A practical solid phase synthesis of oligopeptidosulfonamide foldamers

Article abstract of DOI:10.1016/S0040-4039(00)01387-3

Oligopeptidosulfonamide foldamers were efficiently synthesized on the solid phase using Fmoc protected β-aminoethanesulfonylchlorides in the presence of N-methylmorpholine. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01387-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 7991–7995
A practical solid phase synthesis of oligopeptidosulfonamide
foldamers
Menno C. F. Monnee, Michael F. Marijne, Arwin J. Brouwer and Rob M. J. Liskamp*
Department of Medicinal Chemistry, Utrecht Institute for Pharmaceutical Sciences, Utrecht University,
PO Box 80082, 3508 TB Utrecht, The Netherlands
Received 10 July 2000; revised 9 August 2000; accepted 16 August 2000
Abstract
Oligopeptidosulfonamide foldamers were efficiently synthesized on the solid phase using Fmoc pro-
tected b-aminoethanesulfonylchlorides in the presence of N-methylmorpholine. © 2000 Published by
Elsevier Science Ltd.
1,2
The term ‘foldamer’ was coined by Gellman to designate the class of b-peptides, which has
been shown to fold into defined three dimensional structures similar to those of natural
3
2
peptides. Other foldamer classes include, for example, the vinylogous peptides, but also
oligomers in which the peptide amide bond is replaced by another moiety such as the
sulfonamide moiety, leading to oligopeptidosulfonamides or ‘b-sulfonopeptides’ and vinylogous
2
4
5
sulfonopeptides. In the recent past we and others have synthesized oligopeptidosulfonamides
consisting of up to three residues. Clearly, it will be very interesting to see if oligopeptidosulfon-
2
amides containing a large number of residues adopt defined structures. In order to realize the
synthesis of such large oligomers a solid phase procedure is indispensable. An absolute
prerequisite for the development of a practical solid phase method is the easy availability of the
necessary building blocks. Recently, we have described an efficient synthesis of N-protected
6
b-aminoethane sulfonylchlorides as versatile building blocks for the synthesis of oligopepti-
dosulfonamides. Here we describe the use of these sulfonyl chlorides in a practical and
convenient solid phase synthesis procedure towards oligopeptidosulfonamide foldamers.
7
Figure 1. Oligopeptidosulfonamide analogue 1 of the b-hexapeptide 2 of Seebach et al.
*
Corresponding author. Fax: +31 30 253 6655; e-mail: r.m.j.liskamp@pharm.uu.nl
0
040-4039/00/$ - see front matter © 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)01387-3

7992
As a first target molecule we chose the oligopeptidosulfonamide analogue 1 of the b-hexapep-
7
tide 2 of Seebach et al. (Fig. 1). This b-peptidosulfonamide 1 was assembled on Argogel™-
Rink-NH-Fmoc resin in an iterative manner (Scheme 2). Each cycle consisted of a deprotection
and a coupling step. Deprotection of the Fmoc group was achieved each time with a 20%
solution of piperidine in NMP. The required Fmoc protected b-aminoethane sulfonylchlorides
7
a–c were prepared in four steps starting from Fmoc protected amino acids, as was described
6
recently (Scheme 1). In short, Fmoc amino acids 3a–c were reduced to the corresponding amino
alcohols 4a–c using sodium borohydride. This was followed by mesylation to 5a–c and
subsequent substitution to the thioacetates 6a–c. Oxidation with hydrogen peroxide in acetic
acid gave the sulfonic acids, which were immediately converted into the sulfonylchlorides 7a–c.
Three different sulfonyl chlorides were prepared in overall yields of 30–35% corresponding to an
average yield of 79% per reaction.
6
Scheme 1. Synthesis of the required Fmoc-protected b-aminoethanesulfonylchlorides
For the solid phase procedure the Fmoc protecting group was cleaved from the Rink-linker
with piperidine to give the free amine, which was treated with an excess (4 equiv.) of
Fmoc-Leu-c[CH SO ]-Cl and (6 equiv.) N-methylmorpholine (NMM) to introduce the first
2
2
b-aminoethanesulfonamide residue to give 9. Introduction of the second b-aminoethanesulfon-
amide residue i.e. Fmoc-Ala-c[CH SO ]-Cl was performed in an identical manner and led to 10.
2
2
Repetition of the deprotection and coupling cycle for an additional four times ultimately led to
8
the hexa-b-peptidosulfonamide attached to the solid support, i.e. 11. N-methylmorpholine was
used as a base in each of the coupling reactions. These proceeded smoothly and to a very high
degree of completion, as was clearly observed by the bromophenol blue (BPB) test, which was
9
negative after each coupling and blue colored beads were absent. This observation was
corroborated by measuring the total absorbance of the piperidine–dibenzofulvene adduct (u
max
10
3
01 nm) obtained after cleavage of the Fmoc group after each coupling step. It was found that
the loading of the growing oligopeptidosulfonamide decreased only slightly with each coupling
step (Fig. 2).
After removal of the last Fmoc group, the oligopeptidosulfonamide was cleaved from the
resin with a mixture of TFA and water. Mass spectral studies (ES-MS) unambiguously showed
the presence of the desired oligopeptidosulfonamide 1 in the reaction product, but probably the
low absorbance of the sulfonamide moiety prevented a clear detection of the oligopeptidosulfon-
amide in the HPLC analysis. To circumvent this problem an additional UV-detectable group
was introduced. The p-nitro-benzenesulfonyl- (p-NBS) and the benzyloxycarbonyl- (Cbz) pro-
tective groups were selected for this purpose. The use of an aromatic sulfonyl chloride allowed
detection while at the same time gave rise to an ‘all’ sulfonamide structure. In contradistinction
a Cbz group was introduced solely for the purpose of purification, since it is UV-detectable and
can be cleanly removed afterwards to give the pure oligopeptidosulfonamide 1.

7993
Figure 2.
Both protective groups were introduced after cleavage of the last Fmoc group by treatment
with p-NBS-chloride or Cbz-chloride in the presence of excess NMM. After acidolytic cleavage,
followed by purification by silica gel column chromatography, the desired oligopeptidosulfon-
amides 12 and 13 were obtained in excellent overall yields of 54 and 66%, respectively. These
correspond to excellent average yields of 96 and 97%, respectively, per synthesis step (Scheme 2).
Hydrogenolysis of the Cbz group in the presence of 10% Pd/C gave, after precipitation from
methanol and diethyl ether and lyophilization, the desired compound 1 as a white powder
(
HCl-salt) in 54% yield. The oligopeptidosulfonamides 1, 12 and 13 have all been characterized
1
1
11
by UV, IR, NMR and mass spectral analysis.
Scheme 2. Solid phase synthesis of oligopeptidosulfonamide foldamers
Preliminary circular dichroism (CD) studies so far did not indicate the presence of a defined
helical secondary structure in solution, as has been observed for the corresponding oligo-b-pep-
tides. This might be explained by the poor UV absorbance of the backbone sulfonamide
moieties preventing the observation of a defined structure by this method. Alternatively,
oligopeptidosulfonamides of this length and/or with these residues may fail to assume a defined
structure. Clearly, we have to take recourse to other techniques to investigate the ‘foldamer
behavior’ of these oligopeptidomimetics. Those techniques include, for example, temperature
dependent NMR spectroscopy and exchange rate determination of sulfonamide protons for
3
a
deuterium.
In conclusion, we have described both a practical and efficient solid phase synthesis of
oligopeptidosulfonamide foldamers using Fmoc protected b-aminoethanesulfonylchloride build-
ing blocks. The use of the Fmoc strategy is completely compatible with the Fmoc strategy used
for the construction of peptides and therefore b-peptide-peptidosulfonamide hybrids are easily
accessible.

7994
Under present investigation is the construction and structure of larger oligopeptidosulfon-
amide foldamers, as well as oligopeptidosulfonamides containing functionalized aminosulfonic
acid residues, which might ultimately lead to construction of proteinaceous sulfonamides.
Acknowledgements
Financial support by the council for Chemical Sciences of the Netherlands Organization for
Scientific Research (CW-NWO) (to M.C.F.M.) is gratefully acknowledged. The authors thank
C. Erkelens (NMR facility, Leiden University) and C. Versluis (Department of Biomolecular
Mass Spectrometry, Utrecht University) for recording the 600 MHz NMR spectra and the MS
spectra, respectively. The authors thank Mrs L. J. F. Hofmeyer for assistance with the HPLC
analyses and Dr. E. T. Rump for assistance with the CD-measurements.
References
1
2
3
. Borman, S. C&EN, 1997, June 16, 32.
. Gellman, S. H. Acc. Chem. Res. 1998, 31, 173.
. See for example: (a) Appella, H.; Christianson, L. A.; Klein, D. A.; Richards, M. R.; Powell, D. R.; Gellman,
S. H. J. Am. Chem. Soc. 1996, 118, 13071. (b) Seebach, D.; Ciceri, P.; Overhand, M.; Juan, B.; Rigo, D.; Oberer,
L.; Hommel, U.; Amstutz, R.; Widmer, H. Helv. Chim. Acta 1996, 79, 2043. (c) Seebach, D.; Matthews, J. L.
Chem. Commun. 1997, 2015. (d) Appella, H.; Christianson, L. A.; Klein, D. A.; Richards, M. R.; Powell, D. R.;
Gellman, S. H. J. Am. Chem. Soc. 1999, 121, 7574. (e) Wang, X.; Espinosa, J. F.; Gellman, S. H. J. Am. Chem.
Soc. 2000, 122, 4821.
4
. (a) Moree, W. J.; van der Marel, G. A.; Liskamp, R. M. J. J. Org. Chem. 1995, 60, 5157. (b) De Bont, D. B. A.;
Moree, W. J.; Liskamp, R. M. J. Bioorg. Med. Chem. 1996, 4, 667. (c) De Bont, D. B. A.; Dijkstra, G. D. H.;
Den Hartog, J. A. J.; Liskamp, R. M. J. Bioorg. Med. Chem. Lett. 1996, 6, 3045.
. (a) Gennari, C.; Gude, M.; Potenza, D.; Piarrulli, U. Chem. Eur. J. 1998, 4, 1924. (b) Gude, M.; Piarrulli, U.;
Potenza, D.; Salom, B.; Gennari, C. Tetrahedron Lett. 1996, 37, 8589.
5
6
7
. Brouwer, A. J.; Monnee, M. C. F.; Liskamp, R. M. J. Synthesis 2000, in press.
. Seebach, D.; Overhand, M.; K u¨ hnle, F. N. M.; Martinoni, B.; Oberer, L.; Hommel, U.; Widmer, H. Helv. Chim.
Acta 1996, 79, 913.
8. A typical procedure is as follows: 0.35 g of Argogel™-Rink-NH-Fmoc resin (loading 0.30 mmol/g), swollen in
NMP, was shaken three times, each time for 10 minutes, with 2.1 mL of a 20% solution of piperidine in NMP
(v/v). Subsequently, the resin was successively washed five times, each time for 2 minutes, with NMP and DCM.
Then, the resin was shaken with four equivalents of Fmoc-Leu-c[CH SO ]-Cl and six equivalents NMM in 2.1
2
2
mL DCM for 18 hours at room temperature to introduce the Fmoc protected peptidosulfonamide building block.
Finally, the resin was washed five times, each time for 2 minutes, with DCM, followed by the BPB test. The resin
is then ready for the next deprotection/coupling cycle.
9. Krchnak, V.; Vagner, J.; Safar, P.; Lebl, M. Collect. Czech. Chem. Commun. 1988, 53, 2542.
1
1
0. Meienhofer, J.; Waki, M.; Heimer, E. P.; Lambros, T. J.; Makofske, R. C.; Chang, C.-D. Int. J. Peptide Protein
Res. 1979, 13, 35.
1
1. For example, the p-NBS protected oligopeptidosulfonamide 12: 600 MHz H NMR (CD OH, T=268 K,
3
s
s
s
Watergate CD OH suppression): l 0.85–0.95 (m, 24H, 8×CH , 2×Val , 2×Leu ), 1.26 (d, 3H, CH Ala , J=6.8
3
3
3
s
s
Hz), 1.35 (d, 3H, CH Ala , J=6.7 Hz), 1.44–1.55 (m, 4H, 2×CH CH(CH ) 2×Leu ), 1.77 (m, 2H, CH(CH )
3
2
s
3 2
3 2
s
s
s
2
×Leu ), 1.97 (m, 1H, CH(CH ) Val ), 2.07 (m, 1H, CH(CH ) Val ), 2.96 (dd, 1H, CH SO Val , J =14.5 Hz,
3 2 3 2 2 2 gem
s
s
Jvic=5.6 Hz), 3.08 (dd, 1H, CH SO Ala , J =14.3 Hz, J =5.1 Hz), 3.13 (dd, 1H, CH SO , Val , Jgem=14.6
Hz, Jvic=7.1 Hz), 3.19–3.35 (m, 7H, CH SO Leu , Ala , Val ), 3.41 (dd, 1H, CH SO Ala , Jgem=14.2 Hz,
2
2
gem
vic
2
2
s
s
s
s
2
2
2
2
s
s
Jvic=7.4 Hz), 3.49 (dd, 1H, CH SO Leu , Jgem=14.4 Hz, Jvic=6.5 Hz), 3.60 (m, 1H, NCH Ala ), 3.73 (m, 1H,
2
2
s
s
s
s
NCH Val ), 3.83 (m, 1H, NCH Val ), 3.92 (m, 2H, NCH Leu (2×)), 3.99 (m, 1H, NCH Ala ), 6.90

7995
s
s
s
(
7
bs, 2H, SO NH ), 7.02 (d, 1H, NH Ala , J=8.2 Hz), 7.07 (d, 1H, NH Leu , J=9.0 Hz), 7.18 (bd, 1H, NH Leu ),
2
2
s
s
s
.25 (d, 1H, NH Val , J=9.0 Hz), 7.28 (bd, 1H, NH Ala ), 7.80 (d, 1H, NH Val , J=8.9 Hz), 8.10 (d, 2H,
13
Ar-CH, J=9.0 Hz), 8.39 (d, 2H, Ar-CH, J=9.0 Hz). 150 MHz C NMR (CD OH): l 16.77, 17.48, 18.74 and
3
s
3 3 3 3
s
s
s
1
2
9.15 (CH Val ); 22.12 and 22.29 (CH Leu ); 23.09, 23.23, 23.26 and 23.40 (CH Ala and CH Leu ); 25.37 and
s
s
s
5.47 (CH(CH ) Leu ); 32.86 and 35.54 (CH(CH ) Val ); 46.65 and 46.75 (CH CH(CH ) Leu ); 47.44 (NCH
Ala ), 49.78 and 49.92 (NCH Leu ); 55.78 (CH SO Val ), 55.99 and 56.34 (NCH Val ); 56.64 (CH SO Val ),
3
2
3 2
2
3 2
s
s
s
s
s
2
2
s
2
2
s
6
0.52, 60.89, 60.98 and 61.06 (CH SO Ala , Leu ); 125.27 and 129.54 (Ar-CH); 148.49 and 151.28 (Ar-C).
2 2
+
ESI-MS found: m/z 1091.2689 [M+Na] , calcd 1091.2692.
.
.