C-Backbone branched peptides via reductive amination of cyanomethyleneamino pseudopeptides

Article abstract of DOI:10.1016/S0040-4039(02)00012-6

The synthesis of branched peptides from cyanomethylenamino pseudopeptides, via catalytic hydrogenation in the presence of amino acid derivatives, is described. When the inserted amino acid was a glycine derivative, the resulting branched peptide lactamize

Full text of DOI:10.1016/S0040-4039(02)00012-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1421–1424
C-Backbone branched peptides via reductive amination of
cyanomethyleneamino pseudopeptides
Susana Herrero, M. Luisa Sua´rez-Gea, M. Teresa Garc´ıa-Lo´pez and Rosario Herranz*
Instituto de Qu´ımica Me´dica (CSIC), Juan de la Cierva 3, E-28006 Madrid, Spain
Received 10 October 2001; revised 20 December 2001; accepted 21 December 2001
Abstract—The synthesis of branched peptides from cyanomethylenamino pseudopeptides, via catalytic hydrogenation in the
presence of amino acid derivatives, is described. When the inserted amino acid was a glycine derivative, the resulting branched
peptide lactamized in situ to 2-oxopiperazine derivatives. This new C-backbone peptide branching approach is compatible with
the diversity of amino acid side chains. © 2002 Elsevier Science Ltd. All rights reserved.
Branched peptides have been extensively used in the
preparation of multiple antigen peptides (MAP) for
immunology applications,1 in the synthesis of template-
assembled synthetic proteins (TASP)2,3 and den-
drimers,4,5 or for the introduction of conformational
constrains into peptides through side-chain cycliza-
tions.6 In most of these applications, the branching
points are amino acids containing reactive side chains,
such as Asp, Glu, Lys or Cys. In general, this fact does
not limit the scope of these applications. However,
when the aim is to introduce conformational constrains
by cyclization, the replacement of some amino acids by
those above mentioned is permissible in biological irrel-
evant regions, but it may lead to inactive peptides when
applied to their active regions. To avoid this drawback,
diverse strategies of N-backbone cyclization, involving
introduction of a branching point into the peptide
bond, by N-alkylation with a functionalized carbon
chain linker have been proposed.7–10
cyanomethyleneamino pseudopeptides, via catalytic
hydrogenation, followed by peptide coupling.15 Now,
we have studied and communicate herein a new type of
C-backbone peptide branching, involving insertion of
amino acid derivatives via reductive amination of the
cyano group of C[CH(CN)NH] pseudopeptides, by cat-
alytic hydrogenation in the presence of amino acid
derivatives (Scheme 1).
Catalytic hydrogenation of nitriles may give rise to a
number of products including primary, secondary and
tertiary amines, imines, hydrocarbons, aldehydes,
amides, and alcohols. Despite the complexity of the
reaction, its selectivity, mainly towards primary or sec-
ondary amines, can be controlled with the reaction
conditions, such as catalyst, temperature, solvent, and
addition of amines.16–18 With respect to this last factor,
when the hydrogenation is carried out in the presence
of primary amines, the formation of asymmetrical sec-
ondary amines competes with that of the symmetrical
one. In our case, we carried out an initial study on the
hydrogenation of pseudodipeptides Boc-PheC[CH-
(CN)NH]Leu-OMe [Scheme 1, (R,S)-1, (1:1) epimeric
mixture at the stereogenic center of the peptide bond
surrogate] in the presence of H-Ala-OMe·HCl, to deter-
mine the optimal reaction conditions for the prepara-
tion of the asymmetrical secondary amines (R)- and
(S)-9. As indicated in Table 1, these branched pseu-
dotripeptides were obtained along with the 2-oxo-
piperazine derivatives (R)- and (S)-30, resulting
from lactamization of the corresponding primary
amines A, in different ratios, depending on the hydro-
genation conditions. The corresponding symmetrical
secondary amines were not detected in the crude reac-
tion mixture. Both epimeric mixtures 9 and 30 were
Based on the hypothesis that the [CH(CN)NH] group
could be a good peptide bond surrogate, we developed
a versatile method for the synthesis of cyanomethylene-
amino pseudopeptides,11 which was applied to the syn-
thesis of aminopeptidase inhibitors,12 and neuro-
tensin13 and CCK-4 analogues,14 respectively. On the
other hand, due to its chemical reactivity, the cyano
group is a good tool for C-backbone modification, such
as branching or cyclization reactions. Thus, we have
previously reported the synthesis of branched peptides
and conformationally constrained analogues from
* Corresponding author. Tel.: 34-91-5622900; fax: 34-91-5644853;
e-mail: rosario@iqm.csic.es
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00012-6

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S. Herrero et al. / Tetrahedron Letters 43 (2002) 1421–1424
resolved by flash chromatography. To favor the forma-
tion of the secondary amines, hydrogenations were
carried out at low pressure (1 atm).16,17 Amino acid/
nitrile ratios higher than 2 (entry 2), as well as the
increase in the temperature18 (entry 3) did not lead to
significant increases in the ratio of branched peptides 9.
The most significant improvement in the yield of these
secondary amines was achieved when the amino acid
hydrochloride was neutralized in situ, by adding one
equivalent amount of Et3N to the hydrogenation
medium, (compare entries 1 and 6).† With respect to the
catalyst, the use of 10% Pd(C) led to the highest yield of
branched peptides 9, while Raney Ni gave the highest
ratio of 2-oxopiperazines 30 (entry 4). Although, it has
been described that the use of rhodium generally favors
the formation of secondary amines,16–18 in our case
(entries 8 and 9), only the primary amine derived
2-oxopiperazines 30 were obtained, and in low yield.
The use of EtOAc as solvent (entry 7) led to a decrease
in reduction products, and dirtier reaction mixtures. No
racemization at the Ala residue nor epimerization at the
stereogenic centers of the starting pesudodipeptides 1
were detected in any case.
Branched pseudotripeptides (R)- and (S)-9 could not be
obtained by alkylation of the corresponding primary
amines A [isolated as acetates from the hydrogenation
of (R,S)-1 in the presence of HOAc15] with methyl
(S)-2-bromopropanoate in the presence of Ag2O,19,20 or
with the triflate of (R)-methyl lactate.21
After having determined the optimal reaction condi-
tions for the preparation of branched pseudopeptides
(R)- and (S)-9, we studied the versatility of the method-
ology for different starting amino acids and
cyanomethyleneamino pseudodipeptides (Table 2). In
general, it was necessary to increase the reaction time
from 8 h for the reductive coupling of pseudodipeptides
(R,S)-1 or their amides (R)- and (S)-2 with H-Ala-
OMe, ethylamine or H-Gly-OMe (entries 1–5) to 1 day
for pseudotripeptides (R,S)-3 or (S)-4 (entries 6 and 8),
and 5 days for the rest of the couplings. In most of the
Scheme 1.
reactions,
C[CH(CN)NH]
unchanged, which ranged from 3% in the coupling of
pseudodipeptides (R,S)-7 with H-Phe-NH2 (entry 14) to
100% in the case of the reduction of (R)-4 (entry 7).
a
variable percentage of the starting
pseudopeptide was recovered
The significant different results obtained for the reac-
tion of pseudotripeptides 3 and 4 with H-Ala-OMe
(entries 6–8) could be attributed to a higher steric
hindrance in the Leu-containing pseudotripeptides 4,
which hampered the coupling in the (R)-epimer.
When the inserted amino acid was H-Gly-OMe (entries
3 and 9–12), or when it was replaced by ethylamine
(entry 2), the resulting branched pseudopeptides lac-
tamized in situ to the corresponding 1-substituted-2-
oxopiperazine derivatives 21, 22 and 26.
† Representative procedure: Et3N (0.14 mL, 1 mmol) was added to a
suspension of the amino acid derivative hydrochloride H-Zaa-
R5·HCl (1 mmol) in MeOH (10 mL), and the mixture was stirred at
room temperature for 15 min. Then, the corresponding
C[CH(CN)NH] pseudopeptide 1–10 (0.5 mmol) and 10% Pd(C)
(300 mg) were added, and the mixture was hydrogenated at 1 atm
of H2 and room temperature for 8 h to 5 days, depending on the
starting materials. Afterwards, the catalyst was filtered off and
washed with MeOH (3×5 mL), the solvent was evaporated, and the
reaction crude was dissolved in EtOAc (20 mL). This solution was
washed successively with H2O (2×10 mL), brine (10 mL), dried over
Na2SO4, and evaporated to dryness. The residue were purified by
flash chromatography using 5–30% EtOAc in hexane or 0–5%
MeOH in CH2Cl2 gradients as mobile phases.
Finally, it is worth noting that the presence of free
carboxy groups, either in the C[CH(CN)NH] pseu-
dopeptide or in the amino acid that is inserted, ham-
pered the coupling reaction of reductive amination,
recovering the starting materials unchanged.
In conclusion, the reductive amination of cyanomethile-
neamino pseudopeptides, by catalytic hydrogenation in

S. Herrero et al. / Tetrahedron Letters 43 (2002) 1421–1424
1423
Table 1. Influence of hydrogenation conditions on the product ratio in the reductive amination of (R,S)-1a
Entry
Amino acid
derivative
H-Ala-OMe
equivalentsb
Catalyst
Solvent
T (°C)
[(R)+(S)]-9
[(R)+(S)]-30
(%)c
(%)c
1
2
3
4
5
6
7
8
9
H-Ala-OMe·HCl
H-Ala-OMe·HCl
H-Ala-OMe·HCl
H-Ala-OMe·HCl
H-Ala-OMe·HCl
H-Ala-OMed
2
4
2
2
2
2
2
2
2
10% Pd(C)
10% Pd(C)
10% Pd(C)
Raney Ni
PtO2
10% Pd(C)
10% Pd(C)
5% Rh(C)
5% Rh(Al2O3)
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOAc
MeOH
MeOH
20
20
50
20
20
20
20
20
20
30
32
28
7
64
60
66
81
40
20
15
39
39
0
66
52
0
H-Ala-OMed
H-Ala-OMed
H-Ala-OMed
0
a Hydrogenation at 1 atm of H2 pressure for 8 h.
b Equivalents of the amino acid derivative a with respect to the pseudopeptide (R,S)-1.
c Yield determined by RP-HPLC of the crude reaction mixture [mBondapapk C18 (3.9×300 mm), 40% of CH3CN and 60% of 0.05% solution of
TFA as mobile phase].
d From the hydrochloride by addition of the equivalent amount of Et3N.
Table 2. Results of the reductive amination of C[CH(CN)NH] pseudopeptidesa
Reaction products
1-Substituted
(%)b 2-Oxopiperazine
1-Unsubstituted
(%)b 2-Oxopiperazine (%)c
Entry
Boc-XaaC[CH(CN)NH]Yaa-R3 H-Zaa-R5
Branched
peptide
1
2
3
4
5
6
7
8
(R,S)-1d
(R,S)-1d
(R,S)-1d
(R)-2
H-Ala-OMe
H2N-Et
(R)- and (S)-9
(R,S)-10
(R,S)-11
(S)-12
(R)-12
(R,S)-13
(S)-14
(R)-14
(S)-15
(R)-15
(S)-16
66
0
0
53
56
55
0
44
0
(R,S)-20
0
68
62
0
0
0
(R)- and (S)-30
(R)- and (S)-30
(R)- and (S)-30
(S)-30
(R)-30
(R,S)-31
(S)-32
(R)-32
(S)-33
(R)-33
(S)-33
(R)-33
(R)- and (S)-34
(R)- and (S)-34
(R,S)-35
20
30
35
28
27
0
(R)- and (S)-21
(R)- and (S)-22
(S)-23
(R)-23
(R,S)-24
(S)-25
(R)-25
(S)-26
(R)-26
(S)-26
(R)-26
(R)- and (S)-27
(R)- and (S)-28
(R,S)-29
H-Gly-OMe
H-Ala-OMe
H-Ala-OMe
H-Ala-OMe
H-Ala-OMe
H-Ala-OMe
H-Gly-OMe
H-Gly-OMe
H-Gly-OMe
H-Gly-OMe
H-Phe-OMe
H-Phe-NH2
H-Phe-NH2
(S)-2
(R,S)-3d
(R)-4
0
0
0
0
(S)-4
(R)-5
(S)-5
(R)-6
9
59
62
57
55
0
31
23
26
20
30
22
23
10
11
12
13
14
15
0
0
0
(S)-6
(R)-16
(R)- and (S)-17 55
(R)- and (S)-18 68
(R,S)-7e
(R,S)-7e
(R,S)-8f
0
0
(R,S)-19
71
a (R)- and (S)- refers to the absolute configuration at the peptide bond surrogate. In the specification of this chirality, the ligand preference in
products resulting from hydrogenation of C[CH(CN)NH] pseudopeptides changes. Thus, the configuration of compounds 9–35 resulting from
the reduction of the (R)-epimers of 1–8 is denoted as (S).
b Yield of isolate product.
c Yield determined in the reaction crude by RP-HPLC.
d (1:1) (R)/(S) epimeric ratio. The epimeric mixtures 10, 11, 21, 22, and 30 were resolved by flash chromatography.
e (1:2) (R)/(S) epimeric ratio. The epimeric mixtures resulting from the reduction of (R,S)-7 were resolved by flash chromatography.
f (1:3) (R)/(S) epimeric ratio.
the presence of amino acid derivatives, is a versatile
method for amino acid insertion as C-backbone
branching points, compatible with the presence of
diversity of amino acid side chains. These branched
peptides can lactamize to give 2-oxopiperazine deriva-
tives. This cyclization opens an easy access to confor-
mationally restricted peptide derivatives.
0147), and Consejeria de Educacio´n y Cultura de la
Comunidad de Madrid (CAM-08.5/0006/1998).
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