Expedient synthesis of 1,3-substituted benzene peptidomimetics

Article abstract of DOI:10.1055/s-0030-1258425

A synthetic route for replacing the central amino acid in the tripeptide Thr-Ala-Val (TAV) with a 1,3-substituted benzene ring was developed. l-Threonine was introduced into the benzene ring by a Grignard reaction with protected l-threoninal, where the na

Full text of DOI:10.1055/s-0030-1258425

PAPER
807
Expedient Synthesis of 1,3-Substituted Benzene Peptidomimetics
S
ynthesis of 1,3-S
u
bstituted
B
e
d
nzene
P
eptid
e
omimeticsrs Bach, Kristian Strømgaard*
Department of Medicinal Chemistry, The Faculty of Pharmaceutical Sciences, University of Copenhagen,
Universitetsparken 2, 2100 Copenhagen, Denmark
Fax +4535336041; E-mail: krst@farma.ku.dk
Received 8 December 2010; revised 5 January 2011
HO
HO
Abstract: A synthetic route for replacing the central amino acid in
the tripeptide Thr-Ala-Val (TAV) with a 1,3-substituted benzene
ring was developed. L-Threonine was introduced into the benzene
ring by a Grignard reaction with protected L-threoninal, where the
nature of the side-chain protecting group was found to be of utmost
importance. Subsequently, L-valine was introduced by a copper-
mediated amination. Overall, the tripeptide analogue was obtained
in six steps with an overall yield of 18%. An orthogonal protecting
group strategy allowed attachment of the tripeptide analogue to a
solid support and subsequent preparation of the corresponding pen-
tapeptide analogue. Both compounds were tested in a plasma stabil-
ity assay, showing improved stability compared to their peptide
counterparts.
O
H
R
N
R
N
H
N
COOH
N
N
COOH
H
H
H
O
O
TAV: R = H
IETAV: R = Ile-Glu
1 R = H
2 R = Ile-Glu
Scheme 1 Design of 1 and 2 as mimetics of TAV and IETAV
amino group, respectively, instead of amide bonds.1,6
Also, compared to previously described pyridine-based b-
strand mimetics,1,7–9 compound 1 contains heteroatomic
polar and chiral amino acid side chains and, in addition,
free carboxylic acid and amino groups. Although these
features, together with the meta position of the substitu-
ents, challenged the synthesis, we have developed a feasi-
ble and practical route for obtaining the target
compounds, as described in the following.
Key words: amino acids, peptides, Grignard reaction, nucleophilic
aromatic substitution, copper
Converting bioactive peptides into rigid analogues pro-
vides a general and promising strategy for developing
drug-like inhibitors of protein–protein interactions, pep-
tide–protein interactions or proteolytic enzymes. Several
of these interactions are mediated by peptide b-strands, a
highly abundant structural motif in proteins made by a
continuous stretch of amino acids adopting an extended
conformation.1,2 Constrained scaffolds that are able to
mimic the topographical and pharmacophoric features of
the b-strand will theoretically display increased affinity,
as the entropy penalty is reduced, as well as higher selec-
tivity towards the protein target.1,3 Also, by constraining
the structure or replacing amide bonds reduced suscepti-
bility towards proteolysis is obtained.4
Based on a retrosynthetic analysis of 1 (Scheme 2), we
hypothesized that the threonine moiety could be intro-
duced by a Grignard coupling reaction of the correspond-
ing protected aldehyde and subsequent oxidation,9 while
L-valine could either be incorporated by a Mitsunobu re-
action, which would allow stereospecific alkylation of
amines with the corresponding alcohol,10 or by a direct
amination with L-valine via a mild version of the
Ullmann–Goldberg reaction.11,12
To probe the feasibility of the Mitsunobu reaction in this
process, 3-iodoaniline (3) was applied as a model com-
pound and readily reacted with o-nitrobenzenesulfonyl
chloride (o-NBSCl) to furnish the nitrobenzenesulfona-
mide 4 (Scheme 3).13 Compound 4 was reacted with bu-
tanol or isopropyl alcohol under Mitsunobu conditions,
resulting in successful conversion (~90%) into the desired
products, compounds 5 and 6, respectively (Scheme 3).
In this paper, we explore synthetic routes to peptidomi-
metic 1, where the central amino acid of the tripeptide
Thr-Ala-Val (TAV), an inhibitor of PDZ (postsynaptic
density protein-95/discs large/zonula occludens-1) do-
main mediated interactions,5 is replaced with a 1,3-substi-
tuted benzene ring (Scheme 1). This design provides a
rigid, extended conformation of the two flanking amino
acids, threonine and valine, while the overall proportions
and geometry of 1 are similar to the tripeptide. Also, this
design is applied to the pentapeptide IETAV, as in com-
pound 2 (Scheme 1).
The Grignard reaction was investigated by reacting 3 (1
equiv) with isopropylmagnesium chloride (2 equiv) and
benzaldehyde (1 equiv) at various times and temperatures,
but only starting material was observed. Increasing the
relative amount of isopropylmagnesium chloride, using
tert-butyllithium, or phenylmagnesium chloride followed
by magnesium–halogen exchange with isopropylmagne-
sium chloride14 also failed. It is most likely the electron-
donating effect of the amino group and the absence of
electron-withdrawing groups on the aromatic ring to com-
pensate for this effect that lowers the reactivity of 3.14,15
Carrying out the Grignard reaction using either o-NBS-
protected 3-iodoaniline 4 or the tetramethyldisilylazacy-
Compound 1 deviates from published benzene-based b-
strand mimetics by the two amino acid moieties being
connected to the aromatic ring via a carbonyl and an
SYNTHESIS 2011, No. 5, pp 0807–0815
x
x
.
x
x
.2
0
1
1
Advanced online publication: 08.02.2011
DOI: 10.1055/s-0030-1258425; Art ID: T52010SS
© Georg Thieme Verlag Stuttgart · New York

808
A. Bach, K. Strømgaard
PAPER
Y
COOPG
(Y = OH, NH2)
PGO
PG
+
N
N
COOPG
H
H
PGO
O
PGO
PG
PG
+
H
N
X
N
I
X
H
H
O
O
(X = I, F, NH2)
Scheme 2 Retrosynthetic analysis of target compound 1 (PG: protecting group)
NO2
NO2
O
O
b
O O
a
S
S
I
N
I
N
I
NH2
H
R
3
4
5 R = n-Bu
6 R = i-Pr
Scheme 3 Reagents and conditions: (a) o-NBSCl, NaOAc, EtOH, H2O, 80 °C, 30 min, 60%; (b) ROH, DIAD, Ph3P, toluene, 80 °C, 2 h, 5:
43%, 6: 24%.
clopentane (STABASE) adduct of 3 (see Supporting In- successful in providing 10. This low reactivity of 9 can be
formation),16 also did not provide the desired product. rationalized by the meta positioning of the electron-
Instead, 1-fluoro-3-iodobenzene (7) was used as a model withdrawing carbonyl group relative to the fluorine and,
system in the Grignard reaction, and reaction with benzal- in comparison, pyridine is generally more prone to SNAr
dehyde provided 8, which followed by oxidation gave reactions because of efficient delocalization of electrons
fluorobenzophenone 9 (Scheme 4).
at the nitrogen atom.8
The subsequent nucleophilic aromatic substitution (SNAr) Instead, copper(I) iodide/cesium acetate mediated amina-
reaction with aqueous ammonia under microwave condi- tion was investigated as a means to introduce the valine
tions is feasible with 2-fluoropyridine as the starting ma- moiety into the benzene scaffold. Initially, the feasibility
terial;7 however, the same reaction with 9 was not of the reaction was demonstrated using 3¢-iodoacetophe-
none (11) and butylamine,12 which provided 12 as expect-
ed (Scheme 5). However, the use of tert-butyl-protected
a
b
L-valine (H-Val-O-t-Bu) instead of butylamine did not
provide 13, but when unprotected L-valine and potassium
carbonate were employed,11 the desired product 14 was
obtained.
F
X
I
F
OH
O
7
8
9 X = F
c
X
The Grignard protocol was initially explored using 1,3-di-
iodobenzene (15) and benzaldehyde as model com-
pounds, which furnished the monoalkylated compound,
(3-iodophenyl)(phenyl)methanol, as expected (GCMS).14
For the synthesis of compound 19, tert-butoxycarbonyl-
10 X = NH2
Scheme 4 Reagents and conditions: (a) PhCHO, i-PrMgCl, THF,
0 °C → r.t., 45 min, 86%; (b) DMP, CH2Cl2, r.t., 30 min, 88%; (c)
25% aq NH3, 140 °C, microwave irradiation, 10–180 min.
I
O
11
a
b
c
N
N
COOt-Bu
N
COOH
H
H
H
12
O
O
O
14
13
Scheme 5 Reagents and conditions: (a) BuNH2, CuI, CsOAc, DMF, 90 °C, 5 h, 64%; (b) H-Val-O-t-Bu, CuI, CsOAc, DMF or DMSO, 90 °C,
48 h; (c) L-valine, CuI, K2CO3, DMF, 90 °C, 24 h, 47%.
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York

PAPER
Synthesis of 1,3-Substituted Benzene Peptidomimetics
809
BnO
BnO
BocHN
I
BocHN
I
OH
O
I
I
BnO
15
c
20
19
d
+
+
BocHN
R
H
O
H
O
O
O
O
16 R = CO2H
a
b
N
I
N
I
17 R = CH2OH
H
H
OH
18 R = CHO
21
22
Scheme 6 Reagents and conditions: (a) (i) isobutyl chloroformate, NMM, THF, –20 °C, 30 min; (ii) NaBH4, MeOH, –20 °C, 1 h, 88%; (b)
DMP, 0.1% H2O in CH2Cl2, r.t., 15 min, 93%; (c) 1,3-diiodobenzene (15), i-PrMgCl, THF, –78 °C → r.t., 90 min; (d) DMP, 0.1% H2O in
CH2Cl2, r.t., 15 min, 20: 27% from 18.
and benzyl-protected L-threoninal 18 was prepared, as with L-valine using the mild (90 °C) version of the
previously described for other amino acids,7,17 with the Ullmann–Goldberg reaction11 to provide 28, which upon
modification of adding a catalytic amount of water to ac- treatment with trifluoroacetic acid gave target compound
celerate the Dess–Martin periodinane (DMP) mediated 1 (Scheme 8).
oxidation (Scheme 6).18
t-BuO
t-BuO
t-BuO
The subsequent Grignard reaction with 18 (0.5 equiv) and
15 (1 equiv) provided intermediate 19, which was oxi-
dized in situ to give 20 in modest yield (Scheme 6). We re-
alized that a major side product 21 was produced along
with 19 in the Grignard reaction (~50:50, LC-MS), and al-
though 21 could not be isolated in reasonable purity, the
oxidized product 22 was isolated and structurally eluci-
dated (see Supporting Information). This side reaction
could be a result of a competing E2 elimination initiated
by a reaction of the arylmagnesium species with the Ca
hydrogen atom followed by elimination of the phenyl-
methanolate function as a leaving group (Scheme 7).
d
c
BocHN
R
BocHN
I
BocHN
I
O
OH
26
27
23 R = CO2H
a
b
24 R = CH2OH
e
25 R = CHO
t-BuO
f
1
BocHN
N
COOH
H
O
28
Scheme 8 Reagents and conditions: (a) (i) isobutyl chloroformate,
NMM, THF, –20 °C, 30 min; (ii) NaBH4, MeOH, –20 °C, 1 h, 93%;
(b) DMP, 0.1% H2O in CH2Cl2, r.t., 15 min, 75%; (c) 1,3-diiodoben-
zene (15), i-PrMgCl, THF, –78 °C → r.t., 45 min, 63%; (d) DMP,
0.1% H2O in CH2Cl2, r.t., 15 min, 88%; (e) L-valine, CuI, K2CO3,
DMF, 90 °C, 7 h, 40% (dr 78%); (f) TFA, r.t., 1 h, major epimer: 75%
(dr 98%), minor epimer: 5% (dr 95%).
H
O
O
N
O
H
H
H
O
R
O
DMP
S
22
21
O
N
H
O
18
I
MgCl
A general concern of our synthetic approach was the pres-
ervation of the stereochemistry of the two amino acid
moieties. Up to compound 27 epimerization was prevent-
ed by avoiding flash chromatography of 25 and keeping
the aldehyde intermediate cold during extraction and con-
centration;19 however, LC-MS and analytical HPLC anal-
ysis revealed that 28 was obtained as a diastereomeric
mixture of two species. By monitoring the reaction for the
formation of 28 by LC-MS, we observed a slow but con-
tinuing epimerization of the precursor 27 during the reac-
tion period, probably due to the basic conditions and
elevated temperature. The reaction time for the synthesis
of 28 was therefore settled at 7 hours as a compromise be-
tween optimizing the yield and avoiding excessive
epimerization, which resulted in a 40% yield and a diaste-
reomeric ratio of 78%. Although the use of DMF resulted
in an increased yield of 40%, relative to 14% when DMA
I
MgCl
Scheme 7 Suggested mechanism for the side reaction observed in
the Grignard reaction of 18, resulting in 21 and subsequently 22 after
DMP oxidation
We therefore rationalized that replacing the O-benzyl pro-
tecting group with an O-tert-butyl group should not lead
to the same degree of elimination, since 2-methylpropan-
2-olate would be a much poorer leaving group than phe-
nylmethanolate. Thus, O-tert-butyl-protected L-threoni-
nal 25 was prepared in two steps from 23, and the
subsequent Grignard reaction with 25 and 15 provided 26,
which was oxidized to give 27. The overall yield of 27
from 24 was increased from 42% to 66% by using both 25
and 26 without purification. Finally, 27 was aminated
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York

810
A. Bach, K. Strømgaard
PAPER
was used, and microwave irradiation could reduce reac-
tion time to 50 minutes (90 °C, 36% yield, dr 75%), this
did not reduce the degree of epimerization. Instead, the
epimeric mix of 28 was reacted with trifluoroacetic acid,
and the epimers of 1 were isolated (dr 95–98%) by stan-
dard HPLC methods. Finally, in a principally different at-
tempt to minimize epimerization, amination of alcohol 26
as well as the protected alcohol 30 was explored, assum-
ing that these would be less susceptible to epimerization;
however, none of the desired products, 29 and 31, respec-
tively, was obtained (Scheme 9), indicating the necessity
for a carbonyl group meta to the halogen in order for the
target compound to be obtained.
t-BuO
a
c
28
2
R
N
H
N
COOH
H
O
32 R = H
33 R = Fmoc
b
Scheme 10 Reagents and conditions: (a) HCl, i-PrOH, r.t., 12 h,
78% (dr 75%); (b) Fmoc-OSu, 10% aq Na2CO3, dioxane, MeCN, r.t.,
2 h (>90% conversion); (c) (i) 2-chlorotrityl chloride resin, DIPEA,
CH2Cl2, DMF, r.t., 2 h; (ii) 20% piperidine in DMF, 2 × 10 min; (iii)
Fmoc-Glu(t-Bu)-OH, HATU, collidine, r.t., 30 min; (iv) repeat (ii);
(v) Fmoc-Ile-OH, HATU, collidine, r.t., 30 min; (vi) repeat (ii); (vii)
TFA, r.t., 2 h, major epimer: 39% (dr 99%), epimeric mix: 9.9% (dr
~50%).
t-BuO
a
26
Table 1 Stability in Human Blood Plasma
BocHN
N
H
COOH
Compound
t1/2a (min)
30 1.8
47 3.0
130 5.5
80 1.0
OH
b
29
TAV
IETAV
t-BuO
t-BuO
c
1
2
BocHN
N
COOH
BocHN
I
H
O
O
a Half-lives (t1/2) are indicated as averages SEM, based on three in-
dependent measurements.
31
30
Scheme 9 Reagents and conditions: (a) and (c) L-valine, CuI,
K2CO3, DMF, 90 °C, 24 h; (b) p-methylbenzyl bromide, 50% aq
NaOH, toluene, r.t., 24 h, 81%.
matic cleavage of the amide bond, and since a similar
cleavage is not possible for 1, this rationalizes the in-
creased stability of 1. The degradation of compound 1 was
by a decarboxylation of the C-terminal, as suggested by
LC-MS, and no further degradation was observed (up to
24 h). Compound 2 was somewhat less stable than 1, re-
sulting from a N-terminal cleavage of the isoleucine moi-
ety, again indicating that the N-terminal part is the most
sensitive for proteolytic degradation. The remaining tet-
rapeptide analogue was further decarboxylated, while no
other degradation was observed over 24 hours.
In order to investigate if the tripeptide analogue 1 could be
extended to longer peptide analogues using Fmoc-based
solid-phase peptide chemistry, we selectively deprotected
the N-tert-butoxycarbonyl group on precursor 28
(Scheme 10), without affecting the O-tert-butyl group, by
treatment with HCl in isopropyl alcohol, which we found
was much more selective than HCl in dioxane.20 Subse-
quently, the primary amine was readily Fmoc-protected
and 33 was loaded onto 2-chlorotrityl chloride resin.
Standard Fmoc-based solid-phase peptide synthesis using
O-(7-azabenzotriazol-1-yl)-N,N,N¢,N¢-tetramethyluronium
hexafluorophosphate (HATU) as the coupling reagent,
followed by cleavage from the resin and concomitant
deprotection, and purification by HPLC provided com-
pound 2 (Scheme 10).
In summary, we have established a practical route for ob-
taining benzene-based peptidomimetic scaffolds starting
from 1,3-diiodobenzene, using a Grignard reaction and a
copper-mediated Ullmann–Goldberg-type amination, re-
spectively, for the introduction of the two amino acids into
the aromatic ring system. Selective deprotection allowed
further elaboration of 1 to the pentapeptide analogue 2 us-
ing Fmoc-based solid-phase peptide synthesis. Compared
to their peptide congeners, both 1 and 2 possess increased
proteolytic stability in human blood plasma. Thus, the
synthesis presented herein could contribute to the devel-
opment of future b-strand mimetics.
Since peptide-based ligands generally are subject to enzy-
matic cleavage by proteases in vivo, which is often the
major limiting factor for advanced biological studies,21 we
evaluated the in vitro blood plasma stability of 1, 2, TAV
and IETAV using our previously described protocol.22 We
found that 1 degraded significantly slower than tripeptide
TAV, with a 4.3-fold increase in half-life (t1/2), and a sim-
ilar effect was seen for 2 compared to pentapeptide
IETAV (Table 1). Thus, clearly, the plasma stabilities of
1 and 2 have been significantly improved by the peptido-
mimetic modification.
All reagents were obtained from commercial suppliers and used
without further purification. THF was distilled from sodium/ben-
zophenone and organometallic reagents were titrated prior to use.
Melting points were determined on an MPA100 OptiMelt melting
point apparatus and are presented as averaged values based on at
least two measurements. 1H NMR spectra were recorded on a Vari-
an Mercury Plus (300 MHz) spectrometer and 13C NMR spectra
were recorded on a Varian Gemini 2000 spectrometer (operating at
Interestingly, we found that TAV was readily degraded at
the N-terminal to the dipeptide AV, presumably by enzy-
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York

PAPER
Synthesis of 1,3-Substituted Benzene Peptidomimetics
811
75 MHz), with tetramethylsilane as internal standard. Chemical
shifts (d) are reported as parts per million (ppm) and coupling con-
stants (J) are reported in hertz (Hz). Elemental analyses were per-
formed at the Department of Physical Chemistry, University of
Vienna, Austria. High-resolution mass spectra (HRMS) were ob-
tained using a Micromass Q-TOF 2 instrument and are all within 5
ppm of the theoretical values. LC-MS were obtained with an Agi-
lent 6410 Triple Quadrupole mass spectrometer using electrospray
ionization (ESI), coupled to an Agilent 1200 HPLC system (ESI-
HPLC-MS) with a C18 reverse-phase column (Zorbax Eclipse
XBD-C18, 4.6 × 50 mm), evaporative light-scattering detector
(ELSD, Sedere Sedex 85) and a diode array detector (UV), using a
linear gradient of the solvent system H2O–MeCN–formic acid (A:
95:5:0.1; B: 5:95:0.086) with a flow rate of 1 mL/min. Preparative
HPLC was performed on an Agilent 1100 system using a C18 re-
verse-phase column (Zorbax 300SB-C18, 21.2 × 250 mm) with a
linear gradient of the solvent system H2O–MeCN–TFA (A:
95:5:0.1; B: 5:95:0.1) with a flow rate of 20 mL/min. Analytical
HPLC was performed on an Agilent 1100 system with a C18 re-
verse-phase column (Zorbax 300SB-C18, 4.6 × 150 mm) with a lin-
ear gradient of the above-mentioned solvent system of H2O–
MeCN–TFA with a flow rate of 1 mL/min.
Anal. Calcd for C16H17IN2O4S: C, 41.75; H, 3.72; N, 6.09; S, 6.97.
Found: C, 41.76; H, 3.76; N, 6.04; S, 6.73.
N-(3-Iodophenyl)-N-isopropyl-2-nitrobenzenesulfonamide (6)
The procedure was the same as described for compound 5, except
that i-PrOH (118 mL, 1.54 mmol) was used instead of BuOH. The
reaction mixture was concentrated under reduced pressure and pu-
rified by flash chromatography (linear gradient EtOAc–heptane) to
give 6 (81.4 mg, 24%) as a white fluffy solid.
Mp 124.7–125.6 °C.
1H NMR (CDCl3): d = 1.16 (d, J = 6.7 Hz, 6 H), 4.67 (septet, J = 6.5
Hz, 1 H), 7.06–7.10 (m, 2 H), 7.42–7.44 (m, 1 H), 7.50–7.61 (m, 2
H), 7.66 (t, J = 2.4 Hz, 1 H), 7.67–7.74 (m, 2 H).
13C NMR (CDCl3): d = 22.7 (2 C), 52.6, 93.6, 124.3, 130.5, 131.5,
132.0, 132.6, 133.2, 133.9, 135.1, 138.4, 141.8, 147.9.
MS (EI): m/z (%) = 446 (16) [M+], 431 (69), 404 (20), 245 (51), 186
(100), 133 (40), 91 (69), 76 (52), 64 (33), 51 (29).
Anal. Calcd for C15H15IN2O4S: C, 40.37; H, 3.39; N, 6.28; S, 7.19.
Found: C, 40.43; H, 3.40; N, 6.16; S, 7.06.
(3-Fluorophenyl)(phenyl)methanol (8)
1-Fluoro-3-iodobenzene (7; 352 mL, 3 mmol) and THF (2 mL) were
added to a flame-dried round-bottom flask under N2 and the mixture
was cooled to 0 °C. 1.24 M i-PrMgCl in THF (2.5 mL, 3.1 mmol)
was added dropwise and the mixture was allowed to react for 30
min. PhCHO (273 mL, 2.7 mmol) was added and the mixture was
stirred for 45 min, whilst allowing the temperature to increase to r.t.
The reaction mixture was quenched with equal portions (5 mL) of
sat. aq NH4Cl, sat. aq NaHCO3 and brine, and then extracted twice
with EtOAc (2 × 30 mL). The combined organic layers were dried
over Na2SO4, filtered and concentrated under reduced pressure. The
product was purified by flash chromatography (linear gradient
EtOAc–heptane) to provide 8 (524 mg, 86%) as a colorless oil; pu-
rity >98% (GCMS, TLC).
N-(3-Iodophenyl)-2-nitrobenzenesulfonamide (4)
o-Nitrobenzenesulfonyl chloride (1.11 g, 5 mmol) was added in 10
portions over a period of 30 min under vigorous stirring to a soln of
3 (1.37 g, 6.25 mmol) and NaOAc (554 mg, 6.75 mmol) in EtOH–
H2O (1:1, 50 mL). The mixture was heated to 80 °C over a period
of 30 min, then cooled, diluted with H2O (50 mL) and acidified with
4 M aq HCl to pH ~2. The precipitate was collected by filtration and
crystallized from EtOH (4 °C, 16 h) to give 4 (1.24 g, 60%) as yel-
lowish crystals.
Mp 132.5–132.9 °C.
1H NMR (CDCl3): d = 6.99 (t, J = 7.9 Hz, 1 H), 7.19 (ddd, J = 8.2,
2.2, 0.9 Hz, 1 H), 7.23 (br s, 1 H), 7.49 (ddd, J = 8.0, 1.6, 0.9 Hz, 1
H), 7.54 (t, J = 1.8 Hz, 1 H), 7.62 (td, J = 7.6, 1.5 Hz, 1 H), 7.71 (td,
J = 7.7, 1.5 Hz, 1 H), 7.85 (dd, J = 2.8, 1.5 Hz, 1 H), 7.87 (dd,
J = 2.8, 1.5 Hz, 1 H).
13C NMR (DMSO-d6): d = 95.7, 119.8, 125.4, 128.9, 130.5, 131.6,
131.9, 133.4, 133.8, 135.6, 138.7, 148.5.
1H NMR (CDCl3): d = 2.48 (br s, 1 H), 5.78 (s, 1 H), 6.96 (t, J = 8.4
Hz, 1 H), 7.10–7.15 (m, 2 H), 7.25–7.36 (m, 6 H).23
MS (EI): m/z (%) = 202 (25) [M+], 183 (10), 123 (42), 105 (100), 95
(15), 77 (40), 51 (19).
MS (EI): m/z (%) = 404 (42) [M+], 218 (22), 186 (44), 91 (100), 64
(44).
(3-Fluorophenyl)(phenyl)methanone (9)
H2O (46 mL, 2.54 mmol) was thoroughly mixed with anhyd CH2Cl2
(45 mL), then slowly added dropwise (about 15 min) to a vigorously
stirred soln of alcohol 8 (463.5 mg, 2.29 mmol), CH2Cl2 (1 mL) and
0.3 M DMP in CH2Cl2 (8.4 mL, 2.52 mmol). During the addition the
solution turned white; the reaction was quenched by careful addi-
tion of sodium bisulfite (6 g, 58 mmol) in sat. aq NaHCO3 (10 mL).
The organic phase was washed with sat. aq NaHCO3 (50 mL) and
H2O (50 mL), dried over Na2SO4, filtered, concentrated under re-
duced pressure and purified by flash chromatography (linear gradi-
ent EtOAc–heptane) to provide 9 (405 mg, 88%) as white crystals;
purity >98% (GCMS, TLC).
Anal. Calcd for C12H9IN2O4S: C, 35.66; H, 2.24; N, 6.93; S, 7.93.
Found: C, 35.72; H, 2.22; N, 6.75; S, 7.64.
N-Butyl-N-(3-iodophenyl)-2-nitrobenzenesulfonamide (5)
A flame-dried 2-neck round-bottom flask with a cooling condenser
was charged with sulfonamide 4 (312 mg, 0.772 mmol) and Ph3P
(405 mg, 1.54 mmol), evacuated and a N2 balloon was attached.
Toluene (10 mL) and BuOH (140 mL, 1.54 mmol) were injected into
the reaction mixture, which was then stirred and heated to 80 °C.
Diisopropyl azodicarboxylate (DIAD; 300 mL, 1.54 mmol) was
added slowly and the mixture was stirred for 2 h. The mixture was
concentrated under reduced pressure and purified by flash chroma-
tography (linear gradient EtOAc–heptane) to give 5 (153 mg, 43%)
as a colorless oil.
Mp 51.8–52.2 °C (Lit.24 54.5–55 °C).
1H NMR (CDCl3): d = 7.29 (tdd, J = 8.2, 2.7, 1.1 Hz, 1 H), 7.42–
7.48 (m, 1 H), 7.48–7.53 (m, 3 H), 7.57 (dt, J = 7.6, 1.3 Hz, 1 H),
7.62 (dt, J = 7.3, 1.8 Hz, 1 H), 7.78–7.82 (m, 2 H).
MS (EI): m/z (%) = 200 (45) [M+], 123 (28), 105 (100), 95 (26), 77
(51), 51 (23).
1H NMR (CDCl3): d = 0.83 (t, J = 7.2 Hz, 3 H), 1.20–1.40 (m, 4 H),
3.67 (t, J = 6.9 Hz, 2 H), 6.98 (t, J = 8.0 Hz, 1 H), 7.13 (ddd, J = 8.0,
2.0, 1.2 Hz, 1 H), 7.41–7.45 (m, 3 H), 7.50–7.61 (m, 3 H).
13C NMR (CDCl3): d = 14.1, 19.9, 31.0, 52.3, 94.2, 124.3, 129.3,
1-[3-(Butylamino)phenyl]ethanone (12)
131.1, 131.5, 131.9, 132.2, 134.2, 137.9, 138.5, 139.6, 148.4.
A flame-dried 2-neck round-bottom flask with a cooling condenser
was charged with CsOAc (877 mg, 4.56 mmol), CuI (347 mg, 1.82
mmol) and benzene (3.6 mL). The flask was evacuated and back-
filled with N2. DMF (6 mL), 3¢-iodoacetophenone (125 mL, 0.912
MS (EI): m/z (%) = 460 (6.3) [M+], 417 (10), 232 (17), 186 (100),
105 (10), 91 (15), 77 (17), 64 (8.3), 51 (15).
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York

812
A. Bach, K. Strømgaard
PAPER
mmol) and BuNH2 (356 mL, 3.65 mmol) were added by syringe and
the reaction mixture was stirred and heated to 90 °C for 5 h. The re-
action mixture was cooled to r.t., quenched with sat. aq NH4Cl–
brine–H2O (10:50:40, 100 mL) and extracted with EtOAc (50 mL).
The organic phase was dried over Na2SO4, filtered, concentrated
under reduced pressure and purified by flash chromatography (lin-
ear gradient EtOAc–heptane) to give 12 (112 mg, 64%) as a yellow
solid.
MS (ESI): m/z (%) = 318.2 (100) [M + Na]+, 262.2 (22), 196.2 (81).
tert-Butyl [(2S,3R)-3-(Benzyloxy)-1-oxobutan-2-yl]carbamate
(18)
H2O (100 mL, 5.55 mmol) was thoroughly mixed with anhyd
CH2Cl2 (100 mL), then slowly added dropwise (about 15 min) to a
vigorously stirred soln of alcohol 17 (1.45 g, 5 mmol), CH2Cl2 (2
mL) and 0.3 M DMP in CH2Cl2 (18.3 mL, 5.5 mmol). During the
addition the solution turned white; the reaction was quenched by
careful addition of sodium bisulfite (13 g, 125 mmol) in sat. aq
NaHCO3 (20 mL). The organic phase was washed with sat. aq
NaHCO3 (100 mL), brine (100 mL) and twice with H2O (100 mL),
dried over Na2SO4, filtered and concentrated under reduced pres-
sure at 0 °C. The product was purified by flash chromatography (sil-
ica gel 60 pretreated with 5% Et3N in EtOAc–heptane, linear
gradient EtOAc–heptane) to give aldehyde 18 (1.36 g, 93%) as a
yellowish oil; purity >98% (ELSD, TLC).
Mp 29.8–31.4 °C.
1H NMR (CDCl3): d = 0.97 (t, J = 7.2 Hz, 3 H), 1.39–1.51 (m, 2 H),
1.58–1.68 (m, 2 H), 2.58 (s, 3 H), 3.16 (t, J = 7.0 Hz, 2 H), 6.77–
6.82 (m, 1 H), 7.17–7.27 (m, 3 H).
13C NMR (CDCl3): d = 14.3, 20.7, 27.1, 31.8, 44.0, 111.6, 117.8 (2
C), 129.5, 138.3, 148.7, 198.8.
MS (EI): m/z (%) = 191 (36) [M+], 148 (100), 106 (11), 91 (8.9), 77
(6.7), 65 (6.7), 32 (5.6).
1H NMR (DMSO-d6): d = 1.14 (d, J = 6.2 Hz, 3 H), 1.36 (s, 9 H),
4.01–4.08 (m, 1 H), 4.12–4.18 (m, 1 H), 4.36 (d, J = 11.7 Hz, 1 H),
4.49 (d, J = 12.0 Hz, 1 H), 6.96 (d, J = 8.2 Hz, 1 H), 7.19–7.31 (m,
5 H), 9.47 (s, 1 H).26
Anal. Calcd for C12H17NO: C, 75.35; H, 8.96; N, 7.32. Found: C,
75.11; H, 8.57; N, 7.20.
(S)-2-[(3-Acetylphenyl)amino]-3-methylbutanoic Acid (14)
A flame-dried 2-neck round-bottom flask with a cooling condenser
was charged with 3¢-iodoacetophenone (787 mg, 3.2 mmol), L-va-
line (374 mg, 3.2 mmol), CuI (61 mg, 0.32 mmol) and K2CO3 (663
mg, 4.8 mmol). Anhyd DMF (4 mL) was added under N2 atmo-
sphere by syringe and the reaction was heated to 90 °C for 24 h. The
reaction mixture was cooled to r.t. and quenched with H2O (100
mL), which was followed by the addition of 4 M aq HCl to pH ~2
and dilution with EtOAc (50 mL). The aqueous phase was washed
with EtOAc (50 mL) and the organic phases were combined, dried
over Na2SO4, filtered and concentrated under reduced pressure. The
product was purified by flash chromatography [linear gradient
EtOAc (10% AcOH)–heptane] to provide 14 (354 mg, 47%) as an
orange oil.
1H NMR (CDCl3): d = 1.07 (d, J = 6.7 Hz, 6 H), 2.16–2.27 (m, 1 H),
2.56 (s, 3 H), 3.96 (d, J = 5.3 Hz, 1 H), 6.82 (ddd, J = 7.9, 2.4, 1.2
Hz, 1 H), 7.21–7.33 (m, 3 H).
13C NMR (CDCl3): d = 18.7, 19.6, 27.1, 31.7, 62.4, 112.8, 118.4,
118.9, 129.7, 138.2, 147.6, 178.3, 199.0.
MS (ESI): m/z (%) = 334.2 (9.8), 238.1 (33), 194.1 (100) [M – Boc
+ 2 H]+, 150.1 (42).
tert-Butyl [(2S,3R)-3-(Benzyloxy)-1-(3-iodophenyl)-1-oxobu-
tan-2-yl]carbamate (20) and tert-Butyl [(Z)-1-(3-Iodophenyl)-1-
oxobut-2-en-2-yl]carbamate (22)
1,3-Diiodobenzene (15; 1.036 g, 3.14 mmol) and THF (1.5 mL)
were added to a flame-dried round-bottom flask under N2 and the
mixture was cooled to –78 °C. 2 M i-PrMgCl in THF (1.65 mL, 3.3
mmol) was added dropwise and the mixture was allowed to react for
15 min. Aldehyde 18 (461 mg, 1.57 mmol) in THF (1 mL) was add-
ed and the mixture was allowed to react for 1.5 h while the temper-
ature increased to r.t. The reaction mixture was quenched with sat.
aq NH4Cl (10 mL), sat. aq NaHCO3 (10 mL) and brine (10 mL), and
then extracted with EtOAc (30 mL). The organic phase was dried
over Na2SO4, filtered and concentrated under reduced pressure to
give crude 19/21 (ratio: 45:55, LC-MS).
MS (ESI): m/z (%) = 520.2 (24) [M + Na]+, 398.1 (100) [M – Boc
+ 2 H]+ for 19.
MS (ESI): m/z (%) = 335.2 (11), 236.1 (100) [M + H]+.
MS (ESI): m/z (%) = 412.1 (23) [M + Na]+, 316.0 (100), 238.2 (34),
189.1 (83) for 21.
Anal. Calcd for C13H17NO3: C, 66.36; H, 7.28; N, 5.95. Found: C,
66.01; H, 7.14; N, 6.01.
This 19/21 mix was solubilized in CH2Cl2 (2 mL) and 0.3 M DMP
in CH2Cl2 (5.8 mL, 1.74 mmol). H2O (32 mL, 1.8 mmol) was thor-
oughly mixed with anhyd CH2Cl2 (32 mL) in a separation funnel,
then added dropwise (15 min) to the vigorously stirred mixture of
19/21. During the addition the solution turned white; the reaction
was quenched by careful addition of sodium bisulfite (15 g, 143
mmol) in sat. aq NaHCO3 (30 mL). The organic phase was washed
with sat. aq NaHCO3 (50 mL), dried over Na2SO4, filtered and con-
centrated under reduced pressure. 20 and 22 were separated by flash
chromatography (silica gel 60 pretreated with 5% Et3N in EtOAc–
heptane, linear gradient EtOAc–heptane).
tert-Butyl [(2R,3R)-3-(Benzyloxy)-1-hydroxybutan-2-yl]car-
bamate (17)
Boc-Thr(Bn)-OH (16; 8.7 g, 28 mmol) was dissolved in THF (80
mL). N-Methylmorpholine (3.3 mL, 29 mmol) was added and the
mixture was cooled to –20 °C, treated with isobutyl chloroformate
(4.4 mL, 29 mmol) and stirred for 30 min. The white precipitate was
removed by filtration and rinsed with THF (30 mL). NaBH4 (3.2 g,
84 mmol) was added to the filtrate; MeOH (100 mL) was carefully
added at –20 °C. After 1 h the reaction was quenched with sat. aq
NH4Cl (50 mL), which was followed by the addition of EtOAc (50
mL). The two phases were separated and the organic layer was
washed with brine (100 mL) and H2O (50 mL), dried over Na2SO4,
filtered and concentrated under reduced pressure. The product was
purified by flash chromatography (silica gel 60 pretreated with 5%
Et3N in EtOAc–heptane, linear gradient EtOAc–heptane) to give al-
cohol 17 (7.66 g, 88%) as a colorless oil; purity >98% (ELSD,
TLC).
20 was isolated by HPLC (213 mg, 27%) as a colorless oil.
1H NMR (CDCl3): d = 1.29 (d, J = 6.2 Hz, 3 H), 1.47 (s, 9 H), 3.97
(dq, J = 6.3, 2.6 Hz, 1 H), 4.23 (d, J = 11.7 Hz, 1 H), 4.48 (d,
J = 11.7 Hz, 1 H), 5.17 (dd, J = 8.9, 2.5 Hz, 1 H), 5.63 (d, J = 5.6
Hz, 1 H), 7.06–7.27 (m, 6 H), 7.76–7.87 (m, 2 H), 8.17 (t, J = 1.8
Hz, 1 H).
13C NMR (CDCl3): d = 16.6, 28.8 (3 C), 60.7, 71.1, 74.4, 80.5, 94.9,
127.9, 128.1 (3 C), 128.7 (2 C), 130.6, 137.6, 137.7, 137.8, 142.3,
156.5, 196.9.
1H NMR (DMSO-d6): d = 1.04 (d, J = 6.2 Hz, 3 H), 1.34 (s, 9 H),
3.29–3.37 (m, 1 H), 3.40–3.50 (m, 2 H), 3.56–3.64 (m, 1 H), 4.37
(d, J = 12.0 Hz, 1 H), 4.50 (d, J = 11.7 Hz, 1 H), 6.29 (d, J = 8.2 Hz,
1 H), 7.20–7.25 (m, 2 H), 7.26–7.29 (m, 3 H).25
MS (ESI): m/z (%) = 518.2 (20) [M + Na]+, 396.1 (100) [M – Boc
+ 2 H]+, 352.1 (26).
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York

PAPER
Synthesis of 1,3-Substituted Benzene Peptidomimetics
813
Anal. Calcd for C22H26INO4: C, 53.34; H, 5.29; N, 2.83. Found: C,
53.02; H, 5.01; N, 2.84.
1H NMR (CDCl3): d = 1.14 (d, J = 6.2 Hz, 3 H), 1.20 (s, 9 H), 1.46
(s, 9 H), 4.18 (dd, J = 8.0, 3.5 Hz, 1 H), 4.24–4.31 (m, 1 H), 5.39 (d,
J = 7.0 Hz, 1 H), 9.67 (s, 1 H).
13C NMR (CDCl3): d = 20.0, 28.7 (3 C), 28.8 (3 C), 65.5, 66.7, 74.9,
80.2, 156.2, 201.7.
MS (ESI): m/z (%) = 159 (29) [M – Boc + H]+, 130 (27), 118 (18),
103 (78), 86 (9), 74 (62), 57 (100).
HPLC gave the analytically pure 22 (28 mg, 4.6%) as a white
freeze-dried solid; purity >98% (ELSD, analytical HPLC).
Mp 124.5–127.4 °C.
1H NMR (CDCl3): d = 1.42 [s, 9 H, (CH3)3], 1.89 (d, J = 7.0 Hz, 3
H, =C–CH3), 6.10 (q, J = 6.7 Hz, 1 H, =CH), 6.55 (br s, 1 H, NH),
7.16 (t, J = 7.8 Hz, 1 H, H-5), 7.69 (ddd, J = 7.8, 1.6, 1.0 Hz, 1 H,
H-6), 7.84 (ddd, J = 7.9, 1.7, 1.0 Hz, 1 H, H-4), 8.05 (t, J = 1.6 Hz,
1 H, H-2).
Anal. Calcd for C13H25NO4: C, 60.21; H, 9.72; N, 5.40. Found: C,
60.21; H, 9.78; N, 5.49.
tert-Butyl [(2R,3R)-3-tert-Butoxy-1-hydroxy-1-(3-iodophe-
nyl)butan-2-yl]carbamate (26)
13C NMR (CDCl3): d = 14.6 (s, =C–CH3), 28.5 [s, 3 C, C(CH3)3],
81.6 [s, C(CH3)3], 94.2 (s, C-3), 128.8 (s, C-6), 130.3 (s, C-5), 133.2
(s, =C–CH3), 135.3 (s, C=C–CH3), 138.3 (s, C-2), 139.4 (s, C-1),
141.3 (s, C-4), 153.6 [s, C(=O)NH], 191.9 [s, C(=O)C=C].
1,3-Diiodobenzene (15; 1.98 g, 6 mmol) and THF (3 mL) was added
to a flame-dried round-bottom flask under N2 and the mixture was
cooled to –78 °C. 2 M i-PrMgCl in THF (3.1 mL, 6.2 mmol) was
added dropwise and the mixture was stirred for 15 min. Aldehyde
25 (648 mg, 2.5 mmol) in THF (4 mL) was added, which was fol-
lowed by stirring of the mixture for 45 min while the temperature
increased to r.t. The reaction mixture was quenched with equal por-
tions (20 mL) of sat. aq NH4Cl, sat. aq NaHCO3 and brine, and then
extracted with EtOAc (2 × 40 mL). The combined organic layers
were washed with H2O (100 mL), dried over Na2SO4, filtered and
concentrated under reduced pressure. The product was purified by
flash chromatography (silica gel 60 pretreated with 5% Et3N in
EtOAc–heptane, linear gradient EtOAc–heptane) to give 26 (733
mg, 63%) as a sticky white solid; purity >98% (ELSD, TLC).
MS (ESI): m/z (%) = 410 (5.4) [M + Na]+, 332 (46), 288 (100) [M
– Boc + 2 H]+.
Anal. Calcd for C15H18INO3: C, 46.53; H, 4.69; N, 3.62. Found: C,
46.19; H, 4.30; N, 3.56.
tert-Butyl [(2R,3R)-3-tert-Butoxy-1-hydroxybutan-2-yl]car-
bamate (24)
Boc-Thr(t-Bu)-OH (23; 5 g, 18.2 mmol) was dissolved in THF (50
mL). N-Methylmorpholine (2.1 mL, 18.5 mmol) was added and the
mixture was cooled to –20 °C, treated with isobutyl chloroformate
(2.8 mL, 18.5 mmol) and stirred for 30 min. The white precipitate
was removed by filtration and rinsed with THF (30 mL). NaBH4
(2.06 g, 54 mmol) was added to the filtrate; MeOH (130 mL) was
carefully added to the open flask at –20 °C. After 1 h the reaction
was quenched with sat. aq NH4Cl (50 mL), which was followed by
the addition of EtOAc (50 mL). The two phases were separated and
the organic phase was washed with brine (50 mL) and H2O (100
mL). The combined aqueous phases were washed three times with
EtOAc–heptane (1:1, 50 mL) and the combined organic phases
were dried over Na2SO4, filtered and concentrated under reduced
pressure. The product was purified by flash chromatography (silica
gel 60 pretreated with 5% Et3N in EtOAc–heptane; EtOAc–
heptane, 1:1) to give alcohol 24 (4.42 g, 93%) as white flakes.
Mp 107.2–107.7 °C.
1H NMR (CDCl3): d = 1.22 (d, J = 6.2 Hz, 3 H), 1.28 (s, 9 H), 1.34
(s, 9 H), 3.62 (dt, J = 10.0, 2.9 Hz, 1 H), 4.04–4.11 (m, 1 H), 4.32
(br s, 1 H), 4.87 (d, J = 2.4 Hz, 1 H), 5.20 (d, J = 9.7 Hz, 1 H), 7.02
(t, J = 7.8 Hz, 1 H), 7.26 (d, J = 7.0 Hz, 1 H), 7.55 (d, J = 7.9 Hz, 1
H), 7.73 (s, 1 H).
13C NMR (CDCl3): d = 20.8, 28.7 (3 C), 29.4 (3 C), 60.5, 71.4, 75.1,
75.3, 79.9, 94.7, 125.8, 130.2, 135.5, 136.7, 144.3, 156.9.
MS (ESI): m/z (%) = 949.4 (23) [2 M + Na]+, 486.1 (15) [M + Na]+,
334.0 (100), 290.0 (39).
Mp 95.5–96.4 °C.
tert-Butyl [(2S,3R)-3-tert-Butoxy-1-(3-iodophenyl)-1-oxobutan-
2-yl]carbamate (27)
1H NMR (CDCl3): d = 1.17 (d, J = 6.2 Hz, 3 H), 1.20 (s, 9 H), 1.45
(s, 9 H), 2.92 (br s, 1 H), 3.48–3.57 (m, 1 H), 3.62–3.67 (m, 2 H),
3.88–3.96 (m, 1 H), 5.01 (d, J = 7.3 Hz, 1 H).
13C NMR (CDCl3): d = 20.5, 28.8, 29.1, 57.1, 64.1, 67.3, 74.5, 79.8,
156.9.
MS (ESI): m/z (%) = 545.4 (37) [2 M + Na]+, 284.2 (54) [M + Na]+,
150.1 (100).
H2O (27 mL, 1.5 mmol) was thoroughly mixed with anhyd CH2Cl2
(27 mL), then slowly added dropwise (about 15 min) to a vigorously
stirred soln of alcohol 26 (633 mg, 1.37 mmol), CH2Cl2 (2 mL) and
0.3 M DMP in CH2Cl2 (5 mL, 1.5 mmol). During the addition the
solution turned white; the reaction was quenched by careful addi-
tion of sodium bisulfite (3.5 g, 34 mmol) in sat. aq NaHCO3 (10
mL). The organic phase was washed with sat. aq NaHCO3 (30 mL)
and twice with H2O (30 mL), dried over Na2SO4, filtered and con-
centrated under reduced pressure. The product was purified by flash
chromatography (silica gel 60 pretreated with 5% Et3N in EtOAc–
heptane, linear gradient EtOAc–heptane) to give 27 (556 mg, 88%)
as a thick colorless oil.
1H NMR (CDCl3): d = 1.05 (s, 9 H), 1.19 (d, J = 6.0 Hz, 3 H), 1.45
(s, 9 H), 4.01–4.04 (m, 1 H), 5.08 (dd, J = 8.7, 3.0 Hz, 1 H), 5.59 (d,
J = 8.4 Hz, 1 H), 7.20 (t, J = 7.5 Hz, 1 H), 7.87–7.91 (m, 2 H), 8.28
(s, 1 H).
13C NMR (CDCl3): d = 20.5, 28.7 (3 C), 28.8 (3 C), 61.3, 68.1, 74.7,
80.2, 94.6, 128.3, 130.6, 138.1, 138.3, 142.4, 156.2, 197.5.
Anal. Calcd for C13H27NO4: C, 59.74; H, 10.41; N, 5.36. Found: C,
59.36; H, 10.18; N, 5.49.
tert-Butyl [(2S,3R)-3-tert-Butoxy-1-oxobutan-2-yl]carbamate
(25)
H2O (100 mL, 5.55 mmol) was thoroughly mixed with anhyd
CH2Cl2 (100 mL), then slowly added dropwise (about 15 min) to a
vigorously stirred soln of alcohol 24 (1.31 g, 5 mmol), CH2Cl2 (2
mL) and 0.3 M DMP in CH2Cl2 (18.3 mL, 5.5 mmol). During the
addition the solution turned white; the reaction was quenched by
careful addition of sodium bisulfite (13 g, 125 mmol) in sat. aq
NaHCO3 (20 mL). The organic phase was washed with sat. aq
NaHCO3 (100 mL), brine (100 mL) and twice with H2O (100 mL),
dried over Na2SO4, filtered and concentrated under reduced pres-
sure at 0 °C. The analytically pure product was obtained by flash
chromatography (silica gel 60 pretreated with 5% Et3N in heptane,
heptane as eluent) to give 25 (972 mg, 75%) as a colorless oil.
MS (ESI): m/z (%) = 484.2 (49) [M + Na]+, 306.0 (100) [M – Boc –
t-Bu + 3 H]+.
Anal. Calcd for C19H28INO4: C, 49.47; H, 6.12; N, 3.04. Found: C,
49.16; H, 5.59; N, 2.85.
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York

814
A. Bach, K. Strømgaard
PAPER
(S)-2-(3-[(2S,3R)-3-tert-Butoxy-2-(tert-butoxycarbonylami-
no)butanoyl]phenylamino)-3-methylbutanoic Acid (28)
(d, J = 2.6 Hz, 1 H), 5.12 (d, J = 9.4 Hz, 2 H), 7.06 (t, J = 7.6 Hz, 1
H), 7.13–7.19 (m, 5 H), 7.27 (d, J = 7.6 Hz, 1 H), 7.59 (d, J = 7.6
Hz, 1 H), 7.68 (s, 1 H).
13C NMR (CDCl3): d = 19.3, 21.5, 28.8 (3 C), 29.1 (3 C), 60.7, 67.7,
71.3, 74.2, 78.2, 79.3, 94.7, 126.5, 128.4 (2 C), 129.4 (2 C), 130.5,
135.2, 136.3, 137.0, 137.8, 143.9, 156.0.
A flame-dried round-bottom flask was quickly charged with 27
(677 mg, 1.47 mmol), L-valine (172 mg, 1.47 mmol), CuI (28 mg,
0.15 mmol), K2CO3 (308 mg, 2.2 mmol) and DMF (2 mL). The
flask was fitted with a condenser, evacuated and backfilled with N2.
The reaction mixture was heated to 90 °C for 7 h, which was fol-
lowed by cooling and quenching with H2O (30 mL). EtOAc (30 mL)
was added and the aqueous phase was acidified (pH ~2) and shaken
with the organic phase. The aqueous phase was isolated and washed
twice with EtOAc (30 mL). The combined organic extracts were
dried over Na2SO4, filtered and concentrated under reduced pres-
sure. The product was purified by flash chromatography [linear gra-
dient EtOAc (10% AcOH)–heptane] to give 28 (266 mg, 40%, dr
78%) as a yellow solid; purity >98% (ELSD).
MS (ESI): m/z (%) = 590.2 (28) [M + Na]+, 468.1 (22) [M – Boc +
2 H]+, 412.1 (100) [M – Boc – t-Bu + 3 H]+.
Anal. Calcd for C27H38INO4: C, 57.14; H, 6.75; N, 2.47. Found: C,
57.20; H, 6.58; N, 2.28.
(S)-2-(3-[(2S,3R)-2-Amino-3-tert-butoxybutanoyl]phenylami-
no)-3-methylbutanoic Acid (32)
A mixture of 1.25 M HCl in i-PrOH (100 mL, 125 mmol) and 28
(320 mg, 0.71 mmol) was stirred at r.t. for 12 h, then concentrated
under reduced pressure and coevaporated twice with CH2Cl2 (30
mL). Compound 32 was obtained as the trifluoroacetate salt by
HPLC purification (257 mg, 78%, dr 75%) as a yellow freeze-dried
solid; purity >98% (ELSD).
Mp 77.1–77.8 °C.
1H NMR (CDCl3): d = 0.96 (s, 9 H), 1.05 (d, J = 6.5 Hz, 3 H), 1.08
(d, J = 6.7 Hz, 3 H), 1.23 (d, J = 6.2 Hz, 3 H), 1.46 (s, 9 H), 2.17–
2.28 (m, 1 H), 3.99–4.05 (m, 2 H), 5.21 (dd, J = 9.4, 2.1 Hz, 1 H),
5.72 (d, J = 9.4 Hz, 1 H), 6.81 (dt, J = 6.9, 2.1 Hz, 1 H), 7.21–7.28
(m, 2 H), 7.37 (s, 1 H).
Mp 85.5–86.7 °C.
1H NMR (CDCl3): d = 0.93 (s, 9 H), 1.01 (d, J = 6.7 Hz, 3 H), 1.04
(d, J = 6.7 Hz, 3 H), 1.19 (s, 2 H), 1.35 (d, J = 6.5 Hz, 3 H), 2.08–
2.19 (m, 1 H), 3.81 (d, J = 5.6 Hz, 1 H), 4.10–4.18 (m, 1 H), 4.91
(br s, 1 H), 5.02 (d, J = 4.1 Hz, 1 H), 6.80–6.89 (m, 1 H), 7.04–7.19
(m, 3 H).
13C NMR (CD3OD): d = 18.2, 18.7, 20.5, 27.6 (3 C), 31.2, 61.3,
62.7, 66.4, 74.9, 112.2, 117.6, 119.4, 129.8, 134.7, 149.3, 175.8,
194.3.
13C NMR (CDCl3): d = 18.7, 19.3, 21.3, 28.68 (3 C), 28.73 (3 C),
31.7, 61.2, 62.2, 68.2, 74.5, 80.8, 114.5, 118.3, 119.0, 130.1, 136.7,
147.7, 157.0, 176.9, 198.5.
MS (ESI): m/z (%) = 339.1 (26), 295.1 (100) [M – Boc – t-Bu +
3H]+, 251.1 (15).
HRMS (ESI): m/z [M + H]+ calcd for C24H39N2O6: 451.2808; found:
451.2796.
MS (ESI): m/z (%) = 351.2 (100) [M + H]+, 295.1 (27) [M – t-Bu +
(S)-2-(3-[(2S,3R)-2-Amino-3-hydroxybutanoyl]phenylamino)-
3-methylbutanoic Acid (1)
2 H]+.
HRMS (ESI): m/z [M + H]+ calcd for C19H31N2O4: 351.2284; found:
351.2299.
A mixture of TFA–triisopropylsilane–H2O (90:5:5, 2 mL) and 28
(105 mg, 0.233 mmol) was stirred at r.t. for 1 h, then concentrated,
followed by three cycles of addition/evaporation of Et2O (10 mL).
HPLC gave trifluoroacetate salts of compound 1 (major epimer: 71
mg, 75%, dr 98%; minor epimer: 5.0 mg, 5%, dr 95%) as yellow
freeze-dried solids.
(S)-4-[(2S,3S)-2-Amino-3-methylpentanamido]-5-([(2S,3R)-1-
[3-([(S)-1-carboxy-2-methylpropyl]amino)phenyl]-3-hydroxy-
1-oxobutan-2-yl]amino)-5-oxopentanoic Acid (2)
Compound 32 (451 mg, 0.97 mmol) was dissolved as the trifluoro-
acetate salt in dioxane–MeCN (1:1, 5 mL), then 10% aq Na2CO3 (15
mL) was added under vigorous stirring. Fmoc-OSu (492 mg, 1.46
mmol) in dioxane (3 mL) was added slowly. The orange slurry mix-
ture was stirred for 2 h. H2O (60 mL) was added and the aqueous
layer was extracted twice with EtOAc (60 mL). The aqueous phase
was acidified to pH ~2 with 4 M aq HCl and extracted once with
EtOAc (60 mL). The combined organic layers were concentrated
under reduced pressure, dissolved in H2O–MeCN (1:1, 60 mL) and
freeze-dried to give crude 33 as a yellow solid; purity ~90%
(ELSD).
Data for the major epimer: purity >98% (ELSD).
Mp 56.9–58.0 °C.
1H NMR (D2O): d = 0.89 (d, J = 5.5 Hz, 3 H), 0.92 (d, J = 5.5 Hz, 3
H), 1.19 (d, J = 6.5 Hz, 3 H), 2.00–2.12 (m, 1 H), 3.79 (d, J = 5.9
Hz, 1 H), 4.22–4.29 (m, 1 H), 4.91 (d, J = 2.6 Hz, 1 H), 6.96–7.01
(m, 1 H), 7.12 (s, 1 H), 7.24–7.30 (m, 2 H).
13C NMR (D2O): d = 18.2, 18.6, 19.6, 30.8, 61.2, 63.9, 66.0, 113.6,
120.5, 121.5, 130.6, 134.5, 147.3, 177.5, 196.5.
MS (ESI): m/z (%) = 295.1 (100) [M + H]+, 251.1 (9.6).
HRMS (ESI): m/z [M + H]+ calcd for C15H23N2O4: 295.1658; found:
MS (ESI): m/z (%) = 595.2 (12) [M + Na]+, 517.2 (100) [M – t-Bu
+ 2 H]+, 473.2 (17).
295.1674.
2-Chlorotrityl chloride resin (142 mg, 0.22 mmol) was swelled in
anhyd CH2Cl2 (2 mL) for 15 min, which was followed by draining
and incubation with half the portion of crude 33 (~0.44 mmol) in a
mixture of CH2Cl2–DMF (2:1, 2 mL) and DIPEA (145 mL, 0.87
mmol) for 2 h. The resin was capped with MeOH (CH2Cl2–MeOH–
DIPEA, 17:2:1) three times for 2 min. Fmoc deprotection was per-
formed with 20% piperidine in DMF (2 × 10 min), and coupling of
the consecutive amino acid was carried out with HATU and colli-
dine (resin–amino acid–HATU–collidine 1:4:4:6) for 30 min. The
resin was cleaved with 5% H2O and 5% triisopropylsilane in TFA
for 2 h, which was followed by concentration under reduced pres-
sure and coevaporation with CH2Cl2 (2 × 20 mL) to obtain crude 2
(dr 85%, ELSD). HPLC of half the portion of crude 2 gave the
freeze-dried trifluoroacetate salt of compound 2 as the major epimer
tert-Butyl [(2S,3R)-3-tert-Butoxy-1-(3-iodophenyl)-1-(4-methyl-
benzyloxy)butan-2-yl]carbamate (30)
Toluene (20 mL) and 50% aq NaOH (15 mL) were added to alcohol
26 (576 mg, 1.24 mmol). The solution was vigorously stirred and
treated with p-methylbenzyl bromide (247 mg, 1.34 mmol) and tet-
rabutylammonium hydrogen sulfate (48 mg, 0.14 mmol) for 24 h,
which was followed by the addition of H2O (30 mL) and extraction
with Et2O (50 mL). The organic layer was dried over Na2SO4, fil-
tered and concentrated under reduced pressure. The product was pu-
rified by flash chromatography (silica gel 60 pretreated with 5%
Et3N in EtOAc–heptane, linear gradient EtOAc–heptane) to give 30
(566 mg, 81%) as a colorless oil.
1H NMR (CDCl3): d = 1.16 (d, J = 6.5 Hz, 3 H), 1.20 (s, 9 H), 1.38
(s, 9 H), 2.35 (s, 3 H), 3.58–3.73 (m, 1 H), 4.25–4.34 (m, 1 H), 4.66
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York

PAPER
Synthesis of 1,3-Substituted Benzene Peptidomimetics
815
(28 mg, 39%, dr 99%) and as the epimeric mix (7.1 mg, 9.9%, dr
~50%) as yellowish solids.
(5) Bach, A.; Chi, C. N.; Olsen, T. B.; Pedersen, S. W.; Røder,
M. U.; Pang, G. F.; Clausen, R. P.; Jemth, P.; Strømgaard, K.
J. Med. Chem. 2008, 51, 6450.
Data for major epimer: purity >98% (ELSD).
Mp 111.0–114.5 °C.
(6) Ripka, A. S.; Rich, D. H. Curr. Opin. Chem. Biol. 1998, 2,
441.
1H NMR (D2O): d = 0.78 (t, J = 7.3 Hz, 3 H), 0.85 (d, J = 6.7 Hz, 3
H), 0.91 (d, J = 4.9 Hz, 3 H), 0.93 (d, J = 4.8 Hz, 3 H), 1.07 (d,
J = 6.2 Hz, 3 H), 1.28–1.42 (m, 2 H), 1.76–1.92 (m, 3 H), 2.01–2.11
(m, 1 H), 2.17–2.27 (m, 2 H), 3.75 (d, J = 5.3 Hz, 1 H), 3.77 (d,
J = 5.9 Hz, 1 H), 4.21–4.28 (m, 1 H), 4.40 (t, J = 7.3 Hz, 1 H), 5.21
(d, J = 2.6 Hz, 1 H), 6.91–6.97 (m, 1 H), 7.08 (br s, 1 H), 7.23–7.28
(m, 2 H).
(7) Blomberg, D.; Brickmann, K.; Kihlberg, J. Tetrahedron
2006, 62, 10937.
(8) Blomberg, D.; Fex, T.; Xue, Y.; Brickmann, K.; Kihlberg, J.
Org. Biomol. Chem. 2007, 5, 2599.
(9) Saitton, S.; Kihlberg, J.; Luthman, K. Tetrahedron 2004, 60,
6113.
(10) Mitsunobu, O. Synthesis 1981, 1.
(11) Ma, D.; Zhang, Y.; Yao, J.; Wu, S.; Tao, F. J. Am. Chem.
MS (ESI): m/z (%) = 1073.5 (51) [2 M + H]+, 537.2 (100) [M + H]+,
Soc. 1998, 120, 12459.
(12) Okano, K.; Tokuyama, H.; Fukuyama, T. Org. Lett. 2003, 5,
4987.
269.1 (6) [M + 2 H]2+.
HRMS (ESI): m/z [M + H]+ calcd for C26H41N4O8: 537.2924; found:
537.2922.
(13) Sokolov, V.; Butkevich, A.; Yuskovets, V.; Tomashevskii,
A.; Potekhin, A. Russ. J. Org. Chem. 2005, 41, 1023.
(14) Varchi, G.; Kofink, C.; Lindsay, D. M.; Ricci, A.; Knochel,
P. Chem. Commun. 2003, 396.
(15) (a) Kopp, F.; Krasovskiy, A.; Knochel, P. Chem. Commun.
2004, 2288. (b) Kopp, F.; Wunderlich, S.; Knochel, P.
Chem. Commun. 2007, 2075. (c) Boymond, L.; Rottländer,
M.; Cahiez, G.; Knochel, P. Angew. Chem. Int. Ed. 1998, 37,
1701.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis. Included
are experimental details for the human blood plasma stability assay,
for the alkylation experiments and for the preparation of the
STABASE adduct, H and 13C NMR spectra of all purified com-
1
pounds and 2D NMR spectra of compound 22.
(16) (a) Stoll, A. H.; Knochel, P. Org. Lett. 2008, 10, 113.
(b) Djuric, S.; Venit, J.; Magnus, P. Tetrahedron Lett. 1981,
22, 1787.
Acknowledgment
(17) Wen, J. J.; Crews, C. M. Tetrahedron: Asymmetry 1998, 9,
1855.
(18) (a) Meyer, S. D.; Schreiber, S. L. J. Org. Chem. 1994, 59,
7549. (b) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48,
4155.
This work was supported by the Lundbeck Foundation (K.S.), the
Drug Research Academy, Faculty of Pharmaceutical Sciences, Uni-
versity of Copenhagen, Denmark (A.B.), and the Danish Council
for Independent Research, Technology and Production Sciences
(A.B.). We thank Nicolai Stuhr-Hansen for a critical reading of the
manuscript.
(19) Lubell, W. D.; Rapoport, H. J. Am. Chem. Soc. 1987, 109,
236.
(20) Han, G.; Tamaki, M.; Hruby, V. J. J. Pept. Res. 2001, 58,
338.
(21) Harris, J. M.; Martin, N. E.; Modi, M. Clin. Pharmacokinet.
References
(1) (a) Loughlin, W. A.; Tyndall, J. D.; Glenn, M. P.; Hill, T. A.;
Fairlie, D. P. Chem. Rev. 2004, 104, 6085. (b) Loughlin, W.
A.; Tyndall, J. D.; Glenn, M. P.; Hill, T. A.; Fairlie, D. P.
Chem. Rev. 2010, 110, PR32.
2001, 40, 539.
(22) Bach, A.; Chi, C. N.; Pang, G. F.; Olsen, L.; Kristensen, A.
S.; Jemth, P.; Strømgaard, K. Angew. Chem. Int. Ed. 2009,
48, 9685.
(23) Nolte, C.; Mayr, H. Eur. J. Org. Chem. 2010, 1435.
(24) (a) Garth, R.; Yuho, P.; Taft, T.; Taft, R. W. J. Am. Chem.
Soc. 1967, 89, 2391. (b) Buu-Hoi, N. P.; Lescot, E. Jr.;
Xuong, N. D. J. Org. Chem. 1957, 22, 1057.
(25) Nakamura, Y.; Hirai, M.; Tamotsu, K.; Yonezawa, Y.; Shin,
C. Bull. Chem. Soc. Jpn. 1995, 68, 13699.
(26) Fehrentz, J.-A.; Castro, B. Synthesis 1983, 676.
(2) Remaut, H.; Waksman, G. Trends Biochem. Sci. 2006, 31,
436.
(3) Giannis, A.; Kolter, T. Angew. Chem. Int. Ed. 1993, 32,
1244.
(4) (a) Hruby, V. J.; Balse, P. M. Curr. Med. Chem. 2000, 7,
945. (b) Hruby, V. J. Nat. Rev. Drug Discovery 2002, 1, 847.
Synthesis 2011, No. 5, 807–815 © Thieme Stuttgart · New York