Dipeptidyl Enoates As Potent Rhodesain Inhibitors That Display a Dual Mode of Action

Article abstract of DOI:10.1002/cmdc.201500204

Dipeptidyl enoates were prepared through a high-yielding two-step synthetic route. They have a dipeptidic structure with a 4-oxoenoate moiety as a warhead with multiple reactive sites. Dipeptidyl enoates were screened against rhodesain and human cathepsin

Full text of DOI:10.1002/cmdc.201500204

DOI: 10.1002/cmdc.201500204
Communications
Dipeptidyl Enoates As Potent Rhodesain Inhibitors That
Display a Dual Mode of Action
Santiago Royo,[a] Santiago Rodríguez,[a] Tanja Schirmeister,[b] Jochen Kesselring,[b]
Marcel Kaiser,[c] and Florenci V. Gonzµlez*[a]
Dipeptidyl enoates were prepared through a high-yielding
two-step synthetic route. They have a dipeptidic structure with
a 4-oxoenoate moiety as a warhead with multiple reactive
sites. Dipeptidyl enoates were screened against rhodesain and
human cathepsins B and L, and were found to be potent and
selective inhibitors of rhodesain. Among them (S,E)-ethyl 5-((S)-
2-{[(benzyloxy)carbonyl]amino}-3-phenylpropanamido)-7-
methyl-4-oxooct-2-enoate (6) was the most potent, with an
IC50 value of 16.4 nm and kinact/Ki =1.6”106 mÀ1 sÀ1 against rho-
desain. These dipeptidyl enoates display a reversible mode of
inhibition at very low concentrations and an irreversible mode
at higher concentrations. Inhibition kinetics data, supported by
docking studies, suggest a dual mode of action via attack of
cysteine thiolate at two reactive positions.
Michael acceptors have been used before as rhodesain in-
hibitors.[7] For example, dipeptidyl vinylsulfones such as K11777
(Figure 1) are potent irreversible inhibitors of cysteine proteas-
es and rhodesain in particular.[8] We envisioned inhibitors with
a Michael-acceptor warhead bearing a carbonyl group be-
tween the conjugated double bond and the original a-carbon
atom of the P1 residue. The carbonyl group would be placed
at the same position as the substrate amide carbonyl group
(Figure 1).
A short and efficient synthetic route was developed to pre-
pare the desired inhibitors. The route starts from the corre-
sponding doubly protected dipeptide, which, upon reaction
with lithium phosphonate methylide, affords the correspond-
ing phosphonates 1–3.[9] A Horner–Wadsworth–Emmons-type
reaction between the phosphorous ylide derived from the
phosphonate and ethyl glyoxal furnished the desired inhibitors
4–7 (Scheme 1); trans-alkenes were obtained in all cases.
During the preparation of trans-alkene 6, cis-isomer 7 was also
isolated.
Human African trypanosomiasis (HAT), also known as sleeping
sickness, is a parasitic disease caused by two subspecies of
protozoa of the genus Trypanosoma: T. brucei gambiense and
T. brucei rhodesiense.[1] T. b. gambiense is responsible for the
chronic form of HAT in western and central Africa, whereas
T. b. rhodesiense causes the acute form of the disease in eastern
and southern Africa.[2] Currently more than 60 million people
living in 36 sub-Saharan countries are at risk of contracting the
disease.[3] The estimated number of actual cases is 30000.[4]
Current treatments rely on medications that are toxic, painful,
expensive, and which are becoming ineffective due to parasite
resistance. Thus, owing to limited healthcare and poor treat-
ment options, infection typically results in death. Development
of effective and safe treatments is desperately needed for this
devastating disease.[5] The cathepsin-L-like enzyme rhodesain
has been identified as a potential target in the search for new
drugs against HAT.[6]
The dipeptidyl enoates 4–7 were screened against rhodesain
(Table 1). IC50 determinations indicated that compounds 4, 6,
and 7 are potent inhibitors of rhodesain, with 6 being the
most potent. Compound 5, with an l-leucine residue at the P2
site, was less active. Kinetic analyses were also performed on
the compounds. Compound 6 displayed a second-order rate
constant higher than that of K11777. Inhibitor 5 is one order of
magnitude less active against rhodesain than 4 in terms of
both IC50 and kinact/Ki, denoting a preference for an l-phenylala-
nine residue rather than l-leucine at the P2 site. Inhibitor 6
[a] Dr. S. Royo, Dr. S. Rodríguez, Dr. F. V. Gonzµlez
Departament de Química Inorgànica i Orgànica
Universitat Jaume I, 12080 Castelló (Spain)
E-mail: fgonzale@uji.es
[b] Dr. T. Schirmeister, J. Kesselring
Institute of Pharmacy and Biochemistry
University of Mainz, Staudinger Weg 5, 55099 Mainz (Germany)
[c] Dr. M. Kaiser
Swiss Tropical and Public Health Institute
Socinstrasse 57, 4051 Basel (Switzerland)
and
University of Basel, Petersplatz 1, 4003 Basel (Switzerland)
Scheme 1. Synthesis of inhibitors. Reagents and conditions:
a) CH3C(O)CH2P(O)(OCH3)2, nBuLi, THF, À788C, 92–98%; b) ethyl glyoxal,
EtOH, K2CO3, 76–84%.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cmdc.201500204.
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Table 1. Inhibition parameters for compounds 4–9.[a]
Inhibitor
Rhodesain
kinact/Ki [mÀ1 sÀ1
Cathepsin B
kinact/Ki [mÀ1 sÀ1
Cathepsin L
IC50 [nm]
]
IC50 [nm]
]
IC50 [nm]
50Æ0
kinact/Ki [mÀ1 sÀ1
]
4
5
6
7
8
9
30.7Æ1.6
1270000Æ196000
116000Æ1260
1610000Æ297000
1530000Æ54400
ND[c]
70Æ0
50Æ3
53Æ4
45500Æ5700
42400Æ2400
138000Æ50000
117000Æ16500
ND
45000Æ14000
29900Æ670
759000Æ72000
704000Æ73800
ND
144.6Æ3.3
16.4Æ3.7
26.0Æ1.2
NI[b]
256.7Æ20
9.6Æ0.4
8.15Æ0.8
64.2Æ0.5
52000Æ1400
24000Æ300
ND
46000Æ5900
13000Æ100
ND
9500Æ300
ND
ND
ND
ND
K11777
552000Æ109000[d]
23800Æ840
950000Æ170000
[a] Data are the mean ÆSD of n=3 independent experiments performed in duplicate. [b] No inhibition at 20 mm. [c] Not determined. [d] Published value:
555000.[10]
tested against rhodesain. These
compounds were synthesized
via a synthetic route previously
reported by our research
group[11] (Supporting Informa-
tion). Compound 8 did not in-
hibit rhodesain, and compound
9 inhibited rhodesain in the mi-
cromolar range (Table 1). These
results confirm the carbonyl
group as being key for the inhib-
itory activity of dipeptidyl
enoates. Curiously, compound 9,
with opposite configuration to
that of 8 at C4 is active, whereas
compound 8 is not.
Figure 1. Michael acceptor inhibitors.
shows the best selectivity and the highest reactivity, as it dis-
plays the lowest IC50 and highest kinact/Ki values of all com-
pounds tested. Inhibitor 7, with cis geometry at the C=C
double bond, exhibits a slightly lower activity than its trans-
isomer, 6.
Docking was performed with AutoDock Vina[12] (ver-
sion 1.1.2) between the X-ray crystallographic structure of rho-
desain (PDB ID: 2P7U) and inhibitor 6 as ligand. Comparison
between the docked conformation of 6 and the conformation
of K11777 in complex with rhodesain shows inhibitor 6 to be
oriented similarly to K11777 (Figure 2a).[13] The ester group of
6 is involved in interactions with Gln19, Trp184, and His162
(distances of 3.233, 3.146, and 3.204 Š, respectively) as is sul-
fone group of K11777. The NH group of the modified Phe resi-
due of 6 is involved in hydrogen bonding with the carbonyl
oxygen of Asp161 and the ketone oxygen of 6 (d=3.082 and
2.912 Š, respectively; Figure 2b). Moreover, the NH group of
Phe in compound 6 is hydrogen bonded with the carbonyl
oxygen atom of Gly66 (d=2.890 Š; Figure 2b).[13]
The compounds were also tested against human cysteine
proteases cathepsins B and L, which, like rhodesain, belong to
the papain-family proteases (Table 1). In some cases IC50 values
were similar to those of rhodesain, but in all cases the kinact/Ki
value was found to be one or two orders of magnitude lower
than that against rhodesain.
To confirm irreversibility of inhibition, dilution assays with
rhodesain and the test compounds were carried out; K11777,
a well-known irreversible vinylsulfone, was used as a positive
control (Supporting Information). Inhibitors 4–7 were shown to
display a reversible mode of inhibition at very low concentra-
tions, and then an irreversible mode at higher concentrations.
The antitrypanosomal activity of compounds 4 and 5 against
T. b. rhodesiense trypomastigotes was determined by a serial di-
lution assay. Inhibitors 4 and 5 exhibited IC50 values of 0.505
and 1.178 mgmLÀ1, respectively. The two compounds were also
tested for cytotoxicity, and exhibited IC50 values of 2.775 and
1.735 mgmLÀ1, respectively, against L6 cells.
Based on the kinetics and docking data, a mode of action
for these inhibitors can be suggested. Dipeptidyl enoates dis-
play a reversible mode of inhibition (Scheme 2): a rapid reversi-
ble attack of the thiolate to the ketone carbonyl to give a hemi-
thioacetal intermediate as a reversible E·I complex could be
considered.[14] An irreversible conjugate addition of the Cys25
thiolate followed by protonation of the resulting enolate by
To determine the importance of the carbonyl group for in-
hibitor activity, isomeric alcohols 8 and 9 were prepared and
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ded in a dipeptidic backbone. Kinetics and docking studies
suggest the inhibitors to follow two separate pathways: one
reversible and other irreversible, which finally lead to an irre-
versibly blocked enzyme.
Experimental Section
1
General. H NMR and 13C NMR spectra were measured in CDCl3 (1H:
7.24 ppm, 13C: 77.0 ppm) solution at 308C on 300 or 500 MHz NMR
spectrometers. Mass spectra were measured with a QTOF I (quad-
rupole–hexapole–ToF) mass spectrometer using an orthogonal Z-
spray–electrospray ionization interface. IR spectra were recorded as
oily films on NaCl plates on an FTIR spectrometer. Silica gel 60 (EM
Science) was used for column chromatography, whereas TLC was
performed on pre-coated silica plates (thickness: 0.25 mm). Unless
otherwise specified, all reactions were carried out under N2 atmos-
phere with magnetic stirring.
General procedure for the preparation of phosphonates: n-Butyl-
lithium (1.6m in hexanes; 1.98 mL, 19.17 mmol) was added to a so-
lution of dimethyl methylphosphonate (2.08 mL, 19.17 mmol) in
THF (20 mL) at À708C. The resulting mixture was stirred at À708C
for 15 min, and then a solution of the corresponding doubly pro-
tected dipeptide (previously prepared by standard procedures;
2.40 mmol) in THF (20 mL) was added dropwise. The resulting mix-
ture was stirred at À708C for 2 h and was then quenched with
10% AcOH (20 mL) and extracted with EtOAc (3”30 mL). The or-
ganic layers were washed (saturated NaHCO3 solution), dried
(Na2SO4), and concentrated. The crude oil was submitted directly
to the next step without further purification.
Figure 2. a) Docking pose of inhibitor 6 (magenta) superimposed with
K11777 (orange) in complex with rhodesain (PDB ID: 2P7U). b) Proposed
docking pose of inhibitor 6 (magenta) in complex with rhodesain.
Benzyl ((S)-1-(((S)-1-(dimethoxyphosphoryl)-2-oxo-5-phenylpen-
tan-3-yl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (1). Yield:
98%; 1H NMR (CDCl3, 300 MHz): d=7.48 (d, J=7.8 Hz, 1H), 7.27–
7.08 (m, 15H), 5.68 (d, J=7.2 Hz, 1H), 5.07 (d, J=12.6 Hz, 1H), 5.01
(d, J=12.3 Hz, 1H), 4.63–4.53 (m, 2H), 3.71 (s, 3H), 3.67 (s, 3H),
3.18–2.91 (m, 2H), 2.53 (t, J=7.8 Hz, 2H), 2.24–2.13 (m, 1H), 1.89–
1.80 ppm (m, 1H); 13C NMR (CDCl3, 75 MHz): d=200.3, 170.9, 156.1,
140.7, 136.2, 129.2, 128.6, 128.4, 128.0, 127.9, 126.9, 126.0, 66.9,
58.7, 56.2, 53.1, 53.0, 38.6, 36.9, 31.9, 31.5 ppm.
Benzyl ((S)-1-(((S)-1-(dimethoxyphosphoryl)-2-oxo-5-phenylpen-
tan-3-yl)amino)-4-methyl-1-oxopentan-2-yl)carbamate (2). Yield:
1
89%; H NMR (CDCl3, 500 MHz): d=7.38–7.13 (m, 10H), 5.47 (d, J=
7.0 Hz, 1H), 5.11 (d, J=12.0 Hz, 1H), 5.07 (d, J=12.5 Hz, 1H), 4.61
(dq, J=8.5, 4.5 Hz, 1H), 4.25 (m, 1H), 3.71 (s, 3H), 3.69 (s, 3H), 3.28
(dd, J=22.6, 14.1 Hz, 1H), 3.05 (dd, J=21.9, 14.1 Hz, 1H), 2.60 (m,
2H), 2.23 (m, 1H), 1.92 (m, 1H), 1.69 (m, 2H), 1.52 (m, 1H),
0.93 ppm (m, 6H); 13C NMR (CDCl3, 125 MHz): d=200.8, 172.9,
156.0, 140.6, 136.1, 128.4, 128.0, 127.9, 126.0, 66.8, 58.5, 53.8, 52.1,
52.0, 41.3, 38.6, 36.9, 31.8, 31.6, 24.7, 23.1 ppm.
Scheme 2. Mode of action of dipeptidyl enoates.
His162 might also take place. Regarding the regioselectivity of
the attack, the activation of the ester group by interactions
with Gln19, Trp184, and His162 would support attack of the
a position of the ketone (Scheme 2). It is expected the cis com-
pound 7 would not form a reversible complex as efficiently as
the trans-configured 6, but once the conjugate addition takes
place, the resulting enolate would be the same for both iso-
mers.
Benzyl ((S)-1-(((S)-1-(dimethoxyphosphoryl)-5-methyl-2-oxohex-
an-3-yl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (3). Yield:
92%; H NMR (CDCl3, 500 MHz): d=7.35–7.17 (m, 10H), 5.46 (d, J=
1
7.5 Hz, 1H), 5.08 (d, J=12.0 Hz, 1H), 5.04 (d, J=12.5 Hz, 1H), 4.62
(dt, J=10.0, 4.0 Hz, 1H), 4.50 (m, 1H), 3.74 (d, J=6.5 Hz, 3H), 3.72
(d, J=6.5 Hz, 3H), 3.22–3.06 (m, 3H), 2.97 (dd, J=22.0, 14.0 Hz,
1H), 1.62 (ddd, J=13.6, 9.4, 4.1 Hz, 1H), 1.38–1.53 (m, 3H), 0.88 (d,
J=6.5 Hz, 3H), 0.85 ppm (d, J=6.5 Hz, 3H); 13C NMR (CDCl3,
125 MHz): d=201.4, 171.0, 155.8, 136.1, 129.4, 128.7, 128.6, 128.4,
128.0, 127.1, 67.0, 57.5, 56.2, 53.1, 52.9, 39.6, 38.6, 38.0, 24.6, 23.1,
21.3 ppm.
In summary, we report a new type of potent rhodesain in-
hibitor, displaying a 4-oxoenoate moiety as a warhead embed-
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General procedure for the preparation of dipeptidyl enoates:
K2CO3 (1 mmol) and ethyl glyoxalate (1 mmol) were added to
a stirred solution of the corresponding dipeptidyl phosphonate
(1 mmol) in EtOH (7.5 mL). The resulting mixture was stirred at
room temperature for 2 h and was then filtered, neutralized with
AcOH, and concentrated. The crude oil was purified by silica gel
chromatography (hexanes/EtOAc 7:3).
J=12.0 Hz, 1H), 6.00 (d, J=12.0 Hz, 2H), 5.19 (brs, 1H), 5.10 (s,
2H), 4.77 (td, J=10.0, 4.0 Hz, 1H), 4.47 (q, J=6.5 Hz, 1H), 4.20 (m,
2H), 3.12 (m, 2H), 1.76 (m, 1H), 1.55 (m, 1H), 1.46 (m, 1H), 1.29 (t,
J=7.5 Hz, 3H), 0.92 (d, J=6.5 Hz, 3H), 0.90 ppm (d, J=6.5 Hz, 3H);
13C NMR (CDCl3, 125 MHz): d=197.2, 170.5, 165.1, 155.6, 139.5,
136.0, 132.5, 129.4, 129.2, 128.9, 128.5, 128.2, 128.0, 126.8, 67.3,
61.6, 56.7, 56.1, 40.4, 38.3, 24.9, 23.2, 21.8, 14.1 ppm; HRMS m/z
calcd for C28H35O6N2Na [M+Na]+: 517.2315, found: 517.2318.
(S,E)-Ethyl 5-((S)-2-(((benzyloxy)carbonyl)amino)-3-phenylpropa-
namido)-4-oxo-7-phenylhept-2-enoate (4). Yield: 76%; white solid
(mp: 115–1198C); [a]2D0 =À12.0 (c=0.1, CHCl3); IR (NaCl): n˜ =3619,
3019, 2896, 2399, 1716, 1518, 1385, 1213, 1046, 928, 746, 734, 669,
Acknowledgements
627 cmÀ1 1H NMR (CDCl3, 500 MHz): d=7.35–7.06 (m, 16H), 6.72
;
(d, J=16.0 Hz, 1H), 6.50 (brs, 1H), 5.21 (brs, 1H), 5.13 (d, J=
12.5 Hz, 2H), 5.09 (d, J=12.0 Hz, 2H), 4.84 (q, J=7.0 Hz, 1H), 4.44
(m, 1H), 4.28 (q, J=7.5 Hz, 2H), 3.07 (m, 2H), 2.45–2.58 (m, 2H),
2.18–2.25 (m, 1H), 1.83–1.90 (m, 1H), 1.34 ppm (t, J=7.5 Hz, 3H);
13C NMR (CDCl3, 125 MHz): d=196.3, 170.7, 165.0, 155.9, 140.3,
136.2, 135.8, 132.7, 129.3, 128.7, 128.6, 128.5, 128.4, 128.2, 128.0,
127.1, 126.4, 67.2, 61.7, 56.9, 56.2, 38.4, 32.7, 31.2, 14.1 ppm; HRMS
m/z calcd for C32H35O6N2 [M+H]+: 543.2495, found: 543.2496.
The authors thank Serveis Centrals d’Instrumentació Científica at
Universitat Jaume I for technical support. S.R. thanks the General-
itat Valenciana for a postdoctoral research grant under the
VALi+d Program. T.S. thanks the Deutsche Forschungsgemein-
schaft (DFG) (SFB630) for financial support.
Keywords: dipeptidyl enoates
· inhibitors · rhodesain ·
(S,E)-Ethyl 5-((S)-2-(((benzyloxy)carbonyl)amino)-4-methylpenta-
namido)-4-oxo-7-phenylhept-2-enoate (5). Yield: 80%; colorless
oil; [a]2D0 =À14.5 (c=1.1, CHCl3); IR (NaCl): n˜ =3404, 3064, 2961,
1736, 1679, 1518, 1455, 1369, 1247, 1188, 1029, 979, 764, 726,
sleeping sickness · trypanosomiasis
[1] F. E. G. Cox, Infect. Dis. Clin. North Am. 2004, 18, 231.
[2] M. P. Barrett, R. J. Burchmore, A. Stich, J. O. Lazzari, A. C. Frasch, J. J. Caz-
zulo, S. Krishna, Lancet 2003, 362, 1469–1480.
[3] a) D. Steverding, Parasites Vectors 2008, 1, 3; b) R. Brun, J. Blum, F. Chap-
puis, C. Burri, Lancet 2010, 375, 148.
[4] Trypanosomiasis, human African (sleeping sickness), World Health Or-
ganization (WHO) Fact Sheet #259, updated May 2015: www.who.int/
mediacentre/factsheets/fs259/en/.
[5] Center for Discovery and Innovation in Parasitic Diseases (CDIPD):
www.cdipd.org/index.php (accessed January 2015).
[6] I. D. Kerr, P. Wu, R. Marion-Tsukamaki, Z. B. Mackey, L. S. Brinen, PLoS Ne-
glected Trop. Dis. 2010, 4, e701.
717 cmÀ1 1H NMR (CDCl3, 300 MHz): d=7.33–7.07 (m, 11 H), 6.81
;
(m, 1H), 6.73 (d, J=15.9 Hz, 1H), 5.13 (s, 2H), 4.84–4.90 (m, 1H),
4.26 (q, J=6.6 Hz, 2H), 4.18 (m, 1H), 2.60 (m, 1H), 2.24 (m, 1H),
1.92 (m, 1H), 1.68 (m, 2H), 1.51 (m, 1H), 1.32 (t, J=7.2 Hz, 3H),
0.94 ppm (m, 6H); 13C NMR (CDCl3, 75 MHz): d=196.9, 172.0, 165.2,
156.2, 140.4, 135.8, 132.8, 128.6, 128.6, 128.5, 128.3, 128.1, 128.1,
126.4, 67.2, 61.6, 57.1, 53.6, 41.2, 32.8, 31.3, 24.7, 22.9, 22.0,
14.1 ppm; HRMS m/z calcd for C30H32O6N2 [M+H]+: 509.2652,
found: 509.2657.
(S,E)-Ethyl 5-((S)-2-(((benzyloxy)carbonyl)amino)-3-phenylpropa-
namido)-7-methyl-4-oxooct-2-enoate (6). Yield: 84% for 6 and 7;
white solid (mp: 71–758C); [a]2D0 = +8.0 (c=1.0, CHCl3); IR (NaCl):
n˜ =3618, 3019, 2896, 2436, 2399, 1717, 1519, 1369, 1211, 1046,
[7] R. Ettari, L. Tamborini, I. C. Angelo, N. Micale, A. Pinto, C. De Micheli, P,
Conti, J. Med. Chem. 2013, 56, 5637.
[8] J. T. Palmer, D. Rasnick, J. L. Klaus, D. Brçmme, J. Med. Chem. 1995, 38,
3193.
[9] Y. Torisawa, H. Okabe, M. Shibasaki, S. Ikegami, Chem. Lett. 1984, 1069.
[10] C. Bryant, I. D. Kerr, M. Debnath, K. K. H. Ang, J. Ratnam, R. S. Ferreira, P.
Jaishankar, D. Zhao, M. R. Arkin, J. H. McKerrow, L. S. Brinen, A. R.
Renslo, Bioorg. Med. Chem. Lett. 2009, 19, 6218.
[11] F. V. Gonzµlez, J. Izquierdo, S. Rodríguez, J. H. McKerrow, E. Hansell,
Bioorg. Med. Chem. Lett. 2007, 17, 6697.
[12] O. Trott, A. J. Olson, J. Comput. Chem. 2010, 31, 455.
[13] I. D. Kerr, J. H. Lee, C. J. Farady, R. Marion, M. Rickert, M. Sajid, K. C.
Pandey, C. R. Caffrey, J. Legac, E. Hansell, J. H. McKerrow, C. S. Craik, P. J.
Rosenthal, L. S. Brinen, J. Biol. Chem. 2009, 284, 25697.
[14] In the docking studies, the respective distances between the sulfur
atom of Cys25 and the ketone carbonyl group, Ca to the ketone, and
Ca to the ester in compound 6 are 4.664, 4.153, and 4.504 Š.
929, 793, 765, 755, 747, 731, 719, 625 cmÀ1 1H NMR (CDCl3,
;
500 MHz): d=7.36–7.12 (m, 10H), 6.80 (d, J=16.0 Hz, 1H), 6.61
(brs, 1H), 5.49 (d, J=8.0 Hz, 1H), 5.08 (s, 2H), 4.83 (td, J=9.0,
3.5 Hz, 1H), 4.52 (m, 1H), 4.28 (q, J=7.0 Hz, 2H), 3.07 (m, 2H), 1.55
(m, 2H), 1.34 (t, J=7.0 Hz, 3H), 1.30 (m, 1H), 0.93 (d, J=5.5 Hz,
3H), 0.86 ppm (d, J=6.0 Hz, 3H); 13C NMR (CDCl3, 125 MHz): d=
197.1, 170.6, 165.0, 155.8, 136.2, 136.1, 132.5, 129.3, 128.6, 128.1,
127.1, 67.1, 61.5, 56.9, 56.7, 40.2, 38.2, 24.7, 23.0, 21.7, 14.1 ppm;
HRMS m/z calcd for C28H35O6N2Na [M+Na]+: 517.2315, found:
517.2317.
(S,Z)-Ethyl 5-((S)-2-(((benzyloxy)carbonyl)amino)-3-phenylpropa-
namido)-7-methyl-4-oxooct-2-enoate (7). Yellow oil; [a]2D0 =À28.1
(c=1.2, CHCl3); IR (NaCl): n˜ =3417, 3020, 2961, 1720, 1498, 1386,
;
1302, 1216, 1027, 910, 771, 762, 738, 669 cmÀ1 1H NMR (CDCl3,
Received: May 8, 2015
Revised: June 25, 2015
500 MHz): d=7.39–7.20 (m, 10H), 6.43 (d, J=8.5 Hz, 1H), 6.40 (d,
Published online on July 14, 2015
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