Synthetic studies towards bottromycin

Article abstract of DOI:10.3762/bjoc.8.189

Thio-Ugi reactions are described as an excellent synthetic tool for the synthesis of sterically highly hindered endothiopeptides. S-Methylation and subsequent amidine formation can be carried out in an inter- as well as in an intramolecular fashion. The intramolecular approach allows the synthesis of the bottromycin ring system in a straightforward manner.

Full text of DOI:10.3762/bjoc.8.189

Synthetic studies towards bottromycin
Stefanie Ackermann1, Hans-Georg Lerchen2, Dieter Häbich2,
Angelika Ullrich1 and Uli Kazmaier*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1652–1656.
1Institute of Organic Chemistry, Saarland University, P.O. Box
151150, 66041 Saarbrücken, Germany and 2Bayer Pharma
Aktiengesellschaft, Aprather Weg 18a, 42113 Wuppertal, Germany
doi:10.3762/bjoc.8.189
Received: 16 July 2012
Accepted: 28 August 2012
Published: 01 October 2012
Email:
Uli Kazmaier* - u.kazmaier@mx.uni-saarland.de
This article is part of the Thematic Series "Antibiotic and cytotoxic
peptides".
* Corresponding author
Keywords:
Guest Editor: N. Sewald
amidines; antibiotics; bottromycin; peptides; thiopeptides; Ugi
reactions
© 2012 Ackermann et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Thio-Ugi reactions are described as an excellent synthetic tool for the synthesis of sterically highly hindered endothiopeptides.
S-Methylation and subsequent amidine formation can be carried out in an inter- as well as in an intramolecular fashion. The
intramolecular approach allows the synthesis of the bottromycin ring system in a straightforward manner.
Introduction
Natural products are excellent sources as lead structures for the No. 3668-L2, named bottromycin A and B [13,14]. Acidic
development of new antibiotics. Over millions of years hydrolysis provided a mixture of all-(S)-configured amino acids
microorganisms, such as bacteria and fungi, developed efficient containing 3-methylphenylalanine [15], tert-leucine, valine,
defense strategies against their bacterial competitors [1-3]. Not β-(2-thiazolyl)-β-alanine [16] and glycine. While also proline
surprising, a wide range of common antibiotics such as peni- was found in bottromycin B, cis-3-methylproline is a compo-
cillin or vancomycin are natural products or derivatives thereof. nent in bottromycin A [17]. Originally Nakamura postulated a
linear N-acylated iminohexapeptide structure, a proposal which
In 1957 Waiswisz et al. reported a new antibiotic peptide was revised after synthetic studies [18] as well as NMR spectro-
isolated from the fermentation broth of Streptomyces scopic investigations by Takita et al., which proposed a cyclic
bottropensis, called bottromycin [4-6]. This antibiotic inhibits iminopeptide structure [19]. This proposal was verified by
the growth of a wide range of microorganisms by interfering Schipper [20] and Kaneda [21], based on detailed NMR studies.
with their protein biosynthesis [7-12]. In 1965 Nakamura et al. According to them, the bottromycins are cyclic tetrapeptides,
isolated closely related antibiotics from the strain Streptomyces connected to a tripeptidic side chain through an amidine struc-
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Beilstein J. Org. Chem. 2012, 8, 1652–1656.
ture. The different bottromycins differ only in the substitution Results and Discussion
pattern of the proline (Figure 1). The three-dimensional struc- Our group is also involved in the synthesis of peptide-based
ture was reported recently by Gouda et al. [22]. This structure is natural products [27-32], and of course the structure of
quite unusual, not only because of the amidine moiety but also bottromycin is highly fascinating from a synthetic point of
because most amino acids are found in β-methylated form (tert- view. In our previous investigations we observed that for the
leucine can be seen as β-methylvaline).
synthesis of sterically demanding peptides, especially those
containing N-alkylated amide bonds, Ugi reactions are espe-
cially suited [33,34]. For N-unsubstituted peptides the Ugi reac-
tions should be carried out with ammonia as the amine compo-
nent, which is a protocol that is often accompanied by a range
of side reactions. But in general the yields are good if sterically
demanding aldehydes, such as pivaldehyde are used [35,36].
With thiocarboxylic acids as acid components, this approach
allows also the synthesis of endothiopeptides [37-40], and
therefore this protocol should be extremely suitable for the syn-
thesis of bottromycins.
To prove this option, we reacted Boc-protected (S)-thiovaline
[41] with pivaldehyde, ethyl isocyanoacetate, and a 2 M NH3
solution in CH3OH. In trifluoroethanol, which is the best
Figure 1: Bottromycins.
Although a series of synthetic studies towards linear solvent for ammonia Ugi reactions [35], the expected endothio-
bottromycin sequences have been published [23,24], no total peptide 1 was formed in high yield as a 1:1 diastereomeric mix-
synthesis was reported for a long time. The first and only syn- ture (Scheme 1). With this building block in hand, we were
thesis so far was described by Sunazuka and Ōmura et al. in interested to see whether we would be able to generate the
2009 [25]. Their synthesis was based on the formation of the required cyclic endothiopeptide and if we could even subse-
amidine structure by reaction of the tripeptide side chain with quently connect the side chain.
an endothiopeptide and ring closure between proline and
glycine. The same group also undertook some modifications on Therefore, we prolonged the peptide chain 2 under standard
the natural product [26].
peptide coupling conditions and tried to activate the linear
Scheme 1: Syntheses of endothiopeptides by thio-Ugi reaction.
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Beilstein J. Org. Chem. 2012, 8, 1652–1656.
peptide chain. We chose the pentafluorophenylester protocol
developed by Schmidt et al. for cyclization [42]. But unfortu-
nately this approach failed (as did all other activations investi-
gated) because of the formation of thiazolone side product,
which could not be cyclized.
Table 1: Optimization of amidine formation.
entry Hg salt
equiv
solvent
yield
(%)
(S)-Val-OMe
1
2
3
4
5
Hg(OCOCH3)2 1.3
Hg(OCOCF3)2 1.3
MeCN
MeCN
CH2Cl2
THF
0
24
50
61
72
Hg(OCOCF3)2 1.3
Hg(OCOCF3)2 1.3
Hg(OCOCF3)2 2.0
In parallel, we tried to figure out if the amidine formation is
possible with sterically hindered endothiopeptides (Scheme 2).
As a model substrate we chose thiopeptide 3 [37], easily
obtained by thio-Ugi reaction in excellent yield. The reaction
was very fast and was finished already after 15 min, and peptide
THF
3 crystallized directly from the reaction mixture. Because our This isocyanide was subjected to a thio-Ugi reaction as
first attempts to couple 3 directly with amines to the corres- described before, and the expected sterically highly demanding
ponding amidine 5 failed [43], we decided to convert 3 into the endothiopeptide 8 was obtained in high yield as a 1:1 diastereo-
corresponding thioimidate 4, which was reacted with (S)-valine meric mixture. In this case, the diastereomers could not be sep-
methyl ester as an amine component in the presence of Hg salts arated. Elongation of the peptide chain under standard peptide
[44]. Hg(OCOCF3)2 was found to be more suitable than coupling conditions provided the linear precursor 10 for the
Hg(OCOCH3)2 (Table 1, entries 1 and 2) and from the different ring-closing amidination. After removal of the Boc-protecting
solvents tested, THF was the solvent of choice. With a twofold group the resulting salt showed a low solubility in THF, and
excess of the amine component a yield of up to 72% of 5 could therefore we had to run the amidination in CH3CN, although
be obtained as a 1:1 diastereomeric mixture. The diastereomers this was not the solvent of choice in our model system.
could be separated by flash chromatography, but unfortunately
these reaction conditions could not be transferred to the thio- The amidination was carried out under high-dilution conditions.
imidoester obtained from endothiopeptide 2. Here, the expected The peptide salt was dissolved in CH3CN and was added slowly
amidine was only formed in a trace amount, and a range of side through a syringe to a solution of Hg(OCOCF3)2 in CH3CN at
products was obtained.
50 °C over a period of 10 h. Interestingly, the best result was
obtained not with the free peptide amine but with the
Therefore, we decided to change our strategy and to replace the hydrochloride salt. Here the cyclic amidine was obtained in
intermolecular amidine formation by an intramolecular one. In 51% yield.
this case the peptide ring should be formed in the amidination
step. For this approach we first needed the isocyanide 7 derived Conclusion
from (S)-tert-leucine methyl ester. According to Ugi et al. this In conclusion, we could show that thio-Ugi reactions are an
isocyanide was obtained in enantiomerically pure form from the excellent synthetic tool for the synthesis of highly sterically
corresponding formamide 6 by dehydration with POCl3/NEt3 hindered endothiopeptides. S-Methylation and subsequent
(Scheme 3).
amidine formations can be carried out in an inter- as well as in
Scheme 2: Synthesis of amidine 5 by thio-Ugi reaction.
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Beilstein J. Org. Chem. 2012, 8, 1652–1656.
Scheme 3: Synthesis of the bottromycin ring system 12 by thio-Ugi reaction.
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