Synthesis of enaminone-based vinylogous peptides

Article abstract of DOI:10.1002/ejoc.201402033

Vinylogous peptides 3 with a vinyl fragment inserted into the peptide C-N bond were prepared from ynones 6 and enaminones 7 that are easily available from Boc-protected α-amino acids 4. Coupling at the C terminus was achieved by 1,4-addition of amino esters 5 to the C≡C bond in 6 or by substitution of the NMe₂ group in 7 to give N-terminal vinylogues 3a-p. Coupling at the N terminus of 3 and 7, in contrast, required temporary protection of the acidolytically labile enamino moiety. Thus, cyclization of 6 or 7 with hydroxylamine, removal of the Boc group with HBr-AcOH, acylation of free amine 10 with BocGlyOH (4a), and hydrogenolytic deprotection of the enamino moiety in the presence of GlyOMe (5a) led to tripeptides 3q-s with vinylogous amide as the central building block. Copyright

Full text of DOI:10.1002/ejoc.201402033

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402033
Synthesis of Enaminone-Based Vinylogous Peptides
[a]
[a]
[a]
[a]
[a,b]
ˇ
ˇ
ˇ
ˇ
ˇ
Luka Senica, Uros Groselj, Marta Kasunic, Drago Kocar, Branko Stanovnik,
and
Jurij Svete*[a,b]
Keywords: Peptides / Amino acids / Alkynes / Cyclization
Vinylogous peptides 3 with a vinyl fragment inserted into the
peptide C–N bond were prepared from ynones 6 and en-
aminones 7 that are easily available from Boc-protected α-
amino acids 4. Coupling at the C terminus was achieved by
1,4-addition of amino esters 5 to the CϵC bond in 6 or by
substitution of the NMe2 group in 7 to give N-terminal vin-
ylogues 3a–p. Coupling at the N terminus of 3 and 7, in con-
trast, required temporary protection of the acidolytically la-
bile enamino moiety. Thus, cyclization of 6 or 7 with hydrox-
ylamine, removal of the Boc group with HBr–AcOH, acyl-
ation of free amine 10 with BocGlyOH (4a), and hydro-
genolytic deprotection of the enamino moiety in the presence
of GlyOMe (5a) led to tripeptides 3q–s with vinylogous amide
as the central building block.
Introduction
α-Peptides 1 and their analogues are probably the most
important and so far the most widely explored group of
oligomers that can adopt (or fold into) definite secondary
structures.[1] This ongoing focus on peptides is not surpris-
ing. They play an essential role in life processes, and pro-
found knowledge of the properties of peptides is a crucial
precondition for rational drug design.[1,2] To alter the bio-
logical properties of peptides, numerous modifications of
the amino acid and the peptide backbone structure have
been done.[3] The most prominent examples include incor-
poration of unnatural α-amino acids,[4] α,β-dehydro-α-
amino acids,[5] β-, γ-,[6] and δ-amino acids,[7] α-hydrazino
acids,[8] α-aminoxy acids,[9] N-substituted glycines,[10] and
sulfonamides.[11] Within the plethora of known structural
analogues of peptides, however, vinylogous peptides remain
almost unexplored. In the literature, there are a handful of
reports on vinylogous α-peptides 2, in which the C=C frag-
ment is inserted between the carbonyl group and the α-car-
bon atom. As shown in two recent reports, vinylogues 2
exhibit interesting and promising structural features with
possible applications in protein science, medicinal chemis-
try, and materials science.[12] In contrast, γ-peptides 3 with
a C=C fragment inserted into a peptide C–N bond are, to
the best of our knowledge, unknown (Figure 1).[13]
Figure 1. α-Peptide 1 and its vinylogues 2 and 3.
In continuation of our long-term interest in the chemis-
try of enaminones,[14] we found vinylogous peptides 3 par-
ticularly interesting, as they should be easily available from
N-protected α-amino acids 4 and α-amino esters 5. Herein,
we report preliminary results of the study on the first repre-
sentatives of vinylogous peptides 3: synthesis of building
blocks (BBs) 6 and 7 and their coupling with amino acid
esters 5 to give N-terminal vinylogues 3a–p, temporary pro-
tection of the enaminone moiety in 3 that allows acidolytic
removal of the tert-butoxycarbonyl (Boc) group, and conse-
quently, the synthesis of vinylogous peptides 3q–s with a
vinylogous amide as the central BB.
Results and Discussion
Starting ynones 6a–c were prepared in two steps from
commercially available (S)-N-Boc-α-amino acids 4a–c
through derivatization into the corresponding Weinreb
amides[15] and treatment with ethynylmagnesium bromide
following a slightly modified literature procedure.[16] Ad-
dition of dimethylamine to 6a–c afforded (E)-enaminones
7a–c, whereas treatment of 6a–c with amino esters 5a–g and
[a] Faculty of Chemistry and Chemical Technology, University of
Ljubljana,
Asˇkercˇeva 5, 1000 Ljubljana, Slovenia
E-mail: jurij.svete@fkkt.uni-lj.si
http://www.fkkt.uni-lj.si/en/
[b] Centre of Excellence EN-FIST,
Trg Osvobodilne fronte 13, 1000 Ljubljana, Slovenia
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402033.
Eur. J. Org. Chem. 2014, 3067–3071
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Svete et al.
SHORT COMMUNICATION
Scheme 1. Synthesis of N-terminal vinylogous peptides 3a–m, 3p, 3Јn, and 3Јo; NMM = 4-methylmorpholine, CDI = 1,1Ј-carbonyldiimid-
azole.
GlyGlyOMe (5h) in dichloromethane at room temperature Table 1. Yields of building blocks 6 and 7 and vinylogous peptides
3 and 3Ј.
gave N-terminal vinylogous peptides 3a–m, 3p, 3Јn, and 3Јo
in good yields. Quite expectedly,[14] acid-catalyzed reaction
of 7c with amino esters 5a, 5b, and 5h in methanol at 60 °C
resulted in substitution of the dimethylamino group to give
3b, 3d, and 3Јo, respectively. N,N-Disubstituted vinylogues
7a–c and 3Јn and 3Јo were isolated as pure (E) isomers,
whereas the N-monosubstituted vinylogues were obtained
as mixtures of major (Z) isomers 3a–m and 3p and minor
(E) isomers 3Јa–m and 3Јp (Scheme 1, Table 1).
In contrast to smooth one-step amination at the C ter-
minus of 6 and 7, coupling at the N terminus of vinylogues
3 required prior acidolytic removal of the Boc group. Un-
fortunately, treatment of 3 with HCl gave inseparable mix-
tures of products. We were not able to isolate free amines 8 14
or trap them by in situ acylation. Darkening of the reaction
mixtures also indicated decomposition of 3. Recent results
in closely related enaminone series offered a plausible ra-
tionale through intramolecular cyclization of 8 into highly
reactive 3-hydroxypyrroles (Scheme 2).[17,18] To circumvent
the instability of the enaminone moiety under the acidic
conditions, the enaminone part of the molecule had to be
protected. The idea was that cyclization of 6 or 7 with hy-
droxylamine would produce an isoxazole derivative, which
should “survive” acidolytic Boc deprotection. The enamin-
Entry Reaction
R1 R2
Yield [%] E/Z[a]
1
2
3
4
6aǞ7a
H
–
–
–
H
H
H
60
62
83
66
92
78
67
52
55
65
70
45
54
64
49
54
61
60
46
60
52
52
100:0
100:0
100:0
12:88
13:87
13:87
7:93
15:85
15:85
5:95
6bǞ7b
Me
Bn
Me
Bn
Bn
6cǞ7c
6b+5aǞ3a
6c+5aǞ3b
7c+5aǞ3b
6b+5bǞ3c
6c+5bǞ3d
7c+5bǞ3d
6b+5cǞ3e
6c+5cǞ3f
6b+5dǞ3g
6c+5dǞ3h
6b+5eǞ3i
6c+5eǞ3j
6a+5fǞ3k
6b+5fǞ3l
6c+5fǞ3m
6b+5gǞ3Јn
6c+5gǞ3Јo
7c+5gǞ3Јo
6c+5hǞ3p
5
6
7
8
Me Me
Bn Me
Bn Me
Me iPr
9
10
11
12
13
Bn iPr
5:95
7:93
Me (CH2)2COOMe
Bn (CH2)2COOMe
Me 4-HOC6H4CH2
Bn 4-HOC6H4CH2
22:78
11:89
13:87
14:86
23:77
21:79
100:0
100:0
100:0
19:81
15
16
17
18
19
20
21
22
H
Bn
Me Bn
Bn Bn
Me
Bn
Bn
–
–
–
–
–
[a] In CDCl3.
one moiety would then be regenerated by catalytic hydro- cleavage of the N–O bond to give β-imino ketone 7Јd, which
genation. Proof of concept was obtained by cyclization of tautomerizes into more stable enaminone 7d, followed by
7b with hydroxylamine, which gave a ≈1:1 mixture of dia- substitution of the NH2 group with 5a (Scheme 2).
stereomeric isoxazolines 9b/9Јb.[19] Catalytic hydrogenation
The viability of this approach was confirmed by the syn-
of 9b/9Јb in the presence of GlyOMe·HCl (5a) furnished thesis of tripeptides 3q–s with a vinylogous amide as the
dipeptide 3a. The reaction pathway is explainable by initial central BB. Isoxazolines 9a, 9b/9Јb, and 9c/9Јc were ob-
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Synthesis of Enaminone-Based Vinylogous Peptides
Table 2. Yields of compounds 9a, 9/9Јb,c, 10a–c, 11a–c, and 3q–s.
Entry Reaction
R1
Yield [%]
E/Z[a]
1
2
3
4
5
6
7
8
6aǞ9a
H
H
63
40
70
55
63
100
100
100
88
75
94
–
–
–
–
–
–
–
–
–
7aǞ9a
6bǞ9b/9Јb
7bǞ9b/9Јb
6cǞ9c/9Јc
9aǞ10a
9b/9ЈbǞ10b
9c/9ЈcǞ10c
10aǞ11a
10bǞ11b
10cǞ11c
11aǞ3q
Me
Me
Bn
H
Me
Bn
H
Me
Bn
H
Me
Bn
9
10
11
12
13
14
–
–
58
84
72
19:81
16:84
25:75
11bǞ3r
11cǞ3s
[a] In CDCl3.
Scheme 2. Temporary protection of the enaminone moiety.
C=O···H–N hydrogen bonds (Figure 2).[14,21] The molecular
structures of 9Јb (Figure 3) and 11b (Figure 4) were further
confirmed by X-ray crystal structure analysis.
tained by cyclization of 6a–c with hydroxylamine, and they
were then treated with HBr/AcOH to give 5-aminometh-
ylisoxazoles hydrobromides 10a–c in quantitative yield.[20]
Acylation of 10a–c with BocGlyOH (4a) and CDI gave fully
protected dipeptides 11a–c. Catalytic hydrogenation of 11a–
c in the presence of 5a then furnished desired (Z)-tri-
peptides 3q–s in 58–84% yield (Scheme 3, Table 2).
Figure 2. Configuration around the C=C bond in vinylogous
amides 7 and peptides 3 and 3Ј in CDCl3 and [D6]DMSO solution.
Scheme 3. Synthesis of tripeptides 3q–s with a vinylogous amide as
the central BB.
The structures of 3a–m, 3p–s, 3Јn, 3Јo, 6a, 7a–c, 9a, 9b/
9Јb, 9c/9Јc, 10a–c, and 11a–c were determined by spectro-
scopic/spectrometric methods (IR, 1H NMR, 13C NMR,
and MS) and by elemental analyses for C, H, and N. The
configuration around the C=C bond in 3, 3Ј, and 7 was
determined by NMR spectroscopy on the basis of the vici-
3
nal coupling constant, JH,H. Accordingly, N,N-disubsti-
tuted vinylogues 7a–c, 3Јn, and 3Јo exist in CDCl3 and in
[D6]DMSO solution as the (E) isomers, whereas the pre-
dominance of the (Z) isomers in N-monosubstituted vin-
Figure 3. The molecular structure of 9Јb showing the atom-labeling
scheme. The displacement ellipsoids are drawn with 30% prob-
ability, and the hydrogen atoms are shown as small spheres of arbi-
ylogues 3a–m and 3p–s is explainable by intramolecular trary radii.[22]
Eur. J. Org. Chem. 2014, 3067–3071
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J. Svete et al.
SHORT COMMUNICATION
(5 mL), and the mixture was stirred at room temperature for 3 h.
Then, a solution of ynone 6 (1 mmol) in MeOH (5 mL) was added,
and the reaction mixture was stirred at room temperature for 48 h.
The volatile components were evaporated in vacuo, and the residue
was purified by DVFC (silica gel, EtOAc) to give 9/9Ј.
Synthesis of 10: 33% HBr in acetic acid (1 mL) was added to a
stirred solution of isoxazoline 9 (1 mmol) in AcOH (10 mL), and
the mixture was stirred at room temperature for 0.5 h. The volatile
components were evaporated to give crude 10 in quantitative yield.
Synthesis of 11: CDI (194 mg, 1.2 mmol) was added to a stirred
solution of BocGlyOH (4a; 175 mg, 1.05 mmol) in MeCN (5 mL).
The reaction mixture was stirred at room temperature for 1.5 h,
cooled to 0 °C, and then transferred to a stirred cold (0 °C, ice
bath) mixture of isoxazolium bromide 10 (1 mmol), NMM
(0.55 mL, 5 mmol), and MeCN (10 mL). The ice bath was re-
moved, and the reaction mixture was stirred at room temperature
for 96 h. The volatile components were evaporated in vacuo, and
the residue was purified by column chromatography (CC; silica gel,
EtOAc/hexanes) to give 11.
Figure 4. The molecular structure of 11b showing the atom-labeling
scheme. The displacement ellipsoids are drawn with 30% prob-
ability, and the hydrogen atoms are shown as small spheres of arbi-
trary radii.[22]
Conclusions
Synthesis of Vinylogous Tripeptides 3q–s: Isoxazole 11 was dis-
solved in MeOH (30 mL). Glycine methyl ester hydrochloride (5a;
138 mg, 1.1 mmol) and 10% Pd/C (0.02 g) were added, and the
reaction mixture was hydrogenated (200 kPa H2) for 5 h. The cata-
lyst was removed by filtration through a glass-sintered funnel, and
the volatile components were evaporated in vacuo. The residue was
purified by CC (silica gel, EtOAc/hexanes) to give 3q–s.
In summary, the first representatives of a novel type of
vinylogous peptides 3[13] with a vinyl fragment inserted into
the peptide C–N bond were prepared. The synthesis utilizes
two BBs, ynones 6 and enaminones 7, both easily available
from N-Boc-α-amino acids 4. Coupling at the C terminus
of the BBs was achieved by the addition of amino ester 5
to the CϵC bond of 6 or by substitution of the NMe2
group in 7. For coupling at the N terminus of 3 and 7, the
acidolytically labile enamino moiety was temporary pro-
tected by cyclization with hydroxylamine. Removal of the
Boc group with HBr-AcOH, coupling of free amine 10 with
BocGlyOH (4a), and hydrogenolytic deprotection of the en-
amino moiety in the presence of GlyOMe (5a) led to tri-
peptides 3q–s with a vinylogous amide as the central BB.
Thus, the present method enables incorporation of vinyl-
ogous amide BBs into oligopeptides. However, the prepara-
tion of oligomers with consecutive vinylogous BBs repre-
sents the next synthetic challenge to be met. Nevertheless,
these preliminary results clearly indicate the viability of this
synthetic approach, which enables the incorporation of
vinylogous amides into a peptide chain. Vinylogues 3 might
be interesting or even useful for various applications, for
example, in medicinal chemistry, chemical biology, and in
materials science.
Supporting Information (see footnote on the first page of this arti-
cle): General methods; experimental procedures; characterization
data; copies of the NMR spectra for 3a–m, 3p–s, 3Јn, 3Јo, 6a, 7a–
c, 9a, 9b/9Јb, 9c/9Јc, 10a–c, and 11a–c; and X-ray diffraction data
for 9Јb and 11b.
Acknowledgments
Financial support from the Slovenian Research Agency through
grants P1-0179, J1-6689-0103-04, and L1-4039 is gratefully ac-
knowledged. The authors also thank the Centre of Excellence EN-
FIST for providing a SuperNova diffractometer.
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Experimental Section
Coupling at the C Terminus of BB 6
Synthesis of Vinylogues 3a–p: NMM (0.11 mL, 1 mmol) was added
to a stirred mixture of ynone 6 (1 mmol), amino ester hydrochloride
5 (1 mmol), and CH2Cl2 (10 mL) at 0 °C. The mixture was warmed
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to give 3a–p.
Coupling at the N Terminus of BB 6 Including Temporary Protection
of the Enamino Moiety
Synthesis of 9/9Ј: tBuOK (112 mg, 1 mmol) was added to a stirred
suspension of hydroxylamine hydrochloride 5 (1.1 mmol) in MeOH
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Synthesis of Enaminone-Based Vinylogous Peptides
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[20] We were not able to obtain amines 10 by treatment of 9/9Ј with
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[22] The crystal data and experimental details for the X-ray mea-
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982533 (for 9Јb) and -982534 (for 11b) contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: December 7, 2013
Published Online: March 13, 2014
Eur. J. Org. Chem. 2014, 3067–3071
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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