Synthesis of an anion-binding amino acid

Article abstract of DOI:10.1016/j.tetlet.2011.11.057

We achieved the synthesis of a derivative of phenylalanine with a diazamacrocycle on its side chain by macrocyclization of a dichloride on l-DOPA. We also report its incorporation into peptide structures by solid phase peptide synthesis which will lead to

Full text of DOI:10.1016/j.tetlet.2011.11.057

Tetrahedron Letters 53 (2012) 409–411
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of an anion-binding amino acid
Sylvain Chénard, Claude Barberis, François Otis, Jean-François Paquin, Jonathan Martel, Charles Banville,
⇑
Normand Voyer
Département de Chimie and PROTEO, Faculté des Sciences et de Génie, Université Laval, Québec, Canada G1V 0A6
a r t i c l e i n f o
a b s t r a c t
Article history:
We achieved the synthesis of a derivative of phenylalanine with a diazamacrocycle on its side chain by
Received 26 September 2011
Revised 10 November 2011
Accepted 11 November 2011
Available online 18 November 2011
macrocyclization of a dichloride on
solid phase peptide synthesis which will lead to the development of artificial anion channels.
Ó 2011 Elsevier Ltd. All rights reserved.
L-DOPA. We also report its incorporation into peptide structures by
Keywords:
Unnatural amino acid
Diazamacrocycle
Macrocyclization
Anion binding
The development of peptidic nanostructures that mimic the
biological activity of natural proteins is pursued by many groups
worldwide.1,2 To achieve a variety of biological functions, it is
mandatory to have access to artificial amino acid-bearing effectors
offering new possibilities for interactions.3
methanol to yield the dibenzyl diamine 2 in an 86% yield.
6-Chlorohexan-1-ol was then added to 2 under the conditions of
Finkelstein substitution reaction to form the diol 3 in a 91% yield.
The key intermediate dichloride 4 was obtained in a 92% yield un-
der Vilsmeier conditions using oxalyl chloride and DMF in CH2Cl2.
Our group recently reported the preparation of a cationic por-
phyrin-modified glutamic acid and its use in solid-phase peptide
synthesis (SPPS) for the preparation of selective DNA binders.4,5
We also developed a library of artificial cation channels using
various crown ether-modified phenylalanines.6,7 As the develop-
ment of an anion channel is also an important issue,8–10 it would
be useful to have access to amino acids bearing an anion-binding
macrocycle.
The macrocyclization was then performed on N-Boc-L-DOPA
methyl ester14 5 in DMF at a controlled temperature of 50 °C to
avoid racemization. The macrocyclization by two subsequent
nucleophilic substitutions was promoted under high dilution con-
ditions by a slow addition. CsBr was used for halide conversion to
form the bromide in situ in a Fenkelstein-like substitution, which
reduced the reaction time substantially. The methyl ester macrocy-
clic amino acid 6 was obtained in a 35% yield after purification by
silica gel chromatography. Saponification of the methyl ester with
1 N sodium hydroxide in methanol yielded the desired amino acid
7 in a 66% after a purification on Sephadex G-10 with methanol as
eluent, for an overall yield of 17%.
Herein, we describe the synthesis of
a diazamacrocycle-
modified phenylalanine protected by an N-Boc group, ready to be
used in SPPS. The feasibility of incorporating this new amino acid
in conventional peptide synthesis is also demonstrated by the prep-
aration of a tripeptide and a heptapeptide. The target modified ami-
no acid was designed in such a way to provide a sufficient
electropositive environment for anion when protonated, while
retaining sufficient lipophilicity to incorporate the low polarity core
of a lipid bilayer membrane. Mix N and O macrocycles have been
shown to complex anions including ClÀ within their cavity.11–13
The synthesis strategy, illustrated in Scheme 1, consists of the
All attempts to determine the optical purity of 7 failed. Indeed,
HPLC analysis with a Chiradex column failed to provide separation
of the two enantiomers of 7 prepared from racemic
Likewise, NMR studies using chiral amines ((R)-(+) or (S)-(À)-1-
(1-naphthyl)ethylamine or (R)-(+)- -methylbenzylamine) to form
L-DOPA.
a
diastereoisomeric salts failed to generate signals for both enantio-
mers. Additionally, Mosher derivatives of racemic 7 could not be
differentiated by NMR. Finally, formation of diastereoisomeric
amides using Marfey’s reagent15,16 did not lead to useful HPLC sep-
aration of the mixture. However, it should be noted that under the
reaction conditions used to prepare 7, we demonstrated that race-
mization was negligible in the synthesis of crown ether-modified
macrocyclization of a dichloride on diprotected L-DOPA. First,
1,6-hexanediamine 1 was treated with benzaldehyde, followed
by reducing the resulting diimine with sodium borohydrate in
⇑
L
-phenylalanine.17 Interestingly, 1H NMR studies with 6 in the
presence of HCl in CDCl3 show significant deshielding of protons
Corresponding author. Tel.: +1 418 656 3613; fax: +1 418 656 7916.
E-mail address: normand.voyer@chm.ulaval.ca (N. Voyer).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.057

410
S. Chénard et al. / Tetrahedron Letters 53 (2012) 409–411
a,b
NH2
NH
H2N
HN
1
2
c
Cl
OH
d
N
N
N
N
3
4
Cl
HO
e
OH
OH
OMe
N
N
BocHN
O
5
O
O
OR
BocHN
O
6
R = Me
R = H
f
7
Scheme 1. Reagents, conditions, and yields: (a) PhCHO, H2O, rt, 2 h; (b) NaBH4, MeOH, reflux, 2 h; 86%; (c) ClCH2(CH2)4CH2OH, Na2CO3, NaI cat., n-BuOH, reflux, 24 h; 92%; (d)
(COCl)2, DMF, CH2Cl2, reflux; 91%; (e) Cs2CO3, CsBr cat., DMF, 50 °C, 72 h; 35%; (f) NaOH, MeOH, rt, 4 h; 66%.
from the alkyl chain spacers, which could be attributed to the
presence of ClÀ within the cavity.
in CH2Cl2 and DMF in the presence of diisopropylethylamine
(DIEA). Then the third amino acid, N-Boc-Leu-OH, was introduced
after removing the protecting group and activation as described
above. A portion of the resin was set aside and the tripeptide 8
was obtained from the remainder after cleavage using 1,8-diazabi-
cyclo[5.4.0]undec-7-ene (DBU) in THF/H2O (9:1) in the presence of
lithium bromide.20 The resin-bound tripeptide previously obtained
was deprotected, coupled with N-Boc-Leu-OH to yield a tetrapep-
tide, which was deprotected again and used in a segment conden-
sation with the tripeptide 8 under standard coupling condition
(DIC/HOBt/DIEA). After cleavage with n-propylamine 1 N in CHCl3,
the heptapeptide 9 bearing two diazamacrocycles separated by
three residues was obtained and characterized by ES-MS.
In order to confirm the compatibility of this new artificial amino
acid with solid phase peptide synthesis, we incorporated it in a tri-
peptide and used this tripeptide in a segment condensation to ob-
tain a heptapeptide that could serve as a synthon toward an
artificial anion channel based on our reported strategy.18 Oxime re-
sin19 was used as a solid support, as shown in Scheme 2. The first
amino acid, N-Boc-Leu-OH, was attached to the oxime functional
group of the resin, following an activation with dicyclohexylcarbo-
diimide (DCC) in CH2Cl2. After the usual washings, the remaining
oxime groups were capped with acetic anhydride in DMF. Cleavage
of the N-Boc protecting group was achieved by a 1 h treatment
with 50% trifluoroacetic acid in CH2Cl2. The modified amino acid
7 was coupled to the resin-bound leucine after activation with
diisopropylcarbodiimide (DIC) and hydroxybenzotriazole (HOBt)
In conclusion, we successfully prepared an N-Boc-protected
phenylalanine linked to a diazamacrocycle. The new amino acid
has been effectively incorporated into short peptides using SPPS.

S. Chénard et al. / Tetrahedron Letters 53 (2012) 409–411
411
HO
N
O
polystyrene
BocHN
HO
a
O
O2N
b,c
R
O
O
H
N
O
H
R
b,d
BocHN
N
O
N
H
O
BocHN
O
O
e
O
R
O
H
N
BocHN
b, d, b, f, g
N
H
OH
O
8
R
O
O
O
R
O
H
N
H
H
N
N
BocHN
N
H
N
N
H
N
H
H
O
O
O
9
N
O
O
R=
N
Scheme 2. Synthesis of peptides bearing a macrocyclic side chain: (a) Boc-Leu-OH, DCC, CH2Cl2; (b) TFA/CH2Cl2 (1:1); (c) 7, DIC, HOBt, DIEA, CH2Cl2, DMF; (d) Boc-Leu-OH,
DIC, HOBt, DIEA, CH2Cl2, DMF; (e) DBU, LiBr, THF/H2O (9:1); (f) 8, DIC, HOBt, DIEA, CH2Cl2, DMF; (g) n-propylamine, CHCl3.
3. de Graaf, A. J.; Kooijman, M.; Hennink, W. E.; Mastrobattista, E. Bioconjugate
Chem. 2009, 20, 1281–1295.
peptidic nanostructures that are able to induce chloride transport
4. Biron, E.; Voyer, N. Chem. Commun. 2005, 4652–4654.
We are currently using this amino acid in the preparation of new
across bilayer membranes.
5. Biron, E.; Voyer, N. Org. Biomol. Chem. 2008, 6, 2507–2515.
6. Biron, E.; Otis, F.; Meillon, J. C.; Robitaille, M.; Lamothe, J.; Van Hove, P.;
Cormier, M. E.; Voyer, N. Bioorg. Med. Chem. 2004, 12, 1279–1290.
7. Otis, F.; Racine-Berthiaume, C.; Voyer, N. J. Am. Chem. Soc. 2011, 133, 6481–
6483.
Acknowledgments
8. Davis, J. T.; Okunola, O.; Quesada, R. Chem. Soc. Rev. 2010, 39, 3843–3862.
9. Gokel, G. W.; Barkey, N. New J. Chem. 2009, 33, 947–963.
10. Doval, D. A.; Areephong, J.; Bang, E. K.; Bertone, L.; Charbonnaz, P.; Fin, A.; Lin,
N. T.; Lista, M.; Matile, S.; Montenegro, J.; Orentas, E.; Sakai, N.; Tran, D. H.;
Jentzsch, A. V. Langmuir 2011, 27, 9696–9705.
The financial support of this work by the NSERC of Canada, the
FQRNT of Québec and PROTEO is gratefully acknowledged.
Supplementary data
11. Gokel, G. W. Crown Ethers
& Cryptands; The Royal Society of Chemistry:
Cambridge, 1991. pp 125–126.
Supplementary data (procedures for the synthesis of 2–9;
1H and 13C NMR data for 2–7; MS data for 2–9) associated with this
article can be found, in the online version, at doi:10.1016/j.
tetlet.2011.11.057.
12. Metz, B.; Rosalky, J. M.; Weiss, R. J. Chem. Soc., Chem. Commun. 1976, 533–534.
13. Graf, E.; Lehn, J. M. J. Am. Chem. Soc. 1976, 98, 6403–6405.
14. Banerjee, S. N.; Ressler, C. J. Org. Chem. 1976, 41, 3056–3058.
15. Carroll, A. R.; Feng, Y. J.; Bowden, B. F.; Coll, J. C. J. Org. Chem. 1996, 61, 4059–
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16. Marfey, P. Carlsberg Res. Commun. 1984, 49, 591–596.
17. Voyer, N.; Roby, J.; Deschenes, D.; Bernier, J. Supramol. Chem. 1995, 5, 61–69.
18. Voyer, N. J. Am. Chem. Soc. 1991, 113, 1818–1821.
19. DeGrado, W. F.; Kaiser, E. T. J. Org. Chem. 1980, 45, 1295–1300.
20. Pichette, A.; Voyer, N.; Larouche, R.; Meillon, J. C. Tetrahedron Lett. 1997, 38,
1279–1282.
References and notes
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