Studies on the synthesis of peptides containing dehydrovaline and dehydroisoleucine based on copper-mediated enamide formation

Article abstract of DOI:10.3762/bjoc.12.55

The preparation of peptide fragments containing dehydrovaline and dehydroisoleucine moieties present in the antibiotic myxovalargin is reported. Peptide formation is based on a copper-mediated C-N cross-coupling protocol between an acyl amide and a peptidic vinyl iodide. The presence of a neighboring arginine in the vinyl iodide posed a challenge with respect to the choice of the protecting group and the reaction conditions. It was found that ornithine - a suitable precursor - is better suited than arginine for achieving good yields for the C-N cross-coupling reaction. The optimized conditions were utilized for the synthesis of peptides 32, 33, 39 and 40 containing a neighboring ornithine as well as for the tripeptide 44 containing dehydroisoleucine with the correct stereochemistry.

Full text of DOI:10.3762/bjoc.12.55

Studies on the synthesis of peptides containing
dehydrovaline and dehydroisoleucine based on
copper-mediated enamide formation
Franziska Gille and Andreas Kirschning*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 564–570.
Institute of Organic Chemistry and Center of Biomolecular Drug
Research (BMWZ), Leibniz University Hannover, Schneiderberg 1b,
30167 Hannover, Germany
doi:10.3762/bjoc.12.55
Received: 03 December 2015
Accepted: 02 March 2016
Published: 22 March 2016
Email:
Andreas Kirschning* - andreas.kirschning@oci.uni-hannover.de
This article is part of the Thematic Series "Copper catalysis in organic
synthesis".
* Corresponding author
Keywords:
Guest Editor: S. R. Chemler
catalysis; dehydroamino acids; Hartwig–Buchwald reaction;
myxovalargin; peptides
© 2016 Gille and Kirschning; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The preparation of peptide fragments containing dehydrovaline and dehydroisoleucine moieties present in the antibiotic myxo-
valargin is reported. Peptide formation is based on a copper-mediated C–N cross-coupling protocol between an acyl amide and a
peptidic vinyl iodide. The presence of a neighboring arginine in the vinyl iodide posed a challenge with respect to the choice of the
protecting group and the reaction conditions. It was found that ornithine – a suitable precursor – is better suited than arginine for
achieving good yields for the C–N cross-coupling reaction. The optimized conditions were utilized for the synthesis of peptides 32,
33, 39 and 40 containing a neighboring ornithine as well as for the tripeptide 44 containing dehydroisoleucine with the correct
stereochemistry.
Introduction
Dehydroamino acids [1] are rare amino acids that are carboxylate, dehydroamino acids have hardly employed as
constituents of many oligopeptides from microbial sources. building blocks in peptide synthesis. Therefore, the olefinic
Typical examples are myxovalargin (1), argyrin (2) and nisin double bond is commonly introduced after the peptide back-
(3, Scheme 1).
bone is assembled and typically this is achieved by elimination
when a leaving group occupies the β-position [2]. Especially
Dehydroamino acids and peptides are characterized by the pres- peptides containing dehydroalanine as found in argyrin (2) can
ence of an olefinic double bond conjugated with the carboxyl or be prepared from a precursor that contains a selenide substitu-
peptidic carbonyl group. Besides being α,β-unsaturated acids or ent in the β-position [3]. Peptides that bear the doubly branched
amides, respectively, they can also be regarded as enamines. dehydroamino acids dehydrovaline or dehydroisoleucine, e.g.,
Due to the lack of reactivity of the amino group as well as the found in myxovalargin (1), are much more challenging to
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Beilstein J. Org. Chem. 2016, 12, 564–570.
Scheme 1: Selected examples of oligopeptides bearing dehydroamino acid moieties: myxovalargin (1), argyrin A (2) and nisin (3) (in myxovalargin
dehydroamino acids and neighboring amino acids are numbered).
prepare due to steric hindrance in the β-position and the issue of able catalyst to promote this reaction in the presence of cesium
regiocontrol during elimination [4,5], as β-elimination of a carbonate as base and N-methylpyrrolidone (NMP) as solvent
tertiary alcohol group often leads to the terminal instead of the [7,8]. Buchwald et al. [9] simplified the conditions by demon-
conjugated alkene.
strating that copper(I) iodide, potassium carbonate and the
ligand N,N-dimethylethylenediamine can be used instead.
Principally, enamides can also be prepared by the copper-medi- Finally, Ma and co-workers [10] further modified the condi-
ated C–N coupling between a vinyl halide 6 and an amide 5 as tions using copper(I) iodide, cesium carbonate and N,N-
reported by Ogawa and co-workers in 1991 [6]. Later, the group dimethylglycine in 1,4-dioxane. Recently, Inoue applied this
of Porco showed that copper(I) thiophencarboxylate is a suit- cross-coupling protocol to the synthesis of peptides (Scheme 2)
Scheme 2: The Buchwald cross-coupling reaction in the preparation of peptides containing dehydroamino acids 4.
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Beilstein J. Org. Chem. 2016, 12, 564–570.
[11,12] as demonstrated in the total synthesis of Yaku'amide A resulted from the presence of oxygen in the solvent. Therefore,
[13].
we decreased the temperature and only degassed 1,4-dioxane
was used in order to avoid oxidation and formation of
As part of our study on the total synthesis of myxovalargin (1), copper(II) which can act as a Lewis acid. These changes provi-
a secondary metabolite from Myxococcus fulvus with antibacte- ded peptide 14 (Table 1, entry 1) but this result turned out not to
rial activity [14,15], we report on copper-mediated cross-cou- be reproducible. Instead, when the reaction time was extended,
pling chemistry to create peptide fragments bearing dehydrova- only the formation of the α-ketoamide 15 was encountered
line and dehydroisoleucine. We faced difficulties to use the re- (Table 1, entry 2). Change of the solvent to THF or toluene as
ported conditions for the C–N cross coupling in the preparation well as the use of palladium catalysts [16] or the use of addi-
of myxovalargin peptide fragments mainly because of the steric tives such as HMPA mainly led to substantial decomposition of
hindrance of β,β’-disubstituted dehydroamino acids created. vinyl iodide 12. At room temperature only small amounts of
Additionally, the neighboring amino acid, especially arginine or product were formed but the vinyl iodide was stable (Table 1,
ornithine, a precursor for preparing arginine (amino acid num- entry 3). The presence of the ligand was essential and the choice
ber 9 in 1), can be made responsible. Thus, this report covers of other amines such as tetramethylethylendiamine (TMEDA)
our efforts to optimize the copper-mediated cross-coupling (Table 1, entry 4) or piperidine-2-carboxylate did not lead to
reaction in the preparation of dehydroamino acid containing product formation (Table 1, entry 5).
peptide fragments present in myxovalargin (peptides contain-
ing amino acids 3-methylbutyric acid 1, 2, 7–9 and 11–13).
However, (rac)-trans-N,N-dimethyl-1,2-cyclohexanediamine
(17) [9] in combination with cesium carbonate or potassium
carbonate, the latter being superior to the former base, in 1,4-
Results and Discussion
First vinyl iodide 10, representing dehydrovaline, was prepared dioxane provided conditions that allowed us to prepare the cou-
by an established sequence starting from alkynyl ester 8. Under pling product 14 at room temperature. When raising the temper-
similar conditions vinyl iodide 11 which resembles dehy- ature to 50 °C the desired coupling product was isolated in 35%
droisoleucine was obtained as a single diastereoisomer starting yield along with a substantial amount of starting vinyl iodide
from alkynyl ester 9 (Scheme 3). The stereochemistry of the (Table 1, entry 6). When N,N-dimethylethylenediamine (16)
ethyl ester of 11 was determined NMR spectroscopically in- was employed under these conditions instead, the coupling
cluding nOe experiments (see Supporting Information File 1). product formed only in traces. When potassium carbonate was
Next, vinyl iodide 10 was subjected to amidation with L-alanine exchanged by sodium carbonate the yield dropped to 25% under
methylate (resembling position 2 in myxovalargin) using the the optimized conditions (Table 1, entry 8).
reagent system PyAOP, HOAt, DIPEA to yield amide 12
(Table 1). Now, the stage was set to optimize the C–N coupling Next, we tested these reaction conditions for the preparation of
conditions using amide 13 as coupling partner, the 3-methyl- the other dehydrovaline bearing peptide fragment of myxo-
butyric acid located at the terminus of myxovalargin (1).
valargin 1 (amino acids 7–9). First, dipeptides 18–21 and
methyl esters 22 and 23 were N-acylated with vinyl iodide 10 to
Under the published conditions [13] (vinyl iodide (1.5 equiv), yield peptidic vinyl iodides 24–29 in commonly good yields
amide (1.0 equiv), CuI (0.6 equiv), Cs2CO3 (2.0 equiv), N,N- (Scheme 4). The coupling partners 18–23 (synthesis see Sup-
dimethylethylenediamine 16 (4.05 equiv), 1,4-dioxane, 90 °C) porting Information File 1) all contain arginine or the precursor
we did not encounter the formation of enamide 14 but instead amino acid ornithine and they differ in the choice of the
only the hydrolysis product 15 was isolated which might have protecting group. These variations are important for gaining
Scheme 3: Syntheses of vinyl iodides 10 and 11.
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Beilstein J. Org. Chem. 2016, 12, 564–570.
Table 1: Formation of amide 12 and optimization of C–N cross coupling reaction.
entry
conditions
12 (equiv)
12 (% reisolated)
14 (% yield)
15 (% yield)
1
16, Cs2CO3, 16 h, 50 °C
16, Cs2CO3, 26 h, 50 °C
16, Cs2CO3, 24 h, rt
1.5
1.5
1.5
1.5
1.5
–
31
–
2
–
0
nd
0
3
60
~15
4a
5a
TMEDA, Cs2CO3, 20 h, rt
no reaction
no reaction
piperidine-2-carboxylate,
K2CO3, 20 h, rt
6
7
8
17, K2CO3, 22 h, 50 °C
17, K2CO3, 20 h, 70 °C
17, Na2CO3, 20 h, 70 °C
2.0
2.0
2.0
43
57
66
35
35
25
0
0
0
aCuI (1.0 equiv) was used instead of 0.6 equiv of CuI.
Scheme 4: Preparation of vinyl iodides 24–29 (Cbz = benzyloxycarbonyl, Alloc = allyloxycarbonyl, Boc = tert-butyloxycarbonyl, Pbf = 2,2,4,6,7-
pentamethyldihydrobenzofurane-5-sulfonyl).
flexibility towards the end of the total synthesis when the guani- These results demonstrate that the copper-mediated cross-cou-
dine group has either to be liberated by the removal of the pling reaction depends on the functional groups present in the
protecting groups or used for the introduction to the correspond- peptide containing the vinyl iodide moiety and on the chosen
ing ornithine residue.
protecting groups. Vinyl iodides bearing an arginine moiety
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gave a significantly reduced yield of the desired cross-coupling amide 30 and ornithine-containing vinyl iodides 24 and 28
product compared to the corresponding ornithine derivatives.
(Scheme 5).
Furthermore, it turned out that the Alloc and Pbf protecting It needs to be noted that the presence of a Cbz protecting group
groups are not compatible for this enamide forming protocol. in dehydrooligopeptides is problematic, because it cannot be
The best results were obtained by using the Boc-protected cleaved without simultaneously reducing the acyl enamide. We
Scheme 5: Copper-mediated C–N cross-coupling of dehydropeptides 31–33, 36, 37, and 39–41.
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Beilstein J. Org. Chem. 2016, 12, 564–570.
found that the Teoc protecting group is better suited to be re- erful tool to straight forwardly build-up of sterically hindered
moved from oligoamides 39 and 40, respectively, which were dehydroamino acid-containing peptide fragments.
formed from amide 38 and vinyl iodides 29 and 35 (Scheme 5)
[17]. Facile removal of the Teoc group was achieved within
Supporting Information
24 h at room temperature in quantitative yield using a 1 M solu-
tion in TBAF in THF.
Supporting Information File 1
Syntheses and analytical descriptions of reagents and
With these results in hand we next coupled amide 42 with Boc-
protected valine-derived amide 43. To our delight, the coupling
proceeded in an improved yield of 48% without scrambling of
the stereochemistry of the olefinic double bond (Scheme 6). We
based the stereochemical assignment on NMR spectroscopy
which included nOe experiments.
peptides and copies of 1H and 13C NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-55-S1.pdf]
Acknowledgements
We thank the Beilstein-Institut for generously supporting
Conclusion
Franziska Gille with a Ph.D. scholarship.
In conclusion, we report on the synthesis of dehydrovaline and
dehydroisoleucine-containing oligopeptides as found in the
peptide antibiotic myxovalargin using a C–N cross-coupling ap-
proach. Substantial optimization of the reaction conditions and
the choice of the protecting group became necessary when the
dehydrovaline-containing oligopeptide based on ornithine was
synthesized. For the copper-mediated Buchwald C–N coupling
reaction, (rac)-trans-N,N-dimethyl-1,2-cyclohexandiamine was
the ligand of choice in combination with potassium carbonate as
base. In our case, the usage of Boc and Teoc protection groups
in the cross-coupling reaction gave the best results. By using the
optimized reaction conditions the dehydroisoleucine peptide
was synthesized without scrambling of stereochemistry. In
summary, we showed that the C–N coupling reaction is a pow-
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