Novel route in the synthesis of ψ[CH2NH] amide bond surrogate

Article abstract of DOI:10.1016/j.tetlet.2007.11.112

An alternative method for the synthesis of pseudopeptides containing a ψ[CH₂NH] amide bond surrogate is reported. The synthetic approach is based on a nucleophilic displacement of the chiral N-protected β-iodoamines with conveniently protected

Full text of DOI:10.1016/j.tetlet.2007.11.112

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 731–734
Novel route in the synthesis of w[CH2NH] amide bond surrogate
Pietro Campiglia a, Claudio Aquino b, Alessia Bertamino b, Marina Sala b,
Isabel M. Gomez-Monterrey b, Ettore Novellino b, Paolo Grieco b,*
a Department of Pharmaceutical Science, University of Salerno, I-84084 Fisciano, Italy
b Department of Pharmaceutical and Toxicological Chemistry, University of Naples ‘Federico II’, I-80131 Naples, Italy
Received 9 August 2007; revised 31 October 2007; accepted 20 November 2007
Available online 24 November 2007
Abstract
An alternative method for the synthesis of pseudopeptides containing a w[CH2NH] amide bond surrogate is reported. The synthetic
approach is based on a nucleophilic displacement of the chiral N-protected b-iodoamines with conveniently protected amino acid esters.
The compatibility of this method with both conventional and microwave-assisted peptide synthesis should increase the potentiality of the
w[CH2NH] peptide bond isostere in peptide chemistry.
Ó 2007 Elsevier Ltd. All rights reserved.
Peptides are involved in many important biological pro-
cesses but only very few are used today as pharmaceutical
agents.1 The amide bond is extremely polar and it is a natural
target of many enzymes causing a rapid degradation of pep-
tides. To transform a bioactive peptide into a useful molecule
with potential pharmacological properties, with enhanced
bioavailability and improved transfer rate across cell mem-
branes, modifications of the peptide backbone are applied.
Among the modifications necessary for rendering peptides
more stable are described cyclization,2 unnatural amino
acids insertion,3 and backbone replacements by amide bond
surrogates.4 In particular, the substitution of amide bond by
an aminomethylene group, [–CONH–?–CH2NH–] pro-
duces a reduction of polarity and strengthens resistance
against protease degradation when compared to natural pep-
tides. Pseudopeptides containing w[CH2NH] amide bond
surrogates have been used in design of enzyme inhibitors,5
in the development of antagonists against several receptors,6
and recently as potential agents for gene delivery.7
some negative features relative to the preparation of conve-
niently protected amino aldehydes.10–13 Racemization
through keto–enol tautomerism,14 and undesirable side
reaction as the double alkylation9,15 are often been
observed. An alternative approach, recently described by
Kirillova et al. consists in the formation of an aminomethyl-
ene pseudodipeptide via Mitsunobu condensation of an
alcohol with an acidic component. The reaction proceeds
under a mild condition without racemization.16
We present herein an alternative method for the synthe-
sis of w[CH2NH] amide bond surrogate based on a synthetic
approach involving a nucleophilic displacement of the
chiral N-protected b-iodoamines (1) with conveniently
protected amino acid esters. The N-protected b-iodoamines
represent a potential source of molecular diversity, recently
reported in several communications. In fact, N-Fmoc-b-
iodoamines have been recently used in solid-phase synthesis
of w[CH2S] pseudopeptides,17 in the synthesis of S-Glycoa-
mino acids through their reaction with anchored thioglyco-
sides,18 while N-Boc-protected analogues were used in total
synthesis of efrapeptin C.19
Currently, the reductive alkylation of an a-amino group
by a protected amino acid aldehyde is the main methodol-
ogy to get a w[CH2NH] surrogate in both solution and
solid-phase synthesis.8,9 This methodology is limited for
The w[CH2NH] pseudodipetides of general formula 2
were synthesized according to the synthetic pathway
showed in Scheme 1. Initially, the enantiomeric pure N-
protected-b-iodoamines used in this work were obtained
from the corresponding N-protected b-amino alcohols by
*
Corresponding author. Tel.: +39 081 678620; fax: +39 081 678644.
E-mail address: pagrieco@unina.it (P. Grieco).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.11.112

732
P. Campiglia et al. / Tetrahedron Letters 49 (2008) 731–734
R'
H
(C6H5)3P-I2
imidazole
R
H
H
R
H2N
R
COOR
PHN
H
H
N
COOMe
OH
I
base
PHN
CH2Cl2
reflux, 1h
PHN
H R'
1
2
Scheme 1. General procedure for the synthesis of w[CH2NH] pseudo-dipetides 2.
means of polymer bound triphenylphosphine-iodine com-
plex, or by soluble triphenylphosphine, according to the
previously described method.17,20
Following this synthetic procedure, a series of Fmoc,
Boc and ZN-protected b-iodoamines derived from Ala,
Val, Phe and a,b-diaminopropionic acid were synthesized
(Table 1, 1a–i). The nature of side chains was chosen as a
representative example of aliphatic, aromatic and polar
amino acid residues to address their reactivity.
A preliminary study of the influence of different bases
(Cs2CO3, K2CO3, triethylamine, and DBU) and solvents
(DMF, NMP, DMSO, CH2Cl2, THF, and acetone) on
nucleophilic displacement of the iodine group by a-amino
acid esters was performed using the iodo-derivative 1a
and H-Phe-OMe (Scheme 2).
H3C
H
+
I
ZHN
H2N
COOMe
1a
base
DMF, rt
H3C
N
H3C
H
NHZ
H
H3C
H
COOMe
N
ZHN
COOMe
ZHN
2a
3
Scheme 2. Reaction of 1a with H-Phe-OMe.
As showed in Table 2, after 16 h Cs2CO3 gave the high-
est yields and better selectivity in the formation of pseud-
odipeptide 2a using dimethylformamide as the solvent,
and no byproducts of dialkylation 3 were detected.21
K2CO3 and triethylamine gave low yields with a little per-
centile of dialkylation product, while DBU was ineffective
in degrading the starting iodo-derivates (Table 2).
When various solvents were examinated (Table 3), N,N-
dimethylformamide was found to be the better solvent for
the reaction. N-Methylpyrrolidinone and dimethylsulf-
oxide gave lower yields while other solvents like dichloro-
methane, tetrahydrofuran or acetone were demonstrated
unsuitable for this type of reaction.
Table 2
N-Alkylation of H-Phe-OMe with 1a utilizing different bases
Compound
Yield (%)
K2CO3
Cs2CO3
TEA
DBU
2a
3
85
35
15
15
8
>5
0
Not detected
Table 3
Use of various solvents in cesium base promoted N-alkylation of H-
Phe-OMe
Compound
Yield (%)
DMSO
DMF
NMP
CH2Cl2, THF, acetone
Table 1
2a
3
80
0
70
6
60
0
5
0
N-Protected b-iodoamine derivatives used in this study
H3C
H
1a
1b
I
I
According to these results the reaction of 1a with H-
Phe-OMe in DMF at room temperature using Cs2CO3 as
the base22 generated the Z-Ala-w[CH2NH]Phe-OMe (2a)
in 80% of yield after a flash chromatography purification.
Product of dialkylation of amine (3) or pseudopeptide dia-
stereomers owing to racemization were not detected by
ZHN
H3C
H
I
I
1f
BocHN (S)
H
(S)
ZHN
H
1
HPLC and H NMR analysis of crude product. Based on
1g
1h
1i
H3C
H
(S)
BocHN
MeOOC
BocHN
these preliminary experiments, which optimized the
synthetic parameters, various N-protected b-iodoamines
(1a–i) and amino acid esters (H-Phe-OMe, H-Pro-OBz,
and H-Pro-OMe) were subjected to N-alkylation to vali-
date this new synthetic route. As shown in Table 4 the
new method developed was compatible with all b-iodo-
amines used in this study. The corresponding w[CH2NH]
pseudodipetides (2aa–2i) were obtained in yields in the
range of 65–86%. Variations of reactivity were not detected
using different N-protecting groups, such as Fmoc, Boc,
and Z (entries 1, 4, and 8).
1c
1d
I
FmocHN (S)
H
H3C
I
H
(S)
I
(R)
FmocHN
H
I
N
BocHN
(S)
Boc
1e
H
I
(S)
FmocHN

P. Campiglia et al. / Tetrahedron Letters 49 (2008) 731–734
733
Table 4
The use of b-iodoamine derived from the hindered Val
(1b) afforded the corresponding w[CH2NH] analogue 2b
in good yield (75%, entry 3). Since we were also interested
in demonstrating the compatibility of the optimized
w[CH2NH] procedure with different side chain-functional-
ized building blocks, we performed the N-alkylation using
as substrates, H-Pro-OBz and H-Pro-OMe. It has been
shown that the introduction of a reduced amide linkage
between Phe and Pro residues, by reductive alkylation of
the Pro nitrogen with Boc-Phe-H using NaBH3CN under
acidic conditions leads to epimerization of the Phe
residue.23 Under the same conditions described above,
the pseudodipeptides Fmoc-Ala-w[CH2NH]Pro-OBz (2cc),
Boc-Phew[CH2NH]Pro-OMe (2g), and Boc-Prow[CH2NH]
Pro-OMe (2i) were obtained in 70%, 75%, and 70% yields,
respectively (entries 5, 9, and 11).
Reactivity of b-iodoamines with different amino acid esters
Entry b-
Iodoamines
Amine
2
Yield
(%)
1
2
3
4
5
6
7
1a
H-Phe- 2a
OMe
H-Pro-
OBz
H-Phe- 2b
OMe
H-Phe- 2c
OMe
H-Pro-
OBz
H-Phe- 2d
OMe
Z-Ala-w[CH2NH]Phe-
OMe
80
75
75
86
70
83
70
2aa Z-Ala-w[CH2NH]Pro-
OBz
1b
1c
Z-Val-w[CH2NH]Phe-
OMe
Fmoc-Ala-
w[CH2NH]Phe-OMe
2cc Fmoc-Ala-
w[CH2NH]Pro-OBz
Fmoc-DAla-
w[CH2NH]Phe-OMe
Fmoc-
Lys(Boc)w[CH2NH]Phe-
OMe
1d
1e
H-Phe- 2e
OMe
Finally, the chirality of starting b-iodoamine derivatives
had no influence on N-alkylation reaction (entries 4 and 6).
No racemization was observed under the conditions shown
as evidenced by analytical HPLC (Fig. 1) for compounds
2c and 2d.24 The physicochemical properties and purities
of the final compounds were assessed by TLC, FAB-MS,
8
9
1f
H-Phe- 2f
OMe
Boc-Ala-w[CH2NH]Phe-
OMe
Boc-Phe-w[CH2NH]Pro-
OMe
75
75
83
1g
1h
H-Pro-
OMe
2g
10
H-Phe- 2h
OMe
Boc-
Asp(OMe)w[CH2NH]Pre-
OMe
Boc-Prow[CH2NH]Pro-
OMe
1
analytical RP-HPLC (Table 5), and H NMR.25 Subse-
11
1i
H-Pro-
OBz
2i
70
quently, aiming both to reduce the reaction times and to
2: 254 nm, 4 nm
L-Ala+Phemix
L-Ala+Phemix
2: 254 nm, 4 nm
D-Ala+Phe 17
D-Ala+Phe 17
Iodo-amine
derivatives 1C
1250
2C and 2D
and 1D
1000
750
500
250
0
75 18.00 18.25 18.50 18.75 19
Minutes
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Minutes
Fig. 1. Superimposition of chromatograms relate of crude products Fmoc-LAla-w[CH2NH]Phe-OMe 2c and Fmoc-DAla-w[CH2NH]Phe-OMe 2d. In red
is showed compound Fmoc-LAla-w[CH2NH]Phe-OMe and in black compound Fmoc-DAla-w[CH2NH]Phe-OMe.24
Table 5
Analytical data of the P-Xaa w[CH2NH] Yaa-OR derivatives 2
20
Compound
P
Xaa
Yaa
R
Formula
FAB-MS found
½aꢀD
RP HPLCa tR(min)
2a
2aa
2b
2c
2cc
2d
2e
2f
Z
Ala
Ala
Val
Ala
Phe
Pro
Phe
Phe
Pro
Phe
Phe
Phe
Pro
Phe
Pro
Me
Bz
Me
Me
Bz
Me
Me
Me
Me
Me
Me
C
C
22H28N2O4
24H28N2O5
384.32
424.39
412.37
472.72
512.43
472.69
629.52
350.43
376.49
422.41
312.38
ꢁ1.8 (c 0.1, MeOH)
14.87
11.38
14.68
11.91
12.32
12.26
15.89
13.67
13.65
14.69
12.26
Z
ꢁ15.1 (c 0.1, MeOH)
+3.7 (c 0.2, MeOH)
ꢁ6.8 (c 0.2, MeOH)
ꢁ15.1 (c 0.1, MeOH)
+24.6 (c 0.1, MeOH)
ꢁ12.9 (c 0.3, MeOH)
ꢁ20.0 (c 0.1, MeOH)
ꢁ32.9 (c 0.2, MeOH)
ꢁ22.6 (c 0.2, MeOH)
ꢁ62.16 (c 0.1, MeOH)
Z
C24H32N2O4
C29H32N2O4
C31H32N2O5
C29H32N2O4
C37H47N3O6
Fmoc
Fmoc
Fmoc
Fmoc
Boc
Boc
Boc
Boc
Ala
D-Ala
Lys(Boc)
Ala
Phe
Glu(OMe)
Pro
C19H30N2O4
2g
2h
2i
C21H32N2O4
C22H34N2O6
C16H28N2O4
a
Analytical RP HPLC of final products was performed on a C18 (Vydac 218TP54) column using the gradient 10–90% acetonitrile/0.05% TFA in H2O at
1 mL/min.

734
P. Campiglia et al. / Tetrahedron Letters 49 (2008) 731–734
8. Haaima, G.; Lohse, A.; Buchardt, O.; Nielsen, P. E. Angew. Chem.,
Int. Ed. 1996, 35, 1939–1942.
9. Sasaki, Y.; Coy, D. Peptides 1987, 8, 119–121.
10. Dueholm, K. L.; Egholm, M.; Buchardt, O. Org. Prep. Proced. Int.
1993, 25, 457–462.
improve the yields we performed the syntheses in a micro-
wave dedicated oven (Milestone, CombiChem) and com-
pared the results with the conventional procedure.26 This
further study demonstrated that the microwave-assisted
reaction resulted in a clear advantage only in reduction
on reaction time, being reduced to 1 h from 16 h, without
appreciable variation in yield.
A simple and convenient optimized procedure is
described for the preparation of w[CH2NH] surrogate in
solution. We have demonstrated the feasibility of our
strategy by conventional and microwave synthesis of the
aminomethylene surrogate bond. Every step of the proce-
dure was optimized with respect to time and economy.
The compatibility of this method with conventional
solid phase synthesis is currently under investigation in
our laboratory. Achieving this objective will facilitate the
routine introduction of a w[CH2NH] peptide bond surro-
gate into various biologically active peptides leading to
the synthesis of many important compounds and interest-
ing structure activity relationship studies.
11. Lin, S.; Hanzlik, R. P. J. Med. Chem. 1992, 35, 1067–1075.
12. Mancuso, A. J.; Swern, D. Synthesis 1981, 3, 165–184.
13. (a) Fehrentz, J.-A.; Castro, B. Synthesis 1983, 676–678; (b) Meyer,
J.-P.; Davis, P.; Lee, K. B.; Porreca, F.; Yamamura, H.; Hruby, V. J.
J. Med. Chem. 1995, 38, 3462–3468.
14. (a) Lubell, W. L.; Rapoport, H. J. Am. Chem. Soc. 1987, 109, 236–
239; (b) Coy, D. H.; Hocart, S. J. Tetrahedron 1988, 44, 835–838.
15. Sasaki, Y.; Murphy, W. A.; Heiman, M. L.; Lance, V. A.; Coy, D. H.
J. Med. Chem. 1987, 30, 1162–1166.
16. Boyarskaya, N. P.; Prokhorov, D. I.; Kirillova, Y. G.; Zvonkovab, E.
N.; Shvetsb, V. I. Tetrahedron Lett. 2005, 46, 7359–7361.
17. Campiglia, P.; Gomez-Monterrey, I.; Longobardo, L.; Lama, T.;
Novellino, E.; Greco, P. Tetrahedron Lett. 2004, 45, 1453–1456.
18. Jobron, L.; Hummel, G. Org. Lett. 2000, 2, 2265–2267.
19. Micha Jost, M.; Jorg-Christian Greie, J. C. M.; Nina Stemmer, N.;
Sven David Wilking, S. D.; Karlheinz Altendorf, K.; Sewald, N.
Angew. Chem., Int. Ed. 2002, 41, 4267–4269.
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Acknowledgments
˚
22. Representative experimental procedure: To the activated powdered 4 A
1
molecular sieves (500 mg) in anhydrous N,N-dimethylformamide
(10 mL) was added cesium carbonate (326 mg, 1.0 mmol), and the
suspension was stirred for 10 min. Then phenylalanine methyl ester
hydrochloride (108 mg, 0.5 mmol) was added and followed by
additional 30 min of stirring, b-iodoamine derivative 1a (160 mg,
0.5 mmol) was added. The reaction was stirred for 16 h, filtered to
remove the molecular sieves and undissolved inorganic salts, and
rinsed several times with EtOAc. Then the filtrate was concentrated
and the residue was taken up in 10% NaHCO3 solution and extracted
with CH2Cl2 (4 ꢂ 25 mL). The combined organic layers were washed
with brine, dried over anhydrous sodium sulfate, filtered, and
concentrated. Flash column chromatography (n-hexane–EtOAc, 3:2
v/v) afforded the aminomethylene analogue 2a (128 mg, 80%) as a
colorless oil.
The LC/MS and H NMR spectral data were provided
by Centro di Ricerca Interdipartimentale di Analisi Stru-
mentale, Universita degli Studi di Napoli ‘Federico II’.
The assistance of the staff is gratefully appreciated. This
work was supported by grant from MIUR—PRIN 2005.
`
References and notes
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5. (a) Graham, S. L.; de Solms, S. J.; Giuliani, E. A.; Kohl, N. E.;
Mosser, S. D.; Oliff, A. I.; Pompliano, D. L.; Rands, E.; Breslin, M.
J.; Deana, A. A.; Garsky, V. M.; Scholz, T. H.; Gibbs, J. B.; Smith, R.
L. J. Med. Chem. 1994, 37, 725–732; (b) Gaudron, S.; Grillon, C.;
Thierry, J.; Riches, A.; Wierenga, P. K.; Wdziecazk-Bakala, J. J. Stem
Cells 1999, 17, 100–106.
6. (a) Martinez, J.; Bali, J.-P.; Rodriguez, M.; Castro, B.; Magous, R.;
Laur, J.; Lignon, M.-F. J. Med. Chem. 1985, 28, 1874–1879; (b) Coy,
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Lugrin, D.; Vecchini, F.; Kitabgi, P.; Aumelas, A.; Martinez, J. Pept.
Res. 1992, 5, 30–38; (e) Meyer, J.-P.; Davis, P.; Lee, K. B.; Porreca,
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24. No racemization was confirmed by the HPLC analysis of the crude
products Fmoc-Ala-w[CH2NH]Phe-OMe 2c and Fmoc-DAla-
w[CH2NH]Phe-OMe 2d. This analysis showed the signal correspond-
1
ing to one single isomer (Fig. 1). In addition the H NMR spectra of
the crude products 2c and 2d did not show significant differences of
chemical shifts. Thus, the resonance values of a-H Phe residues were
4.32 for 2c and 4.39 for 2d, while the values for a-H Ala residues were
3.85 and 3.84, respectively.
25. As example, significant 1H NMR analytical data of 2g: 1H NMR
(500 MHz, CD3OD): d, 1.42 ( s, 9H, Boc); 1.94–2.00 (m, 3H, H-c, H-b
Pro); 2.26–2.29 (m, 1H, H-b Pro); 2.72–2.80 (m, 2H, CH–CH2–Ph);
3.47–3.50 (m, 2H, H-d Pro); 3.74 (s, 3H, OCH3); 4.00–4.02 (m, 1H,
H-a Pro); 4.07 (m, 2H, CH2N); 4.37 (m, 1H, CH-CH2-Ph); 4.81 (br s,
1H, NH-Boc); 7.33–7.22 (m, 5H, aryl).
26. General procedure: All experiments were carried out in a Milestone
CombiChem Microwave Synthesizer with vessels of 4-mL volume,
using DMF as the solvent. In all irradiation experiments, rotation of the
rotor, irradiation time, temperature, and power were monitored with
the ‘easyWAVE’ software package. Temperature was monitored with the
aid of an optical fiber inserted into one of the reaction containers. Once
50 °C was reached the reaction mixture was held at this temperature for
10 min and then cooled rapidly to room temperature.