima, I., McCarthy, J. R., Welch, J. T., Eds.;
American Chemical Society: Washington D.C., 1996; pp 129–142.
14. (a) Welch, J. T.; Lin, J. Tetrahedron 1996, 52, 291; (b) Lin, J.; Toscano, P. J.; Welch,
J. T. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 14020; (c) Zhao, K.; Lim, D. S.; Funaki,
T.; Welch, J. T. Bioorg. Med. Chem. Lett. 2003, 11, 207; (d) Van der Veken, P.;
Kertesz, I.; Senten, K.; Haemers, A.; Augustyns, K. Tetrahedron Lett. 2003, 44,
6231; (e) Van der Veken, P.; Senten, K.; Kertesz, I.; De Meester, I.; Lambeir, A.-
M.; Maes, M.-B.; Scharpe, S.; Haemers, A.; Augustyns, K. J. Med. Chem. 2005, 48,
1768; (f) Edmondson, S. D.; Wei, L.; Xu, J.; Shang, J.; Xu, S.; Pang, J.; Chaudhary,
A.; Dean, D. C.; He, H.; Leiting, B.; Lyons, K. A.; Patel, R. A.; Patel, S. B.; Scapin, G.;
Wu, J. K.; Beconi, M. G.; Thornberry, N. A.; Weber, A. E. Bioorg. Med. Chem. Lett.
2008, 18, 2409.
16. Waelchli, R.; Gamse, R.; Bauer, W.; Meigel, H.; Lier, E.; Feyen, J. H. M. Bioorg.
Med. Chem. Lett. 1996, 6, 1151.
17. Mase, N.; Inoue, A.; Nishio, M.; Takabe, K. Bioorg. Med. Chem. Lett. 2009, 19,
3955.
18. (E):(Z) = 1:1.2, determined by 19F NMR of the crude mixture.
19. (a) Robak, M. T.; Herbage, M. A.; Ellman, J. A. Chem. Rev. 2010, 110, 3600; (b)
Ellman, J. A.; Owens, T. D.; Tang, T. P. Acc. Chem. Res. 2002, 35, 984.
20. (a) Pierry, C.; Zoute, L.; Jubault, P.; Pfund, E.; Lequeux, T.; Cahard, D.; Couve-
Bonnaire, S.; Pannecoucke, X. Tetrahedron Lett. 2009, 50, 264; (b) Pierry, C.;
Cahard, D.; Couve-Bonnaire, S.; Pannecoucke, X. Org. Biomol. Chem. 2011, 9,
2378.
21. Schmitz, U.; Tan, S.-L. Recent Pat. Anti-infect. Drug Discov. 2008, 3, 77.
22. McKinney, B. E.; Urban, J. J. J. Phys. Chem. A 2010, 114, 1123.
23. Howard, J. A.; Hoy, W. J.; O’Hagan, D.; Smith, G. T. Tetrahedron 1996, 52, 12613.
15. (a) Allmendinger, T.; Furet, P.; Hungerbuehler, E. Tetrahedron Lett. 1990, 31,
7297; (b) Allmendinger, T.; Furet, P.; Hungerbuehler, E. Tetrahedron Lett. 1990,
31, 7031.