One-pot synthesis of ureido peptides and urea-tethered glycosylated amino acids employing deoxo-fluor and TMSN3

Article abstract of DOI:10.1055/s-0028-1087538

A facile one-pot procedure for the synthesis of urea-linked peptidomimetics and neoglycopeptides under Curtius rearrangement conditions employing Deoxo-Fluor and TMSN₃ is described. The method is efficient and circumvents the isolation of acyl

Full text of DOI:10.1055/s-0028-1087538

LETTER
407
One-Pot Synthesis of Ureido Peptides and Urea-Tethered Glycosylated
Amino Acids Employing Deoxo-Fluor and TMSN3
O
ne-PotSynthesis
.
of
U
reidoPe
P
ptides . Hemantha, G. Chennakrishnareddy, T. M. Vishwanatha, V. V. Sureshbabu*
Peptide Research Laboratory, Department of Studies in Chemistry, Central College Campus, Dr. B. R. Ambedkar Veedhi,
Bangalore University, Bangalore 560001, India
Fax +91(80)22292848; E-mail: hariccb@rediffmail.com
Received 10 November 2008
ureas, ureidopeptides, substituted carbamates, and urea-
linked glycopeptides from N-protected amino acids
through Curtius rearrangement employing the corre-
sponding isocyanates as stable and useful intermedi-
Abstract: A facile one-pot procedure for the synthesis of urea-
linked peptidomimetics and neoglycopeptides under Curtius rear-
rangement conditions employing Deoxo-Fluor and TMSN3 is de-
scribed. The method is efficient and circumvents the isolation of
acyl azide and isocyanate intermediates. The rearrangement of the ates.15–17
in situ generated acyl azide via the acyl fluoride was carried out un-
However, generation of an isocyanate from an amine re-
der ultrasonication followed by amine capture to insert a urea bond.
The protocol worked well with all the common N-urethane-protect-
ed amino acids and also with a sugar-6-acid.
quires handling of hazardous phosgene or triphosgene.11
Methods involving Hoffmann reactions place restrictions
on the nature of N-protecting group, whilst the widely
used and more reliable Curtius rearrangement involves
the isolation and handling of potentially explosive acyl
azides. In this context, a one-pot and high-yielding proce-
dure, which excludes the isolation of hazardous interme-
diates, is desirable for the efficient preparation of
oligoureas. Our group has reported a one-pot synthesis of
N-urethane-protected ureidopeptides employing diphe-
nylphosphoryl azide (DPPA) as azide-transfer reagent.18
Ley and co-workers have recently demonstrated the use of
a mesofluidic flow reactor for performing Curtius rear-
rangements of various carboxylic acids employing DPPA
and in situ trapping of the resultant isocyanate. Several
carbamates, ureas, and semicarbazides were obtained in
good yields.19
Key words: peptidomimetics, neoglycopeptides, ureido linkage,
acyl fluoride, Deoxo-Fluor
Bioisosteric replacement of a peptide bond with various
surrogates is a common method for improving biological
stability, to enhance selectivity, and to induce desired con-
formational features into peptides.1,2 The urea linkage3 is
of special interest in peptidomimetic research as a non-
peptide linkage because of its promising features in drug
discovery and interesting hydrogen-bonding properties.4
The significant degree of conformational restrictions of-
fered by the urea moiety helps in better understanding the
protein secondary structures and provides insights into the
stability of protein b-sheets.5 Due to the various advantag-
es associated with the urea moiety, the generation of mo-
lecular diversity through oligopeptidyl ureas and
ureidopeptides is still a topic of much discussion in spite
of the handful of synthetic methods developed and report-
ed by various groups including ours.6–8 The urea moiety
has been inserted in several biologically active peptides
such as HIV-1 proteases, g-secretaries angiotensin,
[Leu5]enkephalin analogues, TAR binding fragment of
TAT protein, etc.9,10
Over the years, bis(2-methoxy-ethyl)aminosulfur trifluo-
ride (Deoxo-Fluor)20 introduced by Lal et al. has gained
considerable attention as an efficient reagent in various
nucleophilic fluorinating reactions. Being thermally sta-
ble, it has been widely employed for fluorinating various
substrates such as amino alcohols, diols, carbohydrates,
aldehydes, ketones, and carboxylic acids.21,22 Kangani et
al. reported the synthesis of acyl azides, amides, and few
heterocycles employing Deoxo-Fluor.23,24 Georg and co-
workers employed Deoxo-Fluor for the one-pot conver-
sion of carboxylic acids into amides and Weinreb
amides.25 On the other hand, the N-protected amino acid
derived acyl fluorides26 are well known as stable and ac-
tive intermediates in peptide coupling. Their application
in peptide synthesis was first reported by Carpino et al. in
SPPS.27 In continuation of our interest in developing sim-
ple and easier protocols for the preparation of urea-linked
peptides and glycopeptides, we herein report a one-pot
synthesis of such peptidomimetics employing Deoxo-
Fluor and TMSN3. The procedure apparently masks four
independent steps, namely the formation of acyl fluoride,
conversion into azide, then its rearrangement into iso-
cyanate, and finally coupling with the amine to give urea.
Nowick et al. employed isocyanates derived from amino/
peptide acid esters as intermediates to prepare urea-linked
peptides.11 While Liskamp et al. employed p-nitrophenol-
derived N-Fmoc-protected active ethyl carbamates,12
Guichard and co-workers reported the application of O-
succinimidyl(Na-urethane-protected)methyl carbamates13
for the preparation of urea-linked oligomers. Oxidative
Hoffmann rearrangements of Boc/Z-amino acid/peptide
acid amides have also been reported.14 Sureshbabu et al.
reported the preparation of N-Fmoc-protected peptidyl
SYNLETT 2009, No. 3, pp 0407–0410
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Advanced online publication: 21.01.2009
DOI: 10.1055/s-0028-1087538; Art ID: D35608ST
© Georg Thieme Verlag Stuttgart · New York

408
H. P. Hemantha et al.
LETTER
In a typical reaction, Na-Fmoc-Ala-OH dissolved in dry col. The methyl protons of the two ureas showed distinct
CH2Cl2 was chilled in an ice bath, triethyl amine and doublets respectively at d = 1.32, 1.30 and 1.31, 1.29 ppm.
Deoxo-Fluor were added followed by TMSN3. After a The urea obtained by the reaction of Fmoc-Phe-OH
few minutes of ultrasonication, leucine methyl ester (ob- (Table 1) with equimolar mixture of (R)- and (S)-phenyl-
tained by neutralizing the corresponding hydrochloride ethylamines showed two separate doublets for the methyl
1
salt with zinc dust28) was added, and the reaction mixture protons in H NMR, thus showing the absence of any
was maintained under ultrasonication for about 30 min- epimerization.
utes. The isocyanate formed by the rearrangement of acyl
Finally, the versatility of the protocol was demonstrated
azide reacted with the amine component present in the
by the synthesis of a urea through the isocyanate derived
reaction mixture to afford the corresponding urea. This
from a sugar acid. Galactose-6-acid31 was considered as
was evident by TLC analysis and the separation of the
specimen sugar acid, which was treated with Deoxo-
product as solid from the reaction mixture. After comple-
Fluor, Et3N, TMSN3, and amino acid methyl ester as de-
tion of the reaction29 excess hexane was added and the
scribed earlier. Upon ultrasonication for about 40 min-
utes, the corresponding ureas 4a–d were isolated after a
simple workup followed by column chromatography in
precipitated product was collected by filtration. A simple
recrystallization yielded the corresponding peptidyl urea
as white stable solid whose physical constants matched
satisfactory yields and purity (Scheme 3).
with the ones already reported (Scheme 1).
COOMe
R1
Deoxo-Fluor, Et3N
TMSN3
R1
O
R2
HN
O
O
OH
Deoxo-Fluor, Et3N
TMSN3
O
O
OH
HN
R1
PGNH
PGNH
N
N
COOMe
O
O
R2
H
H
O
O
O
R1
1
H2N
COOMe
2
OMe
O
O
O
O
H2N
O
Scheme 1
4
R1
Yield (%)
When the reaction was followed using IR spectroscopic
analysis without the addition of amine, an initial peak was
observed around 2100 cm–1 suggesting the formation of
an acyl azide and in the later stages, a peak around 2245
cm–1 corresponding to the isocyanate. Thus the sequence
of the reaction via Curtius rearrangement was confirmed.
The formation of acyl azide was found to be rapid and
clean in case of the Deoxo-Fluor/TMSN3 reagent system.
The procedure was repeated with several other amino ac-
ids and the resulting urea adducts were isolated in excel-
lent yields. The protocol worked well with Boc- and Z-
protected amino acids also. When the amino acid ester
was replaced by 2,3,4,6-tetra-O-acetyl glycosyl-1-amine,
the reaction went smoothly yielding urea-linked glycosy-
lated amino acids in good yield and purity (Scheme 2).30
4a
4b
4c
4d
Me
Bn
68
72
79
76
i-Pr
CH2OBn
Scheme 3
In summary, we report a one-pot synthesis of urea-linked
peptidomimetics and neoglycopeptides starting from N-
protected amino acids/protected sugar acid. The proce-
dure is based on the use of Deoxo-Fluor and TMSN3 for
the conversion of acid into acyl azide via the in situ gen-
erated acyl fluoride, which rearranges into isocyanate un-
der ultrasonication followed by a reaction with an amine.
The simple, mild conditions and efficacy of the method
makes it an attractive alternative to existing methodolo-
gies.
R2O
Deoxo-Fluor, Et3N
H
H
TMSN3
O
R2O
R2O
N
N
NHPG
1
AcO
R2O
Acknowledgment
O
R1
R2 = Ac, Bz
O
NH2
AcO
AcO
3
The authors thank the Council of Scientific and Industrial Research
[Grant No. 01(2034)/06/EMR-II] and Department of Science and
Technology (Grant No. SR/S1/OC-26/2008), Government of India
for financial support.
AcO
Scheme 2
However, when the reaction was carried out under reflux
conditions, clean conversion, as was the case under ultra-
sonication, was not observed and the yields were not sat-
isfactory.
References and Notes
(1) Liskamp, R. M. J. Angew. Chem., Int. Ed. Engl. 1994, 33,
633.
(2) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173.
(3) Burgess, K.; Linthicum, D. S.; Shin, H. Angew. Chem., Int.
Ed. Engl. 1995, 34, 907.
(4) Nowick, J. S.; Abdi, M.; Bellamo, K. A.; Love, J. A.;
Martinez, E. J.; Noronha, G.; Smith, E. M.; Ziller, J. M.
J. Am. Chem. Soc. 1995, 117, 89.
The mild conditions employed in the protocol were found
to be racemization-free as evident by the 1H NMR analy-
sis. To test this, the urea adducts were prepared by the re-
action of Fmoc-Phe-OH with optically pure (R)-(+)- and
(S)-(–)-1-phenylethylamine employing the present proto-
Synlett 2009, No. 3, 407–410 © Thieme Stuttgart · New York

LETTER
One-Pot Synthesis of Ureido Peptides
409
Table 1 Peptidyl Ureas 2 and Urea-Linked Glycosylated Amino Acids 3
Amino acid 1
Urea
Yield (%)
84
Mpa (°C)
O
O
Fmoc-Ala-OH
2a
200 (199)
184 (184)
FmocHN
FmocHN
N
H
N
H
COOMe
COOBn
Fmoc-Val-OH
2b
83
N
H
N
H
O
Z-Leu-OH
Z-Gly-OH
2c
86
83
177
159
ZNH
ZNH
N
H
N
COOMe
H
O
2d
N
H
N
H
COOMe
COOBn
Boc-Glu(OBz)-OH
Fmoc-Val-OH
2e
3a
75
75
138 (139)
O
BocHN
AcO
N
N
COOMe
H
H
H
N
H
O
AcO
AcO
179
N
NHFmoc
AcO
O
BzO
BzO
OBz
O
H
H
N
N
HNBoc
Boc-Phe-OH
3b
72
155
BzO
O
O
ZN
O
AcO
Z-Asp(Oxa)-OH
Fmoc-Pro-OH
3c
74
69
168
131
H
O
N
NH
AcO
AcO
AcO
AcO
O
AcO
AcO
H
H
O
O
3d
AcO
AcO
N
N
NFmoc
AcO
O
AcO
O
a Values given in parenthesis refer to reported results.
(5) Nowick, J. S. Acc. Chem. Res. 1999, 32, 287.
(12) Boeijen, A.; Ameijde, J. v.; Liskamp, R. M. J. J. Org. Chem.
2001, 6, 8454.
(13) Guichard, G.; Semetey, V.; Didierjean, C.; Aubry, A.;
Briand, J. P.; Rodriguez, M. J. Org. Chem. 1999, 64, 8702.
(14) Myers, A. C.; Kowalski, J. A.; Lipton, M. A. Bioorg. Med.
Chem. Lett. 2004, 14, 5219.
(6) Fischer, L.; Semetey, V.; Lozano, J. M.; Schaffiner, A. P.;
Briand, J. P.; Didierjean, C.; Guichard, G. Eur. J. Org.
Chem. 2007, 3944.
(7) Patil, B. S.; Vasanthakumar, G. R.; Sureshbabu, V. V.
J. Org. Chem. 2003, 68, 7274.
(8) Boeijen, A.; Liskamp, R. M. J. Eur. J. Org. Chem. 1999,
2127.
(15) Sureshbabu, V. V.; Patil, B. S.; Venkataramanarao, R.
J. Org. Chem. 2006, 71, 7697.
(9) Tamilarasu, N.; Huq, F.; Rana, T. M. J. Am. Chem. Soc.
1999, 121, 1579.
(10) Bakshi, P.; Wolfe, M. S. J. Med. Chem. 2004, 47, 6485.
(11) Nowick, J. S.; Holmer, D. L.; Noronha, G.; Smith, E. M.;
Nguyen, J. M.; Huang, S. L. J. Org. Chem. 1996, 61, 3929.
(16) Sureshbabu, V. V.; Venkataramanarao, R.; Hemantha, H. P.
Int. J. Pept. Res. Ther. 2008, 14, 34.
(17) Sureshbabu, V. V.; Kantharaju; Tantry, S. J. Int. J. Pept.
Res. Ther. 2005, 11, 131.
Synlett 2009, No. 3, 407–410 © Thieme Stuttgart · New York

410
H. P. Hemantha et al.
LETTER
(18) Sureshbabu, V. V.; Chennakrishnareddy, G.; Narendra, N.
Tetrahedron Lett. 2008, 49, 1408.
(19) Baumann, M.; Baxendale, I. R.; Ley, S. V.; Nikbin, N.;
Smith, C. D.; Tierney, J. P. Org. Biomol. Chem. 2008, 6,
1577.
(20) (a) Lal, G. S.; Pez, G. P.; Pesaresi, R. J.; Prozonic, F. M.
Chem. Commun. 1999, 215. (b) Lal, G. S.; Pez, G. P.;
Pesaresi, R. J.; Prozonic, F. M.; Cheng, H. J. Org. Chem.
1999, 64, 7048.
(21) Singh, R. P.; Shreeve, J. M. Synthesis 2002, 2561.
(22) Singh, R. P.; Chakraborty, D.; Shreeve, J. M. J. Fluorine
Chem. 2002, 111, 153.
184 °C. 1H NMR (300 MHz, DMSO): d = 0.92 (12 H, m),
1.32–1.85 (4 H, m), 3.10 (2 H, s), 3.70–3.80 (2 H, m), 4.20
(1 H, t), 4.42 (2 H, m), 5.10 (1 H, d), 6.60–6.70 (2 H, m),
7.20–7.85 (13 H, m). 13C NMR (200 MHz, DMSO): d =
18.5, 19.5, 22.0, 23.1, 24.5, 29.2, 40.3, 47.2, 59.0, 66.6,
120.0, 125.1, 126.5, 127.0, 127.2, 128.4, 129.3, 137.6,
141.2, 144.0, 155.4, 156.8, 176.4. HRMS: m/z calcd for
C33H39N3NaO5: 580.2787; found: 580.2774 [M + Na].
Z-Gly-y(NH-CO-NH)-Val-OMe (2d): Off-white solid, mp
159 °C. 1H NMR (300 MHz, DMSO): d = 0.93 (6 H, d), 3.14
(1 H, m), 3.58 (3 H, s), 4.51 (3 H, m), 5.30 (2 H, s), 6.10 (2
H, m), 6.48 (1 H, t), 7.20–7.40 (5 H, m). 13C NMR (200
MHz, DMSO): d = 17.1, 31.1, 52.1, 56.2, 57.8, 65.4, 127.1,
127.2, 128.5, 141.2, 156.8, 157.5, 171.6. HRMS: m/z calcd
for C16H23N3NaO5: 360.1535; found: 360.1513 [M + Na].
Boc-Glu(OBzl)-y(NH-CO-NH)-Ile-OMe (2e): solid, mp
138 °C. 1H NMR (300 MHz, DMSO): d = 0.91 (6 H, d),
1.30–1.45 (11 H, s), 1.65 (1 H, m), 2.55 (2 H, m), 2.90 (2 H,
m), 3.65 (3 H, s), 3.80–3.90 (2 H, m), 5.15 (2 H, s), 5.30 (1
(23) Kangani, C. O.; Day, B. W.; Kelley, D. E. Tetrahedron Lett.
2008, 49, 914.
(24) (a) Kangani, C. O.; Day, B. W.; Kelley, D. E. Tetrahedron
Lett. 2007, 48, 5933. (b) Kangani, C. O.; Kelley, D. E.
Tetrahedron Lett. 2005, 46, 8917.
(25) Tunoori, A. R.; White, J. M.; Georg, G. I. Org Lett. 2000, 2,
4091.
(26) Carpino, L. A.; Mansour, E. M. E.; El-Fahan, A. J. Org.
Chem. 1993, 58, 4162.
H, d), 6.35 (1 H, d), 6.50 (1 H, d), 7.30–7.40 (5 H, m). 13
C
NMR (200 MHz, DMSO): d = 22.1, 23.1, 24.7, 28.6, 37.9,
39.7, 419, 50.9, 51.7, 61.9, 63.1, 78.7, 126.7, 127.6, 128.9,
137.7, 155.3, 156.8, 157.5, 178.1. HRMS: m/z calcd for
C24H37N3NaO7: 502.2529; found: 502.2543 [M + Na].
Fmoc-Val-y(NH-CO-NH)-2,3,4,6-tetra-O-acetyl-b-D-
glucopyranoside (3a): white solid, mp 179 °C. 1H NMR
(300 MHz, DMSO): d = 0.93 (6 H, d), 1.95 (12 H, s), 3.12 (1
H, m), 4.30–4.45 (3 H, m), 4.67 (2 H, d), 4.90–5.20 (5 H, m),
5.40 (1 H, m), 5.70 (2 H, br), 6.90–7.50 (8 H, m). 13C NMR
(200 MHz, DMSO): d = 15.1, 20.8, 21.1, 33.2, 47.5, 59.7,
67.8, 68.0, 69.1, 69.2, 73.0, 74.8, 81.0, 126.8, 128.2, 128.4,
128.6, 141.3, 142.5, 156.0, 158.1, 170.7. HRMS: m/z calcd
for C34H41N3NaO12: 706.2588; found: 706.2601 [M + Na].
(1,2),(3,4)-Diacetylgalactopyranosyl-6-NH-CO-NH-Phe-
OMe (4b): solid, mp 104 °C. 1H NMR (300 MHz, DMSO):
d = 1.25 (12 H, s), 3.21 (2 H, d), 3.85 (3 H, s), 4.30–4.53 (3
H, m), 4.80 (1 H, m), 5.50–5.70 (2 H, m), 6.31 (2 H, s), 7.10–
7.40 (5 H, m). 13C NMR (200 MHz, DMSO): d = 23.5, 34.2,
50.8, 54.2, 67.4, 69.2, 76.8, 77.9, 89.2, 107.6, 113.5, 125.4,
126.7, 127.8, 137.0, 157.2, 171.8. HRMS: m/z calcd for
C22H30N2NaO8: 473.1900; found: 473.1918 [M + Na].
(31) First, a solution of a-D-galactose (5 mmol) and ZnBr2 (5.2
mmol) in acetone (20 mL) was stirred for 12 h and filtered.
The filtrate was concentrated and after a simple workup, the
resulting (1,2),(3,4)-diacetylgalactopyranose was dissolved
in MeCN. Then, TEMPO and Na3PO4 buffer were added and
the reaction mixture was warmed to 35 °C. Sodium chlorite
and bleach were added, and the reaction mixture was stirred
till completion of reaction. A simple workup lead to the
isolation of the desired sugar-6-acid. See: Jhao, M.; Li, J.;
Mano, E.; Song, Z.; Tschaen, D. M.; Grabowski, E. J. J.;
Reider, P. J. J. Org. Chem. 1999, 64, 2564.
(27) Kaduk, C.; Wenschuh, H.; Beyermann, M.; Forner, K.;
Carpino, L. A.; Bienert, M. Lett. Pept. Sci. 1995, 2, 285.
(28) Utility of commercial activated zinc dust to deprotonate
amine hydrochloride salts is documented. Sureshbabu et al.
demonstrated the conversion of amino acid/peptide acid
ester hydrochloride salts into the corresponding free amines,
see: (a) Sureshbabu, V. V.; Ananda, K. J. Pept. Res. 2001,
57, 223. (b) For the use of zinc dust as HCl scavenger in
peptide synthesis via N-Fmoc amino acid chlorides under
non-Schotten–Baumann conditions, see: Gopi, H. N.;
Sureshbabu, V. V. Tetrahedron Lett. 1998, 39, 9769. (c)
For a similar application in N-Boc-Z-Fmoc amino acid
fluoride couplings under neutral conditions, see:
Sureshbabu, V. V.; Ananda, K. Lett. Pept. Sci. 2000, 7, 41.
(d) For the preparation of oligomer-free Z-amino acids
employing ZCl/Zn dust, see: Gopi, H. N.; Ananda, K.;
Sureshbabu, V. V. Protein Pept. Lett. 1999, 6, 233.
(29) Typical Experimental Procedure for 2a
To a stirred solution of Fmoc-Ala-OH (1 mmol) in dry
CH2Cl2, Et3N (2 mmol) and Deoxo-Fluor (1.4 mmol) were
added at 0 °C. After the addition of TMSN3 (1.3 mmol), the
reaction mixture was subjected to ultrasonication. After 10
min, H-Leu-OMe (1.5 mmol) was added, and the
ultrasonication was continued until completion of the
reaction. The reaction mixture was evaporated, hexane was
added, and the residue was filtered. It was washed with H2O,
hexane, and dried under vacuum. Finally, the compound was
recrystallized using DMSO–H2O to afford the urea as a
colorless crystalline solid.
(30) Selected Spectroscopic Data
Fmoc-Val-y(NH-CO-NH)-Leu-OBzl (2b): white solid, mp
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