C2-symmetric macrocyclic carbohydrate/amino acid hybrids through copper(I)-catalyzed formation of 1,2,3-triazoles

Article abstract of DOI:10.1021/jo050585l

An efficient method was developed for the preparation of macrocyclic carbohydrate/amino acid hybrids by macrocyclization with copper(I)-catalyzed 1,2,3-triazole formation. Methyl 2-amino-6-azido-3,4-di-O-benzoyl-2,6-dideoxy- β-D-glucopyranoside was prepar

Full text of DOI:10.1021/jo050585l

ing sugar amino acids. However, these macrocyclic
carbohydrate/amino acid hybrids adopted relatively com-
pact conformations with only small cavities and displayed
only moderate affinities toward various guest biomol-
ecules. Hence, we decided to explore alternative chem-
istries toward conformationally more open macrocycles
presenting topologically different cavities for more ef-
ficient guest molecule binding. Within this context,
copper(I)-catalyzed addition of azides to alkynes to form
1,2,3-triazoles6 appeared attractive as its efficiency in
macrocyclizations was recently demonstrated in the
preparation of cyclodextrin analogues7 and in peptide
cyclizations.8 We decided to exploit this reaction to
prepare C2-symmetric carbohydrate/amino acid macro-
cycles with the expectation that it would lead not only
to an expedient synthesis but also to more rigid macro-
cycles with more open structures that would be interest-
ing for evaluation as artificial receptors.
C2-Symmetric Macrocyclic Carbohydrate/
Amino Acid Hybrids through
Copper(I)-Catalyzed Formation of
1,2,3-Triazoles
Johan F. Billing and Ulf J. Nilsson*
Organic Chemistry, Lund University, P.O. Box 124,
SE-221 00 Lund, Sweden
ulf.nilsson@organic.lu.se
Received March 23, 2005
The synthetic strategy involved the coupling of an
azidoaminoglucopyranoside to dipeptide propiolamides to
obtain bifunctional molecules that carried one azido
group and one terminal alkyne and thus could be cy-
clodimerized to C2-symmetric macrocycles by copper(I)-
catalyzed 1,2,3-triazole formation. Accordingly, the first
task was to prepare the azidoaminoglucopyranoside
derivative (Scheme 1). Compound 14,9,10 was protected
with Boc2O to afford 2 in 95% yield after crystallization.
The protected sugar 2 was deacetylated under Zemple´n
conditions, and the crude product was dissolved in CH2-
Cl2 and pyridine and treated with TsCl overnight followed
An efficient method was developed for the preparation of
macrocyclic carbohydrate/amino acid hybrids by macrocy-
clization with copper(I)-catalyzed 1,2,3-triazole formation.
Methyl 2-amino-6-azido-3,4-di-O-benzoyl-2,6-dideoxy-â-D-
glucopyranoside was prepared and coupled to two different
N-propiolyl dipeptides (propiolyl-Tyr-Tyr-OH and propiolyl-
Arg(Mtr)-Tyr-OH) to obtain bifunctional molecules carrying
one azido group and one terminal alkyne. These bifunctional
molecules were cyclodimerized using Cu(I)-catalyzed 1,3-
dipolar cycloaddition of azides and alkynes to form macro-
cycles containing two 1,2,3-triazoles. Various cyclization
methods were evaluated, and the most efficient conditions
were found to be CuI and N,N-diisopropylethylamine in CH3-
CN.
(2) (a) Graf von Roedern, E.; Lohof, E.; Hessler, G.; Hoffmann, M.;
Kessler, H. J. Am. Chem. Soc. 1996, 118, 10156-10167. (b) van Well,
R. M.; Overkleeft, H. S.; Overhand, M.; Vang Carstenen, E.; van der
Marel, G. A.; van Boom, J. H. Tetrahedron Lett. 2000, 41, 9331-9335.
(c) Lohof, E.; Planker, E.; Mang. C.; Burkhart, F.; Dechantsreiter, M.
A.; Haubner, R.; Wester, H.-J.; Schwaiger, M.; Ho¨lzemann, G.; Good-
man, S. L.; Kessler, H. Angew. Chem., Int. Ed. 2000, 39, 2761-2763.
(d) Gruner, S. A. W.; Ke´ri, G.; Schwab, R.; Venetianer, A.; Kessler, H.
Org. Lett. 2001, 3, 3723-3725. (e) Sto¨ckle, M.; Voll, G.; Gu¨nther, R.;
Lohof, E.; Locardi, E.; Gruner, S.; Kessler, H. Org. Lett. 2002, 4, 2501-
2504. (f) van Well, R. M.; Overkleeft, H. S.; van der Marel, G. A.; Bruss,
D.; Thibault, G.; de Groot, P. G.; van Boom, J. H.; Overhand, M. Bioorg.
Med. Chem. Lett. 2003, 13, 331-334. (g) Grotenbreg, G. M.; Timmer,
M. S. M.; Llamas-Saiz, A. L.; Verdoes, M.; van der Marel, G. A.; van
Raaij, M. J.; Overkleeft, H. S.; Overhand, M. J. Am. Chem. Soc. 2004,
126, 3444-3446. (h) Grotenbreg, G. M.; Kronemeijer, M.; Timmer, M.
S. M.; El Oulaid, F.; van Well, R. M.; Verdoes, M.; Spalburg, E.; van
Hooft, P. A. V.; de Neeling, A. J.; Noort, D.; van Boom, J. H.; van der
Marel, G. A.; Overkleeft, H. S.; Overhand, M. J. Org. Chem. 2004, 69,
7851-7859.
Macrocycles containing amino acids are important as
conformationally constrained peptidomimetics and have
as such found use as pharmaceuticals, e.g., antibiotics.1
Furthermore, macrocycles have been synthesized from
amino acids in combination with other building blocks,
such as carbohydrates.2 Such macrocycles constitute
promising candidates as artificial receptors3 for small
biomolecules as they can contain functionalities that
allow for most types of intermolecular interactions and
can be designed to be water-soluble and conformationally
constrained. Hence, development of methods for efficient
synthesis of macrocycles containing amino acids is an
important area of research. In our ongoing search for
artificial receptors, we have recently prepared macrocy-
clic carbohydrate/amino acids hybrids with both C2 and
C3 symmetry4,5 by cyclization of linear peptides contain-
(3) Hartley, J. H.; James, T. D.; Ward, C. J. J. Chem. Soc., Perkin
Trans. 1 2000, 3155-3184.
(4) Billing, J. F.; Nilsson, U. J. Tetrahedron 2005, 61, 863-874.
(5) Billing, J. F.; Nilsson, U. J. Tetrahedron Lett. 2005, 46, 991-
993.
(6) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K.
B. Angew. Chem., Int. Ed. 2002, 41, 2596-2599. (b) Tornøe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057-3064.
(7) Bodine, K. D.; Gin, D. Y.; Gin, M. S. J. Am. Chem. Soc. 2004,
126, 1638-1639.
(8) Punna, S.; Kuzelka, J.; Wang, Q.; Finn, M. G. Angew. Chem.,
Int. Ed. 2005, 44, 2215-2220.
(9) Irvine, J. C.; McNicoll, D.; Hynd, A. J. Chem. Soc. 1911, 11, 250-
261.
(1) McGeary, R. P.; Fairlie, D. P. Curr. Opin. Drug Discovery Dev.
1998, 1, 208-217.
(10) Yamasaki, T.; Kubota, Y.; Tsuchiya, T.; Umezawa, S. Bull.
Chem. Soc. Jpn. 1976, 49, 3190-3192.
10.1021/jo050585l CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/13/2005
J. Org. Chem. 2005, 70, 4847-4850
4847

SCHEME 1. Synthesis of Methyl 2-Amino-6-azido-
3,4-di-O-benzoyl-2,6-dideoxy-â-D-glucopyranoside 5
SCHEME 3. Synthesis of Macrocycles 11a,b
SCHEME 2. Synthesis of Dipeptide Propiolamides
8a,b
obtained when N,N-diisopropylethylamine (DIPEA) was
used instead of DBU. Compound 9a was only partially
soluble in toluene, and the cyclization was attempted in
THF instead. The reaction was slow, but after 7 days the
starting material 9a had been consumed and cyclodimer-
ized product 10a was obtained in 16% yield along with
an insoluble compound that appeared to be polymerized
starting material.
by BzCl to afford 3 in 52% yield. The tosyl group in 3
was then substituted with an azido group to give 4 in 83%
yield, and finally the Boc group was cleaved using TFA
and Et3SiH in CH2Cl2 in 94% yield to afford the
azidoaminosugar 5.
Although an encouraging result, the reaction condi-
tions needed to be optimized further. The reaction was
repeated at 10 times more dilute conditions, which gave
10a in 45% yield after 28 days. The reaction could be
improved further by using acetonitrile as a solvent, which
resulted in a faster reaction and a better yield. Again,
the yield was better at more dilute conditions, and 64%
of 10a could be obtained after 3 days. It has previously
been noted that the reaction works best in the presence
of acetonitrile when the catalyst is added in the form of
a copper(I) salt,6a and it has been proposed that the
reason for this is that complexation with acetonitrile
prevents oxidation of copper(I) to copper(II).13 The reac-
tion was also attempted with CuSO4 and sodium ascor-
bate,6a which resulted in a very sluggish reaction at room
temperature and only a low yield of 10a at 70 °C.
Optimal cyclization conditions were thus concluded to
be CuI and DIPEA in acetonitrile under dilute conditions.
Compound 9b could also be cyclodimerized to 10b in 33%
yield under these conditions. The formation of the 1,4
regioisomers of 10a,b was confirmed by the presence of
a NOESY cross-peak between triazole H-5 and the
glucose H-6 signals of 10a,b. Macrocycle 10a was depro-
tected with NaOMe in MeOH to afford 11a in 63% yield.
Macrocycle 10b was first treated with neat TFA contain-
ing 5% thioanisol14 to cleave the Mtr group and then with
NaOMe in MeOH to give the deprotected 11b in 58%
yield.
11
The two propiolamides were prepared from dipeptides
6a,b4 (Scheme 2). The Fmoc groups of 6a,b were cleaved
using Bu4NF in THF with 1-octanethiol as a scavenger12
to obtain 7a and previously reported compound 7b.4 The
N-deprotected dipeptides were then coupled to propiolic
acid with 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline
(EEDQ) as activating agent to afford 8a,b in good yields.
The tert-butyl ester of 8a was cleanly cleaved with 33%
TFA and Et3SiH in CH2Cl2,11 and the crude free acid was
directly coupled to 5 using N,N′-diisopropylcarbodiimide
(DIC) and 1-hydroxybenzotriazole (HOBt) to produce
compound 9a in 67% yield (Scheme 3). It has been
reported previously by us4 and others2f that the 4-meth-
oxy-2,3,6-trimethylbenzenesulfonyl (Mtr) group is stable
under these or similar conditions, but when the tert-butyl
ester of 8b was cleaved, partial cleavage of the Mtr group
occurred. Nevertheless, the free acid was purified and
coupled to 5 to afford 9b in 21% over both steps.
The cyclodimerization of 9a to 10a was attempted
under various conditions (Table 1). The previously re-
ported conditions with CuI and DBU in toluene were
sluggish with 9a at low temperatures and failed to give
product at higher temperatures. The same result was
(11) Mehta, A.; Jaouhari, R.; Benson, T. J.; Douglas, K. T. Tetra-
hedron Lett. 1992, 33, 5441-5444.
(12) Ueki, M.; Nishigaki, N.; Aoki, H.; Tsurusaki, T.; Katoh, T.
Chem. Lett. 1993, 721-724.
(13) Sharpless, B. K.; Fokin, V.; Rostovtsev, V.; Green, L.; Himo, F.
PCT Int. Appl. WO200310 1972.
(14) Fujino, M.; Wakimasu, M.; Kitada, C. Chem. Pharm. Bull. 1981,
29, 2825-2831.
4848 J. Org. Chem., Vol. 70, No. 12, 2005

TABLE 1. Cyclodimerization Attempts with 9a To Give 10a
reagents
solvent
concn (mM)
temp (°C)
time
yield (%)
CuI/DBU
toluene
2.5
2.5
rt, then 85
24 h, then 16 h
16 h
0
0
CuI/DBU
toluene
toluene
THF
50
CuI/DIPEA
2.5
45
6 days
0
CuI/DIPEA
2.5
45
7 days
16
45
20
64
9
CuI/DIPEA
THF
0.25
2.5
45
28 days
CuI/DIPEA
CH3CN
CH3CN
t-BuOH/H2O 4:1
45
24 h
CuI/DIPEA
0.25
2.5
45
3 days
CuSO4/sodium ascorbate
rt, then 70
48 h, then 7 days
was concentrated. The product was purified with flash chroma-
tography (toluene/EtOAc 2:5, Rf ) 0.25) followed by size-
exclusion chromatography to give 9a (333 mg, 67%) as a white
foam. [R]22D -16 (c 0.5, DMSO); 1H NMR (MeOH-d4, 400 MHz)
δ 7.87 (m, 4H, Bz-o), 7.52 (m, 2H, Bz-p), 7.37 (m, 4H, Bz-m),
6.98 (d, J ) 8.5 Hz, 2H, Tyr-Hδ), 6.83 (d, J ) 8.5 Hz, 2H, Tyr-
ꢀ
Hδ), 6.68 (d, J ) 8.5 Hz, 2H, Tyr-H ), 6.55 (d, J ) 8.5 Hz, 2H,
ꢀ
Tyr-H ), 5.69 (dd, J ) 10.6 Hz, J ) 9.4 Hz, 1H, H-3), 5.37 (t, J
) 9.6 Hz, 1H, H-4), 4.73 (d, J ) 8.4 Hz, 1H, H-1), 4.51 (dd, J )
9.4 Hz, J ) 5.2 Hz, 1H, Tyr-HR), 4.43 (dd, J ) 9.0 Hz, J ) 4.8
Hz, 1H, Tyr-HR), 4.18 (dd, J ) 10.6 Hz, J ) 8.4 Hz, 1H, H-2),
4.07 (m, 1H, H-5), 3.56 (s, 1H, CH), 3.54 (s, 3H, OMe), 3.50 (m,
1H, H-6), 3.37 (dd, J ) 13.5 Hz, J ) 2.6 Hz, 1H, H-6), 2.91 (dd,
J ) 14.1 Hz, J ) 5.1 Hz, 1H, Tyr-Hâ), 2.80 (dd, J ) 14.1 Hz, J
) 4.8 Hz, 1H, Tyr-Hâ), 2.64 (dd, J ) 14.2 Hz, J ) 9.4 Hz, 1H,
Tyr-Hâ), 2.48 (dd, J ) 14.1 Hz, J ) 9.0 Hz, 1H, Tyr-Hâ); HRMS
(FAB) calcd for C42H40N6O11Na (M + Na) 827.2653, found
827.2666.
Protected Tyr-Tyr-Containing Macrocycle (10a). Com-
pound 9a (50.0 mg, 62.1 µmol) was dissolved in CH3CN (250
mL) and CuI (3.5 mg, 19 µmol) and DIPEA (11 µL, 62 µmol)
were added. The mixture was stirred at 45 °C under N2 for 72
h and then concentrated. The residue was suspended in CH2-
Cl2/MeOH 1:1 and filtered through Celite. The filtrate was
concentrated and the product was purified with flash chroma-
tography (CH2Cl2/MeOH 10:1, Rf ) 0.18) to give 10a (31.9 mg,
64%) as a white amorphous solid. [R]21D -49 (c 0.5, DMSO); 1H
NMR (DMSO-d6, 300 MHz) δ 9.09 (s, 4H, Tyr-OH), 8.58 (d, J )
9.0 Hz, 2H, NH), 8.51 (d, J ) 9.0 Hz, 2H, NH), 8.41 (s, 2H,
triazole), 7.92 (m, 6H, Bz-o + NH), 7.79 (d, J ) 7.2 Hz, 4H, Bz-
o), 7.65 (m, 4H, Bz-p), 7.53 (t, J ) 7.7 Hz, 4H, Bz-m), 7.46 (t, J
) 7.5 Hz, 4H, Bz-m), 6.82 (d, J ) 8.6 Hz, 4H, Tyr-Hδ), 6.79 (d,
FIGURE 1. Superimposed structures of the 25 conformers
with lowest energy found in the conformational search with
11b (left) and the global minimum shown alone (right). Similar
results were obtained with 11a.
Conformational searches carried out on macrocycles
11a,b in MacroModel 8.5 (MMFFs force field in water,
all backbone torsions were selected for variation, 20 000
steps) show that the strategy to incorporate 1,2,3-
triazoles into macrocyclic carbohydrate/amino acid hy-
brids in order to obtain more rigid macrocycles with more
open structures is valid. The low-energy conformers of
macrocycles 11a,b have relatively open structures with
a central cavity (Figure 1), whereas our previous macro-
cycles with three amino acids, instead of one 1,2,3-triazole
and two amino acids, had formed collapsed structures.4
In conclusion, the work herein describes the first
synthesis of C2-symmetic carbohydrate/amino acid hybrid
macrocycles through copper(I)-catalyzed formation of
1,2,3-triazoles. The starting material for the cyclodimer-
ization can be prepared in a relatively small number of
steps, thus making this an efficient synthesis of macro-
cyclic carbohydrate/amino acid hybrid molecules. The
rigidity conferred by the triazole rings leads to open cleft-
like macrocycles as promising candidates for artificial
receptors in water.
ꢀ
J ) 8.7 Hz, 4H, Tyr-Hδ), 6.52 (d, J ) 8.7 Hz, 4H, Tyr-H ), 6.49
ꢀ
(d, J ) 8.6 Hz, 4H, Tyr-H ), 5.45 (t, J ) 9.8 Hz, 2H, H-3), 4.78
(m, 6H, H-4 + H-5 + H-6), 4.71 (d, J ) 8.4 Hz, 2H, H-1), 4.50
(m, 4H, H-6 + Tyr-HR), 4.23 (m, 2H, Tyr-HR), 3.93 (m, 2H, H-2),
3.42 (s, 6H, OMe), 2.40 (m, 6H, Tyr-Hâ), 2.06 (t, J ) 12.3 Hz,
2H, Tyr-Hâ); HRMS (FAB) calcd for C84H80N12O22Na (M + Na)
1631.5408, found 1631.5408.
Deprotected Tyr-Tyr-Containing Macrocycle (11a). Com-
pound 10a (20.2 mg, 12.5 µmol) was dissolved in MeOH (5 mL)
and NaOMe in MeOH (1.0 M, 50 µL) was added. The solution
was stirred for 24 h, then neutralized with AcOH and concen-
trated. The product was purified using preparative HPLC (C18
column, 0 f 30% B in A over 60 min, A ) H2O + 0.1% TFA, B
) CH3CN + 0.1% TFA, tR ) 35 min) to afford 11a (9.5 mg, 63%)
as a fluffy white powder after lyophilization. [R]21 -55 (c 0.3,
D
DMSO); 1H NMR (DMSO-d6, 500 MHz) δ 9.12 (s, 2H, Tyr-OH),
9.10 (s, 2H, Tyr-OH), 8.66 (d, J ) 8.1 Hz, 2H, NH), 8.32 (s, 2H,
triazole), 7.91 (d, J ) 9.0 Hz, 2H, NH), 7.78 (d, J ) 8.9 Hz, 2H,
NH), 7.05 (d, J ) 8.4 Hz, 4H, Tyr-Hδ), 6.89 (d, J ) 8.2 Hz, 4H,
ꢀ
Tyr-Hδ), 6.62 (d, J ) 8.3 Hz, 4H, Tyr-H ), 6.57 (d, J ) 8.3 Hz,
ꢀ
4H, Tyr-H ), 5.39 (d, J ) 5.1 Hz, 2H, OH-4), 4.87 (d, J ) 5.3 Hz,
Experimental Section
2H, OH-3), 4.65 (m, 4H, H-6), 4.46 (m, 2H, Tyr-HR), 4.36 (m,
2H, Tyr-HR), 4.15 (d, J ) 8.1 Hz, 2H, H-1), 3.56 (m, 2H, H-5),
3.30 (m, 4H, H-2+H-3), 3.03 (m, 8H, H-4, OMe), 2.94 (dd, J )
13.9 Hz, J ) 4.2 Hz, 2H, Tyr-Hâ), 2.69 (m, 6H, Tyr-Hâ); 13C NMR
(DMSO-d6, 125 MHz) δ 170.8 (Tyr-C′), 170.6 (Tyr-C′), 159.5
(triazole-C′), 155.70 (Tyr-Cú), 155.68 (Tyr-Cú), 142.0 (triazole-
C4), 130.4 (Tyr-Cδ), 129.7 (Tyr-Cδ), 127.9, 127.7, 127.4 (Tyr-Cγ,
Propiolyl-Tyr-Tyr/Azidoaminosugar Hybrid (9a). Com-
pound 8a (279 mg, 0.615 mmol) was dissolved in CH2Cl2 (9.5
mL) and Et3SiH (245 µL, 1.54 mmol) and TFA (4.7 mL) were
added. The mixture was stirred for 4 h and then concentrated
together with toluene. The crude free acid and compound 5 (262
mg, 0.615 mmol) were dissolved in THF (12 mL) and HOBt (83.1
mg, 0.615 mmol) and DIC (106 µL, 0.677 mmol) were added.
After stirring for 16 h, MeOH (3 mL) was added and the mixture
ꢀ
ꢀ
Tyr-Cγ, triazole-C5), 115.0 (Tyr-C ), 114.7 (Tyr-C ), 101.7 (SAA-
C1), 73.9 (SAA-C3), 72.9 (SAA-C5), 71.0 (SAA-C4), 56.0 (Tyr-CR),
J. Org. Chem, Vol. 70, No. 12, 2005 4849

55.6 (SAA-OMe), 54.8 (SAA-C2), 53.8 (Tyr-CR), 50.3 (SAA-C6),
37.0 (Tyr-Câ), 36.8 (Tyr-Câ); HRMS (FAB) calcd for C56H64N12O18-
Na (M + Na) 1215.4359, found 1215.4340.
Supporting Information Available: General experimen-
tal methods, experimental procedures and physical data for
compounds 2-8 and 9b-11b, 1H NMR spectra for all new
compounds, 13C NMR spectra for compounds 11a,b, and tables
of atom coordinates and pdb files for minimized 11a,b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank the Swedish Research
Council and the program “Glycoconjugates in Biological
Systems” sponsored by the Swedish Strategic Research
Foundation for financial support.
JO050585L
4850 J. Org. Chem., Vol. 70, No. 12, 2005