Chemoselective formation of successive triazole linkages in one pot: "Click-click" chemistry

Article abstract of DOI:10.1021/ol061657d

A methodology for the successive regiospecific "clicking" together of three components in one pot via two triazole linkages is reported. The protocol utilizes copper(I)-mediated alkyne-azide cycloaddition reactions combined with a silver(I)-catalyzed TMS-

Full text of DOI:10.1021/ol061657d

ORGANIC
LETTERS
2006
Vol. 8, No. 20
4505-4507
Chemoselective Formation of
Successive Triazole Linkages in One
Pot: “Click−Click” Chemistry
Vincent Aucagne*,† and David A. Leigh*
School of Chemistry, UniVersity of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh EH9 3JJ, U.K.
aucagne@cnrs-orleans.fr; daVid.leigh@ed.ac.uk
Received July 6, 2006
ABSTRACT
A methodology for the successive regiospecific “clicking” together of three components in one pot via two triazole linkages is reported. The
protocol utilizes copper(I)-mediated alkyne azide cycloaddition reactions combined with a silver(I)-catalyzed TMS-alkyne deprotection under
−
mild hydroalcoholic conditions. We exemplify the approach with peptide-based components to illustrate its compatibility with polyfunctionalized
biomolecules. The method constitutes a promising tool for peptide ligation. We also provide a procedure for directly using TMS-alkynes as
the cycloaddition partner in classical “click” chemistry.
Since the independent discovery of the copper(I)-catalyzed
variant of the Huisgen terminal alkyne-azide 1,3-cycload-
dition1 by the Meldal2 and Sharpless3 groups, hundreds of
papers have appeared describing the use of this simple
“click”4 methodology to link together polyfunctionalized
building blocks.5 Its application in numerous areas has
highlighted the value of its benign reaction conditions and
simple workup and purification procedures. Here we report
a one-pot procedure for the successive copper- and copper-
and-silver-mediated chemoselective formation of two distinct
triazole linkages using the trimethylsilyl (TMS) group as a
temporary masking group for one of two alkyne moieties.
We believe this “click-click” approach could be a potentially
useful variation on standard click chemistry. As well as being
a convenient tool for the selective ligation of multiple
components in a biological environment, it could be used to
generate a library of products whose population grows
according to the square of the number of available azide
building blocks.
† Current address: Centre de Biophysique Mole´culaire, CNRS UPR n°
4301, affiliated to the University of Orle´ans and to Inserm, Rue Charles
Sadron 45071 Orle´ans Cedex 2, France.
(1) Huisgen, R. In 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.;
Wiley: New York, 1984.
(2) Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67,
3057-3064.
(3) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem., Int. Ed. 2002, 41, 2596-2599. (b) Himo, F.; Lovell, T.;
Hilgraf, R.; Rostovtsev, V. V.; Noodleman, L.; Sharpless, K. B.; Fokin, V.
V. J. Am. Chem. Soc. 2005, 127, 210-216.
(4) (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2001, 40, 2004-2021. (b) Kolb, H. C.; Sharpless, K. B. Drug DiscoVery
Today 2003, 8, 1128-1137.
(5) For a recent review, see: Bock, V. D.; Hiemstra, H.; van Maarseveen,
J. H. Eur. J. Org. Chem. 2006, 51-68.
Taking note of recent reports on the Ag(I)-mediated
deprotection of TMS-alkynes under semiaqueous or alcoholic
conditions,6 we wondered whether TMS-alkynes could be
unmasked and directly used for triazole formation without
interim workup or purification. To test this idea, we
(6) (a) Orsini, A.; Vite´risi, A.; Bodlenner, A.; Weibel, J.-M.; Pale, P.
Tetrahedron Lett. 2005, 46, 2259-2262. (b) Carpita, A.; Mannocci, L.;
Rossi, R. Eur. J. Org. Chem. 2005, 1859-1864.
10.1021/ol061657d CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/06/2006

conducted some preliminary experiments using alkyne 1,
TMS-alkyne 2, and azide 3 (Scheme 1).7,8 An equimolar
decided to focus on small peptide derivatives. Such synthons
are easy to prepare, producing polyfunctional building blocks
that could test the functional group tolerance of the desired
click-click protocol.11 Commercially available methyl phe-
nylalanylglycylglycinate hydrochloride was esterified with
propargyl alcohol under acidic conditions and then converted
to the TMS-propargyloxycarbonyl derivative 5 using a tailor-
made (see Supporting Information) 4-nitrophenol (PNP)
carbonate reagent (Scheme 2).
Scheme 1. In Situ Ag(I)-Catalyzed Unmasking and
Cu(I)-Catalyzed 1,3-Cycloaddition of a TMS-Alkyne and an
Azide
Scheme 2. Synthesis of a TMS-Alkyne-Terminal Alkyne
Bisfunctionalized Tripeptide
A series of azide-containing pseudodipeptides (7a-d) were
synthesized via the EDCI-mediated amide coupling of
commercial amino acid methyl esters and chloroacetic acid
(6a-d), followed by nucleophilic displacement of the
chloride by an azide. This two-step strategy was used to avoid
the use of potentially hazardous12 azidoacetic acid or one of
its activated derivatives (Scheme 3).
mixture of the three compounds was reacted in a CH2Cl2/
MeOH mixture in the presence of a catalytic amount (0.2
equiv) of Cu(I). This furnished the expected triazole 4,
resulting from the cycloaddition of 1 and 3, leaving unreacted
TMS-protected 2. Reaction of 2 and 3 in the presence of
AgBF4 led to the unmasked terminal alkyne 1 without
affecting azide 3.9 Finally, simple mixing of 2 and 3 in the
presence of catalytic quantities (0.2 equiv) of both Cu(I) and
Ag(I) salts gave the click product from a one-pot deprotec-
tion/cycloaddition process. This not only indicates the
potential of silyl-protected alkynes for a click-click strategy
but also demonstrates a way of directly employing TMS-
alkynes as a cycloaddition partner in classical click reac-
tions.10
Scheme 3. Synthesis of Azide-Containing Dipeptide
Analoguesa
A molecule was then constructed with both a terminal
alkyne and a TMS-protected one to act as the central scaffold
for attempts to use this methodology to chemoselectively
form successive triazole linkages in one pot. In view of the
need to develop efficient selective ligation tools for the
production of synthetic proteins and glycoproteins, we
a See Supporting Information for experimental details.
Clearly, strict respect of reactant stoichiometry and near-
quantitative conversion during the first cycloaddition is
(7) Aucagne, V.; Ha¨nni, K. D.; Leigh, D. A.; Lusby, P. J.; Walker, D.
B. J. Am. Chem. Soc. 2006, 128, 2186-2187.
(11) Several recent articles illustrate the efficacy of click chemistry for
the ligation of peptide building blocks. See, for example: (a) Horne, W.
S.; Yadav, M. K.; Stout, C. D.; Ghadiri, M. R. J. Am. Chem. Soc. 2004,
126, 15366-15367. (b) Rijkers, D. T. S.; van Esse, G. W.; Merkx, R.;
Brouwer, A. J.; Jacobs, H. J. F.; Pieters, R. J.; Liskamp, R. M. J. Chem.
Commun. 2005, 36, 4581-4583. (c) Franke, R.; Doll, C.; Eichler, J.
Tetrahedron Lett. 2005, 46, 4479-4482. (d) Dirks, A. J. T.; van Berkel, S.
S.; Hatzakis, N. S.; Opsteen, J. A.; van Delft, F. L.; Cornelissen, J. J. L.
M.; Rowan, A. E.; van Hest, J. C. M.; Rutjes, F. P. J. T.; Nolte, R. J. M.
Chem. Commun. 2005, 33, 4172-4174. (e) Jang, H.; Fafarman, A.; Holub,
J. M.; Kirshenbaum, K. Org. Lett. 2005, 7, 1951-1954. (f) Musiol, H.-J.;
Dong, S.; Kaiser, M.; Bausinger, R.; Zumbusch, A.; Bertsch, U.; Moroder,
L. Chem. Bio. Chem. 2005, 625-628. (g) Holub, J. M.; Jang, H.;
Kirshenbaum, K. Org. Biomol. Chem. 2006, 4, 1497-1502. (h) Bock, V.
D.; Perciaccante, R.; Jansen, T. P.; Hiemstra, H.; van Maarseveen, J. H.
Org. Lett. 2006, 8, 919-922.
(8) Most low molecular weight azides and alkynes are rather volatile,
and we found it convenient to use 1-3, which we had in hand from previous
studies.7
(9) The mechanism of Ag(I)-mediated desilylation of TMS-alkynes
involves a silver-alkyne species structurally related to the alkyne cuprates
thought to be an early intermediate in the Cu(I)-catalyzed alkyne-azide
cycloaddition.3b,5 We added azide 3 to the alkyne unmasking reaction to
see if this putative silver intermediate would undergo a copper-free triazole-
forming reaction. However, no traces of triazole products were detected
(1H NMR, ESI-MS).
(10) In many reports, the alkyne moiety is introduced as its TMS
derivative, which is deprotected, worked up, and purified prior to clicking.
See, for example: (a) Malkoch, M.; Thibault, R. J.; Drockenmuller, E.;
Messerschmidt, M.; Voit, B.; Russell, T. P.; Hawker, C. J. J. Am. Chem.
Soc. 2005, 127, 14942-14949. (b) Helms, B.; Mynar, J. L.; Hawker, C. J.;
Fre´chet, J. M. J. J. Am. Chem. Soc. 2005, 127, 15020-15021. (c) Suh,
B.-C.; Jeon, H. B.; Posner, G. H.; Silverman, S. M. Tetrahedron Lett. 2004,
45, 4623-4625.
(12) CAUTION: Low molecular weight azides can be explosive. For a
recent review covering various aspects of azide chemistry, see: Bra¨se, S.;
Gil, C.; Knepper, K.; Zimmermann, V. Angew. Chem., Int. Ed. 2005, 44,
5188-5240.
4506
Org. Lett., Vol. 8, No. 20, 2006

crucial for a one-pot click-click strategy as any remaining
azide or alkyne could react in the second step, complicating
purification and lowering the overall yield.13 We first focused
on the chemoselective cycloaddition of dialkynes 5 and 7a
(Scheme 4). Hydroalcoholic conditions were used to ensure
These optimized conditions for the first cycloaddition were
used as the basis for a second successive triazole-forming
reaction in the same reaction vessel (Scheme 5).
Scheme 5. Chemoselective Click-Click Chemistry:
Successive Triazole Peptide Ligations in One Pot
Scheme 4. Chemoselective Click Cycloadditiona
a See Table 1 for reaction details and yields.
a general procedure that would be compatible with a wide
range of substrates, including complex biomacromolecules.
Although it had proved effective in CH2Cl2/MeOH mixtures
(Scheme 1), Cu(MeCN)4PF6 proved to be a poor catalyst in
tBuOH/water (Table 1, entry 2). However, the widely used
Addition of the second azide (7b, c, or d) and a catalytic
amount (0.2 equiv) of AgBF4 to the product mixture from
Table 1, entry 7, resulted in complete disappearance of 8
and formation of bistriazoles 10a-c (Scheme 5). Additional
CuSO4 (0.1 equiv) and sodium ascorbate (0.2 equiv) were
needed to drive the reaction to completion in a reasonable
time. After 18 h, simple filtering through a silica plug
removed excess azide and metal salts to give the desired
bistriazole pseudononapeptides 10a-c in good purity and
yield (Scheme 5). LC-MS analysis of the crude material,
10a-c, showed only traces (0-4%) of contamination from
reaction of 5 with two molecules of the same azide.
In conclusion, we have described a methodology for the
one-pot chemoselective ligation of three polyfunctionalized
building blocks. The strategy exploits the Cu(I)-catalyzed
alkyne-azide cycloaddition. This efficient reaction, com-
bined with the one-pot Ag(I)-mediated deprotection of a
TMS-protected alkyne moiety, allows for the synthesis of
bistriazole products in high yields in good purity after simple
filtration through a plug of silica. The ease of preparation
(and, in many cases, commercial availability) of unprotected
polyfunctionalized azides, alkynes, and silylated alkynes,
together with the simple conditions for reaction and workup,
suggest this click-click chemoselective ligation strategy
could have wide potential. In particular, it appears promising
as a tool for peptide ligation and, probably, combinatorial
chemistry, allowing direct access to complex structures from
simple building blocks under mild conditions.
Table 1. Optimization of Conditions for the First
Cycloadditiona
yield (%)c
entry
methodb
temp (°C)
duration
5
8
9
1
2
3
4
5
6
7
A
B
C
C
D
C
C
20
20
20
20
20
100e
35
18 h
18 h
18 h
72 h
18 h
10 min
18 h
61
85
8
2
5
39
15
92
98
75
55
98d
0
0
0
0
17
15
0
12
2
a Reactions were carried out using a 1:1 mixture of 6 and 8a at 0.05 M
under a nitrogen athmosphere. b A: Cu(CH3CN)4PF6 (0.1 equiv), CH2Cl2/
MeOH 4:1. B: Cu(CH3CN)4PF6 (0.1 equiv), tBuOH/water 95:5. C: CuSO4
(0.1 equiv), sodium ascorbate (0.2 equiv), tBuOH/water 95:5. D: CuSO4
(0.5 equiv), sodium ascorbate (1 equiv), tBuOH/water 95:5. c Conversions
determined by 1H NMR. d 94% isolated yield. e Microwave irradiation; see
Supporting Information for details.
click conditions which utilize the in situ generation of Cu(I)
(0.1 equiv of CuSO4/sodium ascorbate) proved to be effective
even if the reaction was sluggish (entries 3 and 4). Increasing
the catalyst loading caused the partial deprotection of 8
leading to 9 (entry 5).14 Attempts to use microwave-
accelerated conditions led to the same two compounds,
together with several byproducts, but gentle warming of the
reaction overnight resulted in excellent conversion (entry 7).
Acknowledgment. The authors would like to thank Dr.
R. Viguier, Dr. A. Delmas, and A. Viterisi for helpful
discussions. D.A.L. is an EPSRC Senior Research Fellow
and holds a Royal Society-Wolfson Research Merit Award.
Supporting Information Available: General synthetic
experimental procedure, characterization, and spectroscopic
data for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
(13) Tagged or solid-supported alkynyl “azide-quenching” agents could
allow excess reactants to be used in both cycloaddition steps, probably
enabling the overall yield to be increased still further.
(14) Cu(I)-mediated cleavage of an alkyne C-Si bond has been
reported: Ito, H.; Arimoto, K.; Sensui, H-o; Hosomi, A. Tetrahedron Lett.
1997, 38, 3977-3980. We are currently developing more robust terminal
alkyne protecting groups.
OL061657D
Org. Lett., Vol. 8, No. 20, 2006
4507