First total synthesis of prunustatin A

Article abstract of DOI:10.1016/j.tetlet.2013.12.106

In this study, we achieved the first total synthesis of natural prunustatin A (1), a novel inhibitor of glucose-regulated protein 78 (GRP78) expression. A key feature included a 15-membered ring macrocyclization, which was successfully accomplished by employing Shiina macrolactonization using 2-methyl-6- nitrobenzoic anhydride (MNBA) and 4-dimethylaminopyridine (DMAP).

Full text of DOI:10.1016/j.tetlet.2013.12.106

Accepted Manuscript
First total synthesis of prunustatin A
Shuhei Yamakoshi, Eiji Kawanishi
PII:
S0040-4039(13)02204-1
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.12.106
TETL 44020
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
31 October 2013
24 December 2013
25 December 2013
Please cite this article as: Yamakoshi, S., Kawanishi, E., First total synthesis of prunustatin A, Tetrahedron
Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.12.106
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First total synthesis of prunustatin A
Shuhei Yamakoshi and Eiji Kawanishi

1
Tetrahedron Letters
journal homepage: www.elsevier.com
First total synthesis of prunustatin A
Shuhei Yamakoshi and Eiji Kawanishi
Medicinal Chemistry Laboratories I, Research Division, Mitsubishi Tanabe Pharma Corporation, 1000, Kamoshida-cho, Aoba-ku, Yokohama 227-0033, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
In this study, we achieved the first total synthesis of natural prunustatin A (1), a novel inhibitor
of glucose-regulated protein 78 (GRP78) expression. A key feature included a 15-membered
ring macrocyclization, which was successfully accomplished by employing Shiina
macrolactonization
using
2-methyl-6-nitrobenzoic
anhydride
(MNBA)
and
4-
Available online
dimethylaminopyridine (DMAP).
2009 Elsevier Ltd. All rights reserved.
Keywords:
GRP78
prunustatin A
15-membered ring
Macrocyclization,
Reformatsky reaction
approaches targeted against cancer. In this letter, we report the
first total synthesis of prunustatin A (1) to confirm its absolute
stereochemistry.
Prunustatin A (1) has been isolated from Streptomyces
Violaceoniger 4521-SVS3 as a novel inhibitor of glucose-
regulated protein 78 (GRP78) expression.1,2 The absolute
structure of 1 was determined by Shin-ya et al. in 2007 (Fig. 1).3
The structure of 1 closely resembles that of neoantimycin family
SW-163A (2),3 including a 15-membered macrocycle and 4 ester
groups.
Our retrosynthetic analysis of compound 1 is shown in
Scheme 1. We decided to construct the 15-membered ring by
using macrolactonization of precursor 5, which would be
prepared from esterification of compounds 6 and 7. The strategy
for the construction of the 15-membered ring involve a key step,
i.e., selection of the O4–C5 bond from four cyclized points (O1–
C2, O4–C5, O8–C9, and O11–C12) as appropriate
macrocyclization in precursor 5, based on a conformational
search and molecular dynamics (MD) simulations using MOE.6
Firstly we calculated conformations against four precursors
corresponding to cyclized points (O1–C2, O4–C5, O8–C9, and
O11–C12) with LowModeMD method. Secondary we selected
each the most stable conformation, and then conducted MD
simulations for 2 ns at 320K against four precursors having the
most stable conformation respectively. In this simulation,
tetrahydrofuran molecules were soaked within 20Å of the center
of a compound to set up a virtual macrocyclization reaction
condition. The calculated simulations confirmed that the shortest
distance of O4–C5 was 1.35Å, and the result would be more
promising than other bonds.7 Following, compound 6 was
expected to be obtained by the Reformatsky reaction of bromide
9 and -benzyloxyaldehyde 10, followed by esterification of -
ketoester 8 and alcohol 7.
GRP78 acts as a molecular chaperone in the endoplasmic
reticulum (ER) to promote protein folding. Recently, it was
reported that GRP78 contributes to tumor growth, resistance of
cancer cells to chemotherapy, and hypoglycemic stress in solid
tumors.4 GRP78 is also utilized as a novel predictive biomarker
to guide physicians in choosing the appropriate treatment for
patients.5 Recent discoveries about the role of GRP78 expression
in cancer cells have led to the development of new therapeutic
Figure 1. Structure of prunustatin A (1) and SW-163A (2).
———
 Corresponding author. Tel.: +81-45-963-7243; fax: +81-45-963-7258; e-mail: kawanishi.eiji@mw.mt-pharma.co.jp

2
Tetrahedron Letters
Scheme 1. Retrosynthetic analysis of Prunustatin A.
The Reformatsky reaction of bromide 98 and -
preliminary study, deprotection of the benzyl group in 8 was
benzyloxyaldehyde
benzyloxyphenyllactate
10,9
prepared
from
methyl
-
with
attempted. However, the desired alcohol was not obtained. In this
reaction, 5-membered lactone 14 was produced exclusively. This
was probably because the lactonization proceeded by the
proximity of O1 to C12 under the Thorpe–Ingold effect of
dimethyl substituents on C13.12 The introduction of S-
benzyloxylactate 16 at O1 was conducted after modification of
tert-butylester of 8. tert-Butylester in 8 was deprotected by
trifluoroacetic acid (TFA); subsequently, the esterification
1110
by
reduction
diisobutylaluminium hydride (DIBAL-H) in 88% yield, afforded
two alcohols S-isomer (12a) and R-isomer (12b) in 60% and 28%
yield, respectively. Each stereochemistry was determined by
conversion of 12b3, 11 to the lactone 13. The next oxidation of
both 12a and 12b using Dess–Martin periodinane (DMP)
obtained the same product 8 in high yield (99% and 95%). In the
Scheme 2. Synthesis of the precusor 5.

3
reaction of the L-threonine derivative 7,13 possessing an 2-
chloroform).1 Thus, the first total synthesis of prunustatin A (1)
was achieved in 11 synthetic steps from the known aldehyde 10.
In conclusion, we successfully performed the first total
synthesis of prunustatin A (1) via the Reformatsky reaction
followed by macrocyclization with Shiina’s protocol. The
macrocyclization precursor was successfully selected by studying
its conformation on the basis of molecular dynamics. The
bioactivity of prunustatin A is currently being investigated.
Acknowledgements
We thank Dr. Masako Okamoto for computational analysis.
We also thank Dr. Kunio Saruta and Dr. Tsuyoshi Ogiku for
providing us special advices.
Supplementary data
Supplementary data associated with this article can be found
in the online version.
References and notes
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K. J. Antibiot. 2007, 60, 640–644.
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6. Molecular Operating Environment (MOE 2011.10),
Chemical Computing Group Inc.
7. The detail was described in supplementary data S9.
8. 9 was prepared from (2S,3S)- tert-butyl 2-hydroxy-3-
methylpentanoate and -bromoisobutyryl bromide. (a)
Aurelio, L; Brownlee, R.; Hughes, A. Australian J. Chem.
2008, 61, 615–629. (b) Inaoka, T.; Tei, Y.; Usuki, Y.; Iio, H.
Abstracts of Papers, 457–461, 52th Symposium on the
Chemistry of Natural Products, Shizuoka, Sep 29–Oct 1,
2010.
Scheme 3. Total synthesis of prunustatin A (1).
(trimethylsilyl)ethoxymethyl (SEM) group, with 2-methyl-6-
nitrobenzoic anhydride (MNBA), triethylamine, and 4-
dimethylaminopyridine (DMAP) afforded 6 in 79% yield. The
benzyl group of 6 was removed using Pearlman’s catalyst to
provide 15 in 95% yield. First, an esterification reaction of 15
using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) or
N,N′-carbonyldiimidazole (CDI) was attempted. The desired 17
was not obtained, although 5-membered lactone 14 was
observed. Meanwhile, an esterification reaction of 15 using an
excess amount of acid chloride of S-benzyloxylactate 1614
afforded 17 in 86% yield without the production of lactone 14.
Deprotection of the benzyl group in 17 achieved by
hydrogenation with a stoichiometric amount of palladium on
carbon afforded the macrocyclization precursor 5 in 96% yield,
in which cleavage of the SEM ether took place simultaneously15
(Scheme 2).
9. Lubin, H.; Tessier, A.; Chaume, G.; Pytkowicz, J.; Brigaud,
T. Org. Lett. 2010, 12, 1496–1499.
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Chem. 2001, 66, 7303–7312.
11. Lactone 13 was introduced from 12b by deprotection of
benzyl group. And the 1H NMR spectrum agreed with that in
the report of the degradation study of SW-163A (2).
12. Beesley, R. M.; Ingold, C. K.; Thorpe, J. F. J. Chem. Soc.,
Trans. 1915, 107, 1080–1106.
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1990, 112, 7659–7672.
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17. (a) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.;
Yamaguchi, M. Bull. Chem. Soc. Jpn. 1979, 52, 1989–1993.
Although a large number of methods are available for
macrolactonization reactions, those most commonly employed
include the Corey–Nicolaou,16 Yamaguchi,17 and Shiina18
lactonization procedures. Firstly compound 5 was subjected to
macrolactonization under Yamaguchi’s condition, however, the
product was only diolide. In this study, macrocyclization of 5
was smoothly achieved by treatment with MNBA and DMAP at
50°C under high dilution (1.2 × 10−3 M) and slow addition (13 h)
of 5, affording 4 in 60% yield. After deprotection of 4,
condensation of the amine derivative with 3-aminosalycilic
moiety
319
using
O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-
tetramethyluronium hexafluorophosphate (HATU) afforded 18 in
95% yield. Finally, reductive removal of the benzyl group using
palladium on carbon provided prunustatin A (1) in 88% yield
(Scheme 3). The spectral data (1H and 13C NMR) of the synthetic
1 were identical to those reported for the natural product 1.1,3 The
28
specific rotation of the synthetic 1 ([]
+35.2, c 0.21,
chloroform) agreed with natural sample ([D]2D7 +21.2, c 0.01,

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Tetrahedron Letters
(b) Barbazanges, M.; Meyer, C.; Cossy, J. Org. Lett. 2008,
10, 4489–4492.
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