First total synthesis of aerucyclamide B

Article abstract of DOI:10.1016/j.tetlet.2013.03.060

The first total synthesis of the antimalarial aerucyclamide B has been achieved in 9% overall yield. Two thiazoles and a dipeptide were used to prepare two open precursors of cyclo-Gly-l-allo-Thr-l-Ile-Thz-d-allo-Ile-Thz. Cyclodehydration with Deoxo-Fluor of the β-hydroxyamide present in the macrocycle, rendered aerucyclamide B (67%) and an unexpected fluorous derivative (28%).

Full text of DOI:10.1016/j.tetlet.2013.03.060

Tetrahedron Letters 54 (2013) 2806–2808
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
First total synthesis of aerucyclamide B
⇑
Stella Peña, Laura Scarone, Eduardo Manta, Gloria Serra
Cátedra de Química Farmacéutica, Facultad de Química, Universidad de la República, General Flores 2124, CP 11800 Montevideo, Uruguay
a r t i c l e i n f o
a b s t r a c t
Article history:
The first total synthesis of the antimalarial aerucyclamide B has been achieved in 9% overall yield. Two
Received 13 February 2013
Revised 15 March 2013
Accepted 16 March 2013
Available online 24 March 2013
thiazoles and a dipeptide were used to prepare two open precursors of cyclo-Gly-
-allo-Ile-Thz. Cyclodehydration with Deoxo-Fluor of the b-hydroxyamide present in the macrocycle, ren-
dered aerucyclamide B (67%) and an unexpected fluorous derivative (28%).
Ó 2013 Elsevier Ltd. All rights reserved.
L-allo-Thr-L-Ile-Thz-
D
Keywords:
Aerucyclamide
Macrocyclization
Oxa(thia)zole
Hexacyclopeptide
Cyanobacteria are a rich source of biologically active natural
products. Their secondary metabolites show a wide range of bio-
logical activities including anticancer, antibacterial, antiparasite,
antiviral and protease inhibition activities.1 The cyanobacterium
Microcystis aeruginosa is well-known for the production of the toxic
cyclic peptide microcystins.2 M. aeruginosa also produces several
linear peptides that are potent protease inhibitors,3 and cyclamides
which are cyclic peptides composed of alternating heterocyclic
amino acids.4 Aerucyclamides A, B, C, and D were isolated in
2008 by Gademann and co-workers from the toxic freshwater cya-
nobacterium Microcystis aeruginosa PCC 7806.5 The most active of
the four was aerucyclamide B (1, Scheme 1), displaying a submi-
cromolar IC50 value against Plasmodium falciparum K1. In addition,
this compound displays a large selectivity for the parasite with re-
spect to the L6 rat myoblast cell line.5b Seeking to further explore
their biological activity, a synthesis program was initiated to devel-
op a convergent route to aerucyclamides and analogs. Herein, we
present the total synthesis of aerucyclamide B.
O
H
S
O
N
H
N
N
H
NH
HN
O
D
N
-Ile
L
O
H
S
1
-allo-Ile
O
H
S
HO
O
N
H
NH
NH
N
HN
H
O
H
O
N
S
Our retrosynthetic analysis is shown in Scheme 1. We planned
to obtain first the macrocycle 2 from dipeptide 3,6 and two thiazole
building blocks 4,7 and 5,8 by a convergent macrocycle-assembly
methodology. This strategy was successfully used by Meyers and
co-workers for the synthesis of a structural related product,
bistratamide D using three heterocycle building blocks: a thiazole,
an oxazole and an oxazoline.9 In the paper, the authors described
some problems related with the deprotection of the functional
groups and coupling reactions of the oxazoline ring. In fact, we ob-
tained a mixture of decomposition products during the deprotec-
2
BocHN
S
O
H
HO
O
OMe
N
4
NH
3
CO2Et
NHBoc
MeO2C
BocHN
H
N
H
S
5
⇑
Corresponding author. Tel./fax: +598 29290290.
E-mail address: gserra@fq.edu.uy (G. Serra).
Scheme 1. Retrosynthetic analysis of 1.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.060

S. Peña et al. / Tetrahedron Letters 54 (2013) 2806–2808
2807
O
O
O
NHBoc
S
OMe
H
H
H
S
HO
O
N
H
HO
O
HO
O
OMe
HBTU, DIPEA,
DMAP, CH2Cl2,
Boc-L-Ile-OH
+
NH
NH
N
HN
H
NH
NH
N
NH
CO2Et
3
95%
L
-allo-Thr-OMe
O
BocHN
N
H
O
H
O
NHBoc
N
H
H
S
S
9
11
i. HBTU, DIPEA,
DMAP, CH2Cl2
i) KOHaq/THF
ii) HCl/Dioxane
iii)HBTU,CH2Cl2
12%
i) KOHaq/THF
ii) HCl/Dioxane
iii) HBTU, CH2Cl2
ii. DAST, CH2Cl2
iii. H2S/MeOH:Et3N
Boc-D-allo-Ile-OH
OMe
NHBoc
H
40%
+
O
N
H
L-Ser-OMe
O
iv. DAST, CH2Cl2
v. BrCCl3/DBU,
CH2Cl2
S
HO
O
N
S
H
NH
NH
N
5
80% (five steps)
HN
H
O
H
O
N
Scheme 2. Synthesis of building blocks 3 and 5.
S
2
DeoxoFluor
CH2Cl2, -20°C
tion of the methyl ester of the oxazoline derived from dipeptide 3,
using basic conditions (KOHaq/THF, LiOHaq/THF).
O
Based on these results and the previous reports,8,10 we selected
the cyclodehydration reaction, to obtain the oxazoline ring, as the
last step of our route. The macrolactamization reaction is often a
slow, low yielding procedure and frequently depends of the se-
lected point of cyclization. In our case, macrocycle formation could
be facilitated since the open precursors of 2, contain turn-induc-
ing11 thiazole constraint.12
H
O
H
S
F
O
N
H
S
O
N
H
NH
NH
N
HN
H
N
N
+
H
O
NH
HN
O
H
O
N
N
O
H
S
S
12(28%)
Dipeptide 3 was obtained in excellent yield (95%) from its pro-
tected aminoacids using HBTU as coupling reagent, Scheme 2. Thi-
azole 4 was prepared following our reported procedures.13,14 For
the preparation of thiazole 5 we selected the cyclodehydration of
b-hydroxythioamides and further oxidation methodology. Cou-
1 (67%)
Scheme 4. Synthesis of 2 and aerucyclamide B.
The next steps were the assembling of the key fragments into
the open intermediates of macrocycle 2. In order to investigate if
the point of cyclization is relevant for the formation of 2, we
decided to obtain two open intermediates 9 and 11 (Scheme 3).
We hypothesized that these intermediates could facilitate the mac-
rocyclization since in compound 9 the amino group is derived from
Gly and consequently gave the synthetic advantages of non-epi-
merization and non-steric hindrance; and in compound 11 the
pling reaction between Boc-D-allo-Ile-OH and L-Ser-OMe followed
by cyclodehydration allowed us to obtain the corresponding oxaz-
oline which was used without purification, Scheme 2. Then, thiol-
ysis with H2S/MeOH/Et3N,15 rendered the thioamide. From this
compound, thiazole 5 was obtained employing the one-pot proce-
dure with DAST and then BrCCl3/DBU.16 Following this protocol,
the desired thiazole was obtained in excellent yield (80%, five
steps).
4
5
3
HCl(g)/ Dioxane
route A
HCl (g)
S
quant.
KOH aq
quant.
OMe
quant.
S
BocHN
Dioxane
O
H
THF
N
i) KOHaq/THF
ii) HBTU,CH2Cl2
40%
NH2.HCl
H
HO
O
OMe
N
S
BocHN
NH
O
O
8
HN
N
NH2.HCl
N
H
HBTU, CH2Cl2
94%
MeO2C
H
HO
O
S
7
6
O
NHBoc
H
S
HO
OMe
O
NH
NH
N
i) HCl (g)/ Dioxane
ii) 6, HBTU,CH2Cl2
40%
HN
H
O
H
O
N
S
9
route B
O
H
5
O
i)KOHaq/THF
S
KOHaq
THF
N
H
HO
O
H
ii)HBTU,CH2Cl2
40%
quant.
NH
NH
N
HO
OMe
OH
O
NH
NH
NHBoc
H
CO2Et
S
8, HBTU,
CH2Cl2
80%
ClH.H2N
BocHN
N
N
N
H
O
O
NHBoc
H
H
O
S
EtO
O
H
N
S
S
11
HCl (g)
quant.
10
Dioxane
4
Scheme 3. Synthesis of bis- and tris-heterocycles.

2808
S. Peña et al. / Tetrahedron Letters 54 (2013) 2806–2808
reaction involves a carboxylic acid attached to an aromatic hetero-
cycle and consequently epimerization cannot occur.17
higher yield (40%) than from 9 (12%). Aerucyclamide B was synthe-
sized in 9% overall yield from 5 via intermediate 11, and in 3% over-
all yield from 4 via intermediate 9.
First, for the synthesis of 9 by route A, bis-heterocycle 7 was
prepared in excellent yield (94%) by ethyl ester hydrolysis of 4, fol-
lowed by coupling with the N-deprotected derivative of 5 using
HBTU. Methyl ester hydrolysis of 7, and coupling with the N-
deprotected dipeptide 8, rendered 9 in moderate yield (40%). With
the purpose of investigating another route to 9 (route B), com-
pound 10 was prepared in high yield by coupling dipeptide 8 and
C-deprotected derivative of 5. N-deprotection of 10 and coupling
reaction with 6 rendered 9 in 40% yield.
Acknowledgments
This work was supported by Grants from ANII (FCE2720), CSIC
Grupos (Universidad de la República) and PEDECIBA. We thank
Universidade Estadual de Campinas (Brasil) and Professor Guiller-
mo Moyna for some HRMS analyses. The authors acknowledge a
postgraduate fellowship from ANII (Agencia Nacional de Investiga-
ción e Innovación) (Stella Peña).
For the synthesis of 11, methyl ester hydrolysis of 10, and cou-
pling with the N-deprotected derivative of 4, rendered 11 in mod-
erate yield (40%).
Supplementary data
Macrocycle 2 was obtained in poor yield (12%) by C- and N-
deprotection of 9 followed by coupling using HBTU in diluted con-
ditions (0.005 M), Scheme 4. C- and N-deprotection of the linear
precursor 11 was achieved in quantitative yield using aqueous
KOH and then HCl(g)/dioxane. Macrocyclization was performed
in diluted conditions (0.005 M) using HBTU. The desired macrocy-
cle 2, was obtained from 11 in a higher yield (40%) than from the
open intermediate 9 (12%), suggesting that the selection of the
point for macrolactam formation is relevant in this case.
The last reaction to obtain aerucyclamide B was realized using
the cyclodehydrative reagent Deoxo-Fluor. Previous studies have
demonstrated that the reactivity of Deoxo-Fluor and DAST is sim-
ilar, although Deoxo-Fluor displays increased thermal stability.18
In addition, Deng and Taunton,19 concluded that Deoxo-Fluor effi-
ciently promoted cyclodehydration of an allo-threonine amide of a
macrocycle to afford cis,cis-ceratospongamide in 88% yield. The use
of these conditions, Scheme 4, allowed us to obtain aerucyclamide
B in 67% yield and an unexpected side product in 28% yield. The 1H
NMR of this unexpected compound showed a dqd signal at
4.98 ppm with a coupling constant J = 48.3 Hz. In addition, the
13C NMR spectrum showed a signal at 89.9 ppm (d, J = 171.5 Hz),
that could be assignable to a C–F. These results and the HRMS spec-
trum prompted us to conclude that the side product is the fluorous
derivative of 2 (12). This compound was produced by the intermo-
lecular reaction between a fluoride and the activated intermediate
generated by the reaction of 2 and Deoxo-Fluor. Based on a SN2-
type mechanism, the inversion of the stereochemistry of C–F of
12 is proposed.
Elimination processes as a competition in the oxazoline synthe-
ses using DAST or Deoxo-Fluor were previously reported.20 How-
ever, to the best of our knowledge, competition between the
oxazoline and the fluorous derivative formation was not published
until now. In this case, the formation of 12 should be explained by
a loss of nucleophilicity of the b-hydroxyamide of 2. The conforma-
tion of this macrocycle, imposed by hydrogen bond formation,
would restrict the desired reaction allowing the synthesis of the
fluorous compound.
The spectroscopic data of the synthesized aerucyclamide B are
in agreement with those reported to the natural product.
In conclusion, macrocycle 2 was prepared from two open pre-
cursors 9 and 11 using two heterocycles and a dipeptide as build-
ing blocks. Macrocycle 2 formation from 11 was performed in a
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
03.060.
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