Solid phase synthesis of aromatic oligoamides: Application to helical water-soluble foldamers

Article abstract of DOI:10.1021/jo101360h

Synthetic helical aromatic amide foldamers and in particular those based on quinolines have recently attracted much interest due to their capacity to adopt bioinspired folded conformations that are highly stable and predictable. Additionally, the introduction of water-solubilizing side chains has allowed to evidence promising biological activities. It has also created the need for methods that may allow the parallel synthesis and screening of oligomers. Here, we describe the application of solid phase synthesis to speed up oligomer preparation and allow the introduction of various side chains. The synthesis of quinoline-based monomers bearing protected side chains is described along with conditions for activation, coupling, and deprotection on solid phase, followed by resin cleavage, side-chain deprotection, and HPLC purification. Oligomers having up to 8 units were thus synthesized. We found that solid phase synthesis is notably improved upon reducing resin loading and by applying microwave irradiation. We also demonstrate that the introduction of monomers bearing benzylic amines such as 6-aminomethyl-2-pyridinecarboxylic acid within the sequences of oligoquinolines make it possible to achieve couplings using a standard peptide coupling agent and constitute an interesting alternative to the use of acid chloride activation required by quinoline residues. The synthesis of a tetradecameric sequence was thus smoothly carried out. NMR solution structural studies show that these alternate aminomethyl-pyridine residues do not perturb the canonical helix folding of quinoline monomers in protic solvents, contrary to what was previously observed in nonprotic solvents.

Full text of DOI:10.1021/jo101360h

pubs.acs.org/joc
Solid Phase Synthesis of Aromatic Oligoamides: Application to Helical
Water-Soluble Foldamers
‡
t Baptiste,†,§ Celine Douat-Casassus,‡,§ Katta Laxmi-Reddy,‡,§ Frederic Godde, and
ꢀ
ꢀ ꢀ
Benoıˆ
Ivan Huc*,‡
†
ꢀ ꢀ
Universite de Bordeaux, EA 4138-Pharmacochimie, 146 rue Leo Saignat, 33076 Bordeaux Cedex, France,
and ‡Institut Europeꢀen de Chimie et Biologie, Universiteꢀ de Bordeaux-CNRS UMR 5248,
2 rue Robert Escarpit, 33607 Pessac Cedex, France. §These authors contributed equally to this work.
i.huc@iecb.u-bordeaux.fr
Received July 11, 2010
Synthetic helical aromatic amide foldamers and in particular those based on quinolines have recently
attracted much interest due to their capacity to adopt bioinspired folded conformations that are highly stable
and predictable. Additionally, the introduction of water-solubilizing side chains has allowed to evidence
promising biological activities. It has also created the need for methods that may allow the parallel synthesis
and screening of oligomers. Here, we describe the application of solid phase synthesis to speed up oligomer
preparation and allow the introduction of various side chains. The synthesis of quinoline-based monomers
bearing protected side chains is described along with conditions for activation, coupling, and deprotection on
solid phase, followed by resin cleavage, side-chain deprotection, and HPLC purification. Oligomers having
up to 8 units were thus synthesized. We found that solid phase synthesis is notably improved upon reducing
resin loading and by applying microwave irradiation. We also demonstrate that the introduction of
monomers bearing benzylic amines such as 6-aminomethyl-2-pyridinecarboxylic acid within the sequences
of oligoquinolines make it possible to achieve couplings using a standard peptide coupling agent and
constitute an interesting alternative to the use of acid chloride activation required by quinoline residues. The
synthesis of a tetradecameric sequence was thus smoothly carried out. NMR solution structural studies show
that these alternate aminomethyl-pyridine residues do not perturb the canonical helix folding of quinoline
monomers in protic solvents, contrary to what was previously observed in nonprotic solvents.
Introduction
prepare 14-helical foldamers combining β3-amino acids and
constrained (cyclic) β-amino acids,2 and Royo et al. des-
cribed the SPS of constrained γ-peptides.3 Guichard et al.
reported the SPS of oligoureas4 and the elucidation of their
helical conformation on solid support.5 Peptoids, which are
Solid phase synthesis (SPS) is commonly used to prepare
different kinds of peptidic aliphatic foldamers using proce-
dures similar to those developed for R-peptides. For exam-
ple, Seebach et al. synthesized the first foldamers based on
β3-amino acids on solid support.1 Gellman et al. used SPS to
(3) Farrera-Sinfreu, J.; Zaccaro, L.; Vidal, D.; Salvatella, X.; Giralt, E.;
Pons, M.; Albericio, F.; Royo, M. J. Am. Chem. Soc. 2004, 126, 6048–6057.
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DOI: 10.1021/jo101360h
r
Published on Web 10/14/2010
J. Org. Chem. 2010, 75, 7175–7185 7175
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Baptiste et al.
JOCArticle
also known to fold into helices, have been prepared on solid
support as well.6
matic oligomers is rendered more difficult than for aliphatic
peptides both in solution and in the solid phase because of
the poor nucleophilicity of some aromatic amines. It is not
uncommon that strong acid activation such as an acid
chloride is required for coupling to proceed.15 This difficulty
is particularly serious for helically folded structures for
which folding in the reaction mixture results in steric hin-
drance, leading to slower reactions and generally poorer
yields, in particular when coupling together two long
sequences.16 Synthetic problems associated with folding
and aggregation are notorious in the SPS of long aliphatic
R-peptides as well.17 In the case of aromatic oligoamide
foldamers, these problems are not easy to solve because the
folded structures are generally extremely stable and do not
denature under ambient conditions.14a,15a,16,18 A solution has
been proposed that consists of temporarily changing some
secondary amides into tertiary amides so as to disrupt the folded
structures and reduce hindrance.19 Alternatively, we have
shown that some benzylic amines could be made compatible
with the folding modes of aromatic oligoamides foldamers and
bring both a higher nucleophilicity and a higher flexibility,
which in turn allowed the solution synthesis of long oligomers.20
With the aim of speeding up the preparation of helically
folded quinoline-derived aromatic amide oligomers and to
facilitate the introduction of multiple proteinogenic side
chains in their sequences in view of potential biological
applications, we have investigated and now report the meth-
ods for their synthesis on solid phase. Three different side
chains were introduced within the foldamer sequences: a
leucine-like, an ornithine-like, and an asparate-like side
chain. Protocols for the synthesis of rigid, purely aromatic
backbones have been optimized using acid chloride activa-
tion. Alternate and easier routes that make use of some
aliphatic (benzylic) amines in the sequences and conven-
tional coupling agents have also been validated. Further-
more, because this later approach generates more flexible
backbones, structural studies in solution have been carried
out to verify the helical folding of the resulting oligomers in
protic solvents and thus confirm the appropriateness of
aliphatic amines in this context.
SPS has less commonly been applied to aromatic oligoa-
mide sequences, and in all cases the reduced reactivity of
aromatic amines as compared to aliphatic amines had to be
overcome. In the synthesis of pyrrole- and imidazole-based
oligoamides, Dervan et al. observed incomplete couplings to
imidazole amines when using acids activated with the con-
ventional coupling reagent HBTU. A complete acylation of
the imidazole amine could be restored when the authors
applied the mixed anhydride activations, in the presence of,
e.g., Boc-pyrrole anhydride.7 Similarly, Kilbinger et al. did
not observe successful couplings of aromatic secondary
amines when using traditional solid phase coupling proce-
dures, and in their case, even mixed anhydride activation was
inefficient.8 They finally succeeded by using an activation
method originally reported by Ueda et al.9 involving SOCl2
(1 equiv) in NMP leading to the formation of an acid chloride
under mild conditions. This work was recently extended by
Wilson et al., who prepared aromatic-aliphatic tertiary
amides that project their side chains in a way similar to that
of an R-helix.10
To the best of our knowledge, there has been no report on
the SPS of stable helical aromatic amide foldamers. The need
to develop and extend the scope of SPS of aromatic oligoa-
mides is becoming obvious with the emergence of multiple
potential biological applications of these compounds.11-14
Indeed, the optimization of biological activities often pro-
ceeds through the parallel synthesis of multiple structural
variants possessing different proteinogenic side chains,
which is best achieved on solid phase, as recognized by
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SCHEME 1. Synthesis of N-Fmoc-Quinoline Monomers 3a, 3b, and 3ca
aThroughout the manuscript, quinoline monomers are symbolized by “Q” and their side chains in position 4 by superscript iBu, þp, and -p for
isobutoxy, -O(CH2)3-NHBoc, and -OCH2-CO2tBu, respectively, where p stands for “protected”.
SCHEME 2. Solid Phase Oligoquinoline Synthesis on Bromomethyl Wang Resin
Results and Discussion
The first monomer 3a was anchored to the commercial
bromomethyl Wang resin in the presence of cesium iodide
using conditions reported by Morales et al.21 (Scheme 2). The
Fmoc protecting group was easily removed under standard
deprotection conditions (20% piperidine in DMF). The
introduction of the second monomer gave rise to the problem of
choosing a suitable acid activation method. Our experience
with solution synthesis has been that coupling agents (e.g.,
HBTU, PyBOP) give satisfactory results only when coupling
two monomers and do not perform as well when one of the
reagents is a dimer or longer. Acid chloride activation,
however, though less convenient, is generally high-yielding.
Additionally, it can be carried out under neutral conditions
compatible with the presence of acid-labile side-chain pro-
tecting groups when using the Ghosez reagent (1-chloro-N,
N,2-trimethylpropenylamine).22 Activation via acid chloride
Solid Phase Synthesis of Aromatic, Quinoline-Based Oli-
goamides. Helically folded quinoline oligoamides bearing iso-
butoxy side chains (QiBun) have been synthesized, thoroughly
characterized, and introduced in multiple foldamer architec-
tures.15a,16,20 This motif was thus chosen as a model system for
first investigating SPS. The TFA-labile Wang resin (normal
loading of 1.20 mmol/g) was selected as a starting resin as it
offers the possibility to synthesize oligomers using an Fmoc-type
strategy. For this purpose, we needed to first prepare an Fmoc-
protected 8-amino-2-quinolinecarboxylic acid monomer bearing
an isobutoxy side chain in position 4 (monomer 3a in Scheme 1).
Hence, the previously reported quinoline acid 1a15a was hydro-
genated to give amine 2a, which was then easily protected in the
presence of Fmoc-chloride to give 3a in 89% yield.
(21) Morales, G. A.; Corbett, J. W.; DeGrado, W. F. J. Org. Chem. 1998,
63, 1172–1177.
(22) Ghosez, L.; Haveaux, B.; Viehe, H. G. Angew. Chem., Int. Ed. Engl.
1969, 8, 454–455.
J. Org. Chem. Vol. 75, No. 21, 2010 7177

Baptiste et al.
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formation thus remains well adapted to quinoline amino
acids presenting functional side chains with protecting
groups, such as Boc or tert-butyl ester. Activation of 3a
using the Ghosez reagent and coupling the resulting acid
chloride 5a to the amine residue of the first resin-bound
monomer was complete when using 2 equiv of acid chloride.
Cycles of piperidine deprotection followed by acid chloride
coupling were applied smoothly up to the tetramer, the last
residue bearing a nitro group at the N-terminus. After TFA
cleavage, the tetramer (n = 3 in Scheme 2) QiBu4 was obtained
in good yield (65% from the resin loading) and good purity of
the crude product. Product purity was conveniently assessed
using reverse phase HPLC analysis, even though this technique
is unconventional for such highly hydrophobic compounds (see
Supporting Information for details). Product identity was
confirmed by comparison with a genuine sample obtained via
involved heating at 50 °C for 10 min with 50 W microwave
power. Moreover, to reduce truncated oligomer formation,
capping of potentially uncoupled amines in the presence of
acetic anhydride was implemented after each coupling.
HPLC analysis of QiBu and QiBu synthesized under these
6
8
conditions did not reveal significant improvements in yield
or purity. Nevertheless, the coupling times could be dimin-
ished by a factor of 3, thus rendering possible the synthesis of
an octamer in half a day. The only limitation of microwave-
based synthesis is the resin amount that can be loaded in the
microwave glass reactor, which cannot be scaled up at will.
Encouraged by the successful SPS of QiBu oligomers, we
tested this methodology to prepare oligomers bearing pro-
tected water-solubilizing side chains that may have biological
applications.14 Monomers bearing ornithine-like side chains
(Qþ) protected with a Boc group (Qþp) have already been
described;14 Fmoc-protected monomer 3b was prepared using
the same conditions as for 3a (Scheme 1). In addition, a new
anionic monomer (Q-) bearing an aspartate-like side chain
protected as a tBu ester (Q-p) was also prepared (Scheme 1).
Fmoc-Q-p-OH monomer 3c was prepared starting with a
Williamson alkylation of a nitro-quinolinone precursor in
77% yield. The delicate selective saponification of the methyl
ester in the presence of the tBu ester was conducted under mild
conditions in the presence of LiOH to give 1c in 85% yield,
which was then converted to 3c in 69% yield. Acid chloride
activation of 3b and 3c was achieved with the Ghosez reagent,
which left intact the tBu and Boc side-chain protections,
giving 5b and 5c, respectively.
1
solution phase synthesis, by LC-MS analysis, and by H
NMR (see Supporting Information).
This result prompted us to extend the use of SPS to the
synthesis of longer sequences. However, when this metho-
dology was first applied to the synthesis of the octamer
(O2N-QiBu8-OH), HPLC analysis revealed the presence of
oligomers with one or more deletions in non-negligible
quantity (penta-, hexa-, and heptamers). Even when double
couplings were carried out (i.e., 2 ꢀ 2 equiv of acid chloride
5a without any deprotection in between), deletions persisted
beyond the fourth residue, and the yield and purity of the
octamer were unsatisfactory. It has been previously reported
that the solid phase synthesis of foldamers differs from
standard R-peptides in the sense that during the course of
the synthesis, the increased folding onto the support is
expected to influence the efficiency of coupling and/or
deprotection reactions.23 One possibility to circumvent these
problems of steric hindrance, accessibility, and/or poor
reactivity related to the aggregation/folding of the oligomer
onto the support is to use a resin with a lower loading and a
higher bead size. Thus, we decided to use the commercially
available low loading Wang resin (0.48 mmol/g). First, the
commercial hydroxymethyl group of the Wang resin was
converted to a bromomethyl. Acylation steps were again
optimized by doing a double coupling of each monomer after
the formation of the tetramer. This proved successful as
hexamer 7 and octamer 8 were obtained in this way with
satisfactory yields (60%) and high purities, as confirmed by
HPLC analysis and 1H NMR (Scheme 2). These first results
demonstrate two clear advantages of SPS over solution
phase: the perspective of parallel synthesis of foldamer
libraries for biological screening and rapidity. Indeed, each
deprotection/coupling cycle amounts to 1 h (or 2 for the
double couplings), which makes it possible to prepare an
octamer in only 1 day.
We had planned to carry out SPS of oligoamides using
these monomers on a resin that would allow resin cleavage
under very mildly acidic conditions to avoid side-chain
deprotection. Among the large variety of resins commercially
available, the 2-chlorotrityl resin appeared to be well suited for
this purpose. Oligomers of monomer 3c were prepared to
investigate the efficiency of protected oligoquinoline synthesis
on this support. Monomer 3c was introduced on a chlorotrityl
resin under the conditions reported by Lipton et al.24 After
removal of Fmoc protecting groups, couplings were carried
out as for the Wang resin using, e.g., acid chloride 5c (Scheme 3).
However, oligomer release in AcOH/TFE/DCM showed
that sequences hardly grow beyond four units and that yields are
very poor. It might be that acidity is too high during couplings,
even in the presence of excess DIEA, or that the aromatic acid-
resin bond is simply too labile and resin cleavage occurs during
synthesis. In any case, the trityl resin proved unpractical, and we
returned to the synthesis on low loading Wang resin of homo-
logous oligomer Q-6 (9) and heterologous oligomers (QþQ-)n
(10, n = 2; 11, n = 3). Syntheses were performed under the
conditions optimized for QiBu via acid chloride activation
8
(Scheme 3). In the case of monomer 3b, activation was carried
out following the same procedure, but once formed the corre-
sponding acid chloride 5b was no longer soluble in DCM, and
dry THF was used instead as the solvent for solid phase coupling
(Scheme 3). We noticed that double couplings were useful as
early as the third residue. Cleavage from the resin and side-chain
deprotection was achieved using a TFA/TIS/H2O mixture
(95:2.5:2.5 v/v/v), which proved to be more efficient than the
previously used TFA/DCM (1:1) mixture. Indeed, standard
Inspired by a report by Gellman et al.2a describing that
microwaves improve the synthesis of 14-helical β-peptides on
solid phase, we decided to examine the effects of microwaves
on aromatic amine coupling. Microwaves were applied only
during coupling steps and not during deprotection since the
latter is carried out without any difficulty. THF was used as
solvent in place of DCM as it makes it possible to heat to
higher temperatures. The best microwave conditions found
ꢀ
(24) C-alimsiz, S.; Ramos, A. I. M.; Lipton, M. A. J. Org. Chem. 2006, 71,
6351–6356.
(23) Raguse, T. L.; Porter, E. A.; Weisblum, B.; Gellman, S. H. J. Am.
Chem. Soc. 2002, 124, 12774–12785.
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SCHEME 3. Solid Phase Synthesis of Water-Soluble Oligoquinolines
peptide RP-HPLC analysis revealed fewer byproducts when the
tri-isopropyl-silane (TIS) scavenger was added. The method was
validated to deliver various water-soluble oligomers possessing
nitro (9), amino (10), or Fmoc (11) termini, in 20-30% isolated
(PþP-)4 oligomer. Starting from chelidamic dimethyl ester
12, a reaction scheme similar to that of Boc-PiBu-OH mono-
mer was applied to the synthesis of Fmoc-P-OH monomers
18a and 18b bearing either an asparate-like side chain (Fmoc-
yields after HPLC purification.
P
-p-OH) or an ornithine-like side chain (Fmoc-Pþp-OH)
(Scheme 4). These monomers were purified using preparative
scale HPLC; without this extra purification, we found that
SPS worked very poorly. SPS was then carried out using
classical peptide synthesis conditions. Once the first monomer
18b was anchored on a low loading Wang resin, Fmoc-depro-
tection/coupling cycles were repeated alternatively with 18a and
18b. PyBOP, in combination with HOBt, was selected as a
coupling agent. After TFA cleavage, the (PþP-)4 oligomer 19
was easily isolated in relatively good yield (38%) and with a
good purity as illustrated in the Supporting Information.
Previous structural studies had shown that PiBun oligomers
do not adopt stable folded conformations in CDCl3 despite
the structural similarity between P and Q units, because of
the greater flexibility imparted by the methylene bridges.20a
We sought indications of folding of the (PþP-)4 oligomer 19
in protic solvents (CD3OH and H2O/D2O) with the idea that
solvophobic effects could make aromatic stacking stronger
in these media and promote folding. However, we could not
find any evidence of it. The typical features which indicate
folding of such oligomers were not observed, in particular
upfield shifts of NMR resonances due to ring current effects
and the spreading of NMR signals over a wide chemical shift
range. On the other hand, previous studies have also shown
that P units adopt the same folding mode as Q units provided
that the fraction of the latter in the sequences is sufficiently
large.20a We thus decided to explore the SPS of (PQ)n
oligomers in which PQ segments are first prepared in solu-
tion and then coupled on solid support, presuming that
coupling to aliphatic amines was sufficiently efficient to
incorporate dimeric units at once, hence dividing the number
of coupling steps on solid support by two to reach sequences
of the same length.
This first set of results makes it possible, in principle, to
prepare libraries of short helical quinoline oligoamide
foldamers via parallel synthesis on solid phase. Limitations
reside in the need to use acid chloride activation, which is not
the most convenient, and in average to low yields that do not
allow easy access to oligomers longer than 8-10 units. To
circumvent these limitations, we decided to explore the SPS
of hybrid sequences composed of both fully aromatic amino-
quinolinecarboxylic acid monomers and benzylic 6-amino-
methyl-2-pyridinecarboxylic acid monomers (P), which we
have shown to much improve solution phase synthesis while
being able to adopt folded conformations compatible with
the helices formed by Qn monomers.20
Solid Phase Synthesis with 6-Aminomethyl-2-Pyridinecar-
boxylic Acid Units (P). P units (Scheme 4) are comparable to
Q monomers in that they are δ-amino acids displaying the
same set of atoms toward the inner rim of the folded helices.
However, they introduce two major differences. Firstly, they
provide more conformational flexibility than Q units. They
thus decrease helix stability when introduced into Qn se-
quences because of the loss of conjugation and additional
rotatable bond about the methylene group and because of
the reduced surface for π-π stacking interactions provided
by the pyridine ring as compared to the quinoline ring. Such
a decrease in helix stability is expected to reduce steric
hindrance during synthesis. Secondly, benzylic amines as in
P units are considerably more nucleophilic than those of
8-aminoquinoline monomers, which allows facile synthesis
using standard coupling reagents as was shown in the solu-
tion preparation of sequences composed of up to 40 units. 20
Hence, following the previously reported solution synthe-
sis of PiBu8, we first considered the SPS of a hydrophilic
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SCHEME 4. Synthesis of Fmoc-Aminomethylpyridine Monomers and Rapid Synthesis of 2-Aminomethylpyridine Oligomers on Solid
Support from Fmoc-Aminomethylpyridine Monomers
For this purpose, the water-soluble dimer Fmoc-PþpQ-p
-
sharpness, wide chemical shift distribution, and strong upfield
shifts of 1H NMR signals. Presumably, the solvophobic effects
associated with intramolecular stacking of aromatic rings
prevails in water and compensates for potentially weaker
effects of hydrogen bonds or electrostatic repulsions due to
competing interactions with the solvent. However, as men-
tioned above, solvophobic effects are not strong enough to
promote the folding of oligomers consisting of P units exclu-
sively. We thus considered it important to assess the solution
conformation of (PQ)n oligomers in protic solvents to evaluate
the extent to which P units may replace Q monomers in the
sequences, thus making the SPS method described above a
valid alternative to the more difficult SPS of Qn oligomers. In
chlorinated solvents, (PiBuQiBu)4 has been shown to undergo a
rapid equilibrium between several folded conformations: a
canonical helix similar to that of Qn oligomers and so-called
“herringbone helices” in which consecutive PQ segments are
perpendicular to each other (Figure 1).20a Each of these con-
formations gives rise to some unambiguous intramolecular
NOE correlations, which makes it possible to distinguish them.
Structural studies on water-soluble (PQ)n oligomers were
conducted on the fully cationic octamer H2N-(PþQþ)4-OMe
to allow direct comparison with the structure of Boc-(PiBuQiBu)4-
OMe, whose structure was previously solved in CDCl3 andinthe
solid state.20a H2N-(PþQþ)4-OMe was in fact prepared using
solution phase synthesis, and its conformation was studied prior
to optimizing SPS. Details for the solution phase protocols are
OH 22 was synthesized as a new building block for SPS
(Scheme 5). It was necessary to change the protection of the
quinolinecarboxylic acid function as we encountered diffi-
culties in selectively saponifying the methyl ester of Fmoc-
P
þpQ-p-OMe in the presence of the tBu ester (not shown). We
thus introduced a benzyl ester via a transesterification under
mildly basic conditions (Scheme 5). PQ dimer 22 was obtained
in good yield in two steps from amino-quinoline 20 and was
engaged in SPS. As for P monomers, dimer 22 was HPLC
purified on a preparative scale prior to SPS. After loading the
first dimer onto the resin, six standard couplings proceeded
smoothly up to (PþQ-)7 (Scheme 5). The final amine was then
acetylated. We noticed that the coupling of aminomethyl-
pyridine residues was remarkably efficient, as only 2 equiv of
Fmoc-PþpQ-p-OHwassufficient foreachcoupling step. After
TFA cleavage, the 14-mer 23 was obtained with a crude purity
of 90%. The isolated yield after HPLC purification was only
19%, suggesting that some material was lostin the process and
that room for optimization still exists. Hence, despite the
multiple steps for the preparation of PQ building block, this
strategy allows the preparation of a 14-mer in only 6 efficient
coupling steps on solid phase.
Solution Structural Investigation of PþpQ-p Oligomers.
We have previously described the very high stability of the
helical conformation of sequences consisting exclusively of Q
monomers in chlorinated or aromatic solvents15a,25,26 and also
found indirect evidence that these helices are at least as stable
in protic media.14a,b This is for example illustrated by the
reported in the Supporting Information. The structures of QiBu
,
n
(PiBuQiBu)4, and (PiBuQiBu4)n oligomers have been consistently
investigated in the solid state using X-ray crystallography, taking
advantage of the high crystallizability of these compounds.15,20,25
Solid state structural investigation was thus attempted on water-
soluble oligomers but proved unsuccessful. Crystals with suitable
ꢀ
(25) Jiang, H.; Leger, J.-M.; Huc, I. J. Am. Chem. Soc. 2003, 125, 3448–
3449.
ꢀ
(26) Dolain, C.; Grelard, A.; Laguerre, M.; Jiang, H.; Maurizot, V.; Huc,
I. Chem.;Eur. J. 2005, 11, 6135–6144.
7180 J. Org. Chem. Vol. 75, No. 21, 2010

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SCHEME 5. Preparation of an Fmoc-PþpQ-p-OH Building Block and Preparation of Ac-(PþQ-)7-OH Using Standard SPS
Procedures; RP-HPLC Chromatogram and ESI-MS Spectra of Ac-(PþQ-)7-OH Oligomer after Release from the Support
barriers to crystallization and to long-range order in the solid
state. The structure of H2N-(PþQþ)4-OMe was thus studied in
solution by NMR, both in CD3OHandinH2O/D2O (90:10 v/v).
Spectra were all measured at 50 °C where they were sharper,
some slight broadness being observed at 25 °C. Results are
similar in the two solvents; spectra recorded in H2O/D2O are
slightly complicated by the slow hydrolysis of the methyl
ester function (evidenced by NMR and mass spectrometry)
and thus the emergence of a second set of signals that does
not occur in methanol.
We initially attempted to carry out a full solution structural
investigation. The spin systems of the different residues were
identified in each solvent from combined HMQC, HMBC,
FIGURE 1. Side view and top view of energy minimized conforma-
tions (MacroModel v8.6; force field MM3) of (PQ)8 as a canonical
COSY, and TOCSY experiments as we have previously
reported.20,26 For example, HMQC experiment allowed us to
assign protons to either P or Q monomers (Figure SI-NMR 1 in
Supporting Information). COSY and TOCSY experiments
revealed scalar couplings between H5, H6, and H7 protons of
the quinoline rings and some cross-peaks between H3 and H5
protons of the pyridine moieties (Figure 2a). HMBC experi-
ments (Figure 2b and Figure SI-NMR 3-8 in Supporting
Information) were performed to distinguish H5 from H7 pro-
tons on each quinoline and H3 from H5 on each pyridine ring.
helix (a) and as a noncanonical “herringbone” helix (b). Amide and
quinoline moieties are shown in gray; 6-aminomethyl-pyridine units
are shown in red.
size and shape were obtained for Ac-(PþQ-)7-OH 23 using
various crystallization conditions, but diffraction was too poor
to solve the structure, even under synchrotron radiation.
Similar negative results were obtained with water-soluble Qn
oligomers. This might originate from the flexibility of the
multiple ornithine-like residues susceptible to create entropic
J. Org. Chem. Vol. 75, No. 21, 2010 7181

Baptiste et al.
JOCArticle
FIGURE 2. 2D NMR spectra of H2N-(PþQþ)4-OMe in CH3OD at 323 K from a COSY (a) and an HMBC experiment (b). The structures on
the right show the observed correlations (in bold).
Finally, the following long-range correlations allowed the com-
plete assignment of each spin system: H6-C10 (J3), H6-C8
(J3), H5-C4 (J3) for quinolines and H3-CO (J3) for pyridines
(Figure SI-NMR 3-7 in Supporting Information).
with 4.7-5.6 in CDCl3. These large differences may simply
result from a better folding in protic solvents that would give
rise to a more compact conformation but may also be the result
of the prevalence of the canonical helix in which stacking
interactions are more extensive throughout the structure, parti-
cularly for the benzylic CH2 residues.
However, a few missing or overlapping correlations, in
particular correlations between the amide carbon and amide
proton, did not allow us to reconstitute the sequence of spin
systems and fully assign 1H and 13C spectra, even when
crossing information collected in water and methanol. Chan-
ging mixing times and using a 700 MHz instrument were
unsuccessful. Information from NOE maps could only be
used partially (see below). However, sufficient indirect evi-
dence of folding could be collected to point with a high
confidence to folding into a canonical helix of this sequence.
First of all, 1H NMR signals spread over a wide chemical shift
range and indicate substantial and selective upfield shifts as
expected for a folded structure (Figure 3). Chemical shift values
indicate that ring current effects are stronger in protic solvents
than in CDCl3. The signal of the terminal CH3 ester is found at
3.51 ppm in CDCl3 for (PiBuQiBu)4-OMe and at 3.31 and 3.01
ppm for H2N-(PþQþ)4-OMe in methanol and water, respec-
tively. The signals of the main chain benzylic CH2 signals are
found in the 2.60-5.00 ppm range in protic solvents, compared
Most importantly, the benzylic CH2 protons of the backbone
show a diastereotopic pattern (Figure 3) both in water and
methanol, indicating a slow exchange between right-handed
and left-handed conformations, in contrast with the fast exchange
observed in CDCl3 between the right- and left-handed forms
of both canonical and herringbone helices.20a Diastereotopicity
is expressed by large Δδ values of 0.40, 1.02, and 1.17 ppm
for the inner methylene units of the sequence in MeOH-d3 at
323 K. These values compare well with those observed in
QiBu4PiBunQiBu oligomers, which have been shown to fold
4
exclusively as canonical helices in chloroform.20b The diaster-
otopic patterns of H2N-(PþQþ)4-OMe are very sharp in water
and broader in methanol, suggesting a slower exchange and so
a higher stability in water. The folded structure thus appears to
be substantially more stable in protic solvents than in CDCl3. It
is unlikely, though formally not impossible, that fast equilibria
might exist between several types of folded structures (e.g., a
7182 J. Org. Chem. Vol. 75, No. 21, 2010

Baptiste et al.
JOCArticle
FIGURE 3. 1H NMR spectrum of H2N-(PþQþ)4-OMe in H2O/D2O (90:10 v/v) at 323 K. The arrows indicate clear diastereotopic patterns of
the signals of the backbone benzylic CH2 protons.
canonical helix and herringbone helices), while slow equilibri-
um is observed between the right- and left-handed conforma-
tions. The diastereotopic patterns therefore hint at the strong
prevalence of one stable conformation in protic solutions,
contrary to the mixture of conformers observed in CDCl3.
Further support comes from an intense NOE correlation
between the terminal CH3 ester and a single H3 proton
belonging to a quinoline residue remote in the sequence. Such
a correlation is, in principle, compatible with both a herring-
bone helix and a canonical helix conformation, but it should
occur with two distinct H3 protons. The observation of a single
correlation again suggests the prevalence of one conformation.
Finally, another indication that the canonical helix pre-
vails in protic solvents was provided by the pattern of NOE
correlations between aromatic and aliphatic amide signals
FIGURE 4. NOE correlations between the amide protons derived
from aliphatic amines (NHp) and amide protons derived from
(Figure 4). Two amide protons consecutive in a sequence are
expected to produce an NOE correlation when folded in a
aromatic amines (NHq). The following correlations are identified:
canonical helix, whereas in herringbone helices, this is true
only across Q monomers for which both neighbor amides are
a0-b, a0-d, b0-a, b0-c, c0-c, c0-d).
in the plane of the quinoline ring and not across P monomers.
activity. In any case, the possibility to combine P and Q
The pattern of observed correlations, even though they
monomers expands the range of tools available for solid
cannot be precisely assigned, is fully consistent with a
phase synthesis and provides highly modular sequences in
canonical helix. It shows that two aromatic amides correlate
which side chains can be modified without changing folding
with two aliphatic amides, whereas another two aromatic
propensity and in which backbone stability can be modified
amides (the N- and C-terminal aromatic amides) correlate
without changing the overall conformation and the projec-
with only one aliphatic amide. Conversely, each aliphatic
tion in space of the side chains.
amide correlates with two aromatic amides. Again, if both
canonical and herringbone helices were coexisting, a larger
number of correlations would be expected, and the same set
Conclusion
of correlations could not be produced by a single herring-
bone helix alone.
We have shown that the judicious application of solid phase
synthesis allows the rapid preparation of quinoline oligoamides
equipped with different peptide-like side chains, thus paving the
way for the parallel synthesis of oligomers relevant to screening
assays. The poor nucleophilicity of aromatic amines combined
with the stable folding of Q oligomers on the resin makes com-
plete acylation difficult beyond the fourth residue, thus dimin-
ishing the purity of the crude material. Reducing the loading of
the Wang resin circumvented this problem, and several Qn
oligomers were obtained in good yields. Furthermore, micro-
wave irradiation efficiently speeds up the SPS of aromatic
Altogether, these data demonstrate that the folded struc-
ture of (PQ)n oligomers in protic solvents is both more stable
and better defined than in aprotic medium. All patterns
identified so far support the claim that this folded structure
is a canonical helix. The insertion of P units in Qn sequences
without disturbing the overall helix conformation is thus
better tolerated in water. It remains to be demonstrated
whether changing a Q unit into a P unit having the same
side chain would result in the same potential biological
J. Org. Chem. Vol. 75, No. 21, 2010 7183

Baptiste et al.
JOCArticle
oligoamides with a complete synthesis of QiBu8 oligomer in only
half a day. As a future prospect, the automation of this proce-
dure would offer the possibility of rapidly generating libraries
of Qn oligomers.
Standard Protocol for Fmoc Removal. The Fmoc protecting
group was removed with a solution of piperidine in DMF (1:4)
for 3 min. After filtration, the procedure was repeated for 7 min,
followed by washing (2ꢀ each) with DMF, MeOH, and DCM.
The chloranil test showed the presence of free amine giving a
green coloration to the resin.
General Procedure for Acid Chloride Activation. To a solution
of 3a (0.270 g, 0.56 mmol) in dry DCM (5 mL) under inert
atmosphere was added 1-chloro-N,N,2-trimethyl-1-propenyla-
mine (0.115 mL, 1,12 mmol). The reaction was stirred at room
temperature for 2 h after which time the solvent was removed,
and the sample was dried under vacuum line to give the
corresponding acid chloride in quantitative yield, which was
used without further purification. A proton NMR was generally
recorded to check purity. 1H NMR (CDCl3, 300 MHz) δ 1.16 (d,
J = 6.7, 6H), 2.31 (m, 1H), 4.06 (d, J = 6.7, 2H), 4.39 (t, J = 7.3,
14.9, 1H), 4.55 (d, J = 7.3, 2H), 7.35 (td, J = 7, 14.4, 12H), 7.45
(td, J = 7, 14.4, 2H), 7.65 (td, J = 8.2, 16.1, 1H), 7.72 (d, J =
7.4, 2H), 7.80 (d, J = 7.4, 2H), 7.88 (dd, J = 1.2, 7.3, 1H), 8.46
(bp, 1H, NH), 9.34 (s, 1H).
Moreover, replacing some quinoline residues by 6-amino-
methyl-2-pyridinecarboxylic acid type monomers P rendered
possible the use of classical coupling agents, and the higher
conformational flexibility of this unit abrogates the risk to
observe truncated oligomers, thus extending the scope of
SPS to oligomers much longer than 8 residues. Hence, by
directly coupling presynthesized PQ dimers, we were able to
synthesize a 14-mer with high efficiency as only 2 equiv of PQ
were necessary for each coupling. Finally, a careful NMR
investigation of a hydrophilic (PQ)4 oligomer in protic
solvents highlighted that a stable canonical helix prevails,
unlike what was previously found in nonprotic solvents. This
conformational behavior makes it possible to insert P units
in place of Q without perturbing the conformation of the
generated oligomers in protic solvents and thus validates the
use of SPS with sequences combining both P and Q units
bearing different side chains for biological purposes.
Standard Protocol for TFA Cleavage. The resin was cleaved
with a TFA/DCM mixture (5 mL, 1:1, v/v) for 1 h in the case of
iBu-type oligoquinolines and with a TFA/TIS/H2O mixture
(5 mL, 95:2.5:2.5, v/v/v) for oligoquinolines equipped with
proteogenic-like side chains. After solvent coevaporation the
oligomers were precipitated with MeOH except for oligoquino-
lines bearing ornithine-like side chains, where the precipitation
was carried out with methyl-tert-butyl ether.
Experimental Section
General Procedures. Proton nuclear magnetic resonance (1H
NMR) spectra were recorded in deuterated solvents on 300, 400,
or 700 MHz spectrometers. Chemical shifts are reported in parts
per million (ppm, δ) relative to the signal of the NMR solvent used.
1H NMR splitting patterns with observed first-order coupling are
designated as singlet (s), doublet (d), triplet (t), or quartet (q).
Coupling constants (J) are reported in hertz. Splitting patterns that
could not be interpreted or easily visualized are designated as
multiplet (m) or broad (br). 13C NMR spectra were recorded on 75
or 100 MHz spectrometers. Chemical shifts are reported in ppm (δ)
relative to carbon resonances of the NMR solvent. Mass spectra
(MS) were obtained using electrospray ionization
Dichloromethane was distilled from CaH2. THF was dried by
passing through a neutral alumina column system. Triethyla-
mine (Et3N) and N,N-diisopropylethylamine (DIEA) was dis-
tilled from CaH2. All reagents and solvents were purchased from
commercial suppliers and used without further purification.
Wang resins, HOBT and PyBOP were obtained from Novabio-
chem (Switzerland). Trifluoroacetic acid (TFA) from Sigma-
Aldrich was of spectrophotometric grade (99%). HPLC-quality
acetonitrile (CH3CN) and Milli-Q water were used for HPLC
analyses and purification. Evaporations were conducted under
reduced pressure at temperatures less than 40 °C unless other-
wise noted. Reactions were monitored by analytical thin-layer
chromatography using Merck silica gel 60 F254 plates. Com-
pounds were visualized with a UV lamp (λ 254 nm). Flash
column chromatography was performed under positive pressure
using 40-60 μm silica gel (Merck) and the indicated solvents.
Solid phase reactions were performed manually in glass reactor
with mechanic stirring.
SPS of O2N-(QiBu)8-OH (8). The Wang bromo resin (0.1 g,
0.046 mmol) was reacted with Fmoc-QiBu-OH 3a (0.080 g, 0.092
mmol, 2 equiv relative to the resin loading) in the presence of
cesium iodide (0.024 g, 0.092 mmol) and DIEA (16 μL, 0.092
mmol). The resin was gently shaken overnight. The reaction
solution was removed by filtration. The resin was washed (2ꢀ
each) with DMF, MeOH, and DCM and dried under vacuum.
The Fmoc protecting group was removed according to the
standard protocol. The resin was preswelled with DCM and
acylated with Fmoc-QiBu-Cl 5a (0.090 g, 0.092 mmol) in the
presence of DIEA (33 μL, 0.19 mmol). After 90 min, the resin
was washed (2ꢀ each) with DMF, MeOH, and DCM and then
controlled with the chloranil test. The same acylation step was
repeated five times, and finally the NO2-QiBu-Cl acid chloride
was introduced following the same procedure described above.
Microwave-Assisted SPS of O2N-(QiBu)8-OH. The Wang
bromo resin (0.1 g, 0.046 mmol) was placed in a modified
polypropylene SPE tube (4 mL, Varian, top rim removed with a
razor blade), suspended in DMF (1 mL), and Fmoc-QiBu-OH 3a
(0.043 g, 0.092 mmol, 2 equiv relative to resin loading) following by
cesium iodide (0.024 g, 0.092 mmol) and DIEA (16 μL, 0.092
mmol) were added. The tube was then placed inside a glass 10 mL
microwave reaction vessel. The vessel was placed in the microwave
reactor (CEM Discover) and irradiated (50 W maximum power,
50 °C, ramp 5 min, hold 5 min). All microwave experiments were
conducted at atmospheric pressure. The temperature was con-
trolled by modulation of power, and the sample was cooled with
compressed air. The tube was then removed from the reactor and
washed as for the classical SPS procedure (see above). The removal
of the Fmoc protecting group was done without the assistance of
microwave as described in the standard protocol. The only differ-
ence is that the last washing was done twice with anhydrous THF.
The following acylation steps were achieved by adding to the resin
a solution of Fmoc-QiBu-Cl 5a (0.045 g, 0.092 mmol) and DIEA
(39 μL, 0.23 mmol) in anhydrous THF (2 mL). The tube contain-
ing the resin was again placed inside the glass 10 mL microwave
reaction vessel, itself introduced in the microwave reactor and
again irradiated (50 W maximum power, 50 °C, ramp 5 min, hold
5 min). After coupling, the resin was washed (2ꢀ each) with
DMF, MeOH, DCM, and anhydrous THF. The same microwave
RP-HPLC analyses were carried out on a Thermo Spectra
system using a Chromolith Performance RP-18e column (4.6 mm ꢀ
100 mm, 5 μm) at a flow rate of 3.0 mL min-1 with P1000 XR
pumps, an AS3000 autosampler and a UV 6000 LP diode array
detector. The solvent system was A = 0.1% TFA in H2O and B=
0.1% TFA in CH3CN using the indicated HPLC conditions.
Column effluent was monitored by UV detection at 214 and
254 nm using a diode array detector. Semipreparative purifications
of oligomers were performed on a semi-preparative HPLC using a
˚
C18 column (21.4 mm ꢀ 250 mm, 100 A pore size, 5 μm). Column
effluent was monitored by UV detection at 214 and 254 nm using a
diode array detector.
7184 J. Org. Chem. Vol. 75, No. 21, 2010

Baptiste et al.
JOCArticle
conditions were applied for each acylation step. The efficiency of
coupling was monitored using the chloranil test. After each
coupling, a capping step was carried out with a solution of acetic
anhydride in DCM (2 mL, 1:1, v/v) for 5 min, and the resin was
washed as before. From the tetramer and for longer oligomers, a
double coupling was required to ensure good yields of acylation.
The final coupling was carried out with NO2-QiBu-Cl acid chloride.
SPS of NO2-(Q-p)6. The same procedure as described for the
synthesis of 8 was applied with the exception that the coupling
time was increased to 4 h. NO2-(Q-p)6-OH (9):1H NMR
(CD3OD, 300 MHz) δ 5.02 (2s, 2H), 5.10 (4s, 4H), 5.11 (4s,
4H), 5.28 (s, 2H), 6.39 (s, 1H), 6.42 (s, 1H), 6.51 (s, 1H), 6.71 (s,
1H), 6.82 (s, 1H), 7.12 (s, 1H), 7.38-7.57 (m, 4H), 7.60-7.81 (m,
4H), 7.90 (d, J = 9.82, 1H), 7.99-8.18 (m, 5H), 8.32 (d, J =
6.80, 1H), 8.55 (d, 1H), 8.75 (d, J = 7.74, 1H), 8.90 (d, J = 7.37,
1H), 11.38 (2s, 2H), 11.46 (s, 1H), 11.57 (s, 1H), 11.73 (s, 1H).
HRMS (ESI) m/z calcd for C72H46N12O272-, 1510.2606; found
sulfonylchloride (TNBS) test shows the presence of free primary
amine giving a red coloration to the resin. The resin was then
acylated with Fmoc-P-p-OH 18a (0.046 mg, 0.096 mmol) in the
presence of PyBOP (0.096 g, 0.192 mmol), HOBt (0.025, 0.192
mmol), and DIEA (33 μL, 0.19 mmol). After 4 h, solvents were
removed by filtration. The resin was washed as before, and then
controlled with the TNBS test. The same acylation step was
alternatively repeated six times with either Fmoc-Pþp-OH 18b
or Fmoc-P-p-OH 18a. Finally, the Fmoc protecting group was
removed before TFA cleavage. H2N-(P-Pþ)4-OH (19): 1H NMR
(D2O/H2O 10:90 v/v, 300 MHz) δ 1.79-2.15 (m, 8H), 2.86-3.17
(m, 8H), 3.61-3.85 (m, 8H), 3.86-4.11 (m, 6H), 4.18 (s, 1H),
4.25 (s, 1H), 6.24 (d, J = 1.8, 1H), 6.27 (d, 2H), 6.38 (d, J = 1.9,
1H), 6.49 (d, J = 2, 1H), 6.58 (d, J = 1.8, 1H), 6.68 (d, 2H), 6.74
(d, J = 1.9, 1H), 6.76 (d, 1H), 6.80 (d, 1H), 6.83 (d, 1H), 7.00 (d,
J = 1.9, 1H), 7.13 (d, 1H), 7.33 (d, J = 1.9, 1H), 7.61 (d, 1H),
9.01-9.13 (m, 3H), 9.43 (d, 1H), 9.63 (d, 1H), 9.80 (d, 1H), 9.86
(d, 1H). MS (ESI) m/z calcd for C76H87N20O25þ, 1679.61; found
Mass 1680.81 [M þ D] þ (deuterated compound, see Supporting
Information).
Mass 755.1354 [M - 2H]2-
SPS of H2N-(Q-p þp)2. The same procedure as described for
the synthesis of 8 was applied with the exception that THF was
used as coupling solvent in acylation step involving Fmoc-Qþp
.
Q
-
SPS of Ac-(PþpQ-p)7. The same procedure as described for
the preparation of resin-bound 19 was applied, and Fmoc-
Cl 5c and the coupling time was increased to 4 h. Double
couplings were conducted starting from the third residue.
H2N-(Q-Qþ)2-OH (10): 1H NMR (CD3OD, 300 MHz) δ
2.22-2.56 and 3.20-3.57 (m, 12H), 4.05-4.33 (m, 4H), 5.04
(d, J = 7.05, 1H), 5.99 (d, J = 7.60, 1H), 6.28 (s, 1H), 6.53 (s,
1H), 7.00 (1s, 1H), 7.06 (t, J = 7.57, 15.94, 1H), 7.19 (t, J = 7.92,
15.80, 1H), 7.44 (1d, J = 8.12), 7.61 (1s, 1H), 7.76 (1 m, 4H), 8.12
P
þpQ-P-OH 22 was used as the monomer to be coupled. After
removal of the last Fmoc protecting group the free amine was
acetylated with a solution of acetic anhydride in DCM (1:1, v/v)
1
for 1 h at room temperature. Ac-(PþQ-)7-OH (23): H NMR
(D2O/acetonitrile-d6 (70:30), H2O was used as reference, 300
MHz, 25 °C) δ 1.86 (s, 3H), 1.99-2.69 (m, 22H), 2.69-3.29 (m,
3H) 3.29-3.75 (m, 23H), 4.10-5.42 (m, 22H), 6.56 (s, 1H), 6.58
(2s, 2H), 6.68 (3s, 3H), 6.75 (2s, 2H), 6.78 (2s, 2H), 6.87 (2s, 2H),
6.93 (3s, 3H), 6.99 (2s, 2H), 7.01 (s, 1H), 7.08 (2s, 2H), 7.14 (s,
1H), 7.44-7.71 (13H), 7.79 (t, J = 8.12, 7.37, 1H), 7.99-8.23
(7H), 8.31 (3s, 3H), 8.44 (s, 1H), 8.59 (s, 1H), 8.76 (s, 1H), 9.36 (s,
(1d, J = 8.32, 1H), 8.87 (1 m, 1H), 11.19 (s, 1H), 11.48 (s, 1H),
12.09 (s, 1H). HRMS (ESI) m/z calcd for C50H45N10O13
þ
,
993.3162; found Mass 993.3240 [M þ H] þ
.
SPS of Fmoc-(Q-p þp)3. The same procedure as described
Q
for the preparation of resin-bound 10 was applied. Fmoc-
(Q-Qþ)3-OH (11): 1H NMR (DMSO-d6/acetone-d6 (80:20),
DMSO was used as reference, 300 MHz) δ 1.99-2.11 (m,
12H), 3.07-3.16 (m, 12H), 3.82-5.33 (m, 24H), 6.15 (s, 1H),
6.44 (s, 1H), 6.67 (s, 2H), 6.75 (s, 1H), 6.86 (s, 1H), 6.98 (t, 1H),
7.10 (t, J = 7.37, 1H), 7.19 (s, 1H), 7.24-8.28 (m, 23H), 8.49 (d,
1H), 10.46 (2s. 2H), 10.53 (s, 1H), 10.66 (s, 1H), 10.74 (s, 1H),
11.21 (s, 1H). MS (ESI) m/z calcd for C156H154N35O44
3þ
,
3221.09; found Mass 1074.17 [M þ 3H] 3þ
.
Acknowledgment. This work was supported by the min-
istry of research (predoctoral fellowship to B.B.), by ARC
(postdoctoral fellowship to K.L.-R.), and by ANR (contract
ꢀ
PCV07_185434). Dr. Axelle Grelard is gratefully acknowl-
edged for her help in recording NMR spectra.
1H), 11.16 (s, 1H), 11.32 (s, 1H), 11.38 (s, 1H), 11.52 (s, 1H),
þ
11.60 (s, 1H). HRMS (ESI) m/z calcd for C90H76N15O21
.
,
1702.5335; found Mass 1702.5239 [M þ H] þ
SPS of Fmoc-(P-p þp)4. The Wang bromo resin (0.1 g, 0.048
P
mmol) was acylated with Fmoc-Pþp-OH 18b (0.05 g, 0.096
mmol, 2 equiv relative to the resin loading) in the presence of
cesium iodide (0.024 g, 0.096 mmol) and DIEA (16 μL, 0.096
mmol). The resin was gently shaken overnight. Solvents were
removed by filtration and the resin was washed (2ꢀ each) with
DMF, MeOH, and DCM. The Fmoc protecting group was
removed following the standard protocol The trinitrobenzene-
Supporting Information Available: Experimental details for
the preparation of the different monomers, solution-phase
synthesis of (PiBuQiBu)4-OMe, structural NMR investigation,
and characterization data of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 75, No. 21, 2010 7185