Unprecedented 1,1′-carbonyldiimidazole-mediated amidation of unprotected α-amino acids in water

Article abstract of DOI:10.1055/s-2007-967965

The first amidation reaction of unprotected α-amino acids in water under neutral conditions with various aliphatic, aromatic and heteroaromatic amines in the presence of coupling reagent 1,1′-carbonyldiimidazole at ambient temperature is described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-967965

LETTER
603
Unprecedented 1,1¢-Carbonyldiimidazole-Mediated Amidation of
Unprotected a-Amino Acids in Water
1
R
, 1
¢
-Carbonyldiim
o
idazole-M
e
h
diated
A
mid
i
t
U
nprotected
K
a
-Amino
A
cids . Sharma, Rahul Jain*
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research, Sector 67, S.A.S. Nagar,
Punjab 160062, India
Fax +91(172)2214692; E-mail: rahuljain@niper.ac.in
Received 7 November 2006
recently, chemoselective amide-forming ligation reaction
developed by Bode et al. provides a new way to join
Abstract: The first amidation reaction of unprotected a-amino
acids in water under neutral conditions with various aliphatic, aro-
matic and heteroaromatic amines in the presence of coupling re-
agent 1,1¢-carbonyldiimidazole at ambient temperature is described.
2
4
peptides. The reaction creates an amide bond between
a-keto carboxylic acids and N-alkylhydroxylamines in the
presence of other reactive functional groups and in the
absence of catalysts or coupling reagents. Sufficient data
is available in literature for the synthesis of amino acid
Key words: amidation, 1,1¢-carbonyldiimidazole (CDI), a-amino
acids, amidation reaction in water
amides using protected amino acids in common organic
solvents like DMF and THF.2
5,26
However, no report in
An in-depth analysis of the comprehensive medicinal
chemistry database revealed that the carboxamide group
appears in more than 25% of the known drugs. Similarly,
the importance of amino acid amides cannot be overstated
as they are crucial in architecture of many biological
systems and are prevalent in natural as well as unnatural
the literature could be found pertaining to the amidation of
amino acids using unprotected amino acids. Herein, we
describe an efficient and stereoconservative one-pot
method for the amidation of amino acids using unpro-
tected a-amino acids in water.
1
peptides and peptide drugs. Moreover, the amide func- We required various amides of basic amino acids like
tionality is a common feature in numerous small and arginine and histidine as components of antimicrobial
complex synthetic or natural molecules. For example, it is peptides. Under normal circumstances, the reaction would
ubiquitous in life, as proteins play a crucial role in virtual- involve coupling of suitably protected a-amino acid with
ly all biological processes such as enzymatic catalysis an amine in the presence of a coupling reagent like DCC
(
(
nearly all known enzymes are proteins), transport/storage followed by removal of protective groups to obtain the
hemoglobin), immune protection (antibodies) and me- desired amide in several steps. However, we required
2
chanical support (collagen). This can be attributed to the amides in which a-amino group of the amino acid re-
fact that amide group is almost neutral, stable and has both mained unprotected, and thus initiated a study to achieve
hydrogen-bond-accepting and -donating properties.
amidation reaction on fully unprotected a-amino acids.
Our initial efforts, which involved different conventional
methods including DCC and acid chloride mediated cou-
plings, failed when tried on unprotected arginine and his-
tidine in DMF or dichloroethane (DCE) as a solvent. We
believe this was mainly due to the insolubility of zwitter-
ionic a-amino acid in organic solvents. To compensate for
the insolubility of the reactants in common organic
solvents, we then attempted amidation in the presence of
ionic liquid such as 1-butyl-3-methyl-imidazolium tetra-
fluoroborate. However, reaction again failed presumably
because of insolubility of free amino acid in the ionic
liquid.
For amidation, in general, the carboxyl group needs to be
activated as the OH of the carboxylic acid component is
not a very good leaving group. Many methods have been
reported to activate the carboxyl group that includes
3
–7
8,9
activation via acyl halides, acyl azides, acylimid-
1
0,11
anhydrides (symmetrical and mixed),12–17 and
azoles,
esters (isolated and in situ generated active esters).
18–21
Several other methods have been used to join two func-
tional groups in the presence of other reactive groups to
form a stable amide bond in an aqueous environment
under neutral conditions. For example, Kent ligation in-
volves the chemoselective reaction of two unprotected
peptides in aqueous solution to give a single covalently It is commonly known that the unprotected a-amino acids
2
2
linked ligation product. In this case, a peptide thioester are readily soluble in water. Therefore, coupling of un-
is reacted with a Cys-peptide to produce a polypeptide. protected basic amino acids with amines using coupling
Staudinger ligation exploits the reaction between an azide agents like water-soluble 1-ethyl-3-(3¢-dimethylamino-
2
3
and a phosphane to form a phospha-aza-ylide. This ylide
trapped by an acyl group forms a stable amide bond. More
O
+
H3N
O
CDI
–
H2N R1
R1
2
H N
O
+
H2O, r.t.
–24 h
N
H
R
8
R
SYNLETT 2007, No. 4, pp 0603–0606
0
1.
0
3.
2
0
0
7
1
2
3
Advanced online publication: 21.02.2007
DOI: 10.1055/s-2007-967965; Art ID: G36706ST
Scheme 1
©
Georg Thieme Verlag Stuttgart · New York

6
04
R. K. Sharma, R. Jain
LETTER
propyl)carbodiimide·HCl (EDC·HCl), and water-insolu- To illustrate the generality of the reaction, a variety of
ble N,N¢-diisopropylcarbodiimide (DIC), and O-(benzo- amino acids 1 and amines 2 were reacted and depending
triazol-1-yl)-N,N,N¢,N¢-tetramethyluronium tetrafluoro- upon the nature of amino acid 1 and amine 2, reaction time
borate (TBTU) was tried in water. These attempts did not of 8–24 hours at ambient temperature was required to ob-
yield desired results possibly because of the instability tain the amides 3 in relatively good yields (Table 1).
and/or insolubility of the coupling agent in water.
The important point worth mentioning here is that the
We then turned our attention to 1,1¢-carbonyldiimidazole amidation reaction can be performed on free as well as
1
0
(
CDI), which is a useful coupling reagent for amide for- hydrochloride salts of amino acids and the side-chain
mation reaction in organic solvents. CDI is used exten- functionality was found to be relatively unaffected by the
1
1
sively in peptide chemistry, and is readily soluble in reagent and reaction conditions. Moreover, the reaction
water. To our pleasant surprise, unprotected amino acids was found to be stereoconservative in nature, and no ap-
1
upon reaction with different amines 2 in water in the parent racemization of the final products was observed.33
presence of CDI produced desired amide products 3
Scheme 1). To the best of our knowledge, this represents
the first report on the amidation of a-amino acids using
CDI in water.
As shown in Table 1, arginine amides 3a–f (entries 1–6)
were obtained in better yields as compared to histidine
amides 3g–j (entries 6–10), whereas lysine amide 3k (en-
try 11) and ornithine amide 3i (entry 12) were obtained in
(
Table 1 1,1¢-Carbonyldiimidazole-Mediated Coupling of Amino Acids 1a–e with Primary Amines 2a–f – Synthesis of Amino Acid Amides
27
3
a–m in Water as Solvent
Entry
Amino acid Ra
Amine
R1
Time (h)b
8
Product28–32 Yield (%)c
H2N
1
2
3
4
5
1a
1a
1a
1a
1a
NH(CH ) CH
2
2a
Bn
3a
3b
3c
3d
3e
73
67
62
53
59
2
2
HN
H2N
2b
2c
2d
2e
Me
i-Pr
Ph
8
8
NH(CH2)2CH2
NH(CH2)2CH2
NH(CH2)2CH2
NH(CH2)2CH2
HN
H2N
HN
H2N
12
12
HN
H2N
4-MeOC H
6
4
HN
H2N
HN
6
1a
2f
12
3f
50
NH(CH2)2CH2
CH2
N
7
8
9
1b
1b
1b
1b
2a
2b
2c
2d
Bn
8
12
12
12
3g
3h
3i
60
56
49
43
HN
HN
HN
HN
N
CH2
Me
i-Pr
Ph
N
CH2
N
CH2
1
0
3j
N
1
1
1
1
2
3
1c
1d
1e
H N(CH ) CH
2a
2a
2a
Bn
Bn
Bn
24
24
12
3k
3l
32
27
52
2
2 3
2
2
H N(CH ) CH
2
2 2
H
3m
a
b
c
Glycine, histidine and lysine were used as their hydrochloride salts.
All reactions were performed at r.t. unless indicated otherwise.
Yield of isolated product.
Synlett 2007, No. 4, 603–606 © Thieme Stuttgart · New York

LETTER
1,1¢-Carbonyldiimidazole-Mediated Amidation of Unprotected a-Amino Acids
605
R1 NH2
2
O
O
O
O
–
HN
N
+
H3N
–
O
H2N
N
N
O
N
R
R
N
N
N
4
1
+
H
–
CO2, – HN
N
O
H2N
R1
N
H
R
3
Scheme 2
comparatively lower yields. This is probably due to the an important requirement for the reaction as it allows car-
presence of free NH group on the side chain of lysine and boxylate anion to react directly with CDI. This observa-
2
ornithine which itself may be reacting with CDI, and thus tion is supported by the fact that no product was formed
acting as a crucial factor in limiting the yield. In these cas- when hexanoic acid was reacted with isopropylamine in
es (entries 11, 12) we also isolated amide product formed the presence of CDI in water for 48 hours. We also believe
by a reaction between the side-chain amino group of one that relative unreactivity of the a-NH of the amino acid is
2
amino acid with the a-carboxyl group of the other amino possibly due to its existence in the protonated form, which
acid in the range of 20–30%. While the side-chain func- appreciably reduces its nucleophilicty.
tionalities of arginine and histidine appeared relatively
In our opinion, this remarkable and useful one-pot reac-
less affected by the reaction conditions. The scope of our
tion has several advantages compared to conventional
study was limited to amidation of basic amino acids.
amidation methods: (i) use of unprotected amino acids in
However, to extend and explore the versatility of the reac-
a one-step reaction that increases the yield comprehen-
tion, we reacted glycine·HCl (1e, entry 13) with benzyl-
sively and decreases the cost effectively; (ii) the reaction
is applicable to a wide array of aliphatic, aromatic and
heteroaromatic primary amines; (iii) the reaction is per-
amine (2a) to obtain the desired product 3m in
approximately 50% yield. This observation confirms that
the reaction can also be extended to neutral and hydro-
formed in water that offers environmental and economic
phobic amino acids. Another observation is that the reac-
tion is mainly applicable and limited to primary amines as
exemplified by the reaction of diphenylamine with argin-
ine, which resulted in recovery of about 95% of the start-
ing material along with traces of the desired amide
product. In this case, the steric hindrance due to the pres-
ence of two phenyl rings in diphenylamine possibly also
plays an important role in limiting the reaction. 2-Ami-
nobenzophenone (insoluble in water) upon reaction with
arginine did not produce desired product clearly indicat-
ing that reaction is mainly limited to primary amines
which are readily soluble in water.
36
benefits over the conventional organic solvents; (iv) the
reaction requires approximately 8–24 hours under mild
conditions of ambient temperature; and (v) the reaction is
stereoconservative in nature.
We are exploring the utility of this reaction in peptide
bond formation and results of the study will be reported in
due course.
Acknowledgment
Rohit K. Sharma thanks the Council of Scientific and Industrial
Research (CSIR), New Delhi for Senior Research Fellowship.
Mechanistically, the reaction appears to proceed through Assistance provided by Mr. Ravi P. Reddy is duly acknowledged.
the formation of the intermediate-mixed anhydride 4 ob-
tained by the nucleophilic attack of the carboxylate anion
References and Notes
1
on the electron-deficient carbonyl carbon of CDI
(1) Ghose, A. K.; Viswanadhan, V. N.; Wendoloski, J. J. J.
Comb. Chem. 1999, 1, 55.
(
Scheme 2). This is followed by nucleophilic attack of
amine 2 at the less hindered carbonyl of intermediate 4
facilitating the formation of amides 3.
(
2) Montalbetti, C. A. G. N.; Falque, V. Tetrahedron 2005, 61,
10827.
In practical terms, the amine 2 was added 30 minutes after
the addition of CDI to 1 in order to allow the complete for-
mation of intermediate anhydride 4. This reaction, which
generates imidazole in situ, does not need an additional
base and it has been reported that this type of reaction is
(3) Pearson, A. J.; Roush, W. R. Handbook of Reagents for
Organic Synthesis: Activating Agents and Protecting
Groups; Wiley: New York, 1999, 370–373.
(
4) Chu, W.; Tu, Z.; McElveen, E.; Xu, J.; Taylor, M.; Luedtke,
R. R.; Mach, R. H. Bioorg. Med. Chem. 2005, 13, 77.
5) Klosa, J. J. Prakt. Chem. (Leipzig) 1962, 19, 45.
6) Rayle, H. L.; Fellmeth, L. Org. Process Res. Dev. 1999, 3,
172.
(
(
3
4,35
even compatible with HCl salts of the amine.
We be-
lieve that the zwitterionic nature of the free amino acid is
Synlett 2007, No. 4, 603–606 © Thieme Stuttgart · New York

6
06
R. K. Sharma, R. Jain
LETTER
(7) Venkataraman, K.; Wagle, D. R. Tetrahedron Lett. 1979, 32,
(28) Analytical Data of N1-Benzyl-(2S)-2-amino-5-
3037.
amino(imino)methylaminopentanamide (3a)
(
(
8) Klausner, Y.; Bodansky, M. Synthesis 1974, 549.
9) Shioiri, T.; Ninomiya, K.; Yamada, S. Y. J. Am. Chem. Soc.
Yield 73%; white solid; mp 176–178 °C. MS (APCI): 264.1
+
1
[M + 1] . H NMR (300 MHz, D O): d = 1.61 (m, 2 H), 1.89
2
1972, 94, 6203.
(m, 2 H), 3.16 (t, 2 H, J = 7.3 Hz), 3.69 (t, 1 H, J = 6.8 Hz),
1
3
(
(
10) Paul, R.; Anderson, W. J. Am. Chem. Soc. 1960, 82, 4596.
11) Dale, D. J.; Draper, J.; Dunn, P. J.; Hughes, M. L.; Hussain,
F.; Levett, P. C.; Ward, G. B.; Wood, A. S. Org. Process
Res. Dev. 2002, 6, 767.
4.39 (s, 2 H), 7.23–7.29 (m, 5 H). C NMR (75 MHz, D O):
2
d = 25.53, 30.14, 41.47, 43.59, 54.88, 127.70, 128.15,
128.90, 139.14, 158.44, 175.21. IR (KBr): 3445, 3326, 3033,
–
1
2923, 2872, 1685, 1573, 1458 cm . Anal. Calcd for
(
(
(
(
(
(
(
(
(
(
(
12) Wittenberger, S. J.; McLaughlin, M. A. Tetrahedron Lett.
C H N O: C, 59.32; H, 7.98; N, 26.62. Found: C, 59.24; H,
7.93; N, 26.58.
1
3
21
5
1999, 40, 7175.
13) Chu, W.; Tu, Z.; McElveen, E.; Xu, J.; Taylor, M.; Luedtke,
R. R.; Mach, R. H. Bioorg. Med. Chem. 2005, 13, 77.
14) Mikolajczyk, M.; Kielbasinski, P. Tetrahedron 1981, 37,
(29) Analytical Data of N1-(3-Quinolyl)-(2S)-2-amino-5-
amino(imino)methylaminopentanamide (3f).
Yield 50%; yellow solid; mp 217–219 °C. MS (APCI):
+
1
233.
301.1 [M + 1] . H NMR (300 MHz, D O): d = 1.66 (m, 2
2
15) Fuller, W. D.; Cohen, M. P.; Shabankareh, M.; Blair, R. K.
J. Am. Chem. Soc. 1990, 112, 7414.
16) Ishihara, K.; Ohara, S.; Yamamoto, H. J. Org. Chem. 1996,
H), 1.83 (m, 2 H), 3.09 (t, 2 H, J = 7.1 Hz), 3.69 (t, 1 H,
J = 6.4 Hz), 7.25 (m, 1 H), 7.61 (m, 2 H), 7.79 (m, 1 H), 7.87
1
3
(m, 1 H), 8.55 (m, 1 H). C NMR (75 MHz, D O): d = 25.14,
2
6
1, 4196.
17) Ishihara, K.; Ohara, S.; Yamamoto, H. Macromolecules
000, 33, 3511.
31.97, 41.25, 55.90, 118.21, 127.03, 127.69, 127.92, 137.50,
143.95, 157.34, 173.46. IR (KBr): 3414, 3293, 3019, 2912,
2849, 1679, 1552, 1470, 1386 cm . Anal. Calcd for
–
1
2
18) Gangwar, G. M.; Pauletti, T.; Siahaan, J.; Stella, V. J.;
Borchardt, R. T. J. Org. Chem. 1997, 62, 1356.
19) Jaoudai, M.; Martinez, J.; Castro, B.; Barcelo, G.; Sennyey,
G.; Senet, J.-P. J. Org. Chem. 1987, 52, 2364.
C H N O: C, 60.00; H, 6.67; N, 28.00. Found: C, 59.96; H,
1
5
20
6
6.65; N, 27.95.
(30) Analytical Data of N1-Benzyl-(2S)-2-amino-3-(1H-4-
imidazolyl)propanamide (3g).
20) Wang, W.; McMurray, J. S. Tetrahedron Lett. 1999, 40,
Yield 60%; white solid; mp 193–195 °C. MS (APCI): 245.2
+
1
2501.
[M + 1] . H NMR (300 MHz, D O): d = 3.26 (d, 2 H, J = 7.2
2
21) Kunishima, M.; Kawachi, C.; Iwasaki, F.; Terao, K.; Tani, S.
Tetrahedron Lett. 1999, 40, 5327.
22) (a) Dawson, P. E.; Muir, T. W.; Clark-Lewis, I.; Kent, S. B.
Science 1994, 266, 776. (b) Dawson, P. E.; Churchill, M. J.;
Ghadiri, M. R.; Kent, S. B. J. Am. Chem. Soc. 1997, 119,
Hz), 4.08 (t, 1 H, J = 7.2 Hz), 4.36 (s, 2 H), 6.85 (s, 1 H),
7.18–7.22 (m, 5 H), 7.48 (s, 1 H). C NMR (75 MHz, D O):
1
3
2
d = 33.81, 46.73, 58.87, 122.15, 126.95, 127.07, 127.38,
128.15, 128.46, 133.90, 134.78, 143.56, 176.41. IR (KBr):
3458, 3331, 3046, 2948, 2883, 1692, 1638, 1573, 1535, 1370
–
1
4325. (c) Dawson, P. E.; Kent, S. B. Annu. Rev. Biochem.
cm . Anal. Calcd for C H N O: C, 63.93; H, 6.56; N,
13 16 4
2
000, 69, 923.
22.95. Found: C, 63.88; H, 6.52; N, 22.90.
(31) Analytical Data of N1-Methyl-(2S)-2-amino-3-(1H-4-
imidazolyl)propanamide (3h).
(
(
23) (a) Köhn, M.; Breinbauer, R. Angew. Chem. Int. Ed. 2004,
3, 3106. (b) Fiona, L.; Lin, F. L.; Hoyt, H. T.; Halbeek, H.;
Bergman, R. G.; Bertozzi, C. R. J. Am. Chem. Soc. 2005,
27, 2686. (c) Soellner, M. B.; Tam, A.; Raines, R. T. J.
4
Yield 56%; off-white solid; mp 127–128 °C. MS (APCI):
+
1
1
169.2 [M + 1] . H NMR (300 MHz, D O): d = 2.77 (s, 3 H),
2
Org. Chem. 2006, 71, 9824.
3.38 (d, 2 H, J = 7.2 Hz), 4.18 (t, 1 H, J = 7.2 Hz), 6.91 (s, 1
H), 7.59 (s, 1 H). C NMR (75 MHz, D O): d = 28.13, 36.58,
55.11, 117.74, 131.43, 136.37, 174.72. IR (KBr): 3421,
3305, 2960, 2893, 1694, 1638, 1584, 1541 cm . Anal. Calcd
for C H N O: C, 50.00; H, 7.14; N, 33.33. Found: C, 49.94;
1
3
24) (a) Carrillo, N.; Davalos, E. A.; Russak, J. A.; Bode, J. W. J.
Am. Chem. Soc. 2006, 128, 1452. (b) Bode, J. W.; Fox, R.
M.; Baucom, K. D. Angew. Chem. Int. Ed. 2006, 45, 1248.
25) Somlai, C.; Szokan, G.; Balaspiri, L. Synthesis 1992, 285.
26) Shendage, D. M.; Fröhlich, R.; Haufe, G. Org. Lett. 2004, 6,
2
–
1
(
(
7
12
4
H, 7.09; N, 33.30.
3
675.
(32) Analytical Data of N1-Benzyl-2-aminoacetamide (3m).
Yield 52%; off-white solid; mp 142–144 °C. MS (APCI):
(
27) General Experimental Procedure.
+
1
To a solution of a-amino acid 1a–e (1.73 mmol) in H O (4
165.3 [M + 1] . H NMR (300 MHz, D O): d = 3.67 (s, 2 H),
2
2
1
3
mL), CDI (2.08 mmol) was added at ambient temperature
and the reaction mixture was stirred for 30 min. Aliphatic or
aromatic amine 2a–f (2.08 mmol) was then added and the
stirring continued for another 7.5–23.5 h at ambient
4.41 (s, 2 H), 7.14–7.19 (m, 5 H). C NMR (75 MHz, D O):
2
d = 42.57, 49.60, 126.33, 127.17, 128.41, 143.89, 173.70. IR
–
1
(KBr): 3454, 3319, 2971, 2888, 1687, 1578, 1453 cm .
Anal. Calcd for C H N O: C, 65.85; H, 7.32; N, 17.07.
9
12
2
temperature. The solvent was evaporated under reduced
pressure to afford crude product. Column chromatographic
purification of crude product on basic alumina using CHCl3–
MeOH as eluent produced amides 3a–m in 27–73% yield.
Found: C, 65.78; H, 7.26; N, 16.99.
(33) The optical purity of amide analogues was assessed on
HPLC using CHIRALPAK WH chiral column (ID 0.46 cm,
particle size 10 mm), and results indicate presence of £1.0%
of the D-enantiomer in all cases.
(
(
34) Staab, H. A. Justus Liebigs Ann. Chem. 1957, 609, 75.
35) Staab, H. A.; Lueking, M.; Duerr, F. H. Chem. Ber. 1962, 95,
1
275.
36) Klijn, J. E.; Engberts, J. B. F. N. Nature (London) 2005, 435,
46.
(
7
Synlett 2007, No. 4, 603–606 © Thieme Stuttgart · New York