Unified enantioselective total synthesis of 3,6-dioxygenated diketopiperazine natural products, diatretol and lepistamides A, B and C

Article abstract of DOI:10.1016/j.tetlet.2021.152895

The concise enantioselective total synthesis of Diatretol (1) has been achieved via 9 steps with 30％ yield. The synthetic approach involves stereoselective oxidation and regioselective transacetalization utilizing a folded conformation induced by an intramolecular CH/π interaction. In addition, we have also achieved total synthesis of three diketopiperazine natural products, Lepistamides A (2), B (3) and C (4).

Full text of DOI:10.1016/j.tetlet.2021.152895

Tetrahedron Letters 67 (2021) 152895
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Unified enantioselective total synthesis of 3,6-dioxygenated
diketopiperazine natural products, diatretol and lepistamides A, B and C
Shu Takahashi a, Aoi Kimishima a,b, Tomoyasu Hirose a,b, Takeshi Yamada a,b,1, Akihiro Sugawara a,b,2
,
Tatsuya Shirahata c, Yoshihiko Noguchi a,b, Masato Iwatsuki a,b, Rei Hokari b, Aki Ishiyama a,b
,
Yoshinori Kobayashi c, Toshiaki Sunazuka a,b,
⇑
a Graduate School of Infection Control Sciences, Kitasato University, Japan
b
¯
Omura Satoshi Memorial Institute, Kitasato University, Japan
c School of Pharmaceutical Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The concise enantioselective total synthesis of Diatretol (1) has been achieved via 9 steps with 30％ yield.
Received 30 November 2020
Revised 21 January 2021
Accepted 27 January 2021
Available online 5 February 2021
The synthetic approach involves stereoselective oxidation and regioselective transacetalization utilizing a
folded conformation induced by an intramolecular CH/p interaction. In addition, we have also achieved
total synthesis of three diketopiperazine natural products, Lepistamides A (2), B (3) and C (4).
Ó 2021 Elsevier Ltd. All rights reserved.
Keywords:
Asymmetric total synthesis
Hydroxy diketopiperazine
Sequential oxidation
Antimalarial activity
Introduction
resistance. Therefore, novel antimalarial drugs with new modes of
action are urgently and constantly required to tackle the disease.
Malaria, which is caused by species of Plasmodium parasites, is
one of the world’s three gravest infectious diseases, and according
to the 2019 World Malaria Report, in 2018 there were an estimated
228 million cases of malaria and 405,000 deaths [1]. Antimalarial
drugs are the main weapon to combat malaria, either for prophy-
laxis or treatment. The WHO now recommends artemisinin-based
combination therapies as the preferred treatment option. However,
malaria parasites have historically developed resistance to all
newly introduced antimalarials extremely quickly, and so drug
resistance has remained a continuous problem [2]. For example,
artemisinin is one of the major antimalarial drug. However, plas-
modium falciparum has evolved resistance to Artemisinin [3]. Con-
sequently, there has always been a need to find new antimalarial
drugs, preferably those that have a unique or different structure
or mode of action, to try and offset or delay the emergence of drug
Our research group has strived to develop new medicines from
microbial metabolite. Through the screening of antimalarial agents
from our natural product library in the Kitasato institute, we iden-
tified the potent antimalarial activity of Diatretol (1), [4] originally
isolated from a culture broth of the fungus Clitocybe diatreta by
Arnone and co-workers in 1996 [5]. Other members of the 3,6-
dioxygenated diketopiperazine (DKP) family, similar compounds,
such as the Lepistamides (2–4) [6], don’t demonstrate antimalarial
activity (Fig. 1). The structure of (+)-1 has been clarified by X-ray
crystallography, and the benzyl and methoxy moieties in the DKP
ring show a characteristic folded conformation [7,8] (Fig. 1). Addi-
tionally, the C6 methoxy group in the 1H NMR of (+)-1 was signif-
icantly shifted to lower ppm value (1.97 ppm in DMSO d6) in 1H
NMR analysis data, while the methoxy group is within the shield-
ing region of the phenyl group due to an intramolecular CH/
p
interaction [9,10]. Thereby, the methoxy and benzyl moieties are
cis positioned at the DKP ring. However, the absolute configuration
of (+)-1 remains unknown.
In terms of antimalarial activity, (+)-1 displayed potent in vitro
antimalarial activity against the Plasmodium falciparum K1 strain,
with an IC50 value of 378 ng/ml, as well as in vivo efficacy in a P.
berghei-infected mouse model, with ca. 50% inhibition at 30 mg/
kg (p.o.) [4]. Thus, we envisioned the enantioselective total synthe-
⇑
Corresponding author at: Graduate School of Infection Control Sciences,
Kitasato University, Japan.
E-mail address: sunazuka@lisci.kitasato-u.ac.jp (T. Sunazuka).
Current address: Department of Materials and Chemistry, Kanagawa University,
1
3-27-1, Rokkakubashi, Knagawa-ku, Yokohama 221-8686, Japan.
2
Current address: Graduate School of Pharmaceutical Sciences, Tohoku University,
6-3, Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan.
https://doi.org/10.1016/j.tetlet.2021.152895
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

S. Takahashi, A. Kimishima, T. Hirose et al.
Tetrahedron Letters 67 (2021) 152895
Fig. 1. Structures of Diatretol (1) and Lepistamides (2–4).
sis of (+)-1 to determine its absolute configuration and clarify of its
Structure/Activity relationship (SAR).
Scheme 1. Reported N,O-acetal construction methods for DKP.
Results and discussion
As for the construction of oxygenated DKP skeletons, two ge-
neral methods have been reported (Scheme 1). Method 1 features
halogenation and dehalogenation processes from the dehydrated
intermediate 5 [11], while method 2 afford the N,O-acetal product
8 as a diastereomeric mixture via intramolecular cyclization from
the ketoamide 7 [12]. However, when these methods are applied
for the enantioselective synthesis of (+)-1, induction of the stereo-
centers at C3 and C6, and the chemoselective introduction of the
C6 methoxy group, are problematic tasks. Due to limitations of
the known protocols, we pursued a new synthetic approach for 1
(as shown retrosynthetically in Scheme 2), which features the iso-
merization of the syn compound 9 to a thermodynamically stable
anti compound, followed by removal of N-protection groups to
afford (+)-1. The C6 regioselective transacetalization of 10 using a
folded conformation induced by an CH/p interaction, and the stere-
oselective stepwise oxidation of the known DKP 12 could be
expected to produce the optically active 10. DKP 12 was prepared
from commercially available starting materials by using Xue’s one-
pot procedure [13]. To the best of our knowledge, the use of a
folded conformation induced by an CH/p interaction is unprece-
dented in natural product synthesis.
Scheme 2. Synthetic strategy for Diatretol (1).
Our synthesis began with NAH protection of 12 with a BOM
group to give 13 (Scheme 3). Subsequently, we attempted the
stereoselective oxidation of 13. In the oxidation of the DKP ring,
which consists of phenylalanine as one of amino acid residues,
the corresponding C3 position tends to be predominantly oxidized
over the C6 position [14], due to the lower bond dissociation
energy of the C3-H, which is influenced by the benzyl moiety
[15]. On the basis of this unique chemo selectivity, we postulated
that the diol 10 would be obtained with high enantioselectivity
via a stepwise oxidation process. Prior to the stepwise oxidation
of 13, we undertook a deuterium labeling experiment for the selec-
tive deprotonation of the C3 position. Interestingly, even with the
use of excess amount of KHMDS only the C3 labeled product 14
was obtained as a single isomer, presumably due to the instability
of the corresponding dienolate.
Scheme 3. Synthesis of DKP 13 and deuterium insertion.
With a regio and stereoselective deprotonation product in hand,
we extended the oxidation of 13. Screening various oxidants, it was
progressed under O2 condition with high enantioselectivity. Based
on this expected result, a plausible mechanistic pathway became
clear (Scheme 4). The regio and stereoselective deprotonation at
the C3 position proceeded to generate the enolate 15, which under-
went stereoselective oxidation to afford 16, due to the steric repul-
sion of the isobutyl group. Progression of the 2nd oxidation
subsequently afforded the optical active diol 20, following the
reduction of the peroxide 19. As mentioned above, due to the rel-
ative instability of the dienolate, this remarkable double oxidation
was achieved in a highly enantioselective fashion [16].
In our scenario, syn-20 would take a folded conformation due to
a CH/p interaction, and the lone pair of the C6 adjacent nitrogen
and the C6-OH moiety would adopt an antiperiplanar arrange-
ment. This effect would lead to the regio selective iminium cation
formation, followed by the stereoselective addition of MeOH from
the opposite side of the benzyl moiety to afford the desired
transacetalization product syn-22 as a kinetic product. After testing
a variety of conditions, the regio and diastereoselective transac-
etalization was produced under 10% TFA in MeOH at room temper-
2

S. Takahashi, A. Kimishima, T. Hirose et al.
Tetrahedron Letters 67 (2021) 152895
adjusting the conditions, DBU was found to be most effective for
the isomerization via the ketoamide intermediate 23 to afford
the desired anti-24 as a thermodynamic stable conformer. Then,
anti-24 was protected with TMSOTf and the cleavage of benzyl
moieties was performed to give the hydroxymethyl anti-25.
Finally, an optimized combination of TBAF and DIPEA for global
deprotection provided Diatretol (1) in 9 steps with 30% overall
yield (Scheme 5). Although the NMR spectral data from our syn-
thetic sample of (+)-1 ([a] = +7.4 in MeOH) were in reasonable
D
agreement with those reported for the natural product, the optical
rotation data were not identical with the reported data
([
a] = +44.0 in MeOH) [5]. However, the optical rotation data of
D
the natural sample ([
a
] = +7.8 in MeOH) which was obtained by
D
us is in good agreement with that of the synthetic one. Further-
more, further support for the absolute configuration of natural
(+)-1 was obtained by comparison with the chiral HPLC ret ention
analysis with the synthetic (+)-1 (3S, 6R) [20]. Furthermore, the
synthetic (+)-1 (3S, 6R) and natural (+)-1 showed comparable
activity in the comparison of antimalarial activity in vitro and
in vivo [4].
Scheme 4. Regio and stereoselective sequential oxidation to produce the optically
active diol (À)-20.
This divergent synthesis route allows for production of the
three DKP natural products, Lepistamides A (2), B (3) and C (4)
(Scheme 6). Silylation of syn-22, followed by the manipulations
of syn-22, followed by the manipulations of syn-22, followed by
the manipulations of the protecting group afforded 2, double sily-
lation of syn-20 followed by stepwise deprotections gave 3.
Although the final deprotection step proceeded successfully, the
target product 3 could not be isolated as a pure form due toinsta-
bility of 3. Hence, we confirmed the formation of 3 by the compar-
ison with the reported 1H NMR data of 3 [6] without further
purification. In contrast, acylation of syn-22, with subsequent
introduction of a methoxy group gave syn-28. Removal of the
hydroxymethyl group provided 4 via imine formation. Intriguingly,
Lepistamides A (2), B (3) and C (4) were isolated as racemic natural
products [6], whereas our synthetic route enabled to access to each
enantiomer [21].
Scheme 5. Synthesis of (+)-Diatretol (1).
ature to give syn-22 as a single diastereomer in high yield and 95%
ee [17] (Scheme 5). Racemization occurred slightly during this con-
version from (–)-20 (99% ee) to (–)–22 (95% ee), and we therefore
presumed that there was slight equilibrium between N,O-acetal
and ring-opened form (keto-amide form) at the iminium interme-
diate 21. To prove our concept, this transacetalization was carried
out using a substrate manipulated cyclohexyl moiety instead of
phenyl moiety of (–)-20 (see SI for the preparation of the cyclo-
hexyl derivative). As a result, the regioselectively at C6 was can-
celled and two corresponding mono-methoxy N,O-acetal
products (1.1 : 1 as an inseparable mixture) were obtained [18].
This result indicated that the CH/p interaction has a significant
influence on the regioselectivity in this transacetalization process.
In the end game for the synthesis of (+)-1, the isomerization of
the C6 position was needed. We carried out the structure optimiza-
tion and calculated single point energy of the both syn-22 and anti-
24 by DFT calculation at B3LYP/6-31G* level, and concluded that
the anti-24 is –3.5 kcal/mol more stable than syn-22 [19]. This cal-
culation indicated that the CH/p interaction between benzyl and
methoxy is stronger than that between benzyl and isobutyl. After
Scheme 6. Synthesis of Lepistamides A (2), B (3) and C (4).
3

S. Takahashi, A. Kimishima, T. Hirose et al.
Tetrahedron Letters 67 (2021) 152895
Y. Cheah, T. Sakulthaew, J. Chalk, B. Intharabut, K. Silamut, S.J. Lee, B.
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Conclusions
In conclusion, the first and facile enantioselective total synthe-
sis of Diatretol (1) has been achieved in 9 steps with 30% yield, and
the absolute configuration of 1 was determined. The synthetic
approach involves the stereoselective oxidation and regioselective
transacetalization taking full advantage of a folded conformation.
In addition, we applied the newly established synthetic route to
the total synthesis of three analogs, Lepistamides A (2), B (3) and
C (4). We are currently undertaking SAR studies and creating a
library of derivatives and analogs for further biological evaluations.
The results will be reported in due course. Our findings may facil-
itate the development of much needed novel antimalarial agents.
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Declaration of Competing Interest
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¯
We are grateful to Distinguished Emeritus Professor Satoshi O
mura (Kitasato University) for his helpful supports and valuable
suggestions. We thank Dr K. Nagai and Ms. N. Sato (School of
Pharmacy, Kitasato University) for various instrumental analyses.
This study was also partially supported by the Platform Project
for Supporting Drug Discovery and Life Science Research (Basis
for Supporting Innovative Drug Discovery and Life Science
Research (BINDS)) from the Japan Agency for Medical Research &
Development (AMED) under Grant Number JP19am0101096.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
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[16] See the ESI for the chiral HPLC analytical conditions and chromatograms of (–)-
20 and (+)-20.
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[17] See the ESI for the chiral HPLC analytical conditions and chromatograms of (–)-
22 and (+)-22.
[18] Compound 31 (see the ESI), having no CH/
p interaction between C3 and C6
sidechains, gave two isomers 32 and 33 as an inseparable mixture under
transacetalization condition (10% TFA in MeOH, r.t.).
[19] See the ESI for the DFT calculations by Gaussian 03.
[20] See the ESI for the chiral HPLC analytical conditions and chromatograms of
natural (1), (+)-1 and (–)-1.
[21] See the ESI for the optical rotation data.
4