Microwave-enhanced solid-phase synthesis of N,N′-linked aliphatic oligoureas and related hybrids

Article abstract of DOI:10.1021/ol3012106

A practical and efficient microwave-assisted solid-phase method for the synthesis of N,N′-linked oligoureas and related amide/urea hybrid oligomers, featuring the use of succinimidyl (2-azido-2-substituted ethyl) carbamate monomers, is reported. The rate enhancement of urea formation under microwave irradiation combined with the mild conditions of the phosphine-based azide reduction makes this approach very effective for routine synthesis of oligoureas and possibly for library production.

Full text of DOI:10.1021/ol3012106

ORGANIC
LETTERS
2012
Vol. 14, No. 12
3130–3133
Microwave-Enhanced Solid-Phase
Synthesis of N,N0-Linked Aliphatic
Oligoureas and Related Hybrids
ꢀ
Celine Douat-Casassus, Karolina Pulka, Paul Claudon, and Gilles Guichard*
ꢀ
ꢀ
Universite de BordeauxꢀCNRS UMR 5248, Institut Europeen de Chimie et Biologie,
2 rue Robert Escarpit, 33607 Pessac Cedex, France
g.guichard@iecb.u-bordeaux.fr
Received May 3, 2012
ABSTRACT
A practical and efficient microwave-assisted solid-phase method for the synthesis of N,N0-linked oligoureas and related amide/urea hybrid
oligomers, featuring the use of succinimidyl (2-azido-2-substituted ethyl) carbamate monomers, is reported. The rate enhancement of urea
formation under microwave irradiation combined with the mild conditions of the phosphine-based azide reduction makes this approach very
effective for routine synthesis of oligoureas and possibly for library production.
Since its introduction by Merrifield, solid-phase synth-
esis (SPS) has revolutionized the fields of peptide and
protein chemistry.1 This technology has facilitated the
development of foldamers, i.e., artificial folded archi-
tectures,2 by giving access to oligomers of ever-increasing
size and complexity and by allowing the preparation of
libraries. Aliphatic N,N0-linked oligoureas3 A (Figure 1)
are a class of nonpeptidic helical foldamers that show
promise for biological applications (e.g., as R-peptide
mimetics).4,5 Several SPSmethodologies toaccessaliphatic
oligoureas have been described. All are based on a seq-
uential coupling of activated monomers derived from
monoprotected ethylenediamine derivatives6 such as suc-
cinimyl carbamates 1 developed by our laboratory.7
Although these monomers have proven useful to access
short oligoureas in reasonable yields and purities, they still
suffer some drawbacks: long coupling times (2 ꢁ 120 min)
and a large excess of monomers (at least double couplings
of 3 equiv) are typically needed. Microwave irradiation has
been shown to alleviate some of the hurdles inherent with
SPS (e.g., slow reaction rates associated with folding and
chain aggregation) and to significantly accelerate the
assembly of R-peptides8 and oligoamide foldamers (peptoids,9
β-peptides,10 and aromatic oligoamides11).
Herein we describe our efforts to improve SPS of
aliphatic oligoureas with the assistance of microwave
irradiation. Preliminary investigations were conducted
(1) Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149–2154.
(2) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173–180. (b)
Guichard, G.; Huc, I. Chem. Commun. 2011, 47, 5933–5941.
(3) (a) Burgess, K.; Shin, H.; Linthicum, D. S. Angew. Chem., Int. Ed.
1995, 34, 907–909. (b) Fischer, L.; Guichard, G. Org. Biomol. Chem.
2010, 8, 3101–3117.
(4) (a) Violette, A.; Fournel, S.; Lamour, K.; Chaloin, O.; Frisch, B.;
Briand, J.-P.; Monteil, H.; Guichard, G. Chem. Biol. 2006, 13, 531–538.
(b) Claudon, P.; Violette, A.; Lamour, K.; Decossas, M.; Fournel, S.;
(6) (a) Kim, J.-M.; Bi, Y.; Paikoff, S. J.; Schultz, P. G. Tetrahedron
Lett. 1996, 37, 5305–5308. (b) Burgess, K.; Ibarzo, J.; Linthicum, D. S.;
Russell, D. H.; Shin, H.; Shitangkoon, A.; Totani, R.; Zhang, A. J.
J. Am. Chem. Soc. 1997, 119, 1556–1564. (c) Boeijen, A.; Ameijde, J. V.;
Liskamp, R. M. J. J. Org. Chem. 2001, 66, 8454–8462.
(7) (a) Guichard, G.; Semetey, V.; Rodriguez, M.; Briand, J.-P.
Tetrahedron Lett. 2000, 41, 1553–1557. (b) Guichard, G.; Semetey, V.;
Didierjean, C.; Aubry, A.; Briand, J.-P.; Rodriguez, M. J. Org. Chem.
1999, 64, 8702–8705. (c) Nelli, Y.-R.; Douat-Casassus, C.; Claudon, P.;
Kauffmann, B.; Didierjean, C.; Guichard, G. Tetrahedron 2012, 68,
4492–4500.
ꢀ
ꢀ
Heurtault, B.; Godet, J.; Mely, Y.; Jamart-Gregoire, B.; Averlant-Petit,
M.-C.; Briand, J.-P.; Duportail, G.; Monteil, H.; Guichard, G. Angew.
Chem., Int. Ed. 2010, 49, 333–336.
(5) (a) Hemmerlin, C.; Marraud, M.; Rognan, D.; Graff, R.; Semetey,
V.; Briand, J.-P.; Guichard, G. Helv. Chim. Acta 2002, 85, 3692–3711. (b)
Fremaux, J.; Fischer, L.; Arbogast, T.; Kauffmann, B.; Guichard, G.
Angew. Chem., Int. Ed. 2011, 50, 11382–11385. (c) Fischer, L.; Claudon, P.;
Pendem, N.; Miclet, E.; Didierjean, C.; Ennifar, E.; Guichard, G. Angew.
Chem., Int. Ed. 2010, 49, 1067–1070.
(8) (a) Bacsa, B.; Horvati, K.; Bosze, S.; Andreae, F.; Kappe, O. C. J.
Org. Chem. 2008, 73, 7532–7542. (b) Galanis, A. S.; Albericio, F.; Grøtli,
M. Org. Lett. 2009, 11, 4488–4491. (c) Pedersen, S. L.; Tofteng, A. P.;
Malik, L.; Jensen, K. J. Chem. Soc. Rev. 2012, 41, 1826–1844.
r
10.1021/ol3012106
Published on Web 05/30/2012
2012 American Chemical Society

with trimethylphosphine (PMe3) was successfully applied
to SPPS,16 peptide nucleic acids (PNAs),17 and peptoids.18
On the basis of these promising precedents, succinimidyl
(2-azido-2-substituted ethyl) carbamates 2 (Figure 1) were
selected as new monomers for the microwave-assisted SPS
of oligoureas.
Two synthetic routes were developed as outlined in
Scheme 1, depending on the nature of the side chain of
starting N-protected β-amino alcohols 3.19 Route A in-
volves the conversion of unprotected β-amino alcohol 4 to
its corresponding azide 5 following the method of Wong20
with imidazole-1-sulfonyl azide hydrochloride as the dia-
zo-transfer reagent.21 Next, phthalimide intermediate 6
was formed by substituting alcohol 5 under Mitsunobu
conditions.6b,22 However, because of the high volatility of
azido alcohols 5dꢀe (R = i-Bu, Me, respectively), pre-
ference was given to pathway B in which N-Boc-β-amino
alcohols3dꢀe are first converted totheir phthalimide 8dꢀe
again under Mitsunobu conditions. Removal of phthali-
mide with hydrazine hydrate23 afforded 2-azido-2-substi-
tuted ethylamines 7 which were directly converted to acti-
vated carbamates 2aꢀf in good overall yields (see Table 1)
by treatment with N,N0-disuccinimidyl carbonate (DSC).
Notably, route B was also found to be more efficient in
the preparation of carbamate 2f with indole side chain
(Table 1).
With activated monomers 2aꢀf in hand, we then ex-
plored the coupling/azide reduction cycle on solid support
under microwave assistance. Tetraurea 9 was chosen as a
first model to optimize reaction conditions and screen
methods for reducing azides to amines (Table 2). Synthesis
was performed on a hydrophilic solid support (NovaPEG
Rink amide resin) compatible with a variety of solvents
including aqueous solvent systems typically used for the
mild reduction of azides during Staudinger reaction.16,17
The poly(ethylene glycol) (PEG) matrix is known to
exhibit high swelling properties in water and to diminish
the risk of chain aggregation.24 On-resin urea formation
was performed with monomer 2a by using microwave
irradiation conditions optimized for coupling Boc mono-
mers 1a. Three conditions for on-resin azide reduction
were tested in parallel. Because azide reduction is known to
proceed slowly,6a and in some cases to require heating, we
also contemplated using microwave irradiation for this
step.
Figure 1. Schematic representation of oligoureas A and formu-
las of activated monomers 1 and azide monomer 2.
using monomers 1a and 1b starting from 4-methylbenzhy-
drylamine (MBHA)12 and Rink amide polystyrene (PS)
resins, respectively. It rapidly became apparent that micro-
wave irradiation was not compatible withFmoc chemistry,
leading to degradation of N-Fmoc protected monomers 1b
and uncontrolled oligomerization on resin. Conversely,
excellent results were obtained when N-Boc-protected
monomers 1a were used with microwave irradiation. Un-
der optimized conditions (70 °C, 25 W, dimethylforma-
mide (DMF)), the coupling times and excess monomers
(1.5 equiv of 1a, 10 min, double coupling) were signifi-
cantly reduced at no cost for overall purity and yield (see
the Supporting Information for details). However, while
the Boc strategy is more robust, final HF cleavage from the
resin is impractical for routine use and library production.
In search of an alternative to the Fmoc protecting group
that would allow the use of standard TFA-labile type
resins, we decided to reinvestigate the use of azides as
masked amines for SPS of oligoureas.
Early work by Schultz and collaborators reported the
SPS of short 3-mers using 4-nitrophenyl (2-azido-1-sub-
stituted ethyl) carbamates derived from N-Boc-protected
R-amino acids as activated monomers, the azide reduction
being performed in the presence of a tin reagent.6a,13
Concurrently, Meldal et al. reported a solid-phase peptide
synthesis (SPPS) approach based on the use of R-azido
acids and employing dithiothreitol (DTT) as azide-redu-
cing agent.14 More recently, the Staudinger reduction15
(9) (a) Olivos, H. J.; Alluri, P. G.; Reddy, M. M.; Salony, D.;
Kodadek, T. Org. Lett. 2002, 4, 4057–4059. (b) Gorske, B. C.; Jewell,
S. A.; Guerard, E. J.; Blackwell, H. E. Org. Lett. 2005, 7, 1521–1524.
(10) (a) Murray, J. K.; Gellman, S. H. Org. Lett. 2005, 7, 1517–1520.
(b) Murray, J. K.; Gellman, S. H. Nature Protoc. 2007, 2, 624–631. (c)
Petersson, E. J.; Schepartz, A. J. Am. Chem. Soc. 2008, 130, 821–823.
(11) Baptiste, B.; Douat-Casassus, C.; Katta, L.-R.; Godde, F.; Huc,
I. J. Org. Chem. 2010, 75, 7175–7185.
(12) We previously found that the urea linkage formed by coupling 1a
to MBHA resin does not resist standard conditions for Boc cleavage.4b
To circumvent this problem, oligoureas in Boc chemistry are prepared
with a terminal amide group by attaching an isosteric γ4-amino acid to
the MBHA resin.
(16) Lundquist, J. T.; Pelletier, J. C. Org. Lett. 2001, 3, 781–783.
(17) Debaene, F.; Wissinger, N. Org. Lett. 2003, 5, 4445–4447.
€
(18) Fritz, D.; Brase, S. Synlett 2010, 1544–1548.
(19) Rodriguez, M.; Llinares, M.; Doulut, S.; Heitz, A.; Martinez, J.
Tetrahedron Lett. 1991, 32, 923–926.
(20) Alper, P. B.; Hung, S.-C.; Wong, C.-H. Tetrahedron Lett. 1996,
37, 6029–6032.
(21) (a) Goddard-Borger, E. D.; Stick, R. V. Org. Lett. 2007, 9, 3797–
€
3800. (b) Fischer, N.; Goddard-Borger, E. D.; Greiner, R.; Klapotke,
T. M.; Skelton, B. W.; Stierstorfer, J. J. Org. Chem. 2012, 74, 1760–1764.
(22) Zhang, Z.; Fan, E. J. Org. Chem. 2005, 70, 8801–8810.
(23) Sheehan, J. C.; Chapman, D. W.; Roth, R. W. J. Am. Chem. Soc.
1952, 74, 3822–3825.
(13) (a) Bartra, M.; Romea, P.; Urpı, F.; Vilarrasa, J. Tetrahedron
1990, 46, 587–594. (b) Kick, E. K.; Ellman, J. A. J. Med. Chem. 1995, 38,
1427–1430.
(14) Meldal, M.; Juliano, M. A.; Janason, A. M. Tetrahedron Lett.
1997, 38, 2531–2534.
(24) (a) Garcia-Martin, F.; Quintanar-Audelo, M.; Garcia-Ramos,
^ ꢀ
Y.; Cruz, L. J.; Gravel, C.; Furic, R.; Cote, S.; Tulla-Puche, J.; Albericio,
F. J. Comb. Chem. 2006, 6, 213–220. (b) Renil, M.; Meldal, M. Tetra-
hedron Lett. 1996, 37, 6185–6188.
(15) Staudinger, H.; Meyer, J. Helv. Chim. Acta 1919, 2, 635.
Org. Lett., Vol. 14, No. 12, 2012
3131

Scheme 1. General Synthetic Procedure for the Preparation of Azido Carbamates 2 Depending on the Nature of the Residue Side Chain
Table 1. Succinimidyl (2-Azido-2-substituted ethyl) Carba-
mates 2aꢀf Prepared According to Scheme 1
routea yieldb (%) HPLC tR
c
compounds
side chain (R)
2a
2b
2c
2d
2e
2f
Bn
A
A
A
B
B
B
36
34
33
43
27
34
7.25
8.25
6.88
7.25
4.83
7.09
p-t-BuOC6H4CH2
BocNH(CH2)3
i-Bu
Me
1H-indol-3-yl-CH2
a See Scheme 1. b The yield given corresponds to the overall yield.
c RP-HPLC system: linear gradient 10 to 100% of B in 10 min with
A (0.1% TFA in H2O) and B (0.1% TFA in CH3CN), UV detection
200 nm.
Figure 2. RP-HPLC profiles (UV detection at 200 nm) of 10
prepared by Fmoc strategy using monomers 1b (gray trace,
purity 32%) and by microwave-assisted azide strategy (black
trace, purity 70%). HPLC conditions: see the footnote in
Table 1.
Table 2. Microwave-Assisted SPS of Urea-Based Tetramer 9
and Azide Reduction Conditions Screened on Solid Support
As outlined in Table 2, the best results were obtained
when conversion of azides into amines was performed with
PMe3 (10 equiv) in a mixture of dioxane/water (70:30)
under microwave irradiation (70 °C, 25 W, 2 ꢁ 30 min).
These microwave-SPS conditions provided oligourea 9 in
73% purity (based on RP-HPLC) and good recovery yield
(86%). In comparison, the purity of crude product after
cleavage was much lower when SnCl2/PhSH/Et3N was
employed. The same observation was made when DTT
wasused, with the presence of many sideproductsresulting
from incomplete azide reductions.
These optimized conditions for azide reduction using
the Staudinger reaction with microwave irradiation were
further tested for the preparation of longer oligomers (e.g.,
heptaurea 10). Oligourea 10 was obtained in 70% purity
(based on RP-HPLC) and fair yield (57%) after TFA
cleavage and precipitation of the cleaved product with
purityc
entry azide reducing agent
conditions
(%)
1
2
3
SnCl2, PhSH, Et3N
DTT, DIEA
PMe3
THF, 25 W, 50 °Ca
40
34
73
DMF, 25 W, 70 °Cb
dioxane/H2O, 25 W, 70 °Cb
a 2 ꢁ 10 min. b 2 ꢁ 30 min. c Purity of crude 9 analyzed by RP-HPLC:
linear gradient 50ꢀ100% of B in 10 min (UV detection 200 nm). DIEA:
diisopropylethylamine. TFA: trifluoroacetic acid.
3132
Org. Lett., Vol. 14, No. 12, 2012

Scheme 2. Microwave-Assisted SPS of Oligourea Hybrid 11 Using Azide Monomers 2a
a Insets: RP-HPLC chromatograms (UV detection at 220 nm) of 11 prepared using the azide strategy (black trace, purity 86%) and by using standard
N-Fmoc monomers 1b7a (gray trace, purity 30%). HPLC conditions: see the footnote in Table 1.
methyl tert-butyl ether (Figure 2). This result confirms the
possibility to perform multiple successive azide reductions
on solid support (up to seven for 10) without compromis-
ing the final purity of the oligoureas.
revealed truncated sequences lacking urea linkages. In
contrast, the combined use of azido monomers 2 and
microwave irradiation resulted in a marked improvement.
The deletion impurities were completely eliminated and
oligomer 11 was recovered in high purity (86%) as judged
by HPLC analysis of the crude product (Scheme 2).
In conclusion, we have developed the synthesis of succi-
nimidyl (2-azido-2-substituted ethyl) carbamate mono-
mers and have validated the clear benefits of their
combined use with microwave irradiation on solid support
to quickly access urea-based foldamers. Under microwave
assistance, the azide reduction using PMe3 was highly
reproducible and efficient, leading to the preparation of
oligoureas and related hybrids in high purity and good
yields. This improved procedure is likely to facilitate the
preparation of libraries of urea-based foldamers for bio-
logical evaluation as well as the construction of larger and
more sophisticated foldamer architectures.
For comparison, we also prepared 10 using standard
Fmoc chemistry (monomer 1b without microwave
assistance) starting from the same NovaPEG resin. The
mass recovery of crude product based on resin loading was
71%.7a The RP-HPLC purity of the cleaved material was
only 32%, highlighting the overall superiority of the azide
strategy combined with microwave irradiation (Figure 2).
Furthermore, the total time to assemble 10 was consider-
ably reduced under microwave assistance (11 h versus at
least 30 h using Fmoc chemistry).
To further demonstrate the general applicability of this
methodology, we evaluated the SPS of heterogeneous
foldamer backbones made of alternating urea and amide
linkages (e.g., 11). The microwave-assisted SPS of hepta-
mer11isdetailedinScheme 2. γ-Aminoacids wereinserted
in the chain as their N-Fmoc-protected derivatives in the
presence of HBTU/HOBt as coupling reagents with assis-
tance of microwave irradiation. We also examined the
synthesis of 11 using standard Fmoc chemistry without
application of microwave irradiation (see the Supporting
Information for details).7a Mass recovery of crude 11
based on resin loading, following TFA cleavage and
lyophilization, was similar in both cases (46% for the
Fmoc strategy, 50% for the azide strategy). However,
purity based on HPLC analysis of crude 11 was only 30%
when the synthesis was performed using N-Fmoc-pro-
tected monomers 1b (Scheme 2). Analysis of byproducts
Acknowledgment. This work was supported by the
Centre National de la Recherche Scientifique (CNRS),
the Region Aquitaine, the University of Bordeaux, the
European Union (Marie Curie PEOPLE-2010-IEF-
273224-FOLDAPOP, postdoctoral fellowship to K.P.),
and ImmuPharma (CIFRE Fellowship to P.C.).
Supporting Information Available. Experimental pro-
cedures and characterization data for 2ꢀ11. This material
is available free of charge via the Internet at http://pubs.
acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 12, 2012
3133