Efficient microwave-assisted synthesis of 4-amino-2-benzazepin-3-ones as conformationally restricted dipeptide mimetics

Article abstract of DOI:10.1016/j.tet.2008.10.072

In this paper, we describe the synthesis of conformationally constrained dipeptide mimetic derivatives. Microwave flash heating was used in several synthetic steps providing the opportunity to perform the reactions in dramatically shortened time as well as to increase the obtained yields. The efficiency of the methodology makes it useful in order to prepare other dipeptides containing the 4-amino-tetrahydro-2-benzazepin-3-one motif.

Full text of DOI:10.1016/j.tet.2008.10.072

Tetrahedron 65 (2009) 206–211
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Efficient microwave-assisted synthesis of 4-amino-2-benzazepin-3-ones as
conformationally restricted dipeptide mimetics
Beatrice Severino, Ferdinando Fiorino, Antonella Esposito, Francesco Frecentese, Francesca De Angelis,
*
Elisa Perissutti, Giuseppe Caliendo, Vincenzo Santagada
`
Dipartimento di Chimica Farmaceutica e Tossicologica, Universita di Napoli, Federico II, Via D. Montesano, 49, 80131 Napoli, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2008
Received in revised form 3 October 2008
Accepted 23 October 2008
Available online 26 October 2008
In this paper, we describe the synthesis of conformationally constrained dipeptide mimetic derivatives.
Microwave flash heating was used in several synthetic steps providing the opportunity to perform the
reactions in dramatically shortened time as well as to increase the obtained yields. The efficiency of the
methodology makes it useful in order to prepare other dipeptides containing the 4-amino-tetrahydro-2-
benzazepin-3-one motif.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Peptide mimetic
Constrained amino acids
Microwave irradiation
Intramolecular
Friedel–Crafts cyclization
1. Introduction
intermediate oxazolidinone that affords the benzazepinone nu-
cleus by means of an intramolecular Friedel–Crafts cyclization via
Introduction of conformational restrictions into the sequence of
bioactive peptides is one of the most useful strategies in the design
and preparation of more selective and/or more potent peptide
mimetics as enzyme inhibitors or receptor agonists and/or antag-
onists.1 Moreover, the so-obtained pseudopeptides display several
important advantages compared to natural peptides such as in-
creased bio-stability and improved selectivity toward the natural
biological target. Due to the flexibility of the peptide side chains,
the proper topography during receptor interaction can only be
studied by constraining or fixing of the side-chain conformers.2
A reduced conformational flexibility of phenylalanine-containing
peptides can be obtained embodying a rigidified skeleton deriving
from the cyclization of the aromatic side chain that is anchored to
an N-acyliminium ion (Scheme 1).3,6,7 Several laboratories have
exploited N-acyliminium ion cyclization, especially for the syn-
thesis of natural products.8,9 N-Acyliminium species, in fact, have
demonstrated a high reactivity toward a wide variety of
p-nucle-
O
O
O
O
N
N
N
N
OH
OH
O
O
R
R
O
O
Scheme 1. Formation of benzazepinone nucleus via an N-acyliminium ion.
the a-amine of the next residue by means of a methylene bridge.
This structural modification leads to the 4-amino-tetrahydro-2-
benzazepin-3-one (Aba) nucleus, which has proven to be an ex-
cellent tool for the design and preparation of several different
peptide mimetics such as renin inhibitors,3 opioid peptides,2,4 and
human melanocortin-3 receptor ligands.5 The previously reported
synthetic strategy adopted for the preparation of this kind of
moiety is based on the reaction, in acidic conditions, of an
ophiles including alkenes, allenes, alkynes, and aromatic or heter-
oaromatic systems.9 Benzazepinone ring formation has been
performed using acid, such as trifluoromethanesulfonic acid
(TFMSA), or Lewis acids, such as titanium(IV) chloride or tin(IV)
chloride, at room temperature or under reflux, and required several
hours (usually 12–24 h). In later years, microwave irradiation in
chemical reaction enhancement has been widely applied in various
synthetic fields.10 Replacing the oil bath with a dedicated micro-
wave reactor allows using conditions not attainable under
* Corresponding author. Tel.: þ39 81 678648; fax: þ39 81 678649.
E-mail address: santagad@unina.it (V. Santagada).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.10.072

B. Severino et al. / Tetrahedron 65 (2009) 206–211
207
Table 1
Structures of dipeptide mimetics 6a–f
O
i
OH
O
OH
H2N
N
O
O
O
O
N
O
N
1
2
OH
O
R
O
O
O
O
H
N
ii
H
iii
N
N
OH
N
O
Compound
R
Compound
R
O
R
O
R
3a-f
O
O
6a
–H
6d
4a-f
6b
6c
6e
6f
O
R
O
iv
O
v
O
O
N
N
N
N
OH
O
O
R
O
O
5a-f
6a-f
Scheme 2. (i) Phthalic anhydride, TEA, toluene, my; (ii) H-aa-O-t-Bu, DCC, HOBt, DMAP,
DMF; (iii) 40% TFA in DCM, my; (iv) (CH2O)n, p-TsOH, toluene; (v) Methods A–D.
conventional heating, providing the opportunity to perform re-
actions in dramatically shortened time as well as increasing yields.
Enhancements of the reaction rate and product selectivity under
microwave dielectric heating have been attributed to thermal ef-
fects, which may result because of differences between the real
reaction temperature at the reaction sites and the observed average
temperature. Our special interest in applying the potential of mi-
crowave irradiation to the synthesis of peptide and peptide mi-
metic structures11 prompted us to use this technology for the
preparation of several dipeptide mimetics with the general struc-
ture Pht-Aba-X-OH, where X is represented by the amino acids Gly,
Ala, Val, Leu, Ile and Phe, in order to develop new series of com-
pounds in our ongoing research for novel protease activated re-
ceptor-1 (PAR-1) antagonists;12 particularly, we planned the
replacement of the Phe2 of the sequence of PAR-1AP (human
SFLLRN or rat SFFLRN) by these conformationally restricted de-
rivatives. In this paper, we describe the application of microwave
flash heating to the synthesis of conformationally constrained di-
peptide mimetic derivatives. The efficiency of the methodology
makes it useful in order to prepare other dipeptides containing the
4-amino-tetrahydro-2-benzazepin-3-one motif.
The synthetic procedure started with the protection of phenylala-
nine
a-amino group obtained using phthalic anhydride in the
presence of TEA. The reaction has been performed in several dif-
ferent anhydrous solvents, such as dioxane, DMF, and toluene,
obtaining the best results (yield 85%) when using anhydrous tolu-
ene and irradiating the solution at 500 W (temperature 120 ꢀC) for
1 h. Pht-Phe-OH 2 was then condensed with the appropriate tert-
butyl ester amino acid, employing standard solution-phase peptide
synthesis method (such as DCC/HOBt/TEA), affording the in-
termediates 3a–f. The tert-butyl protection was removed by 40%
TFA in DCM; the irradiation at 500 W (temperature 60 ꢀC) for
10 min gave excellent yields (ranging between 80 and 98%) of the
desired acids 4a–f. The oxazolidinones 5a–f were formed by re-
action of the starting acids 4a–f with paraformaldehyde in the
presence of catalytic p-toluenesulfonic acid. Attempts to perform
this step under microwave irradiation furnished low yields because
it was impossible for us to achieve the water azeotropic removal
needed to reach the completion of the reaction. Compounds 5c and
5e were obtained with yields lower than those accomplished with
the other derivatives (32% and 25%, respectively); this result could
2. Results and discussion
be addressed to the b-branch of Val and Ile that hinders the oxa-
In short peptides, different orientation of the side-chain group
zolidinone formation. No detectable epimerization occurred at the
chiral centers during these transformations as measured by 1H
NMR spectroscopy. The crucial cyclization needed for obtaining the
desired final dipeptide mimetic analogues 6a–f was performed
employing several acidic conditions such as TFMSA, BF3$OEt2, AlCl3,
and ZnCl2 achieving the best results when titanium(IV) and tin(IV)
chloride were used. The obtained yields and the reaction conditions
are summarized in Table 2. As shown, the cyclization reaction,
based on the key formation of an N-acyliminium ion that evolves
through a Friedel–Crafts intramolecular reaction to the desired
benzazepinone derivative, was accomplished with high yields us-
ing TiCl4 (10 equiv of 1 M solution in anhydrous DCM) using mi-
crowave irradiation at 600 W for 1 h (temperature 80 ꢀC in method
A and 50 ꢀC in method C). The final products were obtained with
yields ranging between 10% and 89% when the reaction was per-
formed at 80 ꢀC, and 6%–89% when the reaction was performed at
50 ꢀC. The accomplished yields, reported in Table 2, show that the
operative temperature does not have a critical effect in the prepa-
ration of 6a, 6d, and 6f (that were obtained with comparable yields
when performing the reaction both at 50 ꢀC and at 80 ꢀC) while
method A (80 ꢀC) affords better results for compounds 6b and 6c.
of an L-amino acid residue in terms of c
1 space can lead to a relevant
difference of the chemical surfaces, often corresponding to a dra-
matically different biological profile. The staggered conformations
around Ca–Cb for Phe, such as gauche (þ) (c1¼60ꢀ) and trans
(
c1¼180ꢀ), can be fixed by means of the 4-amino-tetrahydro-2-
benzazepin-3-one nucleus. The dipeptide mimetic derivatives 6a–f,
reported in Table 1, have been prepared following a previously
described procedure2–7 summarized in Scheme 2. Phthaloylation of
Phe, the –t-Bu removal from intermediates 3a–f, and the crucial
cyclization of oxazolidinones 5a–f to benzazepinones 6a–f have
been performed by microwave flash heating. In recent years, the
application of microwave-assisted reactions in organic synthesis
has received considerable attention. Microwave irradiation com-
monly increases the reaction rates above those obtained by con-
ventional heating and leads to the production of fewer by-products,
making necessary minimum efforts for the purification of the final
products. In contrast to conventional heating performed in blocks
or ovens, where heat must initially be transferred to the medium
via metal parts, air, or vessels, microwave irradiation directly heats
the medium, resulting in significant time savings in several fields.

208
B. Severino et al. / Tetrahedron 65 (2009) 206–211
Table 2
used for reactions were dried over 3 Å molecular sieves. All solvents
were filtered and degassed prior the use. Reagent grade materials
were purchased from Bachem (Bubendorf, Switzerland) and from
Aldrich (Milan, Italy) and were used without further purification.
All reactions were followed both by TLC, carried out on precoated
silica gel Kieselgel 60F254 (Merck, A.G., Darmstadt, Germany)
plates with fluorescent indicator, and by RP-HPLC using a Beckman
Comparison of TiCl4 and SnCl4 in the synthesis of 4-amino-2-benzazepin-3-ones 6a–
f from oxazolidinones 5a–f
Reaction conditions
Products formed (% yield)
6a
6b
6c
6d
6e
6f
Method Aa
Method Ba
Method Ca
Method Da
82
58
88
70
68
64
53
42
62b
78
89
56
86
69
10b
61b
6b
85
56
89
46
16b
48
C18 column (5
mm, 4.6ꢁ250 mm) employing the following condi-
58b
tions: A, 0.05% TFA (v/v) in water; B, 0.05% TFA (v/v) in acetonitrile;
UV detection at 220 nm, flow rate 1 mL/min. Preparative chro-
matographic purifications were performed using a Kieselgel 60
silica gel column and, when necessary, RP-HPLC purifications were
performed on a Waters Delta Prep 4000 system equipped with
a Waters 484 multi-wavelength detector on a Vydac C18 column
a
Method A: TiCl4 (1 M in CH2Cl2), 80 ꢀC, 600 W, 1 h; method B: SnCl4 (1 M in
CH2Cl2), 80 ꢀC, 600 W, 1 h; method C: TiCl4 (1 M in CH2Cl2), 50 ꢀC, 600 W, 1 h;
method D: SnCl4 (1 M in CH2Cl2), 50 ꢀC, 600 W, 1 h.
b
The reaction was run for 2 h.
(15–20
m
m, 22ꢁ5000 mm). The operational flow rate was 30 mL/
The same reaction, performed with SnCl4 (10 equiv of 1 M solution
in anhydrous DCM), irradiating at 600 W for 1 h (temperature 80 ꢀC
in method B and 50 ꢀC in method D), afforded yields lower than
TiCl4, except for compounds 6c and 6e. In these cases, the Lewis
acids show specific reactivity; in fact, the preparation of 6c using
SnCl4 furnished yields higher than those obtained with TiCl4 (78%
vs 62% at 80 ꢀC and 48% vs 16% at 50 ꢀC) while compound 6e was
prepared with satisfactory yields (61% at 80 ꢀC and 58% at 50 ꢀC)
only when SnCl4 was employed as catalyst. The efficiency of the
here-described microwave-assisted synthesis was also confirmed
by examining the extent of racemization occurring during the final
Friedel–Crafts intramolecular cyclization. An additional di-
astereomer was observed in the synthesis of compound 6b, per-
formed employing both SnCl4 and TiCl4, while compounds 6c and
6e furnished the diastereomeric peak by only carrying out the re-
action via SnCl4. It was detected by 1H NMR spectroscopy and its
amount was quantified by HPLC. In all the observed cases, the
measured epimerization was less than 5%, placing the microwave-
assisted approach as an advantageous alternative to the previously
described synthetic procedure.
min. Capacity factor (K0) and homogeneity of the final compounds
were assessed by analytical RP-HPLC using both Vydac C18 column
(5
mm, 4.6ꢁ250 mm) and Beckman C18 column (5
mm,
4.6ꢁ250 mm) employing the following conditions: eluent A, 0.05%
TFA (v/v) in water; eluent B, 0.05% TFA (v/v) in acetonitrile; gradient
20–50% B over 25 min on the Vydac C18 column (system 1) and 30–
60% B over 25 min on the Beckman C18 column (system 2), UV
detection at 254 nm, flow rate 1 mL/min. The column was con-
nected to a Rheodyne model 7725 injector, a Waters 600 HPLC
system, a Waters 486 tunable absorbance detector, and a Waters
746 chart recorder. The melting points, measured on a Buchi B-540
instrument, represent values obtained on recrystallized or chro-
matographically purified material. Elemental analyses were carried
out on a Carlo Erba model 1106. Specific rotations were measured
on a Perkin–Elmer 243 B polarimeter. High resolution ESI-MS
spectra were performed with a Micromass QTOF Micromass spec-
trometer. ESI-MS experiments were performed on an Applied
Biosystem API 2000 triple-quadrupole mass spectrometer. IR
spectra were recorded on Thermo Nicolet 5700 FT-IR spectrometer.
The 1H and 13C NMR spectra were recorded on a Varian Mercury
Plus 400 MHz instrument. Chemical shifts are reported in parts per
million using Me4Si as an internal standard.
3. Conclusion
We have described the synthesis of dipeptide mimetic ana-
logues 6a–f as useful scaffolds to be introduced in different bi-
ologically active peptide sequences. Some synthetic steps have
been accomplished by microwave flash heating in order to explore
the application of this methodology to this kind of reaction. Even
in this case, microwave irradiation has shown its potential as
technological tool able to improve on the reaction conditions
allowing the synthesis of the final compounds 6a–f with high se-
lectivity and less than 5% racemization, occurring during the final
synthetic step, as judged by HPLC profile and 1H NMR spectros-
copy. The previously reported synthesis of compound 6a,1,3 al-
though characterized with high yields, required several hours (12–
16 h). Using our methods, we obtained very good yields (82% at
80 ꢀC and 88% at 50 ꢀC) in just 1 h, and good results were obtained
in the preparation of the other derivatives. TiCl4 was the best
catalyst in most of the cases except for compounds 6c and 6e that
were prepared in acceptable yields only when SnCl4 was used.
Further development regarding the synthesis of several sub-
stituted benzazepinone derivatives is currently in progress in our
laboratory for potential applications in medicinal chemistry and
peptide mimetics synthesis.
4.2. Microwave equipment and conditions
The synthetic steps performed by microwave irradiation were
carried out using a microwave oven (ETHOS 1600 Milestone) es-
pecially designed for organic synthesis. Microwave reactions were
performed in sealed tubes and a microwave program that was
composed by appropriate ramping and holding steps was selected.
The temperature of the stirred reaction mixture was monitored
directly by a microwave-transparent fluoroptic probe inserted into
the reaction mixture; irradiation time and power were monitored
with the ‘easyWAVE’ software package.
4.3. Na-Phthaloyl-
L-phenylalanine (2)
A solution of -phenylalanine 1 (2.5 g, 0.015 mol), phthalic an-
L
hydride (2.2 g, 0.015 mol), and triethylamine (3.6 mL, 0.026 mol) in
30 mL of anhydrous toluene over molecular sieves (3 Å) was placed
in a closed reaction vessel, equipped with temperature control unit,
and irradiated according to the following parameters: initial power,
500 W; initial time, 1 min (ramping); final power, 500 W; T, 120 ꢀC;
reaction time, 1 h. The reaction mixture was filtered and the
obtained solution was extracted with 1 N HCl (3ꢁ40 mL). The or-
ganic phase was dried over anhydrous Na2SO4, filtered, and con-
centrated in vacuo to give a residue, which was crystallized with
4. Experimental
4.1. General
ethyl acetate/n-hexane furnishing the desired product 2 as a white
20
solid (3.8 g, 85%); mp 183–185 ꢀC (lit.13 mp 183–185 ꢀC); [
a]
ꢂ164
D
20
All solvents were purchased from Carlo Erba (Rodano, Milan,
Italy). Extraction solvents were dried over sodium sulfate. Solvents
(c 1.1, EtOH) (lit.13
[
a]
ꢂ212); nmax (KBr) 3300, 1780, 1760, 1690,
D
1400 cmꢂ1; NMR (400 MHz, CDCl3)
d
3.56 (d, 2H, J¼8.2 Hz), 5.11

B. Severino et al. / Tetrahedron 65 (2009) 206–211
209
(dd, 1H, J¼10, 6.8 Hz), 7.10–7.15 (m, 5H), 7.63–7.66 (m, 2H), 7.73–
7.75 (m, 2H); 13C NMR (CDCl3)
34.8, 53.6, 123.6, 126.9, 128.7, 129.0,
131.9, 134.2, 137.4, 167.8, 171.3. Anal. Calcd (%) for C17H13NO4: C,
69.15; H, 4.44; N, 4.74. Found C, 69.27; H, 4.43; N, 4.75. ESI-MS:
calcd 295.3; found [MþH]þ 296, [MþNa]þ 318. HRMS (ESI): calcd
295.0845; found 295.0846.
J¼14.0 Hz), 3.48 (dd, 1H, J¼14.0, 4.4 Hz), 4.17 (t, 1H, J¼7.4 Hz), 4.99
(dd, 1H, J¼11.6, 4.4 Hz), 7.04–7.14 (m, 5H), 7.76–7.78 (m, 4H), 8.40
d
(d, 1H, J¼8.8 Hz); 13C NMR (400 MHz, DMSO-d6)
d 19.1, 19.9, 30.3,
34.8, 54.9, 58.6, 123.7, 127.1, 128.9, 129.4, 132.0, 135.1, 138.3, 168.2,
168.6, 173.6. Anal. Calcd (%) for C22H22N2O5: C, 66.99; H, 5.62; N,
7.10. Found: C, 66.87; H, 5.61; N, 7.09. ESI-MS: calcd 394.4; found
[MþH]þ 395.4, [MþNa]þ 417.4, [MþK]þ 433.4. HRMS (ESI): calcd
394.1529; found 394.1532.
4.4. General procedure for the synthesis of intermediates 3a–f
Pht-Phe-OH (2, 1.0 mmol), DCC (1.1 mmol), and HOBt (1.1 mmol)
were dissolved in DMF (20 mL) and the solution was ice cooled.
After 30 min the opportune H-aa-O-t-Bu/HCl and TEA (1.0 mmol)
were added and the reaction mixture was stirred for 4 h at room
temperature. After DCU removal, the filtrate was dried in vacuo and
the obtained residue was dissolved in ethyl acetate (20 mL). The
organic layer was extracted with 5% NaHCO3 (3ꢁ10 mL), 10% citric
acid (3ꢁ10 mL), and brine, then dried over Na2SO4, filtered, and
evaporated under reduced pressure. The desired products 3a–f
were obtained by crystallization with diethyl ether/n-hexane with
yield ranging between 87% and 95%.
4.5.4. Pht-Phe-Leu-OH (4d)
Yield 419 mg, 95%; white solid, mp 163–165 ꢀC; Rf 0.33 (DCM/
20
MeOH 9/1); [
a
]
ꢂ73.3 (c 1.1, MeOH); nmax (KBr) 1702, 1663, 1514,
1382, 716 cmꢂ1D
;
1H NMR (400 MHz, DMSO-d6)
d
0.80 (d, 6H,
J¼5.2 Hz), 1.43–1.52 (m, 3H), 3.24 (t, 1H, J¼13.6 Hz), 3.47 (dd, 1H,
J¼14.0, 4.4 Hz), 4.29–4.31 (m, 1H), 4.92 (dd, 1H, J¼11.6, 4.4 Hz),
7.05–7.13 (m, 5H), 7.77–7.84 (m, 4H), 8.41 (d, 1H, J¼8.0 Hz); 13C
NMR (400 MHz, DMSO-d6)
d 21.8, 23.6, 24.7, 34.7, 51.2, 54.8, 123.7,
127.1, 128.9, 129.4, 132.0, 135.1, 138.3, 168.1, 168.5, 174.7. Anal. Calcd
(%) for C23H24N2O5: C, 67.63; H, 5.92; N, 6.86. Found: C, 67.71; H,
5.91; N, 6.86. ESI-MS: calcd 408.4; found [MþH]þ 409.4, [MþNa]þ
431.4, [MþK]þ 447.4. HRMS (ESI): calcd 408.1685; found 408.1672.
4.5. Procedure for the microwave-assisted synthesis of
intermediates 4a–f
4.5.5. Pht-Phe-Ile-OH (4e)
Yield 397 mg, 90%; white solid, mp 151–153 ꢀC; Rf 0.32 (DCM/
20
4.5.1. Pht-Phe-Gly-OH (4a)
MeOH 9/1); [
a]
ꢂ86.0 (c 1.0, MeOH); nmax (KBr) 1698, 1651, 1517,
D
Na-Phthaloyl-phenylalanylglycine tert-butyl ester 3a (500 mg,
1.2 mmol) was dissolved in 10 mL of 40% TFA in CH2Cl2, and the
mixture was placed in a closed reaction vessel, equipped with
temperature and pressure control units, and irradiated according to
the following parameters: initial power, 500 W; initial time, 1 min
(ramping); final power, 500 W; T, 60 ꢀC; reaction time,10 min. After
evaporation of the solvent, the desired product 4a was obtained by
precipitation with diethyl ether as a white solid. Yield 385 mg, 91%;
mp 177–179 ꢀC (lit.7 mp 184–185 ꢀC); Rf 0.13 (DCM/MeOH 9/1);
1376, 1359, 1100 cmꢂ1; 1H NMR (400 MHz, DMSO-d6)
d 0.75 (t, 3H,
J¼7.4 Hz), 0.82 (d, 3H, J¼6.8 Hz), 1.08–1.12 (m, 2H), 1.32–1.34 (m,
1H), 3.30 (t, 1H, J¼12.8 Hz), 3.47 (dd, 1H, J¼14.0, 4.4 Hz), 4.22 (t, 1H,
J¼7.6 Hz), 4.99 (dd, 1H, J¼11.6, 4.4 Hz), 7.06–7.12 (m, 5H), 7.73–7.75
(m, 4H), 8.39 (d, 1H, J¼8.4 Hz); 13C NMR (400 MHz, DMSO-d6)
d
11.6, 16.2, 25.3, 34.8, 36.6, 54.9, 57.4, 123.7, 127.1, 128.9, 129.4,
132.0, 135.1, 138.3, 168.2, 168.6, 173.7. Anal. Calcd (%) for
C23H24N2O5: C, 67.63; H, 5.92; N, 6.86. Found: C, 67.52; H, 5.92; N,
6.87. ESI-MS: calcd 408.4; found [MþH]þ 409.4, [MþNa]þ 431.4,
[MþK]þ 447.4. HRMS (ESI): calcd 408.1685; found 408.1671.
20
20
[
a]
ꢂ155 (c 0.8, EtOH) (lit.7
[
a
]
ꢂ153.7); nmax (KBr) 1730, 1705,
D
D
1665, 1630, 1600, 750 cmꢂ1 1H NMR (400 MHz, DMSO-d6)
; d 3.31
(d, 1H, J¼14.0 Hz), 3.49 (dd, 1H, J¼14.0, 4.4 Hz), 3.61 (dd, 1H, J¼17.2,
5.6 Hz), 3.77 (dd, 1H, J¼17.2, 6.4 Hz), 4.96 (dd, 1H, J¼12.0, 4.4 Hz),
7.05–7.14 (m, 5H), 7.76–7.78 (m, 4H), 8.59 (d, 1H, J¼8.4 Hz); 13C
4.5.6. Pht-Phe-Phe-OH (4f)
Yield 420 mg, 95%; white solid, mp 166–168 ꢀC; Rf 0.30 (DCM/
20
MeOH 9/1); [
a
]
ꢂ77.4 (c 1.2, MeOH); nmax (KBr) 1706, 1511, 1374,
D
NMR (400 MHz, DMSO-d6)
d
34.4, 42.3, 54.9, 123.8, 127.2, 128.9,
1179, 1093, 861, 691 cmꢂ1 1H NMR (400 MHz, DMSO-d6)
; d 2.82–
129.3, 131.9, 135.2, 138.2, 168.0, 168.9, 174.6. Anal. Calcd (%) for
C19H16N2O5: C, 64.77; H, 4.58; N, 7.95. Found: C, 64.83; H, 4.42; N,
7.96. ESI-MS: calcd 352.3; found [MþH]þ 353.3, [MþNa]þ 375.2,
[MþK]þ 391.2. HRMS (ESI): calcd 352.1059; found 352.1062.
Using the procedure described above for the preparation of 4a,
the following additional intermediates were synthesized using as
starting material compounds 3b–f.
2.88 (m, 1H), 2.97 (dd, 1H, J¼13.8, 5.0 Hz), 3.24 (t, 1H, J¼13.6 Hz),
3.41 (dd, 1H, J¼14.2, 4.4 Hz), 4.39–4.42 (m, 1H), 4.91 (dd, 1H, J¼11.6,
4.4 Hz), 7.03–7.20 (m, 10H), 7.76–7.78 (m, 4H), 8.53 (d, 1H,
J¼8.0 Hz); 13C NMR (400 MHz, DMSO-d6)
d 34.5, 36.9, 54.8, 56.6,
123.7, 126.9, 127.1, 128.8, 128.9, 129.4, 129.7,131.9,135.1,138.2, 138.3,
167.9, 168.5, 173.5. Anal. Calcd (%) for C26H22N2O5: C, 70.58; H, 5.01;
N, 6.33. Found: C, 70.49; H, 5.02; N, 6.33. ESI-MS: calcd 442.5, found
[MþH]þ 443.4, [MþNa]þ 465.4, [MþK]þ 481.4. HRMS (ESI): calcd
442.1529; found 442.1536.
4.5.2. Pht-Phe-Ala-OH (4b)
Yield 386 mg, 89%; white solid, mp 197–199 ꢀC; Rf 0.19 (DCM/
20
MeOH 9/1); [
a]
ꢂ130.1 (c 1.1, MeOH); nmax (KBr) 1706, 1648, 1524,
4.6. Procedure for the synthesis of intermediates 5a–f
D
1449, 1382, 712 cmꢂ1; 1H NMR (400 MHz, DMSO-d6)
d 1.35 (d, 3H,
J¼7.2 Hz), 3.48 (t, 2H, J¼5.2 Hz), 4.48–4.51 (m, 1H), 5.04 (dd, 1H,
4.6.1. (S)-2-(1-Oxo-1-(5-oxooxazolidin-3-yl)-3-phenylpropan-2-yl)
isoindoline-1,3-dione (5a)
J¼10.0, 6.4 Hz), 7.05–7.10 (m, 5H), 7.62–7.63 (m, 2H), 7.64–7.71 (m,
2H), 8.50 (d, 1H, J¼8.4 Hz); 13C NMR (400 MHz, DMSO-d6)
d
18.4,
A solution of Pht-Phe-Gly-OH 4a (2.5 g, 7.1 mmol), para-
34.9, 48.7, 55.6, 123.6, 127.0, 128.7, 129.1, 131.7, 134.3, 137.1, 168.0,
168.4, 174.7. Anal. Calcd (%) for C20H18N2O5: C, 65.57; H, 4.95; N,
7.65. Found: C, 65.43; H, 4.95; N, 7.65. ESI-MS: calcd 366.4; found
[MþH]þ 367.3, [MþNa]þ 389.3, [MþK]þ 405.3. HRMS (ESI): calcd
366.1216; found 366.1218.
formaldehyde (4.3 g, 20 equiv), and p-toluenesulfonic acid (270 mg,
0.2 equiv) in anhydrous toluene (75 mL) was refluxed in a Dean–
Stark apparatus for 4 h and then the solvent was evaporated. The
residue was dissolved in EtOAc (100 mL), washed with NaHCO3
saturated (2ꢁ30 mL), saturated NaCl (1ꢁ30 mL), and then the col-
lected organic phase was dried over Na2SO4. After evaporation, the
solid was crystallized as a white solid in toluene. Yield 2.1 g, 80%, mp
4.5.3. Pht-Phe-Val-OH (4c)
Yield 403 mg, 92%; white solid, mp 127–133 ꢀC; Rf 0.36 (DCM/
165–167 ꢀC (lit.4 mp 176–178 ꢀC); Rf 0.19 (n-hexane/EtOAc, 7/3);
20
20
MeOH 9/1); [
a
]
ꢂ44.3 (c 1.0, MeOH); nmax (KBr) 1707, 1666, 1534,
[a
]
ꢂ139.2 (c 1.0, CHCl3); nmax (KBr) 1805, 1706, 1664, 1374, 1238,
1378, 718 cmꢂ1D
;
1H NMR (400 MHz, DMSO-d6)
d
0.78 (d, 3H,
1167, 716 cmꢂ1
;
1H NMR (400 MHz, CDCl3)
d
3.44 (dd, 1H, J¼14.2,
D
J¼6.4 Hz), 0.84 (d, 3H, J¼6.8 Hz), 1.97–2.01 (m, 1H), 3.29 (t, 1H,
8.8 Hz), 3.63 (dd, 1H, J¼14.0, 6.8 Hz), 3.80–4.05 (m, 2H), 4.97 (t, 1H,

210
B. Severino et al. / Tetrahedron 65 (2009) 206–211
J¼8.0 Hz), 5.40–5.58 (m, 2H), 7.17–7.26 (m, 5H), 7.73–7.75 (m, 2H),
7.80–7.82 (m, 2H); 13C NMR (400 MHz, CDCl3)
34.9, 51.7, 53.6, 79.8,
4.6.6. 2-((S)-1-((S)-4-Benzyl-5-oxooxazolidin-3-yl)-1-oxo-3-
d
phenylpropan-2-yl)isoindoline-1,3-dione (5f)
124.0,127.6, 129.0, 129.4, 131.3,134.9, 136.3, 167.5, 168.9, 170.3. Anal.
Calcd (%) for C20H16N2O5: C, 65.93; H, 4.43; N, 7.69. Found: C, 65.82;
H, 4.42; N, 7.70. ESI-MS: calcd 364.3; found [MþH]þ 365.0, [MþNa]þ
387.1, [MþK]þ 403.0. HRMS (ESI): calcd 364.1059; found 364.1061.
Using the procedure described above for the preparation of 5a,
the following additional intermediates were synthesized using as
starting material compounds 4b–f.
Yield 1.4 g, 56%, white powder, mp 124–126 ꢀC; Rf 0.42 (n-
20
hexane/EtOAc, 7/3); [
a
]
þ7.7 (c 1.0, CHCl3); nmax (KBr) 1802,
1H NMR (400 MHz, CDCl3)
3.34–3.37 (m, 2H), 3.48–3.53 (m, 1H), 3.60–3.64 (m, 2H), 4.86–
D
1710, 1680, 1377, 1238, 1181 cmꢂ1
;
d
4.89 (m, 1H), 5.00 (t, 1H, J¼7.6 Hz), 5.10–5.15 (m, 1H), 7.00–7.26
(m, 10H), 7.78–7.81 (m, 2H), 7.83–7.84 (m, 2H); 13C NMR
(400 MHz, CDCl3)
d 34.6, 35.4, 53.0, 56.6, 78.2, 124.0, 127.4, 127.5,
128.7, 128.9, 129.4, 130.1, 131.1, 134.7, 134.9, 136.3, 167.6, 169.3,
171.6. Anal. Calcd (%) for C27H22N2O5: C, 71.35; H, 4.88; N, 6.16.
Found: C, 71.41; H, 4.89; N, 6.16. ESI-MS: calcd 454.5; found
[MþH]þ 455.3, [MþNa]þ 477.2, [MþK]þ 493.3. HRMS (ESI): calcd
454.1529; found 454.1512.
4.6.2. 2-((S)-1-((S)-4-Methyl-5-oxooxazolidin-3-yl)-1-oxo-3-
phenylpropan-2-yl)isoindoline-1,3-dione (5b)
Yield 1.7 g, 65%, white powder, mp 180–182 ꢀC; Rf 0.31 (n-hex-
20
ane/EtOAc, 7/3); [
a
]
ꢂ137.9 (c 1.1, CHCl3); nmax (KBr) 1792, 1708,
D
1678, 1379, 1238, 952 cmꢂ1 1H NMR (400 MHz, CDCl3)
; d 1.45 (m,
3H), 3.30–3.45 (m, 1H), 3.58–3.70 (m, 1H), 4.52–4.61 (m, 1H), 4.98
(dd, 1H, J¼8.6, 6.6 Hz), 5.05–5.10 (m, 1H), 5.20–5.29 (m, 1H), 7.16–
7.17 (m, 5H), 7.74–7.80 (m, 2H), 7.81–7.82 (m, 2H); 13C NMR
4.7. Procedure for the synthesis of final compounds 6a–f
4.7.1. Reaction with TiCl4 (methods A and C)
(400 MHz, CDCl3)
d
15.7, 35.2, 50.6, 53.5, 78.9, 124.1, 127.4, 128.9,
The appropriate oxazolidinone (5a–f, 100 mg) was dissolved
in 10 mL of anhydrous CH2Cl2; subsequent addition of 1 M TiCl4
in CH2Cl2 (10 equiv) gave a fluorescent yellow solution that
was placed in a closed reaction vessel, equipped with tem-
perature and pressure control units, and irradiated according
to the following parameters: initial power, 600 W; initial time,
3 min (ramping); final power, 600 W; T, 80 ꢀC (method A)
or 50 ꢀC (method C); reaction time, 1 h. The mixture was
cooled down to 0 ꢀC in an ice bath and quenched with
0.5 N HClaq. The two phases were separated and the water
phase was extracted twice with CH2Cl2 (10 mL). By addition of
acetone (10 mL) to the combined organic phases, a clear solu-
tion was obtained, which was dried over MgSO4, filtered, and
evaporated.
129.5, 131.2, 134.9, 136.4, 167.9, 170.0, 172.4. Anal. Calcd (%) for
C21H18N2O5: C, 66.66; H, 4.79; N, 7.40. Found: C, 66.53; H, 4.80; N,
7.40. ESI-MS: calcd 378.4; found [MþH]þ 379.3, [MþNa]þ 401.3,
[MþK]þ 417.1. HRMS (ESI): calcd 378.1216; found 378.1223.
4.6.3. 2-((S)-1-((S)-4-Isopropyl-5-oxooxazolidin-3-yl)-1-oxo-3-
phenylpropan-2-yl)isoindoline-1,3-dione (5c)
Yield 0.82 g, 32%, white powder, mp 192–194 ꢀC; Rf 0.40 (n-
20
hexane/EtOAc, 7/3); [
a]
ꢂ121 (c 1.1, MeOH); nmax (KBr) 1796, 1714,
D
1681, 1508, 1380, 1243,1104 cmꢂ1; 1H NMR (400 MHz, CDCl3)
d 0.80
(dd, 6H, J¼6.8 Hz), 2.08–2.12 (m, 1H), 3.33–3.41 (m, 1H), 3.59–3.61
(m, 1H), 4.58–4.63 (m, 1H), 4.95–4.99 (m, 1H), 5.01 (t, 1H, J¼6.8 Hz),
5.15–5.20 (m, 1H), 7.16–7.28 (m, 5H), 7.76–7.81 (m, 4H); 13C NMR
(400 MHz, CDCl3)
d 19.2, 31.6, 35.3, 53.4, 56.2, 59.6, 124.1, 127.2,
128.7,129.5, 131.3, 134.5, 136.4, 168.2,168.6,171.7. Anal. Calcd (%) for
C23H22N2O5: C, 67.97; H, 5.46; N, 6.89. Found: C, 68.01; H, 5.45; N,
6.90. ESI-MS: calcd 406.4; found [MþH]þ 407.3, [MþNa]þ 429.1,
[MþK]þ 445.1. HRMS (ESI): calcd 406.1529; found 406.1531.
4.7.2. Reaction with SnCl4 (methods B and D)
The appropriate oxazolidinones (5a–f, 100 mg) were dis-
solved in 10 mL of anhydrous CH2Cl2; after addition of 1 M SnCl4
in CH2Cl2 (10 equiv), the solution was placed in a closed re-
action vessel, equipped with temperature and pressure control
units, and irradiated according to the following parameters:
initial power, 600 W; initial time, 3 min (ramping); final power,
600 W; T, 80 ꢀC (method B) or 50 ꢀC (method D); reaction time,
1 h. The mixture was cooled down to 0 ꢀC in an ice bath and
quenched with 0.5 N HClaq. The two phases were separated and
the water phase was extracted twice with CH2Cl2 (10 mL). The
combined organic phases were dried over MgSO4, filtered, and
evaporated.
4.6.4. 2-((S)-1-((S)-4-Isobutyl-5-oxooxazolidin-3-yl)-1-oxo-3-
phenylpropan-2-yl)isoindoline-1,3-dione (5d)
Yield 1.9 g, 76%, white powder, mp 129–131 ꢀC; Rf 0.46 (n-hex-
20
ane/EtOAc, 7/3); [
a
]
ꢂ126.4 (c 0.9, CHCl3); nmax (KBr) 1800, 1711,
D
1676,1380,1194, 705 cmꢂ1; 1H NMR (400 MHz, CDCl3)
d 0.90 (d, 6H,
J¼6.8 Hz), 1.66 (t, 2H, J¼6.6 Hz), 1.81 (m, 1H), 3.36 (t, 2H, J¼11.6 Hz),
3.58–3.63 (m, 1H), 4.60–4.68 (m, 1H), 5.01 (t, 1H, J¼6.0 Hz), 5.25–
5.30 (m, 1H), 7.15–7.31 (m, 5H), 7.73–7.82 (m, 4H); 13C NMR
(400 MHz, CDCl3)
d
22.5, 22.8, 24.5, 35.2, 39.3, 53.0, 53.3, 124.1,
The desired final products 6a–f were recovered by crystalliza-
tion from diethyl ether/n-hexane or, when necessary, by purifica-
tion on silica gel column employing CH2Cl2/EtOH/CH3COOH (9.5/
0.5/0.25) as eluent. The yields obtained for compounds 6a–f
employing methods A–D are reported in Table 2.
127.4, 128.9, 129.5, 131.1, 134.9, 136.4, 166.5, 167.0, 172.1. Anal. Calcd
(%) for C24H24N2O5: C, 68.56; H, 5.75; N, 6.66. Found: C, 68.51; H,
5.75; N, 6.67. ESI-MS: calcd 420.5; found [MþH]þ 421.3, [MþNa]þ
443.1, [MþK]þ 459.1. HRMS (ESI): calcd 420.1685; found 420.1696.
4.6.5. 2-((S)-1-((S)-4-sec-Butyl-5-oxooxazolidin-3-yl)-1-oxo-3-
4.7.3. (S)-2-(4-(1,3-Dioxoisoindolin-2-yl)-3-oxo-4,5-dihydro-1H-
phenylpropan-2-yl)isoindoline-1,3-dione (5e)
benzo[c]azepin-2(3H)-yl)acetic acid (6a)
Yield 0.64 g, 25%, white powder, mp 156–158 ꢀC; Rf 0.70 (n-
White solid; mp 203–205 ꢀC (lit.2 mp 197–200 ꢀC); Rf 0.10
20
20
hexane/EtOAc, 7/3); [
a]
ꢂ86.9 (c 0.9, CHCl3); nmax (KBr) 1801,1710,
(DCM/MeOH, 9/1); K0 (HPLC): 5.1 (system 1), 4.3 (system 2); [
a]
D
D
1674, 1375, 1048, 701 cmꢂ1; 1H NMR (400 MHz, CDCl3)
d
0.85 (t, 3H,
þ6.6 (c 1.1, MeOH); nmax (KBr) 2922, 1704, 1666, 1383, 1339, 1195,
1168 cmꢂ1 1H NMR (400 MHz, CDCl3)
J¼5.4 Hz), 0.91 (d, 3H, J¼7.2 Hz), 1.54–1.56 (m, 2H), 2.50–2.55 (m,
1H), 3.25–3.33 (m, 1H), 3.50–3.55 (m, 1H), 4.64 (m, 1H), 4.94–4.97
(m, 1H), 5.02 (t, 1H, J¼7.8 Hz), 5.25–5.29 (m, 1H), 7.13–7.31 (m, 5H),
;
d
3.08 (dd, 1H, J¼15.2,
4.4 Hz), 4.08 (t, 1H, J¼14.0 Hz), 4.22 (m, 2H), 4.63 (m, 2H), 5.32
(dd, 1H, J¼12.8, 4.4 Hz), 7.19–7.31 (m, 4H), 7.71–7.73 (m, 2H),
7.73–7.83 (m, 4H); 13C NMR (400 MHz, CDCl3)
d
12.0,15.1, 25.2, 35.3,
7.85–7.87 (m, 2H); 13C NMR (400 MHz, CDCl3)
d 34.5, 51.7, 52.2,
37.6, 53.4, 58.7, 78.3, 124.1, 127.4, 128.9, 129.3, 131.0, 135.0, 136.4,
166.9, 167.0, 172.2. Anal. Calcd (%) for C24H24N2O5: C, 68.56; H, 5.75;
N, 6.66. Found: C, 68.61; H, 5.74; N, 6.66. ESI-MS: calcd 420.5; found
[MþH]þ 421.3, [MþNa]þ 443.2, [MþK]þ 459.3. HRMS (ESI): calcd
420.1685; found 420.1695.
53.6, 123.9, 127.6, 128.7, 129.2, 130.1, 132.0, 134.6, 135.0, 135.9,
168.1, 170.4, 171.2. Anal. Calcd (%) for C20H16N2O5: C, 65.93; H,
4.43; N, 7.69. Found: C, 65.81; H, 4.42; N, 7.67. ESI-MS: calcd
364.4; found [MþH]þ 365.0. HRMS (ESI): calcd 364.1059; found
364.1065.

B. Severino et al. / Tetrahedron 65 (2009) 206–211
211
4.7.4. (S)-2-((S)-4-(1,3-Dioxoisoindolin-2-yl)-3-oxo-4,5-dihydro-
130.2, 132.1, 134.5, 134.6, 135.8, 168.1, 170.7, 172.4. Anal. Calcd (%) for
C24H24N2O5: C, 68.56; H, 5.75; N, 6.66. Found: C, 68.60; H, 5.76; N,
6.66. ESI-MS: calcd 420.5; found [MþH]þ 421.2. HRMS (ESI): calcd
420.1685; found 420.1678.
1H-benzo[c]azepin-2(3H)-yl)propanoic acid (6b)
White solid; mp 165–167 ꢀC; Rf 0.18 (DCM/MeOH, 9/1); K0
20
(HPLC): 5.6 (system 1), 4.6 (system 2); [
a]
ꢂ36.3 (c 0.9, MeOH);
D
nmax (KBr) 2913, 1706, 1637, 1391, 1201 cmꢂ1
CDCl3)
;
1H NMR (400 MHz,
d
1.51 (d, 3H, J¼6.8 Hz), 3.04 (dd, 1H, J¼14.8, 4.8 Hz), 3.93 (t,
4.7.8. (S)-2-((S)-4-(1,3-Dioxoisoindolin-2-yl)-3-oxo-4,5-dihydro-
1H, J¼13.6 Hz), 4.40 (d, 1H, J¼15.6 Hz), 4.81 (d, 1H, J¼16 Hz), 5.16
(dd, 1H, J¼12.8, 4.8 Hz), 5.29–5.31 (m, 1H), 7.23–7.31 (m, 4H), 7.72–
1H-benzo[c]azepin-2(3H)-yl)-3-phenylpropanoic acid (6f)
White solid; mp 180–182 ꢀC; Rf 0.24 (DCM/MeOH, 9/1); K0
20
7.73 (m, 2H), 7.86–7.88 (m, 2H); 13C NMR (400 MHz, CDCl3)
d
14.8,
(HPLC): 8.8 (system 1), 7.0 (system 2); [
a]
ꢂ161 (c 1.1, MeOH); nmax
D
34.9, 47.9, 52.8, 54.9, 123.9, 127.9, 128.2, 129.1, 129.9, 132.1, 134.5,
136.1, 136.6, 168.1, 169.4, 171.3. Anal. Calcd (%) for C21H18N2O5: C,
66.66; H, 4.79; N, 7.40. Found: C, 66.81; H, 4.78; N, 7.38. ESI-MS:
calcd 378.4; found [MþH]þ 379.3. HRMS (ESI): calcd 378.1216;
found 378.1227.
(KBr) 2932, 2855, 1709, 1650, 1425, 1384, 1200, 1172 cmꢂ1; 1H NMR
(400 MHz, CDCl3)
d
3.11 (dd, 1H, J¼15.2, 6.4 Hz), 3.21 (dd, 1H,
J¼15.2, 5.2 Hz), 3.52–3.58 (m, 2H), 4.49 (s, 2H), 5.20 (t, 1H,
J¼5.4 Hz), 5.43–5.44 (m, 1H), 6.81 (d, 1H, J¼7.6 Hz), 6.87–7.26 (m,
8H), 7.64–7.70 (m, 4H); 13C NMR (400 MHz, CDCl3)
d 30.3, 35.1, 45.7,
53.0, 53.4, 123.6, 125.7, 126.9, 127.2, 127.8, 128.5, 128.8, 129.4, 131.5,
132.1, 132.7, 134.4, 136.9, 167.5, 167.8, 172.3. Anal. Calcd (%) for
C27H22N2O5: C, 71.35; H, 4.88; N, 6.16. Found: C, 71.48; H, 4.88; N,
6.14. ESI-MS: calcd 454.5; found [MþH]þ 455.3. HRMS (ESI): calcd
454.1529; found 454.1538.
4.7.5. (S)-2-((S)-4-(1,3-Dioxoisoindolin-2-yl)-3-oxo-4,5-dihydro-
1H-benzo[c]azepin-2(3H)-yl)-3-methylbutanoic acid (6c)
White solid; mp 224–226 ꢀC; Rf 0.32 (DCM/MeOH, 9/1); K0
20
(HPLC): 7.7 (system 1), 6.8 (system 2);
[
a
]
D
ꢂ39.6 (c 1.0,
MeOH);nmax (KBr) 2935, 1703, 1656, 1391, 1184 cmꢂ1
(400 MHz, CDCl3)
;
1H NMR
d
0.75 (d, 3H, J¼6.8 Hz), 1.03 (d, 3H, J¼6.4 Hz),
Acknowledgements
2.39–2.50 (m, 1H), 3.15 (dd,1H, J¼15.8, 4.4 Hz), 4.11 (m, 2H), 4.58 (d,
1H, J¼14.8 Hz), 4.79 (d, 1H, J¼16.4 Hz), 5.44 (dd, 1H, J¼10.0, 4.4 Hz),
7.21–7.26 (m, 4H), 7.70–7.73 (m, 2H), 7.85–7.87 (m, 2H); 13C NMR
The authors are grateful to Maria Rubino from ‘Dipartimento di
Chimica Organica e Biochimica’ for recording IR spectra.
(400 MHz, CDCl3)
d 19.4, 31.8, 34.2, 51.1, 55.9, 58.6, 123.8, 127.9,
128.2, 128.9, 130.2, 132.3, 134.5, 136.2, 136.5, 168.2, 169.6, 171.4.
Anal. Calcd (%) for C23H22N2O5: C, 67.97; H, 5.46; N, 6.89. Found: C,
68.15; H, 5.45; N, 6.91. ESI-MS: calcd 406.4; found [MþH]þ 407.3.
HRMS (ESI): calcd 406.1529; found 406.1537.
References and notes
1. (a) Haack, M.; Beck-Sickinger, A. G. Top. Curr. Chem. 2007, 278, 243–288; (b)
Janecka, A.; Kruszynski, R. Curr. Med. Chem. 2005, 12, 471–481; (c) Reissmann,
S.; Imhof, D. Curr. Med. Chem. 2004, 11, 2823–2844.
2. Tourwe`, D.; Verschueren, K.; Frycia, A.; Davis, P.; Porreca, F.; Hruby, V. J.; Toth,
G.; Jaspers, H.; Verheyden, P.; Van Binst, G. Biopolymers 1996, 38, 1–12.
3. de Laszlo, S. E.; Bush, B. L.; Doyle, J. J.; Greenlee, W. J.; Hangauer, D. G.; Halgren,
T. A.; Lynch, R. J.; Schorn, T. W.; Siegl, P. K. S. J. Med. Chem. 1992, 35, 833–846.
4. Ballet, S.; Frycia, A.; Piron, J.; Chung, N. N.; Schiller, P. W.; Kosson, P.; Lipkowski,
A. W.; Tourwe`, D. J. Peptide Res. 2005, 66, 222–230.
5. Ballet, S.; Mayorov, A. V.; Cai, M.; Tymecka, D.; Chandler, K. B.; Palmer, E. S.; Van
Rompaey, K.; Misicka, A.; Tourwe`, D.; Hruby, V. J. Bioorg. Med. Chem. Lett. 2007,
17, 2492–2498.
6. Casimir, J. R.; Tourwe´, D.; Iterbeke, K.; Guichard, G.; Briand, J. P. J. Org. Chem.
2000, 65, 6487–6492.
7. Flynn, G. A.; Burkholder, T. P.; Huber, E. W.; Bey, P. Bioorg. Med. Chem. Lett. 1991,
1, 309–312.
4.7.6. (S)-2-((S)-4-(1,3-Dioxoisoindolin-2-yl)-3-oxo-4,5-dihydro-
1H-benzo[c]azepin-2(3H)-yl)-4-methylpentanoic acid (6d)
White solid; mp 232–234 ꢀC; Rf 0.33 (DCM/MeOH, 9/1); K0
20
(HPLC): 9.3 (system 1), 8.0 (system 2); [
a
]
ꢂ43.1 (c 1.0, MeOH);
D
nmax (KBr) 2958, 1709, 1645, 1389 cmꢂ1; 1H NMR (400 MHz, CDCl3)
d
0.80 (dd, 6H, J¼6.4 Hz), 1.26–1.33 (m, 1H), 1.84 (t, 2H, J¼7.2 Hz),
3.02 (dd, 1H, J¼15.2, 4.8 Hz), 3.96 (t, 1H, J¼14.0 Hz), 4.32 (d, 1H,
J¼15.6 Hz), 4.86 (d,1H, J¼15.2 Hz), 5.16 (dd,1H, J¼12.8, 4.8 Hz), 5.33
(t, 1H, J¼7.6 Hz), 7.19–7.33 (m, 4H), 7.71–7.73 (m, 2H), 7.86–7.89 (m,
2H); 13C NMR (400 MHz, CDCl3)
d
21.7, 23.2, 24.9, 34.9, 37.6, 48.0,
8. Speckamp, W. N.; Hiemstra, H. Tetrahedron 1985, 41, 4367–4416.
9. Maryanoff, B. E.; Zhang, H. C.; Cohen, J. H.; Turchi, I. J.; Maryanoff, C. Chem. Rev.
2004, 104, 1431–1628.
52.8, 57.5, 124.0, 127.8, 128.2, 129.2, 130.0, 132.0, 134.6, 136.1, 136.3,
168.1, 169.3, 171.2. Anal. Calcd (%) for C24H24N2O5: C, 68.56; H, 5.75;
N, 6.66. Found: C, 68.44; H, 5.76; N, 6.68. ESI-MS: calcd 420.5; found
[MþH]þ 421.2. HRMS (ESI): calcd 420.1685; found 420.1679.
10. (a) Loupy, A.; Petit, A.; Hamelin, J.; Texier-Boullet, F.; Jacquault, P.; Mathe, D.
Synthesis 1998, 1213–1234; (b) Larhed, M.; Hallberg, A. Drug Discov. Today 2001,
6, 406–416; (c) Kaval, N.; Dehaen, W.; Van der Eycken, E. J. Comb. Chem. 2005, 7,
90–95; (d) Kappe, C. O.; Dallinger, D. Nat. Rev. Drug Discov. 2006, 5, 51–63.
11. (a) Perissutti, E.; Frecentese, F.; Lavecchia, A.; Fiorino, F.; Severino, B.; De An-
gelis, F.; Santagada, V.; Caliendo, G. Tetrahedron 2007, 63, 12779–12785; (b)
Santagada, V.; Frecentese, F.; Perissutti, E.; Fiorino, F.; Severino, B.; Cirillo, D.;
Terracciano, S.; Caliendo, G. J. Comb. Chem. 2005, 7, 618–621; (c) Santagada, V.;
Frecentese, F.; Perissutti, E.; Cirillo, D.; Terracciano, S.; Caliendo, G. Bioorg. Med.
Chem. Lett. 2004, 14, 4491–4493; (d) Santagada, V.; Frecentese, F.; Fiorino, F.;
Cirillo, D.; Perissutti, E.; Severino, B.; Terracciano, S.; Caliendo, G. QSAR Comb.
Sci. 2004, 23, 899–901; (e) Severino, B.; Santagada, V.; Frecentese, F.; Perissutti,
E.; Terracciano, S.; Fiorino, F.; Cirillo, D.; Salvadori, S.; Balboni, G.; Caliendo, G.
Synthesis 2004, 18, 3011–3016.
4.7.7. (2S,3S)-2-((S)-4-(1,3-Dioxoisoindolin-2-yl)-3-oxo-4,5-
dihydro-1H-benzo[c]azepin-2(3H)-yl)-3-methylpentanoic acid (6e)
White solid; mp 220–222 ꢀC; Rf 0.34 (DCM/MeOH, 9/1); K0
20
(HPLC): 8.7 (system 1), 7.5 (system 2); [
a]
ꢂ32.2 (c 1.7, MeOH);
D
nmax (KBr) 2961, 1708, 1671, 1395 cmꢂ1
;
1H NMR (400 MHz, CDCl3)
d
0.78 (t, 3H, J¼7.2 Hz), 0.93 (d, 3H, J¼7.6 Hz), 1.25–1.32 (m, 2H),
2.22–2.24 (m, 1H), 3.13 (dd, 1H, J¼15.8, 4.4 Hz), 4.16 (t, 1H,
J¼14.6 Hz), 4.60 (d, 1H, J¼15.6 Hz), 4.73 (d, 1H, J¼10.8 Hz), 4.78 (d,
1H, J¼15.6 Hz), 5.43 (dd, 1H, J¼13.2, 4.4 Hz), 7.16–7.29 (m, 4H),
7.73–7.75 (m, 2H), 7.86–7.88 (m, 2H); 13C NMR (400 MHz, CDCl3)
12. (a) Fiorucci, S.; Antonelli, E.; Distrutti, E.; Severino, B.; Roviezzo, F.; Baldoni, M.;
Caliendo, G.; Santagada, V.; Morelli, A.; Cirino, G. Hepatology 2004, 39, 365–
375; (b) Severino, B.; Fiorino, F.; Perissutti, E.; Frecentese, F.; Cirino, G.; Ro-
viezzo, F.; Santagada, V.; Caliendo, G. Bioorg. Med. Chem. 2008, 16, 6009–6020.
13. Casimir, J. R.; Guichard, G.; Briand, J. P. J. Org. Chem. 2002, 67, 3764–3768.
d
10.7,15.8, 25.2, 33.3, 34.3, 49.4, 52.3, 62.1,123.9,127.3,128.9,129.0,