Synthesis of Macrocyclic Peptide Analogues of Proteasome Inhibitor TMC-95A

Article abstract of DOI:10.1021/jo035256c

The synthesis of three constrained macrocyclic peptide analogues 1 of TMC-95A as potential proteasome inhibitors is described. The key step involves a Ni(O)-mediated macrocyclization of tripeptides 2 bearing halogenated aromatic side chains for the formation of the biaryl junction. In addition, an enantioselective preparation of L-7-bromotryptophan methyl ester 3 using a Corey-O'Donnell alkylation of the glycine benzophenone imine was achieved in good overall yield with very high ee (>85%) on a multigram scale.

Full text of DOI:10.1021/jo035256c

Syn th esis of Ma cr ocyclic P ep tid e
An a logu es of P r otea som e In h ibitor
TMC-95A
Alexandra Berthelot, Sandrine Piguel,
Gwennae¨l Le Dour, and J oe¨lle Vidal*
Institut de chimie de Rennes, Synthe`se et e´lectrosynthe`se
organiques, UMR 6510, Universite´ de Rennes I, Campus de
Beaulieu, F 35042 Rennes Cedex, France
joelle.vidal@univ-rennes1.fr
Received August 28, 2003
F IGURE 1. Structure of TMC-95A, -B, -C, and -D.
of action.9 The proteasome10 is a multicatalytic protease
which plays a major role in intracellular processes and
its inhibition represents a promising target for drug
development.11,12
TMC-95A has stimulated a great interest in the
scientific community mainly as the result of the impor-
tance of proteasome inhibition but also for its synthetic
challenge. In fact, during the course of our work, several
groups developed either routes to the unusual highly
oxidized L-tryptophan fragment13 or strategies based on
macrolactamization to perform the ring closure of TMC-
95A.14 In 2002, Danishefsky et al. published the first total
synthesis of TMC-95A15,16 while Moroder et al. described
the synthesis of the first analogue.17
As starting point of our work, we decided to focus on
the synthesis of TMC-95A analogues in order to study
their structure-activity relationship. Recently, a crystal
structure showed that TMC-95A was bound to the
trypsin-like site of proteasome by a network of five
hydrogen bonds involving four centers of the TMC-95A
peptide backbone and the C-23 carbonyl of the oxindole.18
Moreover, the two hydroxyl groups at C-6 and C-7 as well
as the ketoamide side chain (C-34 and C-35) do not seem
to be essential for recognition. However the interactions
between TMC-95A and the other active sites still remain
to be established. Therefore, we based the design of TMC-
Abstr a ct: The synthesis of three constrained macrocyclic
peptide analogues 1 of TMC-95A as potential proteasome
inhibitors is described. The key step involves a Ni(0)-
mediated macrocyclization of tripeptides 2 bearing haloge-
nated aromatic side chains for the formation of the biaryl
junction. In addition, an enantioselective preparation of L-7-
bromotryptophan methyl ester 3 using a Corey-O’Donnell
alkylation of the glycine benzophenone imine was achieved
in good overall yield with very high ee (>85%) on
multigram scale.
a
TMC-95A and its diasteromers TMC-95B, -C, and -D,
novel 17-membered macrocyclic peptides, have been
recently isolated from the fermentation broth of Ap-
iospora montagnei Sacc. TC10931 (Figure 1). They con-
tain L-tyrosine, L-asparagine, a highly oxidized L-tryp-
tophan, a C-terminal (Z)-1-propenylamine, an N-terminal
3-methyl-2-oxopentanoyl moiety, and a phenol-oxindole
ring junction. Such a biaryl link is found only in the
natural products neuroprotectin2 or complestatins A and
B.3 The related phenyl-indole ring attachment encoun-
tered in natural products such as chloropeptin,4 kista-
micin,5 complestatin,6 SCH 212394,7 and diazonamide8
is more widespread.
TMC-95A was found to be a very potent, reversible,
and noncovalent inhibitor of the peptidase activities
(chymotrypsin-like, trypsin-like, and caspase-like) of the
proteasome 20S, the last being an unprecedented mode
(9) For reviews on proteasome inhibitors, see: Kisselev, A. F.;
Goldberg, A. L. Chem. Biol. 2001, 8, 739-758; Myung, J .; Kim, K. B.;
Crews, C. M. Med. Res. Rev. 2001, 21, 245-273.
(10) For review on proteasome, see: Groll, M.; Huber, R. Int. J .
Biochem. Cell Biol. 2003, 35, 606-616 and references therein.
(11) Adams, J . Drug Discovery Today 2003, 8, 307-315.
(12) Recently, proteasome inhibitor VELCADE (PS-341 or bort-
ezomib) received FDA approval in the treatment of multiple myeloma,
a type of blood cancer. See http://millennium.com/.
(13) (a) Ma, D.; Wu, Q. Tetrahedron Lett. 2000, 41, 9089-9093. (b)
Albrecht, B. K.; Williams, R. M. Tetrahedron Lett. 2001, 42, 2755-
2757. (c) Ma, D.; Wu, Q. Tetrahedron Lett. 2001, 42, 5279-5281. (d)
Inoue, M.; Furuyama, H.; Sakazaki, H.; Hirama, M. Org. Lett. 2001,
3, 2863-2865.
(1) (a) Koguchi, Y.; Kohno, J .; Nishio, M.; Takahashi, K.; Okuda,
T.; Ohnuki, T.; Komatsubara, S. J . Antibiot. 2000, 53, 105-109. (b)
Kohno, J .; Koguchi, Y.; Nishio, M.; Nakao, K.; Kuroda, M.; Shimizu,
R.; Ohnuki, T.; Komatsubara, S. J . Org. Chem. 2000, 65, 990-995.
(2) Kobayashi, H.; Shin-Ya, K.; Furihata, K.; Najai, K.; Suzuki, K.
I.; Hayakawa, Y.; Seto, H.; Yun, B. S.; Ryoo, I. J .; Kim, J . S.; Kim, C.
J .; Yoo, I. D. J . Antibiot. 2001, 54, 1019-1024.
(3) Singh, S. B.; J ayasuriya, H.; Salituro, G. M.; Zink, D. L.; Shafiee,
A.; Heimbuch, B.; Silverman, K. C.; Lingham, R. B.; Genilloud, O.;
Teran, A.; Vilella, D.; Felock, P.; Hazuda D. J . Nat. Prod. 2001, 64,
874-882.
(4) Matsuzaki, K.; Ikeda, H.; Ogino, T.; Matsumoto, A.; Woodruff,
H. B.; Tanaka, H.; Omura, S. J . Antibiot. 1994, 47, 1173-1174.
(5) (a) Naruse, N.; Tenmyo, O.; Kobaru, S., Hatori, M.; Tomika, K.;
Hamagishi, Y.; Oki, T. J . Antibiot. 1993, 46, 1804-1811. (b) Naruse,
N.; Oka, M.; Konishi, M.; Oki, T. J . Antibiot. 1993, 46, 1812-1818.
(6) Seto, H.; Fujioka, T.; Furihata, K.; Kaneko, I.; Takahashi, S.
Tetrahedron Lett. 1989, 30, 4987-4990.
(7) Hedge, V. R.; Puar, M. S.; Patel, M.; Gullo, V. P.; Pramanick,
B.; J enh C. H. Tetrahedron Lett. 2002, 43, 5339-5341.
(8) Lindquist, N.; Fenical, W.; Van Duyne, G. D.; Clardy, J . J . Am.
Chem. Soc. 1991, 113, 2303-2304.
(14) (a) Lin, S.; Danishefsky, S. J . Angew. Chem., Int. Ed. 2001, 40,
1967-1970. (b) Albrecht, B. K.; Williams, R. M. Org. Lett. 2003, 5,
197-200.
(15) Lin, S.; Danishefsky, S. J . Angew. Chem., Int. Ed. 2002, 41,
512-515.
(16) During the preparation of our manuscript, a second total
synthesis of TMC-95A was published. Inoue, M.; Sakazaki, H.;
Furuyama, H.; Hirama, M. Angew. Chem., Int. Ed. 2003, 42, 2654-
2657.
(17) Kaiser, M.; Groll, M.; Renner, C.; Huber, R.; Moroder, L. Angew.
Chem., Int. Ed. 2002, 41, 780-783.
(18) Groll, M.; Koguchi, Y.; Huber, R.; Kohno, J . J . Mol. Biol. 2001,
311, 543-548.
10.1021/jo035256c CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/11/2003
J . Org. Chem. 2003, 68, 9835-9838
9835

SCHEME 1. Retr osyn th etic P a th w a y
SCHEME 2. Syn th esis of th e P r ecu r sor 8
SCHEME 3. Syn th esis of 7-Br om otr yp top h a n
Der iva tive 3
95 analogues on retaining the peptide structure but
replacing the oxindole with a simple indole. We identified
the general target structures 1 with two main degrees
R1 and R2 of molecular diversity. Our retrosynthetic
strategy for the macrocycles 1 is indicated in Scheme 1.
It is relied on assembling the peptide backbone 2 first
and then closing the ring by an intramolecular biaryl
carbon-carbon reaction. This route requires three R-ami-
no acids building blocks (3-5) as starting materials and
therefore opens the possibility of generating a library of
analogues for further biological assays by varying R1, R2,
R3, and R4.
Here, we present our contribution to the chemistry of
TMC-95s and describe the convergent synthesis of three
novel macrocycles TMC-95A analogues 1 using an in-
tramolecular biaryl coupling as the key step for perform-
ing the macrocyclization.
In the beginning, our efforts were focused on the
enantioselective synthesis of unusual tryptophan deriva-
tive 3. Our strategy was to form the indole nucleus of 3
prior to introduce the asymmetric carbon via an enanti-
oselective alkylation by phase-transfer catalysis.
As depicted in Scheme 2,7-bromoindole 6 was syn-
thesized by treatment of 1-bromo-2-nitrobenzene with
vinylmagnesium bromide in 46% yield according to
the Bartoli procedure.19 Subsequent formylation under
Vilsmeier-Haack conditions led to 7-bromo-3-formyl-
indole 7 in 89% yield.20 Following the initial protection
of the indole nitrogen by a Boc group, reduction of
the aldehyde function afforded alcohol 8 in 84% overall
yield.
The protection of the indole nitrogen proved to be
crucial for the stability in the subsequent steps. Bromi-
nation of the alcohol derivative 8 was performed with Br2/
PPh3 in CH2Cl2 (Scheme 3). The resulting bromo com-
pound 9 was found to be extremely unstable and was
always freshly prepared just before use. According to
Corey’s procedure,21 alkylation of N-(diphenylmethylene)-
glycine tert-butyl ester with bromide 9 in the presence
of O-9-allyl-N-(9-anthracenylmethyl)cinchonidinium bro-
mide 10 gave the imine 11. Then, hydrolysis of the
imine function under acidic conditions (AcOH/THF/H2O)
provided the fully protected 7-bromotryptophan 12. Start-
ing from the alcohol 8, the yield over three steps was
excellent: 75% with an enantiomeric excess of 87%.22
This sequence was scaled up to 5 g. The absolute
configuration was established experimentally as being S
by conversion of 12 to the known L-7-bromotrypto-
(19) (a) Bartoli, G.; Palmieri, G. Tetrahedron Lett. 1989, 30, 2129-
2132. (b) Bartoli, G.; Bosco, M.; Dalpozzo, R.; Palmieri, G.; Marcantoni,
E. J . Chem. Soc., Perkin Trans. 1 1991, 2757-2761.
(20) Sundberg R. J . Best Synthetic Methods: Indoles Academic
Press: New York, 1996. (b) Ohta, T.; Yamato, Y.; Tahira, H.; Somei,
M. Heterocycles 1987, 26, 2817-2822.
(21) (a) Corey, E. J .; Xu, F.; Noe, M. C. J . Am. Chem. Soc. 1997,
119, 12414-12415. (b) O’Donnell, M. J . Aldrichim. Acta 2001, 34,
3-15.
(22) Enantiopurity was determined by HPLC; see the Supporting
Information.
9836 J . Org. Chem., Vol. 68, No. 25, 2003

SCHEME 4. P r ep a r a tion of Dip ep tid es 16
SCHEME 5. Access to th e Ma cr ocycles 1
phan and comparison of its optical rotation with the
literature value.23
HCl-saturated MeOH was then used to remove the tert-
butyl protecting groups, and transesterification proceeded
to give the amino methyl ester 3 as its hydrochloride salt
quantitatively.
system of complestatin.29 Alternatively, we investigated
a nickel(0) intramolecular cross-coupling reaction to form
the C-C bond between the two aromatic rings. The
reaction was carried out at 50 °C using freshly prepared
Ni(0).30 The outcome of the reaction was encouraging
since two desired constrained 17-membered cycles, 1a a
and 1ba , were formed in 13% and 4% respectively.
Varying the conditions (high dilution or ligands) did not
improve the yields, and reduced compounds were ob-
tained as byproducts. Application of the same conditions
to the Asn-derivative 2ca gave only a mixture of reduced
linear tripeptides.31 Because of the steric hindrance of
the benzyl protecting groups,32 we turned our attention
to less bulky substituted derivatives such as the methyl-
protected tripeptide 2bb. Treatment of the linear precur-
sor 2bb with the zerovalent nickel complex at 50 °C in
DMF afforded the TMC-95A analogue 1bb with 8% yield.
The constrained nature of the macrocycles 1 probably due
to the sp2 carbon at C-6 position can explain the low
yields observed during the macrocyclization.33 These
results also reflect the difficulty of macrocyclization by
biaryl coupling which had been already reported regard-
ing similar structures.34
The second key intermediates of our convergent syn-
thesis, dipeptides 16, were synthesized by standard
procedures of peptide chemistry24 (Scheme 4).
Commercially available activated ester 13a 25 was
coupled to the methyl amino ester hydrochloride salts
14a , b, or c to form the corresponding dipeptides 15a a ,
15ba , and 15ca in, respectively, 100%, 96%, and 95%
yields. Subsequent selective iodination ortho to the
benzyloxy group was achieved in good yields using I2/
Ag2SO4 in ethanol26 to afford the resulting iodo com-
pounds 16a a , 16ba , and 16ca .
Quantitative saponification of esters 16 followed by
coupling with protected bromotryptophan 3 in the pres-
ence of EDC/HOBt afforded the linear tripeptides 2a a ,
2ba , 2ca , and 2bb in, respectively, 70%, 81%, 47%, and
71% (Scheme 5).
With the acyclic precursors 2 in hand, we next turned
our attention to the formation of the 17-membered
macrocycle by biaryl junction. First, the macrocyclization
of 2a a or 2ba was attempted in a one-pot Suzuki27 cross-
coupling reaction via in situ arylboronate formation.28
Under a set of conditions varying the source of palladium,
the solvent, and the base in the presence of the pinacol
ester of diboronic acid, no cyclized product could be
detected although biaryl coupling under Suzuki condi-
tions was successful with the similar 17-membered ring
The biological activity of the macrocycles 1 and acyclic
precursors 2 is currently under evaluation. Further
studies on the structure-activity relationship are under
investigation in order to probe the importance of the
(29) Elder, A. M.; Rich, D. H. Org. Lett. 1999, 1, 1443-1446.
(30) (a) Semmelhack, M. F.; Helquist, P.; J ones, L. D.; Keller, L.;
Menelson, L.; Speltz Ryono, L.; Gorzynski Smith, J .; Stauffer, R. D. J .
Am. Chem. Soc. 1981, 103, 6460-6471. (b) Kende, A. S.; Liebeskind
L. S.; Braitsch, D. M. Tetrahedron Lett. 1975, 39, 3375-3378.
(31) HRMS (ESI) measurements of the mixture showed two peaks
corresponding to the mono- (calcd for [C37H42N5O8Br + Na]+ 786.2114,
found 786.2119) and direduced (calcd for [C37H43N5O8 + Na]+ 708.3009,
found 708.3008) compounds.
(23) (a) Allen, M. C.; Brundish, D. E.; Wade, R. J . Chem. Soc., Perkin
Trans. 1 1980, 1928-1932. (b) Konda-Yamada, Y.; Okada, C.; Yoshida,
K.; Umeda, Y.; Arima, S.; Sato, N.; Kai, T.; Takayanagi, H.; Harigaya,
Y. Tetrahedron 2002, 58, 7851-7861.
(24) Bodansky, M.; Bodansky, A. The practice of peptide synthesis,
2nd rev. ed.; Springer-Verlag: Berlin, New York, 1994.
(25) Williams, E. B.; Krishnaswamy, S.; Mann, K. G. J . Biol. Chem.
1989, 264, 7536-7545.
(26) (a) Sy W.-W. Tetrahedron Lett. 1993, 34, 6223-6224.
(27) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457-2483.
(28) (a) Giroux, A.; Han, Y.; Prasit, P. Tetrahedron Lett. 1997, 38,
3841-3844. (b) Baudoin, O.; Gue´nard D.; Gue´ritte, F. J . Org. Chem.
2000, 65, 9268-9271. (c) Carbonnelle, A.-C.; Zhu, J . Org. Lett. 2000,
2, 3477-3480.
(32) Carruthers, W.; Coggins, P.; Weston, J . B. J . Chem. Soc., Perkin
Trans. 1 1991, 611-616.
(33) The presence of an sp2 carbon at the C-6 position in TMC-95
precursor or analogue prevented macrolactamization from occurring.
See ref 14a and: Kaiser, M.; Milbradt, A. G.; Moroder, L. Lett. Pept.
Sci. 2002, 9, 65-70.
(34) See, for instance: (a) Beugelmans, R.; Roussi, G.; Gonza´lez
Zamora, E.; Carbonnelle, A.-C. Tetrahedron 1999, 55, 5089-5112. (b)
Roussi, G.; Gonza´lez Zamora, E.; Carbonnelle, A.-C.; Beugelmans, R.
Heterocycles 1999, 51, 2041-2063. (c) Bagley, M. C.; Hind, S. L.; Moody,
C. J . Tetrahedron Lett. 2000, 41, 6897-6900.
J . Org. Chem, Vol. 68, No. 25, 2003 9837

macrocyclic structure and the presence of the oxindole
moiety in the binding properties to the proteasome.
In summary, we have synthesized three novel TMC-
95A analogues using a Ni(0)-mediated ring-closure strat-
egy. Further optimization of the key step and application
to the synthesis of more TMC-95A derivatives are being
actively pursued in our laboratory.
de la jeunesse, de l’e´ducation nationale et de la recher-
che for a doctoral fellowship. We also thank the Centre
re´gional de mesures physiques de l’Ouest for its prompt-
ness in HR-mass spectra measurements.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and compound characterization data for all new
compounds. Copies of 1H and 13C NMR spectra of compounds
7, 12, 3, 2a a , 2ba , 2ca , 2bb, 1a a , 1ba , and 1bb. This material
is available free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. CNRS (UMR 6510) and Univer-
site´ de Rennes 1 (BQR grant) are gratefully acknowl-
edged for financial support. A.B. thanks the Ministe`re
J O035256C
9838 J . Org. Chem., Vol. 68, No. 25, 2003