Highly efficient coupling of β-substituted aminoethane sulfonyl azides with thio acids, toward a new chemical ligation reaction

Article abstract of DOI:10.1021/ol0501119

(Chemical Equation Presented) A highly efficient coupling of protected β-substituted aminoethane sulfonyl azides with thio acids is reported. In the case of peptide thio acids, this method encompasses a new chemoselective ligation method. Furthermore, the

Full text of DOI:10.1021/ol0501119

ORGANIC
LETTERS
2005
Vol. 7, No. 6
1125-1128
Highly Efficient Coupling of
-Substituted Aminoethane Sulfonyl
Azides with Thio Acids, toward a New
Chemical Ligation Reaction
â
Remco Merkx, Arwin J. Brouwer, Dirk T. S. Rijkers, and Rob M. J. Liskamp*
Department of Medicinal Chemistry, Faculty of Pharmaceutical Sciences,
Utrecht Institute for Pharmaceutical Sciences, Utrecht UniVersity, P.O. Box 80082,
3508 TB Utrecht, The Netherlands
r.m.j.liskamp@pharm.uu.nl
Received January 19, 2005
ABSTRACT
A highly efficient coupling of protected
â-substituted aminoethane sulfonyl azides with thio acids is reported. In the case of peptide thio
acids, this method encompasses a new chemoselective ligation method. Furthermore, the resulting
r-amino acyl sulfonamides can be alkylated
with suitable electrophiles to obtain densely functionalized sulfonamide scaffolds.
Today, there is tremendous interest in the development of
new chemoselective (bio)conjugation or ligation reactions
for coupling of large and complex molecules.1 Among these
are the reaction of an aldehyde/ketone with a hydrazide to
yield a hydrazone,2 an aldehyde/ketone with an amino-oxy
moiety to yield an oxime,3 a (peptide) thio ester with a
cysteine to yield a native amide bond,4 a triphenylphosphine
(thio) ester with an azide to yield an amide bond,5 and the
reaction between an acetylene with organic azides to yield
the corresponding 1,4-disubstituted 1,2,3-triazoles. The latter
reaction is also known from click chemistry, as was recently
independently developed by Meldal6 and Sharpless.7
The reaction of an azide with a thio acid has been
described in the literature.8 Recently, the reaction between
a thio acid and a sulfonyl azide has been reinvestigated by
Williams et al.9 It was found that thio acids react chemo-
selectively with azides to R-amino acyl sulfonamides via a
concerted [2 + 3] cycloaddition and formation of a thia-
triazoline intermediate.
(5) (a) Saxon, E.; Bertozzi, C. R. Science 2000, 287, 2007. (b) Saxon,
E.; Armstrong, J. I.; Bertozzi, C. R. Org. Lett. 2000, 2, 2141. (c) Nilsson,
B. L.; Kiessling, L. L.; Raines, R. T. Org. Lett. 2000, 2, 1939. (d) Nilsson,
B. L.; Kiessling, L. L.; Raines, R. T. Org. Lett. 2001, 3, 9. (e) Soellner, M.
B.; Nilsson, B. L.; Raines, R. T. J. Org. Chem. 2002, 67, 4993. (f) Nilsson,
B. L.; Hondal, R. J.; Soellner, M. B.; Raines, R. T. J. Am. Chem. Soc.
2003, 125, 5268. (g) Merkx, R.; Rijkers, D. T. S.; Kemmink, J.; Liskamp,
R. M. J. Tetrahedron Lett. 2003, 44, 4515. (h) David, O.; Meester, W. J.
N.; Bieraugel, H.; Schoemaker, H. E.; Hiemstra, H.; van Maarseveen, J. H.
Angew. Chem., Int. Ed. 2003, 42, 4373.
(6) Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67,
3057.
(7) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem., Int. Ed. 2002, 41, 2596.
(1) (a) Hang, H. C.; Bertozzi, C. R. Acc. Chem. Res. 2001, 34, 727. (b)
Breinbauer, R.; Ko¨hn, M. ChemBioChem 2003, 4, 1147. (c) Link, A. J.;
Mock, M. L.; Tirrell, D. A. Curr. Opin. Biotechnol. 2003, 14, 603. (d)
Ko¨hn, M.; Breinbauer, R. Angew. Chem., Int. Ed. 2004, 43, 3106.
(2) Lemieux, G. A.; Bertozzi, C. R. TIBTECH 1998, 16, 506.
(3) Lelie`vre, D.; Turpin, O.; El Kazzouli, S.; Delmas, A. Tetrahedron
2002, 58, 5525.
(4) (a) Dawson, P. E.; Muir, T. W.; Clark-Lewis, I.; Kent S. B. H. Science
1994, 266, 776. (b) Dawson, P. E.; Kent, S. B. H. Annu. ReV. Biochem.
2000, 69, 923.
(8) Rosen, T.; Lico, I. M.; Chu, D. T. W. J. Org. Chem. 1988, 53, 1580.
(9) Shangguan, N.; Katukojvala, S.; Greenberg, R.; Williams, L. J. J.
Am. Chem. Soc. 2003, 125, 7754.
10.1021/ol0501119 CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/17/2005

The application of peptide-based thio acids and amino
acid-derived sulfonyl azides in this chemoselective coupling
reaction would provide an entry toward large peptide mimic
systems with a high degree of chemical diversity because
of the presence a â-substituted aminoethane sulfonamide
residue in the ligated product (Figure 1). We have previously
Table 1. Synthesis of N-â-Protected Acyl Sulfonamides
Figure 1. General structure of the R-amino acyl/peptidyl sulfon-
amides.
reported the synthesis of N-protected â-aminoethane sulfonyl
chlorides.10 Recently, we also found that the conversion of
the chlorides into the corresponding azides proceeded with
good to excellent yields.
First, the coupling of either Cbz- or Fmoc-protected
â-aminoethane sulfonyl azides was investigated, which were
derived from the amino acids glycine (entries 1-4), phenyl-
alanine (entries 5-8), valine (entries 9 and 10), and
(protected) serine (entries 11 and 12) (Scheme 1, Table 1).11
Scheme 1. Synthesis of N-â-Protected Acyl Sulfonamides
a Reagents and conditions: 1 equiv of sulfonyl azide, 1.3 equiv of thio
acid, 1.3 equiv of 2,6-lutidine, solvent, rt, 15 min. b Yield of isolated product.
These sulfonyl azides reacted smoothly with thioacetic acid
or thiobenzoic acid at room temperature: the reaction was
complete in 15 min, and the resulting acyl sulfonamides (1-
12) could be isolated in good to excellent yields (Table 1).
As a solvent, CHCl3 was used in entries 1-4, but because
of the low solubility of the other sulfonyl azides, DMF was
a good alternative for reactions in entries 5-12.
was used (Scheme 2). Boc-Leu-SH (13) was synthesized
from Boc-protected leucine hydroxysuccinimide ester (Boc-
Leu-ONSu) by treatment with sodium hydrogensulfide
(NaHS) as was essentially described by Goldstein and Gelb.12
In our hands, the reaction workup and purification of 13 was
Then, as a first step toward chemical ligation of amino
acids and peptides, the thio acid derived from Boc-leucine
Scheme 2. Synthesis of (R-Amino) Acyl Sulfonamide 14a
(10) Brouwer, A. J.; Monnee, M. C. F.; Liskamp, R. M. J. Synthesis
2000, 1579.
(11) General Procedure for the Synthesis of N-â-Protected Acyl
Sulfonamides. To a mixture of Fmoc-Phe-Ψ[CH2SO2]-N3 (248 mg, 0.54
mmol) and 2,6-lutidine (81 µL, 0.70 mmol, 1.3 equiv) in DMF (2.5 mL)
was added dropwise thiobenzoic acid (82 µL, 0.70 mmol, 1.3 equiv). After
the addition of each drop, the evolution of N2 could be observed and the
color of the reaction mixture turned to pale yellow. After completion of
the addition, the reaction mixture was allowed to react for 15 min at room
temperature. Then, the reaction mixture was evaporated to dryness, and
the residue was crystallized from EtOAc/hexane. Compound 6 was obtained
as a white solid in 87% yield. Rf ) 0.61 (EtOAc/hexane, 4:1 v/v). Mp:
234 °C (dec). [R]23D -21.8 (c 0.1 DMF). 1H NMR (300 MHz, DMSO-d6):
δ 12.18 (1H, broad s), 7.91 (4H, m), 7.58-7.19 (15H, broad m), 4.19-
4.03 (4H, m), 3.82 (1H, m), 3.63 (1H, m), 2.85 (2H, m). 13C NMR (75
MHz, DMSO-d6): δ 166.8, 155.6, 144.3, 141.1, 138.1, 133.6, 132.1, 129.7,
129.0, 128.7, 128.1, 127.5, 126.8, 125.6, 120.6, 65.7, 56.2, 48.9, 47.0.
ESMS: calcd for C31H28N2O5S 540.17, found 541.55 [M + H]+, 563.30
[M + Na]+, 539.45 [M - H]-. Anal. Calcd for C31H28N2O5S: C, 68.87;
H, 5.22; N, 5.18. Found: C, 68.97; H, 5.16; N, 5.06.
a Reagents and conditions: 1 equiv of sulfonyl azide, 1.3 equiv
of dipeptide thio acid, 1.3 equiv of 2,6-lutidine, DMF, rt, 15 min.
Yield is based on isolated product.
1126
Org. Lett., Vol. 7, No. 6, 2005

very troublesome, and it was decided to use the crude Boc-
Leu-SH. Nevertheless, its coupling to sulfonyl azide Fmoc-
Gly-Ψ[CH2SO2]-N3 proceeded very well, and R-(amino) acyl
sulfonamide 14 was obtained in 81% yield.
Scheme 4. N-Alkylation of Acyl Sulfonamide 3a
Next, we attempted the reaction of a dipeptide thio acid
1512 with Cbz-Phe-Ψ[CH2SO2]-N3 to obtain tripeptide mimic
16 (Scheme 3). Under identical conditions, the azide reacted
Scheme 3. Synthesis of Peptidyl Sulfonamide 16a
a Reagents and conditions: 1 equiv of 3, 1.05 equiv of bromide,
2 equiv of DIPEA, DMF, rt, 16 h. Yields are based on isolated
product.
However, the yield of 17 was only 17%. Increasing the
amount of K2CO3 to 3 equiv, in the presence of 2 equiv of
allylbromide, also resulted in alkylation of the urethane NH,
and the dialkylated product was isolated in 15% yield. These
conditions clearly eradicated any chemoselectivity. Fortu-
nately, when DIPEA (2 equiv) was used, the chemoselective
alkylation product 17 was obtained in 75% yield. Employing
this chemoselective alkylation protocol, we employed tert-
butyl bromoacetate as an electrophile, and alkylation product
18 was obtained in 94% yield (Scheme 4).
a Reagents and conditions: 1 equiv of sulfonyl azide, 1.3 equiv
of dipeptide thio acid, 1.3 equiv of 2,6-lutidine, DMF, rt, 15 min.
Yield is based on isolated product.
Finally, when we tried to alkylate tripeptide mimic 16 with
allyl chloroacetate, thus introducing a third orthogonality in
a protecting group strategy (Scheme 5), alkylation product
smoothly with the dipeptide thio acid to give the orthogonally
protected tripeptide mimic 16 containing a sulfonamide in
99% yield. Both amino groups can be selectively deprotected,
thereby allowing also a selective elongation of the peptide
chain in either direction.
Scheme 5. N-Alkylation of Tripeptide Mimic 16a
Furthermore, to investigate the potential of this reaction
as a novel chemoselective (bio)conjugation or chemical
ligation reaction, the reaction of entry 2 (Table 1) was also
carried out in a more aqueous system, that is, THF/H2O 2:1
v/v. Successfully, acyl sulfonamide 2 was obtained in a
quantitative yield. Moreover, neither Cbz-GlyΨ[CH2SO2)-
NH2 nor Cbz-GlyΨ[CH2SO2)-N3 reacted (after 3 days of
stirring) with thioacetic acid and acetic acid, respectively,
to give acyl sulfonamide 3. These experiments underline the
chemoselective character of the sulfonyl azide/thio acid
reaction pair.
We took advantage of the increased acidity of the
sulfonamide NH13 in a chemoselective N-alkylation reaction
(Scheme 4).14 First, alkylation of 3 was attempted with
allylbromide in the presence of 1 equiv of K2CO3 in DMF.
a Reagents and conditions: 1 equiv of 16, 2 equiv of allyl
chloroacetate, 1 equiv of KI, 4 equiv of DIPEA, DMF, 40 °C, 16
h. Yield is based on isolated product.
(12) For the synthesis of peptide thio acids: (a) Goldstein, A. S.; Gelb,
M. S. Tetrahedron Lett. 2000, 41, 2797. (b) Yamashiro, D.; Li, C.-H. Int.
J. Pept. Protein Res. 1988, 31, 322.
(13) Increased acidity of the acyl sulfon amide functionality make it
useful as a carboxylic acid bioisostere: (a) Patani, G. A.; LaVoie, E. J.
Chem. ReV. 1996, 96, 3147. (b) Johansson, A.; Poliakov, A.; Åkerblom,
E.; Wiklund, K.; Lindeberg, G.; Winiwarter, S.; Danielson, U. H.;
Samuelsson, B.; Hallberg, A. Bioorg. Med. Chem. 2003, 11, 2551.
(14) Modification of the acylsulfon amide is based on Kenner’s safety-
catch linker ((a) Kenner, G. W.; McDermott, J. R.; Sheppard, R. C. J. Chem.
Soc., Chem. Commun. 1971, 636) as modified by: (b) Backes, B. J.; Virgilio,
A. A.; Ellman, J. A. J. Am. Chem. Soc. 1996, 118, 3055. (c) Backes, B. J.;
Ellman, J. A. J. Org. Chem. 1999, 64, 2322.
19 could only be isolated in a very small amount. However,
the addition of KI (1 equiv) and increasing the amount of
electrophile (2 equiv) and base (4 equiv) at a reaction
temperature of 40 °C allowed the alkylation product 19
to be isolated in 35% yield. Mass spectrometric analysis
showed that only monoalkylated product was present, and
Org. Lett., Vol. 7, No. 6, 2005
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1
NMR analysis (500 MHz, H, COSY, TOCSY, HSQC,
The chemical ligation of larger peptide thio acids and
peptido-peptidosulfonamide azides is currently under inves-
tigation.
and HMBC) verified the correct position of the N-alkyl
group.
In conclusion, we have developed a high-yielding synthesis
of (R-amino) acyl sulfonamides and peptidyl sulfonamides
featuring the use of N-protected â-aminoethane sulfonyl
azides and (amino) thio acids or peptide thio acids. The high
yields, also in aqueous solvents, points to the possibility of
using this reaction in bioconjugation and chemical ligation
protocols. Furthermore, the efficient chemoselective N-
alkylation procedure may provide an entry into attachment
of an additional peptide chain and thereby knotted peptide
systems.
Acknowledgment. We thank Dr. Johan Kemmink for
measuring the 500 MHz NMR spectra and Hans Jacobs for
assistance with the HPLC analyses.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data (1H and 13C), and HPLC data
for compounds 1-19. This material is available free of
charge via the Internet at http://pubs.acs.org.
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