Total synthesis of chrysamide B

Article abstract of DOI:10.1016/j.tetlet.2017.04.079

We report an efficient synthesis of the dimeric trans-epoxyamide chrysamide B, recently isolated from the deep-sea-derived fungus Penicillium chrysogenum SCSIO41001. Our synthetic strategy exploits a convergent approach using solid-phase peptide synthesis for the piperazine core and a Sharpless-Katsuki epoxidation to prepare the chiral epoxyacid. The double amidation final step provides chrysamide B that was thoroughly characterized with all spectra identical to those of the natural sample. The approach was devised to facilitate the preparation of a library of analogs of chrysamide B.

Full text of DOI:10.1016/j.tetlet.2017.04.079

Accepted Manuscript
Total synthesis of chrysamide B
Christopher Bérubé, Claudia Carpentier, Normand Voyer
PII:
S0040-4039(17)30533-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.04.079
TETL 48871
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
16 March 2017
14 April 2017
24 April 2017
Please cite this article as: Bérubé, C., Carpentier, C., Voyer, N., Total synthesis of chrysamide B, Tetrahedron
Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.04.079
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Tetrahedron Letters
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Total synthesis of chrysamide B
Christopher Bérubéa†, Claudia Carpentiera†1and Normand Voyera*
aDépartement de chimie, PROTEO, Université Laval, Faculté des sciences et de génie, 1045 avenue de la Médecine, Québec, QC, G1V 0A6, Canada
ARTICLE INFO
ABSTRACT
We report an efficient synthesis of the dimeric trans-epoxyamide chrysamide B, recently
isolated from the deep-sea-derived fungus Penicillium chrysogenum SCSIO41001. Our
synthetic strategy exploits a convergent approach using solid-phase peptide synthesis for the
piperazine core and a Sharpless-Katsuki epoxidation to prepare the chiral epoxyacid. The
double amidation final step provides chrysamide B that was thouroughly characterized with all
spectra identical to those of the natural sample. The approach was devised to facilitate the
preparation of a library of analogs of chrysamide B.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
Chrysamide
Chiral piperazines
Nitro natural products
Chiral epoxides
2009 Elsevier Ltd. All rights reserved
.
Natural product synthesis
———
∗ Corresponding author. Tel.: +1-418-656-3613; fax: +1-418-656-7916; e-mail: normand.voyer@chm.ulaval.ca
† These authors contributed equally to the manuscript

2
Introduction
Results and discussion
Marine microorganisms are well-known sources of novel
bioactive natural products (Fig.1).1-6 For example, piperazine
derivatives are of particular interest exhibiting anticancer,
antiviral, antibacterial, antimalaria, anti-HIV and antifungal
Preparation of chiral 1,4-piperazine 2
Taking advantage of the availability of Boc protected natural
and unnatural amino acids as chiral synthons, the chiral 1,4-
piperazine ring (2) was synthesized via diamide reduction of D-
alanine 2,5-diketopiperazine derivative (4). Among various
possible synthetic strategies to access 2,5-diketopiperazines, we
chose a solid-phase synthesis on oxime resin shown in Figure
3.14-17 The first N-Boc-protected D-alanine was coupled for three
hours using diisopropylcarbodiimide (DIC) and 6-chloro-
hydroxybenzotriazole (6-Cl-HOBt) as coupling reagents. The
Boc protecting group was removed using a mixture of 1:1
trifluoroacetic acid (TFA)/ dichloromethane, while the second
amino acid was activated with 6-Cl-HOBt and 2-(6-Chloro-1H-
benzotriazole-1-yl)-1,1,3,3-tetramethylaminium
activities.4,7 Also, the piperazine moiety is widely used as
a
scaffold in the preparation of bioactive compounds due to its
ability to bind multiple receptors with high affinity and to orient
divergently two lateral chains.7-10
O
NO2
O
O
N
N
O
O2N
O
Chrysamide A
O
NO2
O
N
N
hexafluorophosphate (HCTU). After coupling and deprotection,
the linear dipeptide was simultaneously cyclized and cleaved
from the resin in the presence of diisopropylethylamine (DIEA;
2.5 equiv.) and acetic acid (AcOH; 5 equiv.) in dichloromethane,
leading very efficiently to 2,5-diketopiperazine 4 (92% yield)
without epimerization. Based on work done in our lab,18 we
chose lithium aluminum hydride (1 M in THF) in dry THF under
an inert atmosphere as optimal reducing conditions. After 48
hours, sodium sulfate decahydrate was added to neutralize
residual lithium aluminum hydride and other active species, and
the mixture was refluxed for 30 minutes.19,20 A simple filtration
eliminated the residual solids and led to the desired D-alanine
derived 1,4-piperazine (2) in 74% yield. NMR studies showed
6% epimerization during the overall reduction procedure.18,21 The
1,4-piperazine 2 was obtained with an overall yield of 68% in six
steps starting from commercially available oxime resin and N-
Boc protected D-alanine precursors with the solid-phase peptide
synthesis. The separation of the contaminant epimer proved
difficult at this stage with the piperazine degrading slowly during
chromatography on silica. We decided to use piperazine 2 as is in
the next steps and to perform purification on the final product.
O
O2N
O
OMe
Chrysamide B
HO
Me
OAc
H
N
Me
NH
N
Me
S
R3
N
O
Br
Br
H
O
OH
N
O
R2
O
R1
HN
MeO
NH
Dragmacidin
R1: Me R2: Br R3: OH
Dragmacidon A R1: H R2: H R3: H
Dragmacidon B R1: Me R2: H R3: H
HO
Ecteinaiscidin
Figure 1. Structures of chiral piperazine derivatives isolated from
marine microorganisms
In 2016, three new dimeric nitrophenyl trans-epoxyamides,
chrysamides A-C, were isolated from the deep-sea-derived
fungus Penicillium chrysogenum SCSIO41001.5,11 Chrysamides
A and B (Fig. 1) possess a unique symmetrical dimer skeleton
featuring a chiral piperazine central core. Such dimeric natural
products are attractive to medicinal chemists due to their
pharmacological potential.11-13
A
preliminary biological
screening showed that chrysamide A exhibits inhibitory effect on
the production of proinflammatory cytokine IL-17.11 However,
no biological activity has been associated to chrysamide B so far,
perhaps due to the lack of sufficient material for biological
testing. To further investigate the pharmacological potential of
chrysamide B, we have developed a synthetic methodology
which gives access rapidly to this unique compound. The main
step involves a double amidation of the chiral D-alanine derived
1,4-piperazine ring (2) with the epoxy-acid (3) as final step (Fig.
2).
Preparation of epoxy-acid 3
The synthesis of key epoxy-acid building block 3 is shown in
Figure 4. It is prepared in four steps from commercially available
p-nitrobenzaldehyde. This aldehyde reacted with trietyl-2-
phosphonopropionate in the presence sodium hydride in a
Horner-Wadsworth-Emmons olefination22,23 to obtain the
cinnamate derivative 5. The ester 5 was then reduced with
DIBAL at -78oC in dichloromethane giving the allylic alcohol
6.23-25 We envisioned that the epoxy-alcohol could be prepared in
a
highly enantioselective fashion by a Sharpless-Katsuki
epoxidation reaction on the tri-substituted alkene 6.25 Several
experiments led to the optimal conditions. During our exploration
of the allyl alcohol epoxidation, we observed lower chemical
yields and
Figure 2. Retrosynthetic approach to chrysamide B
Figure 3. D-alanine derived 1,4-piperazine 2 synthetic procedure

3
details). Spectroscopic data of synthetic chrysamide B (1)
matched entirely those of the isolated natural product (see ESI for
details).
In conclusion, we achieved the first total synthesis of
chrysamide B (1) in six steps with an overall yield of 32% taking
advantage of solid-phase synthesis of
a
chiral 2,5-
diketopiperazine and the highly enantioselective Sharpless-
Katsuki epoxidation reaction. The high convergency of the
synthetic methodology paves the way to a rapid access of a wide
variety of enantiopure chrysamide analogs with stereocontrol at
all chiral centers. Work is currently underway to study the
bioactivity of chrysamide B, as well as to prepare its analogs and
chrysamide A containing a unique bicyclic bis-hemiaminal
structure.
Acknowledgments
Figure 4. Epoxy-acid 3 and chrysamide B synthetic procedures
This work was supported by the NSERC of Canada, the
FRQNT of Quebec, PROTEO and Université Laval. CC and CB
wish to thank PROTEO, FRQNT and NSERC for postgraduate
scholarships. The authors are also thankful to Jean-François
Parent and François Otis for useful advice.
enantioselectivities when employing diethyl tartrate ((-)-DET).
Better results were obtained with titanium isopropoxide Ti(OiPr)4
and diisopropyl tartrate ((-)-DIPT). We tried increasing from 5
mol% and 7.5 mol% to 10 and 15 mol% of Ti(OiPr)4 and (-)-
DIPT respectively to verify the impact on yield and
enantioselectivity of the corresponding epoxy-alcohol 7. The best
yield was obtained with the latter conditions at -23oC providing
enantioselectivity of 98% with the desired stereochemistry, in
agreement with Sharpless proposed mechanism.26-28 The
enantiomeric excess was determined by HPLC analysis using a
chiral AD-H column by comparison with the other enantiomer
obtained using the same epoxidation conditions but with (+)-
DIPT.21 The synthesis of the key epoxy-acid 3 was then achieved
using first 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) and
sodium hypochlorite (NaOCl) to convert the epoxy-alcohol to
epoxy-aldehyde without purification as green procedure.29 The
crude product was further treated with sodium chlorite (NaOCl2),
sodium dihydrogenophosphate (NaH2PO4) and 2-methylbut-2-
ene.25,30 Simple acid-base extractions gave the pure epoxy-acid 3.
It is noteworthy of mention, oxidation of the primary alcohol
with PDC31 or by a ruthenium catalyzed process25,32 provided the
desired carboxylic acid in poor yield with significant degradation
into undesired side products.25
Supplementary Material
Supplementary material (including chiral HPLC analyses,
HSQC, COSY, spectra of compound 1, as well as complete
characterization and 1H and 13C-NMR spectra of all prepared
compounds) associated with this article can be found in the
online version.
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Preparation of chrysamide B 1
Previous examples of related double amidation are reported in
the literature using Schotten-Baumann reaction,33 mixed
anhydride32 and peptide coupling agents34 to assemble carboxylic
acids onto 1,4-piperazine secondary amines. Having more
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b]pyridinium 3-oxide hexafluorophosphate (HATU). The
piperazine was then added to the reaction mixture to provide
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4
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Total synthesis of chrysamide B
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Christopher Bérubé, Claudia Carpentier and Normand Voyer

6
Tetrahedron
Highlights
•
•
•
Convergent approach exploiting peptide
synthesis towards piperazine core
Concise chrysamide B synthesis by double
amidation
Devised approach to facilitate chrysamide
B analogs preparation