Development of a practical solid-phase synthesis approach to 1,3,5-triazepan-2,6-diones

Article abstract of DOI:10.1016/j.tet.2012.06.051

1,3,5-Triazepan-2,6-diones are a class of conformationally restricted heterocycles derived from dipeptides. With the aim to develop a general and practical method useful for library production, three polymer-assisted syntheses, all based on a catch and release approach, have been evaluated and compared. The method involving a Hofmann rearrangement of N-Boc dipeptide carboxamides and subsequent trapping of the isocyanate on polymer-supported N-hydroxysuccinimide (PS-HOSu) was found to be the most reliable and versatile, allowing rapid access to the 1,3,5-triazepan-2,6-dione skeleton.

Full text of DOI:10.1016/j.tet.2012.06.051

Tetrahedron 68 (2012) 7472e7478
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Development of a practical solid-phase synthesis approach to
1,3,5-triazepan-2,6-diones
,
b
,
,
,
Joel Boeglin a y, Claire Venin a y, Gilles Guichard a
*
a
ꢀ
ꢀ
Universite de Bordeaux‒CNRS UMR5248, Institut Europeen de Chimie et Biologie, CBMN, 2 rue Robert Escarpit, Pessac 33607, France
CNRS, Institut de Biologie Moleculaire et Cellulaire, Laboratoire d’Immunologie et Chimie Therapeutiques, Strasbourg 67000, France
b
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2012
Received in revised form 11 June 2012
Accepted 12 June 2012
Available online 19 June 2012
1,3,5-Triazepan-2,6-diones are
a class of conformationally restricted heterocycles derived from
dipeptides. With the aim to develop a general and practical method useful for library production, three
polymer-assisted syntheses, all based on a catch and release approach, have been evaluated and com-
pared. The method involving a Hofmann rearrangement of N-Boc dipeptide carboxamides and sub-
sequent trapping of the isocyanate on polymer-supported N-hydroxysuccinimide (PS-HOSu) was found
to be the most reliable and versatile, allowing rapid access to the 1,3,5-triazepan-2,6-dione skeleton.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Peptidomimetics
Heterocycles
Ureas
Peptides
Solid phase synthesis
1. Introduction
We have started to explore the potential applications of 1,3,5-
triazepan-2,6-diones in biology. Biological screening of a small pi-
Compounds with triazepine skeletons have attracted much at-
tention as a result of their interesting biological properties.1 Tri-
azepandiones represent a particular subset, of which a number of
functional isomers have been synthesized.2e4 In the search for
novel heterocyclic scaffolds derived from peptides, we became in-
terested by the 1,3,5-triazepan-2,6-dione skeleton 1 (Scheme 1)
whose main features are: (i) a short reaction sequence with N-
protected dipeptides as starting materials; (ii) a densely function-
alized core with up to 5 points of diversity; (iii) possibilities for
further functionalization at urea nitrogens by post-cyclization re-
actions (e.g., mono- and di-alkylation, mono-acylation); (iv) three
dominant ring conformations depending on the substitution
patterns.5e7 The 1,3,5-triazepan-2,6-dione system whose ring
structure shows overall similarity with that of 2,5-diketopiperazine
(DKP) may thus be seen as a rigid peptide mimetic scaffold. In
addition, the ring conformation of 4,5-fused 1,2,5-triazepane-3,6-
diones, originally developed as constrained cis-peptidyl prolina-
mide mimetics3 and 1,7-fused-1,3,5-triazepan-2,6-diones match
perfectly,5 thus suggesting that 1,3,5-triazepan-2,6-diones could
also be used as mimics of native cis-peptidyl conformations.
lot library led to the identification of compounds with some activity
against the malaria liver stage.5 Concurrently, a collection of 2150
druggable active sites from the Protein Data Bank was screened by
high-throughput docking to identify putative targets for a small set
of representative 1,3,5-triazepan-2,6-diones. This led to the dis-
covery of inhibitors of human group V (hgV) and human group X
(hGX) secreted phospholipases A2 (sPLA2).8
In previous work, 1,3,5-triazepan-2,6-diones have been pre-
pared in solution from N-Boc dipeptides in four steps. The dipeptide
was first converted to its acyl azide derivative. The isocyanate
generated in situ upon Curtius rearrangement of the corresponding
dipeptidyl azide was trapped by N-hydroxysuccinimide to give the
corresponding activated succinimidyl carbamate.5,9 Following Boc
removal, cyclization of the resulting salt was performed in the
presence of a tertiary amine base. Efficient ring formation requires
the cis-conformation around the amide bond to be populated (i.e.,
R3sH). Although this solution synthesis approach is robust, a faster
access to the 1,3,5-triazepan-2,6-dione skeleton is needed to gen-
erate librairies and fully address the potential of this scaffold in
biological applications. A general polymer-assisted solution-phase
synthesis approach involving cyclative cleavage and employing
polymer bound carbamate B as a key intermediate is outlined in
Scheme 1. Previously, route A, which parallels the solution phase
synthesis has been positively evaluated for two heterocycles
starting from purified N-Boc dipeptides (i.e., BocePheeSareOH and
* Corresponding author. Tel.: þ33 540 003 020; fax: þ33 540 002 215; e-mail
address: g.guichard@iecb.u-bordeaux.fr (G. Guichard).
y
These authors contributed equally to the work.
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.06.051

J. Boeglin et al. / Tetrahedron 68 (2012) 7472e7478
7473
Scheme 1. Polymer-assisted synthesis approaches to 1,3,5-triazepan-2,6-diones evaluated in this study.
BocePheeNMePheeOH synthesized in solution). However, to be
practically useful for the preparation of compounds in a library
format, the synthesis needs to work with crude dipeptide arrays
accessible by parallel solid-phase techniques.
To meet these specifications, we have now decided to re-
evaluate route A in comparison with two new alternative routes
involving either the Hofmann rearrangement of N-Boc dipeptide
uncontrolled evaporation of the solvent (THF) during the Curtius
rearrangement (T¼70 ꢀC). Substituting DMF for THF gave consis-
tently better results, triazepan-2,6-diones 1a and 1b being obtained
in good yield (>75%) and purity (>81% based on C18 RP-HPLC)
when starting from purified N-Bocedipeptides (Scheme 2). We
carboxamides (Scheme 1, route B) or trapping of N-protected
amino isocyanates derived from commercially available N-pro-
tected -amino acids (Scheme 1, route C).
a-
a
2. Results and discussion
All reaction sequences corresponding to routes AeC have been
evaluated using a compact multiparallel synthesizer MiniblockÔ
BodhanÒ from Mettler Toledo filled with 4 mL polypropylene re-
action tubes and have been optimized independently. In all cases,
the formation of the key polymer bound carbamate intermediates B
was monitored by FTIR spectroscopy.
Scheme 2. Reagents and conditions. (a) (i) DPPA (1.1 equiv), TEA (1.1 equiv), DMF, rt,
30 min; (ii) 70 ꢀC, 1.5 h; (iii) PS-SuOH 1.54 mmol/g (0.3 equiv), 70 ꢀC, 40 min;
(b) TFA/CH2Cl2 (1:3, v/v), 2ꢁ25 min, rt; (c) DIPEA (1.2 equiv), CH2Cl2, 40 ꢀC 1 h.
2.1. Synthesis via Curtius rearrangement of dipeptidylacyl
azides (route A)
We
selected
cyclo(PheegSareCO)
(1a)
and
cyclo(-
found however, that the timing to dispense the polystyrene
polymer-supported N-hydroxysuccinimide (PS-HOSu)11 in the re-
action mixture with respect to isocyanate formation was critical. If
the resin was added prior to Curtius rearrangement, a fraction of
dipeptidyl acyl azide was trapped by PS-HOSu to form the
PheegNMePheeCO) (1b)10 as reference 1,3,5-triazepan-2,6-diones
to evaluate this route. The experimental procedure previously set
up5 to access 1a and 1b by route A needed first to be slightly refined
because of the limited airtightness of the system, resulting in

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J. Boeglin et al. / Tetrahedron 68 (2012) 7472e7478
corresponding polymer-bound succinimidyl ester and sub-
sequently the corresponding six-membered 2,5-diketopiperazine
as a cyclocleavage byproduct (See Supplementary data for de-
tails). The resin was thus introduced after 1.5 h to ensure comple-
tion of the Curtius rearrangement.
The next step was to validate the method starting from N-Boc-
protected dipeptides directly prepared by conventional solid-phase
peptide synthesis (SPPS) on the MiniBlockÔ. 2-Chlorotrityl chloride
resin (CleTrt resin) was selected for SPPS as it allows the release of
N-Boc protected dipeptides under very mild conditions (hexa-
fluoroisopropanol (HFIP) in dry CH2Cl2) (Scheme 3). The purity of
crude dipeptides was found to be over 90% based on C18 RP-HPLC.
preparation of polymer-supported carbamates starting from
dipeptidyl carboxamides.
N-Boc protected dipeptides were readily assembled by solid-
phase techniques starting from the acid labile Sieber amide resin
(i.e., amino-xanthen-3-yloxy-Merrifield resin).16 Cleavage from the
resin was performed by exposure to a 2% TFA solution in CH2Cl2.
After neutralization of TFA with pyridine and concentration on
a centrifugal evaporator, the crude peptide mixtures were used
directly in the next step by assuming quantitative peptide forma-
tion. The protected peptide carboxamides were then dissolved in
THF and treated with PIFA (0.8 equiv) and pyridine (1.6 equiv)
under anhydrous conditions for
(0.2 equiv) was added (Scheme 4).
1 h, after which PS-HOSu
Scheme 3. Reagents and conditions. (a) N-Fmoc-N(alkyl)-a-amino acid (2 equiv),
DIPEA (6 equiv), CH2Cl2, rt, 4 h; (b) (i) 25% piperidine in DMF, 2ꢁ20 min; (ii) N-Boc-
a-
amino acid (5 equiv), BOP (5 equiv), HOBt (5 equiv), DIEA (15 equiv), DMF, 2ꢁ30 min;
(c) HFIP/CH2Cl2 (3:2, v/v), 2 h.
However, direct transformation of the crude peptides produced
by SPPS into corresponding polymer bound succinimidyl carba-
mates 3 by treatment with diphenylphosphoryl azide (DPPA) and
subsequent heating (Scheme 2) proved to be unreliable. FTIR
spectra of the resin were indeed lacking the characteristic stretch at
Scheme 4. Reagents and conditions. (a) N-FmoceXbbeOH (2 equiv), DIC (2 equiv),
HOBt (2 equiv), rt, 30 min; (b) 20% piperidine in DMF, rt, 2ꢁ30 min; (c)
N-BoceXaaeOH (2 equiv), DIC (2 equiv), HOBt (2 equiv), rt, 30 min; (d) TFA/CH2Cl2
(2:98, v/v), rt, 6ꢁ10 min (e) (i) PIFA (0.8 equiv), pyridine (1.6 equiv), THF, rt, 1 h; (ii)
PS-HOSu (0.2 equiv), rt, 5 h; (f) TFA/CH2Cl2 (1:3, v/v), 2ꢁ25 min, rt, washings; (g) DIPEA
(5 equiv), THF, 35 ꢀC, 5 h.
ca.
n
¼1740 cmꢂ1. One possible reason for this inconsistency is the
presence of traces of HFIP in the crude peptides even after pro-
longed drying. These traces (which would be difficult to remove
systematically in the context of library production) were found to
interfere in the activation step. Despite several attempts to solve
this problem and optimize the reaction conditions, this route
was found to be insufficiently robust to allow a rapid synthesis of
1,3,5-triazepan-2,6-diones in a parallel format, and therefore was
abandoned.
FTIR analysis of the resin after treatment of model sequences i.e.,
BocePheeSareNH2 (5a) and BocePheeNMePheeNH2 (5b) with
PIFA and subsequent PS-HOSu trapping revealed the expected
three bands characteristic of the polymer bound succinimidyl car-
bamate at ca.
n
¼1785, 1740 and 1651 cmꢂ1 (C]O bond stretching
frequency) (Fig. 1). Attempt to introduce PIFA and PS-HOSu at the
same time resulted in lower on-resin carbamate formation (see
Supplementary Data for details).
2.2. Synthesis via Hofmann rearrangement of dipeptide
carboxamides (route B)
After Boc removal and cyclorelease, the remaining TFA salts
were removed from the crude product by treatment with (poly-
styrylmethyl)trimethylammonium bicarbonate) in CH2Cl2. Analysis
by C18 RP-HPLC of the crude materials revealed the formation of the
expected 1,3,5-triazepan-2,6-diones 1a and 1b in high purity (93%
and ꢃ95%, respectively). Identification of the products was con-
firmed by ESI-MS and 1H NMR analyses (See Supplementary Data
for details). To confirm the potential utility of this route, four ad-
ditional peptide carboxamide sequences were subjected to this
one-pot oxidative rearrangement and catch step (Table 1). The
expected FTIR signature of the polymer-supported succinimidyl
carbamate was observed in all cases. Target heterocycles 1ce1f
were recovered in purities ranging from 90% to ꢃ95% (based on C18
RP-HPLC or NMR analysis, see Supplementary Data for details). The
crude products were further purified by trituration with dieth-
ylether. These results demonstrate the superiority of route B over
route A for accessing the 1,3,5-triazepan-2,6-dione skeleton in
a parallel format. N-Boc protected dipeptide carboxamides can be
engaged in the Hofmann rearrangement directly after cleavage from
We then considered the possibility to generate dipeptidyl iso-
cyanates and subsequently the polymer supported carbamate B via
Hofmann rearrangement of N-Bocedipeptide carboxamides.
Bis(trifluoroacetoxy)eiodobenzene (PIFA) is an effective reagent for
the oxidative rearrangement of aliphatic amides to amines in mixed
aqueouseorganic media.12 This reaction, which proceeds with re-
tention of configuration at the migrating carbon has been applied
extensively to amides derived from peptides and amino acid in the
context of retroeinverso peptide modifications.13,14 Under such
mildly acidic conditions, the intermediate isocyanate is rapidly
hydrolyzed and the amine product protonated, thus preventing the
formation of urea byproducts. Recently, Lipton and co-workers
have reported a one-pot synthesis of ureidopeptides by Hofmann
rearrangement of protected amino amides using PIFA under
anhydrous conditions, followed by reaction of the isocyanate
intermediate with the amine function of an amino acid.15 We
have tested whether these conditions can be employed for the

J. Boeglin et al. / Tetrahedron 68 (2012) 7472e7478
7475
This route was explored for the synthesis of several cyclo(-
XaaegSareCO) derivatives. Their synthesis started with the three-
step one-pot transformation of a commercially available N-Boc-
protected a-amino acid (i.e., BoceSareOH) into the corresponding
polymer-supported oxime carbamate 6 (Scheme 5).
The reaction was monitored by FTIR spectroscopy (Fig. 2). After
2 h, the resin was found to display an FTIR spectrum characteristic
Scheme 5. Reagents and conditions. (a) (i) DPPA (1 equiv), TEA (1.1 equiv), DMF, rt,
30 min; (ii) 70 ꢀC, 1.5 h; (iii) PS-oxime (0.2 equiv) rt, 1 h; (b) (i) TFA/CH2Cl2 (0.5:1.5, v/v),
30 min; (ii) N-BoceXaaeOH (5 equiv), HOBt (5 equiv), DIC (5 equiv), 2ꢁ1 h, rt; (c) (i)
TFA/CH2Cl2 (1:3, v/v), 2ꢁ25 min; (ii) DIPEA (5 equiv), toluene, 80 ꢀC, 48 h.
Fig. 1. FTIR spectra of (a) PS-HOSu and (b) expected polymer-bound succinimidyl
carbamate 3a.
Table 1
Synthesis of 1,3,5-triazepan-2,6-diones by oxidative rearrangement of dipeptidyl
carboxamides prepared on solid support according to route B in Scheme 3
a
Compd
Xaa
Xbb
tR (min)
Purity (%)
Mass
recovery (%)e
1a
1b
1c
1d
1e
1f
Phe
Phe
Ala
Ser(OBn)
Glu(OBn)
Phe
Sar
NMePhe
Sar
Sar
Sar
3.38
7.00
db
93c
ꢃ95c
ꢃ90d
ꢃ95c
95c
71
31
44
33
28
88
3.98
4.80
3.91
Pro
ꢃ95c
a
Retention time by C18 RP-HPLC (see supplementary data for details).
Hardly detectable by UV absorbance.
b
c
Purity determined by HPLC analysis at 200 nm.
d
e
Purity estimated by 1H NMR.
Mass recovery of crude 1 relative to the amount of resin PS-HOSu (loading of
1.54 mmol/g) added to the reaction mixture.
the resin and do not need additional purification. However, one
limitation is that route B will not be amenable to sequences con-
taining oxidation sensitive side chains, such as Met, as well as
unprotected Asn, Gln, Trp and Tyr.
2.3. Synthesis via Curtius rearrangement of a-aminoacyl
azides (route C)
Route B was challenged with an alternative procedure involving
formation of the polymer-bound activated carbamate at an earlier
stage, prior to the coupling of the second amino acid (route C). In
this procedure, PS-HOSu was replaced by p-nitrobenzhydryl oxime
polystyrene resin (PS-oxime or Kaiser oxime resin).17 Polymer-
supported oxime-derived carbamates are attractive because they
can serve as heat-labile isocyanate equivalents on resin, but do not
react with amines under conditions of standard peptide bond for-
mation. Oxime carbamates have been used previously for the
preparation of nonsymmetric ureas, acyclic
a-ureidoacetamides,
Fig. 2. FT-IR spectra of (a) PS-oxime, (b) polymer supported oxime-derived carbamate
6 (c) polymer supported oxime-derived carbamate 7a.
3-aminohydantoins as well as 1,2,4-triazine-3,6-diones.18

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J. Boeglin et al. / Tetrahedron 68 (2012) 7472e7478
of the expected oxime carbamate with bands at
n
¼1755 and
spectrometer (Bruker Biospin) 300 MHz, DPX-300, DPX-400, DPX-
500 NMR spectrometer (Bruker Biospin). RP-HPLC analyses were
performed on a Dionex U3000SD using a MachereyeNagel Nucle-
1694 cmꢂ1. Resin 6 was deprotected in 25% TFA in CH2Cl2 and
coupled with BocePheeOH under standard coupling conditions
(DIC/HOBt in DMF) to give resin 7a (R1¼H, R2¼Bn). The FTIR
odur column (4.6ꢁ100 mm, 3
m
m) at a flow rate of 1 mL minꢂ1. The
spectrum of resin 7a display an additional band at ca.
n
¼1655 cmꢂ1
mobile phase was composed of 0.1% (v/v) TFA in H2O (Solvent A)
and 0.1% (v/v) TFA in CH3CN (Solvent B). Mass spectra have been
recorded using an ESI-TOF apparatus (Bruker micro-TOF) and HMRS
were recorded with LCT Premier XE KE483. Specific rotations have
been measured using a Jasco P-2000 polarimeter at 25 ꢀC with Na
lamp.
attributable to the amide bond. The Boc group was removed and
thermolytic intramolecular cyclization was conducted in toluene
containing 5 equiv DIPEA at 80 ꢀC. As judged from 1H NMR and C18
RP-HPLC, purity of crude 1a and 1c after trituration with dieth-
ylether was excellent (ꢃ95% based on HPLC). However, mass re-
covery based on oxime resin was modest (23%). Starting from resin
6, the procedure was repeated with BoceSer(OBn)eOH and
BoceAlaeOH. Corresponding 1,3,5-triazepan-2,6-diones 1c and 1d
were recovered in high purity (ꢃ95% based on HPLC) albeit in
modest yield (See Supplementary data for details). Overall, this
method appears to work relatively well for 1,3,5-triazepan-2,6-
diones incorporating a gem-diamino residue derived from sarco-
sine (R4¼H). However, the presence of an alkyl side chain at this
position (R4sH, Scheme 4) may increase the sensivity to hydrolysis
of the intermediate monocarbamoylated gem-diamino de-
rivative9,19 formed upon Boc deprotection of 6 precluding access to
a diverse set of 1,3,5-triazepan-2,6-diones.
4.2. General procedures for solid-phase synthesis via route A
4.2.1. Solid phase synthesis of dipeptide (2) (exemplified for 2a). 2-
CleChlorotrityl resin (316 mg, 0.57 mmol) was rinsed with dis-
tilled CH2Cl2 (ꢁ3) and swollen in distilled CH2Cl2. A solution of N-
FmoceSareOH (355 mg, 1.14 mmol, 2 equiv) and DIPEA (0.6 mL,
3.42 mmol, 6 equiv) was added onto the resin. The mixture was
gently stirred for 4 h at room temperature. Solvent was removed
by filtration and the resin was washed with distilled CH2Cl2 (ꢁ4),
MeOH (ꢁ2), DMF (ꢁ2), distilled CH2Cl2 (ꢁ2) and Et2O (ꢁ2). After
drying, the N-FmoceSar resin was swollen in distilled DCM and
deprotected using DMF/piperidine (3:1). The mixture is stirred at
room temperature for 20 min then the solvent was removed by
filtration and the resin washed with DMF (3ꢁ6 min). This step
was repeated once. The loading was determined by UV spec-
trometry: 0.74 mmol. A mixture of BocePheeOH (450 mg,
1.7 mmol, 5 equiv), Benzotriazole-1-yl-oxy-tris-(dimethylamino)-
phosphonium hexa-fluorophosphate (BOP) (752 mg, 1.7 mmol,
3. Conclusion
We have investigated three different approaches for rapid syn-
thesis of 1,3,5-triazepan-2,6-diones in high purity by solid-phase
techniques. Routes A and B both feature the trapping of an in-
termediate dipeptidyl isocyanate by a polymer supported N-
hydroxysuccinimide and
a
cyclorelease step following Boc-
5 equiv), HOBt (230 mg, 1.7 mmol, 5 equiv) and DIPEA (891 mL,
deprotection. Route B, which involves Hofmann rearrangement of
N-Boc dipeptide carboxamides, was found to be superior and more
reliable than route A based on Curtius rearrangement of N-Boc
protected dipeptidyl acyl azides.
5.1 mmol, 15 equiv) in DMF (5 mL) was added to the Sar resin and
stirred for 30 min at room temperature. The solvent was removed
by filtration, the resin was rinsed with DMF (ꢁ2) and the coupling
step was repeated under the same conditions. After filtration, the
resin was washed with DMF (3ꢁ6 min) and distilled CH2Cl2
(1ꢁ6 min). Cleavage was performed by treatment of resins with
a mixture of distillated CH2Cl2/hexafluoroisopropanol (2:3, v/v)
for 2 h at room temperature. The purity was checked by HPLC and
was above 90%.
Trapping of N-protected
commercially available N-protected
a
-amino isocyanates derived from
-amino acids (route C) was
a
also tested, but the scope of this method is more limited. Alto-
gether, our results point to route B as the most useful approach for
the production of 1,3,5-triazepan-2,6-dione librairies using parallel
synthesis. Our efforts in this direction together with biological
screening of resulting libraries will be reported in due course.
4.2.2. Preparation of PS-HOSu. PS-SH (5 g, 10 mmol) was swollen in
dry DMF (150 mL) and N-hydroxymaleimide (2.26 g, 20 mmol) fol-
lowed by pyridine (4.85 mL, 60 mmol) were added. The mixture was
stirred for 24 h at room temperature and then heated at 55 ꢀC for 3 h
under an inert atmosphere. The resin was filtered and washed with
DMF (ꢁ3), H2O (ꢁ3), MeOH(ꢁ3), and CH2Cl2 (ꢁ4). Oncethewashings
were complete, the resin was dried under high vacuum for 16 h
(mass¼5.99 g, 98%). Based on mass recovery the loading of PS-HOSu
was determined to be 1.54 mmol/g. IR (solid): nmax (cmꢂ1)¼1715 (CO).
4. Experimental section
4.1. General methods
Commercially available reagents were used throughout
without purification. N-protected amino acids were purchased
from PolyPeptide Laboratories France. Fmoc Sieber resin
(loading¼0.67 mmol/g, 75e150
Kaiser oxime resin (PS-oxime, loading¼1.3 mmol/g, 75e150
and 4-mercaptomethyl polystyrene (PS-SH, 1.87 mmol/g,
75e150 m) were purchased from Polymer Laboratories, the (pol-
mm) was purchased from varian,
mm)
4.2.3. Synthesis of BocePheegXbbeCOOSuePS (4) (exemplified for
4a). BocePheeSareCOOH (245 mg, 0.73 mmol) was dissolved in
distilled DMF (1.5 mL) under inert atmosphere, then triethylamine
m
ystyrylmethyl)etrimethylammonium bicarbonate (5.9 mmol/g)
was purchased from EMD Biosciences and the 2-chlorotrityl chlo-
ride resin was purchased from CBL Patras. All organic solvents were
of analytical quality and Milli-Q (Millipore) water was used for RP-
HPLC analyses and purifications. Dry THF was purchased from
SigmaeAldrich and CH2Cl2 was distilled from CaH2. Thin layer
chromatography (TLC) was performed on silica gel 60 F254 (Merck)
with detection by UV light and charring with 1% ninhydrin in
ethanol followed by heating. Flash column chromatography was
(TEA) (110 mL, 0.79 mmol, 1.1 equiv) and DPPA (157 mL, 0.73 mmol,
1.1 equiv) were added. The mixture was stirred 30 min at room
temperature followed by heating at 70 ꢀC for 1.5 h. Addition of PS-
HOSu (143 mg, 0.22 mmol, 0.3 equiv) was followed by stirring at
70 ꢀC for 40 min. The resin was filtred, washed with DMF (ꢁ3),
distilled THF (ꢁ3) and distilled CH2Cl2 (ꢁ3) then dry under high
vacuum for 16 h and analyzed by FTIR. IR (solid): nmax (cmꢂ1)¼1787
(CO, Boc), 1742 (CO, HNCOOSu), 1711 (CO, Suc), 1650 (CO, amide).
carried out on silica gel (40e63
m
m, Merck). IR spectra were
4.2.4. Cyclorelease procedure (exemplified for 1a). BocePheegXbbe
COOSuePS (4) was treated with TFA/CH2Cl2 (1:3, v/v) (1.5 mL) during
25 min. This step was repeated once. The resin was swollen in dry
recorded with a Perkin Elmer Spectrum One spectrometer with an
ATR (Attenued Total Reflexion) accessory. 1H NMR spectra were
recorded on different NMR spectrometers: an Avance II NMR
CH2Cl2 (2 mL), and DIPEA (45 mL, 0.26 mmol, 1.2 equiv) was added.

J. Boeglin et al. / Tetrahedron 68 (2012) 7472e7478
7477
The resin was vortexed at 40 ꢀC for 1 h before filtred and rinsed with
CH2Cl2 (2ꢁ1 mL). Filtrate was evaporated and gave cyclo(-
PheegSareCO) 1a with 77% yield and 89% purity based on C18 RP-
HPLC as a white solid.
(172 mg, 0.65 mmol, 5 equiv), HOBt (88 mg, 0.65 mmol, 5 equiv) and
DIC (100 L, 0.65 mmol, 5 equiv) in DMF (2 mL) was added then the
resin was vortexed for 1 h at room temperature. This step was re-
peated once then the resin was filtered and washed with DMF (ꢁ6),
CH2Cl2 (ꢁ6) and Et2O (ꢁ6). IR (solid) : nmax (cmꢂ1)¼3420, 3329
(NH), 1751, 1710, 1655 (CO), 1205, 1163 (NCOO).
m
4.3. General procedures for solid-phase synthesis via route B
4.3.1. Synthesis of BoceXaaeXbbeNH2 (exemplified for BocePheeSare
NH2 (5a)). The Fmoc Sieber resin (670 mg, 0.45 mmol) was swollen in
DMF then treated with 20% piperidine in DMF (2.5 ml). After several
washings with DMF, FmoceSareOH (280 mg, 0.9 mmol, 2 equiv),
4.4.3. Cyclorelease
(exemplified
for
1a). The
resin
for
BoceXaaegSareCOOePS 7a was swollen in CH2Cl2 for 5 min. Boc
removal was then performed with TFA/CH2Cl2 (1:3, v/v) (2.5 mL) for
25 min. The resin was washed with CH2Cl2 (ꢁ6) and with a mixture
HOBt (135 mg, 0.9 mmol, 2 equiv) and DIC (140
mL, 0.9 mmol, 2 equiv)
of DIPEA (111
mL, 0.65 mmol, 5 equiv) in CH2Cl2. Then toluene
were added. The resin was vortexed at room temperature for 30 min
before being washed with DMF (ꢁ6) and treated with 20% piperidine
in DMF (2.5 ml) for 30 min. This step was repeated once. Then
BocePheeOH (240 mg, 0.9 mmol, 2 equiv), HOBt (135 mg, 0.9 mmol,
(2.5 ml) and DIPEA (111 mL, 0.65 mmol, 5 equiv) were added and the
mixture was stirring at 80 ꢀC for 48 h. The heterocycle 1a was
obtained by filtration and evaporation under high pressure with
23% yield and ꢃ95% purity by HPLC. Spectrometric data: identical to
that of compound described in ref 5.
2 equiv) and DIC (140 mL, 0.9 mmol, 2 equiv) were added and the
mixture was shaken at room temperature for 30 min. The resin was
washed with DMF (ꢁ6), MeOH (ꢁ6), DMF (ꢁ6), CH2Cl2 (ꢁ6) and Et2O
(ꢁ6). Cleavage was performed by treatment of resins with a mixture of
distilled CH2Cl2/TFA (98:2, v/v) at room temperature for 10 min (ꢁ6).
The TFA was neutralized by pyridine then the mixture was concen-
trated under high vaccum. Filtrate was evaporated and gave
0.33 mmol of dipeptide 5a.
4.5. Characterization of 1,3,5-triazepan-2,6-diones
4.5.1. cyclo(PheegSareCO) (1a). Compound obtained by routes AeC.
Spectrometric data: identical to that of compound described in Ref. 5.
4.5.2. cyclo(PheegNMePheeCO) (1b). Compound obtained by route
B. Spectrometric data: identical to that of compound described in Ref. 5.
4.3.2. Synthesis of BoceXaaegXbbeCOOSuePS (exemplified for
BocePheegSareCOOSuePS (3a)). To a solution of BocePheeSare
NH2 (0.33 mmol) in dry THF, PIFA (110 mg, 0.26 mmol, 0.8 equiv) and
4.5.3. cyclo(AlaegSareCO) (1c). Compound obtained by route B
and route C. ½a 2D5
ꢄ
¼ꢂ6.09 (c 0.2, CH3CN); 1H NMR (CDCl3, 300 MHz,
pyridine (43
mL, 0.53 mmol, 1.6 equiv) were added and the mixture
298 K)
d
¼1.41 (d, J¼6.7 Hz, 3H, CHCH3), 3.13 (s, 3H, NCH3), 4.05 (dd,
was stirred at room temperature. After 1 h, PS-HOSu (45 mg,
0.07 mmol, 0.2 equiv) was added and the mixture was vortexed 5 h at
room temperature. The resin was washed with THF (ꢁ6), DMF (ꢁ6),
CH2Cl2 (ꢁ6) and Et2O (ꢁ6). IR (solid): nmax (cmꢂ1)¼1785 (CO, Boc),
1740 (CO, HNCOOSu), 1715 (CO, Suc), 1651 (CO, amide).
J¼7.1, 15.4 Hz, 1H, NCH2N), 4.56 (m, 1H, NCHCO), 4.74 (s, 1H, NH),
5.31 (dd, J¼5.5, 15.6 Hz, 1H, NCH2N), 6.03 (s, 1H, NH); 13C NMR
(CDCl3, 400 MHz, 298 K)
d¼17.14 (CH3), 34.21 (CH3), 49.27 (CH),
56.59 (CH2), 157.87, 171.18 (CO); HRMS(ESI): m/z: calculated for
C6H10N3O2 [MꢂH]ꢂ: 156.0773, found: 156.0767.
4.3.3. Cyclorelease procedure (exemplified for 1a). The resin
BocePheegSareCOOSuePS (3a) was swollen in CH2Cl2 for 5 min. Boc
removal was then performed with TFA/CH2Cl2 (1:3, v/v) (2.5 mL) for
25 min. This step was repeated once and the resin was washed with
CH2Cl2 (ꢁ6) and THF(ꢁ2). The resin was then swollen in THF, and
4.5.4. cyclo(Ser(OBn)egSareCO) (1d). Compound obtained by route
B and C. C18 RP-HPLC (A: 0.1% TFA in H2O, B: 0.1% TFA in MeCN,
20e80% B, 1 mL/min, 10 min): tR¼3.98 min; ½a D25
ꢄ ¼ꢂ22.85 (c 0.2,
CH3CN); 1H NMR (CDCl3, 300 MHz, 298 K)
d
¼3.07 (s, 3H, NCH3), 3.72
(t, J¼9.7 Hz,1H, CHCH2O), 3.92 (dd, J¼4.0,10.2 Hz,1H, CHCH2O), 4.03
(dd, J¼7.0, 15.6 Hz, 1H, NCH2N), 4.59 (d, J¼2.3 Hz, 2H, CH2Ph), 4.66
(m,1H, NCHCO), 5.22 (dd, J¼5.3,15.6 Hz,1H, NCH2N), 5.25 (s,1H, NH),
6.49 (m,1H, NH), 7.36 (m, 5H, Ph); 13C NMR (CDCl3, 400 MHz, 298 K)
DIPEA (61 mL, 0.35 mmol, 5 equiv) was added and the mixture was
vortexed at 35 ꢀC for 5 h. The mixture was filtered and concentrated
under high vaccum. The crude cycle was purified using (poly-
styrylmethyl)etrimethylammonium bicarbonate (36 mg, 0.21 mmol,
3 equiv) in CH2Cl2 for 3 h. After evaporation of the filtrate, cyclo(-
PheegSareCO) was obtained in 71% yield and 93% purity by HPLC.
Spectrometric data: identical to that of compound described in Ref. 5.
d
¼33.80 (CH3),53.37 (CH),56.36 (CH2), 69.81(CH2), 74.03 (CH2),
128.41, 128.52, 129.01, 137,59 (Ph), 158.25, 169.17 (CO); HRMS(ESI):
m/z: calculated for C13H17N3NaO3: 286.116 [MþNa]þ, found: 286.115.
4.5.5. cyclo(Glu(OBn)egSareCO) (1e). compound obtained by route
B and C. C18 RP-HPLC (A: 0.1% TFA in H2O, B: 0.1% TFA in MeCN,
4.4. General procedure for solid-phase synthesis via route C
20e80% B, 1 mL/min, 10 min): tR¼4.80 min;½a D25
ꢄ ¼ꢂ10.36 (c 0.1,
4.4.1. BocegSareCOOePS
BoceSareOH (123 mg, 0.65 mmol) was dissolved in dry DMF (1 mL)
thenTEA (100 L, 0.72 mmol,1.1 equiv) and DPPA (141 L, 0.65 mmol,
(6). Under
inert
atmosphere,
CH3CN); 1H NMR (CDCl3, 300 MHz, 298 K)
d
¼1.41 (d, J¼6.7 Hz, 3H,
CHCH3), 3.13 (s, 3H, NCH3), 4.05 (dd, J¼7.1, 15.4 Hz, 1H, NCH2N), 4.56
(m, 1H, NCHCO), 4.74 (s, 1H, NH), 5.31 (dd, J¼5.5, 15.6 Hz, 1H,
m
m
1 equiv) were added. The mixture was stirred 30 min at room tem-
perature followed by heating at 70 ꢀC for 1.5 h. Then the heating was
stopped and the oxime resin (100 mg, 0.13 mmol, 0.2 equiv) was
added. The resin was shaken for 1 h at room temperature before
being filtered off and washed with DMF (ꢁ6) and CH2Cl2 (ꢁ6). The
resin BocegSareCOOePS (6) was finally driedunderhigh vacuum. IR
(solid): nmax (cmꢂ1)¼3511 (NH), 1755,1694 (CO),1203,1150 (NCOO).
NCH2N), 6.03 (s,1H, NH); 13C NMR (CDCl3, 400 MHz, 298 K)
d¼26.87,
30.63 (CH2), 34.17 (CH3), 52.73 (CH), 56.63, 67.07 (CH2), 128.76,
128.82, 129.06, 136.15 (Ph), 157.64, 170.34, 173.34 (CO); HRMS(ESI):
m/z: calculated for C15H20N3O4: 306.1454 [MþH]þ, found 306.1445.
4.5.6. cyclo(PheegProeCO) (1f). Compound obtained by route B.
Spectrometric data: identical to that of compound described in Ref. 5.
4.4.2. General procedure for BoceXaaegSareCOOePS (exemplified
for 7a). The resin BocegSareCOOePS (6) (0.13 mmol) was swollen
in distilled CH2Cl2 then the Boc deprotection was performed with
TFA/CH2Cl2 (1:3, v/v) (2.5 mL) during 30 min. After several washings
with CH2Cl2 (ꢁ6) and DMF (ꢁ6), a mixture containing BocePheeOH
Acknowledgements
This work was supported in part by the Centre National de la
Recherche Scientifique (CNRS) and ImmuPharma France. CIFRE
fellowships from ImmuPharma France and the Association

7478
J. Boeglin et al. / Tetrahedron 68 (2012) 7472e7478
Durain, E.; Guichard, G.; Didierjean, C. Acta Cryst. Section
o2306eo2308; (c) Lena, G.; Wenger, E.; Guichard, G.; Didierjean, C. Acta Cryst.
Section E 2006, 62, o518eo520.
8. Muller, P.; Lena, G.; Boilard, E.; Bezzine, S.; Lambeau, G.; Guichard, G.; Rognan,
D. J. Med. Chem. 2006, 49, 6768e6778.
E 2007, 63,
Nationale de la Recherche et de la Technologie (ANRT) for C.V. and
J.B. are gratefully acknowledged. We thank Gersande Lena for the
initial synthetic efforts on route A.
9. Fischer, L.; Semetey, V.; Lozano, J.-M.; Schaffner, A.-P.; Briand, J.-P.; Didierjean,
C.; Guichard, G. Eur. J. Org. Chem. 2007, 2511e2525.
Supplementary data
10. g¼gem, and refers to the 2-alkyl gem-diamino derivative of the corresponding
amino acid, according to the nomenclature introduced by Goodman and Chorev.
Goodman, M.; Chorev, M. Acc. Chem. Res. 1979, 12, 1e7.
11. Adamczyk, M.; Fishpaugh, J. R.; Mattingly, P. G. Tetrahedron Lett. 1999, 40,
463e466.
12. (a) Radhakrishna, A. S.; Parham, M. E.; Riggs, R. M.; Loudon, G. M. J. Org. Chem.
1979, 44, 1746e1747; (b) Loudon, G. M.; Radhakrishna, A. S.; Almond, M. R.;
Blodgett, J. K.; Boutin, R. H. J. Org. Chem. 1984, 49, 4272e4276; (c) Boutin, R. H.;
Loudon, G. M. J. Org. Chem. 1984, 49, 4277e4284.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.06.051.
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