Total Synthesis of Tunicamycin v

Article abstract of DOI:10.1021/acs.orglett.7b03623

The total synthesis of tunicamycin V is described. This strategy is based on the initial construction of tunicaminyluracil, which is regarded to play an important role in the observed biological activities. The key to the synthesis was a Mukaiyama aldol reaction followed by a furan-oxidation to construct the undecose skeleton, a [3,3] sigmatropic rearrangement of a cyanate, and a highly selective trehalose-type glycosylation.

Full text of DOI:10.1021/acs.orglett.7b03623

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis of Tunicamycin V
Kazuki Yamamoto,† Fumika Yakushiji,†,‡ Takanori Matsumaru,†,‡ and Satoshi Ichikawa*,†,‡
†Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan
‡Center for Research and Education on Drug Discovery, Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6,
Kita-ku, Sapporo 060-0812, Japan
S
* Supporting Information
ABSTRACT: The total synthesis of tunicamycin V is described.
This strategy is based on the initial construction of
tunicaminyluracil, which is regarded to play an important role
in the observed biological activities. The key to the synthesis was
a Mukaiyama aldol reaction followed by a furan-oxidation to
construct the undecose skeleton, a [3,3] sigmatropic rearrange-
ment of a cyanate, and a highly selective trehalose-type
glycosylation.
unicamycins1−3 (Figure 1) are nucleoside natural products
Scheme 1. Retrosynthetic Analysis of Tunicamycin V (1)
T
isolated from the fermentation broths of Streptomyces
Figure 1. Structure of tunicamycins.
lysosuperficus in 1971 and exhibit a variety of biological properties
including antibacterial, antiviral, antifungal, and antitumor
activities. Tunicamycins strongly inhibit UDP-N-acetylglucos-
amine (GlcNAc): polyprenol phosphate translocase, the enzyme
responsible for the first N-acetylglucosamination of the N-linked
glycopeptide in endothelial reticulum (ER).4,5 Treatment of
eukaryotic cells with tunicamycins results in a complete
truncation of the oligosaccharides from N-linked glycopeptides
causing ER stress and ultimately cell death. Tunicamycins also
inhibit prokaryotic phospho-N-acetylmuraminic acid (Mur-
NAc)-pentapeptide translocase (MraY) responsible for the
biosynthesis of peptidoglycan and undecaprenyl-phosphate α-
N-acetylglucosaminyl 1-phosphate transferase (WecA) for
lipopolysaccharide and enterobacterial common antigen syn-
thesis. MraY catalyzes the reaction between UDP-MurNAc-
pentapeptide (Park’s nucleotide) and undecaprenyl mono-
phosphate, providing lipid I. MraY is an essential enzyme in
bacteria and a good target for antibacterial drug discovery.6 Their
chemical structure is divided into three moieties that include
GlcNAc, an amide-linked fatty acyl side chain, and tunicaminy-
luracil (2, Scheme 1), where a uracil base is attached to an
aminoundecose constructing a linked ribofuranosylgalactopyr-
anosamine. The tunicaminyluracil and GlcNAc are connected by
an 11′-β-1″-α-trehalose-type linkage. It is suggested that the
structure of tunicamycins closely resembles the transition state of
the transfer reaction of UDP-sugar onto a phospholipid catalyzed
by the translocases. Therefore, the tunicaminyluracil moiety is
Received: November 21, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03623
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Letter
Scheme 2. Total Synthesis of Tunicamycin V (1)
regarded as a key scaffold of tunicamycins in biological
properties, and the fatty acyl side chain as well as the GlcNAc
moiety act as accessory motifs. Their structural complexity
renders them worthy targets for organic synthesis. The total
synthesis of tunicamycins has been accomplished by the groups
of Suami,7 Myers,8 and Li,9 and other synthetic studies of related
compounds have also been reported.10−15 In spite of much effort
to synthesize tunicamycins, there are only a few studies regarding
their analogue synthesis, and their structure−activity relation-
ships have yet to be properly explored.16 In all the previous total
syntheses of 1, the galactosamine moiety was derived from D-
galactose.7−9 We planned de novo construction of the galactos-
amine moiety, which could provide novel access to both
functionality and stereochemistry unavailable in past syntheses.
Here, we describe an efficient total synthesis of tunicamycin V
(1). Our retrosynthetic analysis of 1 is shown in Scheme 1.
Considering the tunicaminyluracil moiety acting as a mimic of
UDP, it was desirable to first construct 2 and subsequently install
the GlcNAc moiety 3 and the fatty acyl side chain 48 in the final
stages of the synthesis from the viewpoint of medicinal
chemistry. We planned to construct the galactosamine moiety
from furyl alcohol 7 via oxidative reconstruction of the
dihydropyran (Achmatowicz rearrangement17) followed by
[3,3] sigmatropic rearrangement of the allylcyanate generated
from allylcarbamate 6. The furan 7 was disconnected to the silyl
enol ether 818 and uridine-5′-aldehyde derivative 9.
B
DOI: 10.1021/acs.orglett.7b03623
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The synthesis of 1 is shown in Scheme 2. IBX oxidation of the
5′-hydroxy group of the suitably protected uridine derivative
1019 provided the corresponding aldehyde 9. Mukaiyama-type
aldol reaction of 9 was conducted with the silyl enol ether 8 and
BF3·OEt2 in CH2Cl2 at −78 °C to afford the desired 5′R-product
11 in 64% yield over two steps with high stereoselectivity (5′R/
5′S = 98:2). Presumably, the Mukaiyama-type aldol reaction of 8
and 9 promoted by BF3·OEt2 proceeded via an open transition
state of the Felkin−Ahn model.20 In the case of the reaction using
SnCl4 as a Lewis acid, the undesired 5′S-product was obtained as
a major product (75% yield, 5′R/5′S = 4:96, see Supporting
Information (SI)). It should be noted that a Mukaiyama-type
aldol reaction was utilized to form the C5′−C6′ bond in the Li’s
synthesis.9 Interestingly, the reaction with SnCl4 provided the
desired 5′R-product with their substrates, in contrast with our
findings. Although anti-reduction21 of the 7′-oxo group of 11 was
first conducted by a treatment with NaBH(OAc)3 in CH2Cl2, the
desired anti-diol was not obtained because of the decomposition
of the substrate. Extensive efforts were conducted to find that
after protection of the 5′-hydroxy group by the MOM group, the
7′-oxo group of 12 could be stereoselectively reduced by (S)-Me-
CBS22 and BH3·SMe2 in THF to give the anti-product 7 in 93%
as a sole product. At this stage, the absolute stereochemistry at
the 7′-position was determined by the Mosher’s method for 7
(for details, see SI). Oxidation of the furan ring in 7 by m-CPBA
cleanly proceeded to provide the lactol 13 in 92% yield, and
protection of the resulting 11′-hydroxyl group by the TBS group
followed by Luche reduction23 of the enone gave allyl alcohol 14
in 85% yield over two steps as a mixture of anomers at the 11′-
position. The resulting allyl alcohol 14 was converted to the
carbamate 6 by way of N-trichloroacetylcarbamate
(CCl3CONCO, CH2Cl2, 0 °C) upon deacylation (K2CO3, aq.
MeOH).24 Dehydration of 6 resulted in clean conversion to the
corresponding allylcyanate ((CF3CO)2O, Et3N, CH2Cl2, 0 °C),
which underwent [3,3] sigmatropic rearrangement to give the
10′-isocyanate 5. To achieve a high facial selectivity in the
following dihydroxylation, a construction of the cis-cyclic
carbamate-fused pyranose was planned to hinder the α-face of
the pyrane ring. Entrapment of the isocyanate in 5 by the
adjacent hydroxyl group upon removal of the TBS group by
tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF)
afforded 15 in 71% yield over four steps from 14. The following
dihydroxylation proceeded stereoselectively at the convex face to
give the diol 16 (K2OsO4, NMO, aq. acetone, 71%). The
stereochemistry of 16 was determined by its conversion to the
known heptaacetate of tunicaminyluracil8 (see SI).
Scheme 3. Dihydroxylation of 23
the 11′-β-1″-α-trehalose-type linkage out of four possible
anomers is a challenging task. Tunicaminyluracil derivative 18
exists as an equilibrium mixture of anomers at the 11′-position,
and the 11′-β-anomer, which is the same stereochemistry as 1, is
predominant in CDCl3 (α/β = 1:10). Thus, 18 was used as a
glycosyl acceptor and reacted with a suitably protected 1-O-
trichloroacetimido-2-azidoglucose derivative 19 as a glycosyl
donor to construct the 11′-β-glycoside bond via neighboring
group participation and the 1″-α-glycoside bond via anomeric
effect as previously investigated by Myers.8 Optimization of the
glycosylation conditions was investigated (see SI). The use of
Et2O as a solvent improved the selectivity and treatment of 18
and 19 with TfOH in Et2O at 0 °C provided the desired 11′-β-1″-
α-glycoside 20 in 84% yield in a highly stereoselective manner
(11′-β-1″-α/11′-β-1″-β = 14:1). The observed stereoselectivity
in this study was the highest among previously reported total
synthesis of 1.7−9 Alternatively, reversal of a glycosyl donor/
acceptor was also investigated to construct the 11′-β-1″-α-
trehalose-type linkage (Scheme 4). The hemiacetal 18 was
Scheme 4. Alternative Glycosylation
Dihydroxylation of the N-benzylcarbamate 23, which was
obtained by treating 5 with BnOH, was also conducted.
However, its facial selectivity was poor, and the desired
diastereomer 24 was obtained as a minor product (Scheme 3).
As expected, the observed high facial selectivity in the
dihydroxylation of 15 was induced by the cis-cyclic carbamate
moiety, which hinders the α-face of the pyran ring. The diol and
the carbamate in 16 were sequentially protected with the
isopropylidene (2,2-dimethoxypropane, BF3·OEt2, acetone,
75%) and Ns25 groups (NsCl, NaH, THF, 97%), respectively,
to give 17. Hydrolysis of the cyclic carbamate of 17 (LiOH, aq.
THF) followed by protecting group manipulation (PhSH,
K2CO3, MeCN, 86% over two steps, N-EtO2C-phthalimide,
Et3N, THF, 82%) gave an orthogonally protected tunicaminylur-
acil 18.
transformed to a trichloroacetimidate 26, which is a glycosyl
donor (Cl3CCN, DBU, MeCN). The trichloroacetimidate 26
was reacted with a suitably protected 2-azidoglucose 27 (α/β =
3:4 in CDCl3) in the presence of TfOH in CH2Cl2. However, the
undesired 11′-β-1″-β glycoside 28 was obtained in 54% yield, and
the desired 20 was not observed at all. The azide group of 20 was
transformed to an acetamide group (AcSH, pyridine, 94%) to
give 21 with clean conversion. The phthaloyl group at the
GalNAc moiety of 21 was removed by ethylenediamine in EtOH,
With 18 in hand, the union of three components to complete
the total synthesis of 1 was investigated. Selective construction of
C
DOI: 10.1021/acs.orglett.7b03623
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Letter
and the liberated amine was acylated with 48 using EDCI and
DMAP in CH2Cl2 to give 22 in 54% yield over two steps. Finally,
global deprotection of six acid labile protecting groups by BCl3 in
CH2Cl2 followed by quenching with NaOMe resulted in clean
conversion and successfully afforded 1 in 97% yield. The
analytical data for synthetic 1 were in good agreement with the
previously reported data.8,9
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In conclusion, a total synthesis of tunicamycin V has been
accomplished. The key to the synthesis was a diastereoselective
Mukaiyama aldol reaction followed by furan-oxidation to
construct an undecose skeleton, cyclic carbamate formation by
[3,3] sigmatropic rearrangement of a cyanate followed by
intramolecular entrapment of the resulting isocyanate, and
stereoselective construction of the 11′-β-1″-α-trehalose-type
linkage. Tunicamycin V is readily accessible via the longest linear
sequence of 24 synthetic steps from uridine and commercially
available simple materials, in an overall chemical yield of 3.9%.
The complex structure with tunicamycins binding to MraY from
Clostridium bolteae has recently been elucidated,26 and this could
be utilized for the development of analogues. Our strategy is
based on initial construction of tunicaminyluracil, which is
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a variety of biological activities. Alterations to the uridine moiety9
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lipid moieties in the last stage of the synthesis could provide a
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ASSOCIATED CONTENT
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* Supporting Information
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Detailed experimental procedures and characterization of
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ichikawa@pharm.hokudai.ac.jp.
ORCID
(b) Miguel
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Satoshi Ichikawa: 0000-0001-5345-5007
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We wish to thank Ms. S. Oka (Center for Instrumental Analysis,
Hokkaido University) for measurement of the mass spectra. This
research was supported by JSPS Grant-in-Aid for Scientific
Research (B) (to S.I., Grant Number 16H05097) and by Astellas
Foundation for Research on Metabolic Disorders and partly
supported by Hokkaido University, Global Facility Center
(GFC), Pharma Science Open Unit (PSOU), funded by
MEXT under “Support Program for Implementation of New
Equipment Sharing System”, 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 and Development
(AMED).
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, G.; Chen, H.; Snijder, A.; Ek,
̈
D
DOI: 10.1021/acs.orglett.7b03623
Org. Lett. XXXX, XXX, XXX−XXX