Synthesis of retro-inverso peptides employing isocyanates of Nα-Fmoc-amino acids/peptide acids catalyzed by DMAP

Article abstract of DOI:10.1016/j.tetlet.2006.10.066

The Goldschmidt-Wick type reaction between isocyanates of N^α-Fmoc-amino acids/peptide acids and N^α-Boc-/Z-/Bsmoc-amino acids catalyzed by DMAP leads to the incorporation of a reversed peptide bond. It was found to be a simple, effici

Full text of DOI:10.1016/j.tetlet.2006.10.066

Tetrahedron Letters 47 (2006) 9139–9141
Synthesis of retro-inverso peptides employing isocyanates of
Na-Fmoc-amino acids/peptide acids catalyzed by DMAPI
Rao Venkataramanarao and Vommina V. Sureshbabu*
Department of Studies in Chemistry, Central College Campus, Bangalore University, Dr. B.R. Ambedkar Veedhi,
Bangalore 560 001, India
Received 14 June 2006; revised 27 September 2006; accepted 12 October 2006
Available online 2 November 2006
Abstract—The Goldschmidt–Wick type reaction between isocyanates of Na-Fmoc-amino acids/peptide acids and Na-Boc-/Z-/
Bsmoc-amino acids catalyzed by DMAP leads to the incorporation of a reversed peptide bond. It was found to be a simple, efficient
and clean reaction. All the retro-inverso peptides made were obtained as crystalline compounds in 70–92% yields.
Ó 2006 Elsevier Ltd. All rights reserved.
The concept of retro-inverso peptides with free and
blocked C- and N-termini has led to new molecules with
improved biological activity based on conformational
and topological properties.1 Important classes of mole-
cules studied include neurotransmitters, hormones,
inhibitors of proteases and protein kinases, sweeteners,
antimicrobial peptides, adhesion molecules, antigenic
epitopes, immunomodulators and immunological probes.2
A large number of molecules have been synthesized by
two approaches.3,4 The first protocol developed by Cho-
rev et al. involves the preferential use of acetyl protecting
groups rather than carbamates (specifically Boc or Z moi-
eties) due to the stability of isocyanate, which avoids the
displacement reactions.5,6
The more commonly used route involves a Hofmann
rearrangement of Na-protected amino acid amides using
iodobenzene bis(trifluoroacetate) (IBTFA) as an oxidiz-
ing agent to obtain mono N-acetylated, gem-diamino-
alkyl trifluoroacetates as key intermediates.7–9 As
concluded recently by Chorev, rigorous purification is
required for building blocks; thus selection of protecting
groups and the presence of reactive side-chains are key
factors which cannot be overlooked.3 Further, IBTFA
oxidation is not compatible with Fmoc chemistry.10
Acid azides11,12 and isocyanates13 derived from Fmoc
protected natural amino acids can be prepared easily
and can also be isolated, characterized and employed
as key intermediates at ambient temperature in the syn-
R1
R1
R1
N3
OH
b
a
+
PhCH2OH
Fmoc-NH
NH-Z
Fmoc-NH
Fmoc-NH
O
O
c
R1
O
NH-Boc/Z
Fmoc-
HN
N
H
R2
1
Scheme 1. Reagents and conditions: (a) NMM, IBC-Cl, 0 °C, aq NaN3, 30 min; (b) microwave irradiation for four 15 min intervals; (c) (i) Pd/C, H2,
(ii) Boc-/Z-amino acid, HBTU, DIEA.
q Presented at the 42nd Japanese Peptide Society, Osaka, Japan, held in 2005.
*
Corresponding author. Tel.: +91 0822961339; e-mail: hariccb@rediffmail.com
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.066

9140
R. Venkataramanarao, V. V. Sureshbabu / Tetrahedron Letters 47 (2006) 9139–9141
R1
R1
R1
O
a
b
NH-Bsmoc-/Z-/Boc
Fmoc-
HN
N
COOH
Fmoc-
Fmoc-HN
HN
NCO
H
R2
2
Scheme 2. Reagents and conditions: (a) NMM, IBC-Cl, 0 °C, aq NaN3, 30 min, microwave irradiation in toluene, 1 min or reflux 1 h; (b) Boc-/Z-/
Bsmoc-amino acid, cat. DMAP, temp 0 °C to rt.
R3
R1
R1
O
H
H
H
N
N
a, b
N
OH
NH-Boc/Z
Fmoc-NH
Fmoc-NH
O
O
R2
O
R2
2
2
3
Scheme 3. Reagents and conditions: (a) NMM, IBC-Cl, 0 °C, aq NaN3, 30 min, microwave irradiation in toluene, 1 min or reflux for 1 h; (b) Boc-/Z-
amino acid, cat. DMAP, temp 0 °C to rt.
Table 1. Retro-inverso peptides synthesized through the DMAP catalyzed method
Entry Compound
Mp (°C) Yield Mass
1H NMR
(%)
a
Fmoc-gLeu-rPhe-Boc
174
190
90
594.7032 d 0.93 (d, J = 5.1 Hz, 6H), 1.33 (m, 2H), 1.37 (s, 9H), 1.62 (m, 1H), 3.10 (d,
J = 6.2 Hz, 2H), 3.77 (m, 1H), 3.96 (m, 1H), 4.19 (t, J = 6.6 Hz, 1H), 4.42
(d, J = 5.9 Hz, 2H), 5.08 (br s, 1H), 5.62 (br s, 1H), 6.20 (br s, 1H), 7.17–
7.77 (m, 13H)
b
c
Fmoc-gAla-rLeu-Boc
92
75
79
518.2631 0.82 (m, J = 5.2 Hz, 6H), 1.21 (d, J = 6.2 Hz, 3H), 1.34 (s, 9H), 1.42 (m,
2H), 4.18 (t, J = 6.6 Hz, 1H), 4.21 (d, J = 6.5 Hz, 2H), 5.10 (d, J = 8.8 Hz,
1H), 6.10 (d, J = 6.6 Hz, 1H), 6.35 (d, J = 9.2 Hz, 1H), 7.31–7.87 (m, 8H)
621.7504 0.91 (m, 6H), 1.33 (s, 9H), 1.41 (m, 1H), 1.56 (m, 2H), 2.30 (s, 3H), 3.11 (d,
J = 6.2 Hz, 2H), 3.62 (m, 1H), 4.19 (t, J = 6.6 Hz, 1H), 4.42 (d, J = 6.5 Hz,
2H), 4.27 (m, 1H), 5.12 (br d, J = 9.2 Hz, 2H), 7.27–7.77 (m, 13H)
652.7466 0.82 (m, J = 5.1 Hz, 6H), 1.34 (s, 9H), 1.42 (m, 2H), 2.21–2.56 (m, 1H),
3.00–3.33 (m, 1H), 4.21 (t, J = 6.6 Hz, 1H), 4.42 (d, J = 6.9 Hz, 2H), 5.11
(m, 2H), 5.21 (d, J = 8.8 Hz, 1H), 6.20 (d, J = 6.6 Hz, 1H), 6.38 (d,
J = 9.2 Hz, 1H), 7.24–7.87 (m, 13H)
Fmoc-gIle-rCys(Acm)-Boc 202
Fmoc-gLeu-rAsp(Bzl)-Boc 198
d
e
f
Fmoc-gPhe-rVal-Bsmoc
Fmoc-gAla-rPhe-Bsmoc
Fmoc-gAla-rVal-Boc
Fmoc-gGly-rPhe-Boc
Fmoc-gSer(tBu)-rLeu-Z
172
180
218
166
171
89
85
88
85
87
88
91
89
82
702.7854 0.94 (t, J = 13.2 Hz, 6H), 1.89 (m, 1H), 3.19 (d, J = 6.1 Hz, 2H), 3.91 (br s,
1H), 4.19 (t, J = 6.6 Hz, 1H), 4.25 (br s, 1H), 4.42 (d, J = 6.5 Hz, 2H), 5.21
(s, 2H), 7.13–7.79 (m, 18H)
674.7295 1.30 (d, J = 6.3 Hz, 3H), 2.96 (d, J = 6.0 Hz, 2H), 3.33 (m, 1H), 3.88 (m,
1H), 4.19 (t, J = 6.6 Hz, 1H), 4.42 (d, J = 6.9 Hz, 2H), 5.21 (d, J = 8.8 Hz,
2H), 5.91 (br s, 1H), 6.71 (br s, 1H), 7.13–7.90 (m, 17H)
504.5642 0.94 (t, J = 13.2 Hz, 6H), 1.33 (s, 9H), 1.39 (d, J = 6.4 Hz, 3H), 1.89 (m,
1H), 3.87 (br s, 1H), 4.19 (t, J = 6.6 Hz, 1H), 4.26 (br s, 1H), 4.42 (d,
J = 6.5 Hz, 2H), 7.39–7.78 (m, 8H)
538.6246 1.33 (s, 9H), 3.21 (d, J = 4.4 Hz, 1H), 3.88 (br s, 1H), 4.19 (t, J = 6.6 Hz,
1H), 4.30 (br d, J = 9.2 Hz, 2H), 4.42 (d, J = 6.5 Hz, 2H), 7.13–7.79 (m,
13H)
624.7435 0.92 (d, J = 5.2 Hz, 6H), 1.33 (m, 2H), 1.38 (s, 9H), 1.62 (m, 1H), 3.55 (br
m, 2H), 3.91 (br s, 1H), 4.10 (m, 1H), 4.19 (t, J = 6.6 Hz, 1H), 4.42 (d,
J = 6.9 Hz, 2H), 5.10 (s, 2H), 7.13–7.79 (m, 13H)
665.7854 0.90 (d, J = 5.4 Hz, 6H), 1.33 (m, J = 6.0 Hz, 2H), 1.37 (s, 9H), 1.62 (m,
1H), 3.19 (d, J = 6.2 Hz, 2H), 3.88 (m, 1H), 4.10 (m, 1H), 4.19 (t, J =
6.6 Hz, 1H), 4.42 (d, J = 6.9 Hz, 2H), 7.13–7.79 (m, 13H)
691.3472 0.88 (d, J = 5.4 Hz, 6H), 1.25 (m, 2H), 1.41 (s, 9H), 1.55 (m, 1H), 1.78 (m,
1H), 1.97 (m, 4H), 3.18 (d, J = 6.2 Hz, 2H), 3.35 (m, 2H), 3.88 (br s, 1H),
4.19 (t, J = 6.6 Hz, 1H), 4.42 (d, J = 6.9 Hz, 2H), 7.13–7.79 (m, 13H)
685.7735 0.93 (t, J = 13.2 Hz, 6H), 1.39 (d, J = 6.6 Hz, 3H), 1.89 (m, 1H), 3.20 (m,
2H), 3.81 (br s, 1H), 4.19 (t, J = 6.6 Hz, 1H), 4.42 (d, J = 6.9 Hz, 2H), 5.12
(d, J = 9.2 Hz, 2H), 7.21–7.81 (m, 18H)
637.7371 0.92 (d, J = 5.2 Hz, 6H), 1.32 (m, J = 6.4 Hz, 2H), 1.39 (d, J = 6.6 Hz, 3H),
1.43 (s, 6H), 3.90 (m, 1H), 4.19 (t, J = 6.6 Hz, 1H), 4.42 (d, J = 6.5 Hz, 2H),
5.12 (d, J = 9.2 Hz, 2H), 7.21–7.81 (m, 13H)
g
h
i
j
Fmoc-Phe-gAla-rLeu-Boc 110
Fmoc-Leu-gPhe-rPro-Boc 105
k
l
Fmoc-Val-gAla-rPhe-Z
Fmoc-Ala-gLeu-rAib-Z
123
134
m

R. Venkataramanarao, V. V. Sureshbabu / Tetrahedron Letters 47 (2006) 9139–9141
9141
thesis of peptides, peptidomimetics and various methyl
carbamates.14 This letter demonstrates the utility of
isocyanates derived from Fmoc-a-amino acids for the
synthesis of retro-inverso peptides by the Goldsch-
midt–Wick type reaction.
financial assistance. R. V. R. Rao thanks the CSIR,
New Delhi, for the award of a fellowship.
References and notes
In the context of our ongoing interest in the incorpora-
tion of retro-inverso bonds in peptides, we initially
envisaged a route involving the preparation of N,N0-bis-
protected gem-diamines (Scheme 1). After deprotection
of the Z group employing Pd/C, the coupling of Boc-/
Z-amino acids with Fmoc–NH–CHR1–NH2 using
HBTU was explored. In addition to the multi-step
protocol, the alcoholysis of isocyanates has to be carried
out under microwave irradiation for about 15 min.
Recently, Gurtler et al. developed a Mg catalyst system
for the reaction of aliphatic isocyanates and blocked
isocyanates with carboxylic acids. However, our efforts
to couple the isocyanate of Fmoc-Leu-OH with Boc-
Phe-OH in the presence of Mg as a catalyst was
unsuccessful.15
1. Chorev, M. Biopolymers 2005, 80, 67–84.
2. Chorev, M.; Goodman, M. Acc. Chem. Res. 1993, 26,
266–273.
3. Scheibler, L.; Chorev, M. Methods of Organic Chemistry,
Synthesis of Peptides and Peptidomimetics; Houben-Weyl:
New York, 2002, E 22c, pp 528–550.
4. For other protocols for the synthesis of retro-inverso
peptides, see Ref. 3.
5. Chorev, M.; MacDonald, S. A.; Goodman, M. J. Org.
Chem. 1984, 49, 821–827.
6. Fletcher, M. D.; Campbell, M. M. Chem. Rev. 1998, 98,
763–795.
7. Sisto, A.; Verdini, A. S.; Virdia, A. Synthesis 1985, 294–
296.
8. Boutin, R. H.; Loudon, G. M. J. Org. Chem. 1984, 49,
4277–4284.
9. Yamamoto, Y.; Yatagai, H.; Maruyama, K. J. Org. Chem.
1979, 44, 1746–1747.
10. Englund, E. A.; Gopi, H. N.; Appella, D. H. Org. Lett.
2004, 6, 213–215.
11. Suresh Babu, V. V.; Ananda, K.; Vasanthakumar, G. R.
J. Chem. Soc., Perkin. Trans. 1 2000, 4328–4331.
12. Vasanthakumar, G. R.; Ananda, K.; Sureshbabu, V. V.
Indian J. Chem. 2002, 41B, 1733–1735.
13. Patil, B. S.; Vasanthakumar, G. R.; Suresh Babu, V. V. J.
Org. Chem. 2003, 68, 7274–7280.
14. Patil, B. S.; Suresh Babu, V. V. Lett. Pept. Sci. 2004, 10,
93–97.
15. (a) Gurtler, C.; Danielmeier, K. Tetrahedron Lett. 2004,
45, 2515–2521; (b) Gertzmann, R.; Gurtler, C. Tetra-
hedron Lett. 2005, 46, 6659–6662.
16. Schuemacher, A. C.; Hoffmann, R. W. Synthesis 2001,
243–246.
17. Ramiah, M.; Scriven, E. F. V. Aldrichim. Acta 2003, 36,
21–27.
Later we found that a catalytic amount of DMAP16,17
accelerates the coupling leading to the formation of a
retro-inverso peptide bond. In a typical reaction,
Fmoc-amino acid azides were prepared by generating
a mixed anhydride of Fmoc-amino acid and then reac-
tion with NaN3. The resulting azide was dissolved in tol-
uene and subjected to Curtius rearrangement.18 After
evaporation of toluene under reduced pressure, it was
dissolved in DCM and a mixture of Boc-/Z-/Bsmoc-
amino acid and a catalytic amount of DMAP were
added at 0 °C and the mixture was stirred for 30 min,
allowed to come to rt and stirring was continued for
another 2 h. A simple work-up and recrystallization
led to the isolation of product 2 in 70–92% yield
(Scheme 2). The same methodology has also been ap-
plied to Fmoc-peptide acids which resulted in products
3 (Scheme 3).19 The entire course of the reaction can
be completed in about 4 h. All the retro-inverso peptides
made were isolated by a single recrystallization (Table 1)
18. General procedure for the synthesis of isocyanates of Na-
Fmoc-amino acids/peptide acids: To an ice-cold solution
of Na-Fmoc-amino acid or peptide acid (1 mmol) in dry
THF (5 mL) were added N-methylmorpholine (NMM)
(0.11 mL, 1 mmol) and IBC-Cl (0.135 mL, 1 mmol) and
the mixture was stirred at ꢀ10 °C for 5 min. The resulting
reaction mixture was treated with aq NaN3 (0.098 g,
1.5 mmol in 1 mL) and stirred for another 30 min. After
completion of the reaction, the organic layer was evapo-
rated and the residue was dissolved in CH2Cl2 (30 mL),
washed with 10 mL each of 5% HCl, 5% aq NaHCO3 and
brine and dried over anhydrous Na2SO4. The solvent was
evaporated in vacuo and the residue dissolved in toluene
(10 mL) and heated at 90 °C under nitrogen. After 1 h, the
solvent was removed under reduced pressure to give the
isocyanate, which was used for the next step.
19. General procedure for the synthesis of retro-inverso pep-
tides: To a mixture of Na-Fmoc-amino acid/peptide
isocyanate (1 mmol) and Boc-/Z-amino acid (1.2 mmol)
in dry CH2Cl2 (20 mL) at 0 °C was added DMAP
(0.3 mmol). After 30 min at 0 °C, stirring was continued
at rt for another 2 h. The solvent was evaporated in vacuo
and the residue was dissolved in ethyl acetate, washed with
10 mL each of 5% Na2CO3, 5% citric acid and brine and
dried over anhydrous Na2SO4. The solvent was evapo-
rated and the resulting residue crystallized using ethyl
acetate/hexane (2:8).
1
and were fully characterized by H NMR, 13C NMR
and mass spectroscopic measurements.
In conclusion, the Goldschmidt–Wick type reaction
between isocyanates of Fmoc-amino acids and Na-
protected amino acids catalyzed by DMAP results in
retro-inverso peptides. The protocol is simple, efficient
and scale-up of the reaction up to 25 mmol per batch
did not pose any problems. Thus, it is now demon-
strated that the Fmoc group for Na-protection during
the synthesis of retro-inverso peptides permits the use
of a urethane as a protecting group which can be easily
deprotected, unlike an N-acetyl, and can also allow
further extension of the peptide chain.
Acknowledgements
The authors thank Professor Fred Naider, The College
of Staten Island, City University of New York, New
York for useful discussions. We also thank the Depart-
ment of Biotechnology, Government of India for the