A concise formal total synthesis of TMC-95A/B proteasome inhibitors.

Article abstract of DOI:10.1021/ol0272545

[reaction: see text] A formal total synthesis of proteasome inhibitors TMC-95A/B is described. The synthesis features a stereoselective modified Julia olefination and a diastereoselective dihydroxylation to construct the highly oxidized tryptophan residue.

Full text of DOI:10.1021/ol0272545

ORGANIC
LETTERS
2003
Vol. 5, No. 2
197-200
A Concise Formal Total Synthesis of
TMC-95A/B Proteasome Inhibitors
Brian K. Albrecht and Robert M. Williams*
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523
rmw@chem.colostate.edu
Received November 8, 2002
ABSTRACT
A formal total synthesis of proteasome inhibitors TMC-95A/B is described. The synthesis features a stereoselective modified Julia olefination
and a diastereoselective dihydroxylation to construct the highly oxidized tryptophan residue.
TMC-95 A-D (1-4, Figure 1) are potent proteasome
inhibitors isolated from the fermentation broth of Apiospora
cally active against chymotrypsin-like, trypsin-like, and
peptidylglutamyl-peptide hydrolyzing proteases.1b Protea-
some inhibitors have received considerable attention recently
due to the role they play in intracellular processes such as
cell progression, antigen presentation, and cytokine-stimu-
lated signal transduction. In addition, proteasome inhibitors
are proving to be valuable tools for probing the function of
the proteasome in cells.2
The great interest emerging in the field of proteasome
inhibition, the considerable biological activity, and the
distinctive structures of the TMC-95 class of natural products
have provided motivation to contemplate a total synthesis
of these compounds. Recently, it has been determined that
TMC-95A displays noncovalent and reversible inhibition of
the proteasome, a mode of action not observed until recently
with other inhibitors.3 With this in mind, our goal was to
establish a concise and convergent total synthesis that would
be ammenable to the preparation of a variety of analogues
that could exploit TMC-95s mode of action. Immediately
following the publication of the structures of these novel
Figure 1. Structures of TMC-95 A-D.
montagnei Sacc. TC 1093, derived from soil samples.1 These
natural products are unique cyclic peptides containing
L-tyrosine, L-asparagine, a highly oxidized L-tryptophan, (Z)-
1-propenylamine, and 3-methyl-2-oxopentanoic acid units.
It has been demonstrated that these compounds are biologi-
(2) (a) Groll, M.; Kim, K. B.; Kairies, N.; Huber, R.; Crews, C. M. J.
Am. Chem. Soc. 2000, 122, 1237. (b) Peters, J. M. Trends. Biochem. Sci.
1994, 19, 377. (c) Kisselev, A. F.; Goldberg, A. L. Chem. Biol. 2001, 8,
739.
(3) Groll, M.; Koguchi, Y.; Huber, R.; Kohno, J. J. Mol. Biol. 2001,
311, 543.
(1) (a) Khono, J.; Koguchi, Y.; Nishio, M.; Najao, K.; Juroda, M.;
Shimizu, R.; Ohnuki, T.; Komatsubara, S. J. Org. Chem. 2000, 65, 990.
(b) Koguchi, Y.; Khono, J.; Nishio, M.; Takahashi, K.; Okuda, T.; Ohnuki,
T.; Komatsubara, S. J. Antibiot. 2000, 53, 105.
10.1021/ol0272545 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/21/2002

cyclic peptide natural products, significant synthetic activity
in this field commenced.4
commercially available 3-iodotyrosine 8. Modified Julia6
olefination of a heteroaromatic sulfone 10 and readily
available 7-substituted isatin7 11 would in turn furnish
oxindolene 7.
Synthesis of the highly oxidized tryptophan moiety began
with treatment of readily available N-Cbz-serine methyl ester
12 under Mitsunobu8 conditions with 2-mercaptobenza-
thiazole (BTSH), DIAD, and PPh3 to furnish S-heteroaro-
matic cysteine derivative 13 (Scheme 1). Completion of the
In this paper, we describe a stereocontrolled approach to
the core macrocycle of TMC-95A/B. Although the pioneer-
ing work of Danishefsky, Hirama, and Ma proved to be an
invaluable resource in our synthesis, we felt that the number
of synthetic steps4c and the lack of stereocontrol in the
construction of the oxidized tryptophan moiety had to be
addressed.4a,b,e,f Our approach is concise and provides a
stereocontrolled route to the dihydroxylated oxindole frag-
ment. Also, we have been able to intercept a late-stage
intermediate in the Danishefsky synthesis and this report thus
constitutes a formal total synthesis of TMC-95A/B.
Scheme 1a
When contemplating the total synthesis of TMC-95A/B,
we felt that these natural products could ultimately be
prepared from a macrocylic peptide such as 5 (Figure 2).
a Reaction conditions: (a) BTSH, DIAD, PPh3, THF, rt, 89%;
(b) CaCl2, NaBH4, THF, 0 °C, and then 13, 95%; (c) 2,2-
dimethoxypropane, p-TsOH, CH2Cl2, rt; (d) Mo7O24(NH4)6‚4H2O,
H2O2, EtOH, 77%, two steps; (e) LiHMDS, DMF, DMPU, 0 °C,
79%, E:Z ) 5:1.
modified Julia coupling partner was accomplished by (1)
reduction of the methyl ester with Ca(BH4)2, (2) blocking
of the carbamate nitrogen and the primary alcohol as the
acetonide with DMP and p-toluenesulfonic acid, and (3)
oxidation9 of the thioether to sulfone 14.
Next, our efforts were focused on optimizing the modified
Julia coupling reaction between sulfone 14 and 7-iodoisatin
15. It was determined that conditions similar to those reported
by Jacobsen10 and co-workers gave the best selectivity in
the modified Julia olefination, furnishing the desired oxin-
dolene 16 in 79% yield. By increasing the reaction temper-
ature to 0 °C, we were able to increase the selectivity to 5:1
(E:Z), yielding the thermodynamically favored product.
With alkene 16 and aryl stannane 64d in hand, attempts
were made at constructing the biaryl moiety of the TMC-95
proteasome inhibitors under the Stille conditions developed
earlier in our laboratory.4d Despite extensive experimentation,
we found that numerous combinations of Pd-catalyst and
ligand gave unsatisfactory yields of biaryl product 18
Figure 2. Retrosynthetic Analysis.
Stille coupling5 of aryl stannane 6 with tryptophan moiety 7
followed by oxidation to the diol, peptide formation with
asparagine derivative 9, deprotection, and macrolactamization
was anticipated to furnish the requisite macrocycle 5. We
envisioned that aryl stannane 6 would be derived from
(4) For synthetic efforts on TMC-95s, see: (a) Lin, S.; Danishefsky, S.
J. Angew. Chem., Int. Ed. 2002, 41, 512. (b) Lin, S.; Danishefsky, S. J.
Angew. Chem., Int. Ed. 2001, 40, 1967. (c) Inoue, M.; Furuyama, H.;
Sakazaki, H.; Hirama, M. Org. Lett. 2001, 3, 2863. (d) Albrecht, B. K.;
Williams, R. M. Tetrahedron Lett. 2001, 42, 2755. (e) Ma, D.; Wu, Q.
Tetrahedron Lett. 2001, 42, 5279. (f) Ma, D.; Wu, Q. Tetrahedron Lett.
2000, 41, 9089. (g) Karatjas, A. G.; Feldman, K. S. Abstracts of Papers,
223rd National Meeting of the American Chemical Society, Orlando, FL,
April 7-11, 2002; American Chemical Society: Washington, DC; ORGN-
400. (h) Albrecht, B. K.; Williams, R. M. Abstracts of Papers, 224th ACS
National Meeting, Boston, MA, United States, August 18-22, 2002, ORGN-
819.
(6) (a) Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morley, A. Synlett
1998, 28. (b) Baudin, J. B.; Hareau, G.; Julia, S. A.; Ruel, O. Tetrahedron
Lett. 1991, 32, 1175. (c) Julia, M.; Paris, J.-M. Tetrahedron Lett. 1973, 14,
4833.
(7) (a) Sandmeyer, T. HelV. Chim. Acta 1919, 2, 234. (b) Marvel, C. S.;
Hiers, G. S. Organic Syntheses; Wiley: New York, 1941; Collect. Vol. I,
p 327. (c) Lisowski, V.; Robba, M.; Rault, S. J. Org. Chem. 2000, 65,
4193.
(8) Mitsunobu, O. Synthesis 1981, 1.
(9) Schultz, H. S.; Freyermuth, H. B.; Buc, S. R. J. Org. Chem. 1963,
28, 1140.
(5) (a) Farina, V.; Krishnamurthy, V.; Scott, W. J. 1997, 50, 1-652. (b)
Stille, J. K., Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(10) Liu, P.; Jacobsen, E. N. J. Am. Chem. Soc. 2001, 123, 10772.
198
Org. Lett., Vol. 5, No. 2, 2003

Incorporation of the asparagine residue was readily ac-
complished first via saponification of methyl ester 18. The
resulting carboxylic acid was coupled to NH2-Asn-OBn
ester13 mediated by HOAt and EDCI in CH2Cl2 to give the
pseudotripeptide 23 (98% yield from 18).
Scheme 2
Treatment of alkene 23 with OsO4 in aqueous pyridine
afforded diol 24 as a single diastereomer in 87% yield
(Scheme 4). Treatment of this substance with a 1:1 mixture
Scheme 4a
(Scheme 2). The best isolated yield of coupled product 18
was ∼20%, which was routinely accompanied by side-
products resulting from alkyl group transfer from the
stannane (19) and reductive removal of the iodine atom (20).
Due to the fact that the Stille coupling gave undesired side
products and insufficient yields, we decided that the
Suzuki4a,b,11 coupling was the next logical choice for con-
structing the biaryl bond.
Treatment of tyrosine derivative 214d with bis(pinacolato)-
diboron, Pd(dppf)Cl2, and KOAc in DMSO via the Miyaura
protocol12 gave boronic ester 22 (Scheme 3). Treatment of
boronic ester 22 under Suzuki conditions with aryl iodide
16 and K2CO3 in refluxing aqueous DME catalyzed by
Pd(dppf)Cl2 gave the desired biaryl product 18 in 90% yield.
a Reaction conditions: (a) OsO4, py., H2O, 0 °C, then NaHSO3,
THF, MeOH, (87%); (b) TFA, H2O, 1:1; (c) 3-methyl-2-oxopen-
tanoic acid, HOAt, EDCI, THF, 98%, two steps; (d) Pd black, H2,
EtOH; (e) EDCI, HOAt, CH2Cl2, DMF (1:1), 1 µM, 49%, two steps.
of trifluoroacetic acid-water resulted in the liberation of the
acid-labile protecting groups. Coupling of the resultant free
amine salt with d,l-3-methyl-2-oxo-pentanoic acid gave
ketoamide 25 as a mixture of inseparable diastereomers.
Since it is known that the ketoamide residue is labile to
epimerization,4 no attempt was made to effect the coupling
of either (R)- or (S)-3-methyl-2-oxo-pentanoic acid because
the same diastereomeric mixture would result.
Scheme 3a
Hydrogenolysis of the benzyloxy carbamate and the benzyl
ester residues of 25 produced the requisite amino acid
substrate for macrocyclization. Subjecting this substance to
EDCI and HOAt afforded the TMC-95 macrocyclic core
structure 26 in 49% overall yield from 25. It was previously
known4a,b that macrocyclization would only provide the
desired atropisomer; therefore, this was of no synthetic
concern. The structure of this substance was secured by
conversion into a late-stage intermediate reported by Lin and
Danishefsky.4a
The completion of the formal synthesis was accomplished
by treatment of macrocyclic tetraol 26 with TESOTf and
a Reaction conditions: (a) bis(pinacolato)diboron, KOAc, Pd-
(dppf)Cl2, DMSO, 80 °C, 4 h, (80%); (b) 16, K2CO3, Pd(dppf)Cl2,
aqueous DME, 90%; (c) LiOH, THF, H2O, 0 °C; (d) H2N-Asn-
OBn, HOAt, EDCI, NMM, CH2Cl2, 0 °C, 4 h, 98%, two steps.
(11) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
(12) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60,
7508.
(13) Yoshimura, S.; Miki, M.; Ikemura, H.; Aimoto, S.; Shimonishi, Y.;
Takeda, T.; Takeda, Y.; Miwatani, T. Bull. Chem. Soc. Jpn. 1984, 57, 125.
Org. Lett., Vol. 5, No. 2, 2003
199

In summary, we have effectively applied a stereoselective
modified Julia olefination reaction, followed by a diastereo-
selective dihydroxylation and macrocyclization with limited
use of protecting group chemistry as key transformations in
a concise formal total synthesis of the TMC-95 A/B
proteasome inhibitors. It is felt that an efficient total synthesis
of TMC-95A/B can be accomplished via elaboration of the
unprotected macrocycle 26. These efforts along with studies
focused on preparing novel TMC-95 analogues are currently
under investigation in our laboratories.
Scheme 5a
Acknowledgment. This material is based upon work
supported by the National Science Foundation under Grant
0202827 and the National Institutes of Health. We are also
grateful to Boehringer-Ingelheim Pharmaceuticals for partial
support of this work. Mass spectra were obtained on
instruments supported by the NIH Shared Instrumentation
Grant GM49631. We are indebted to Dr. Jun Kohno, Tanabe
Seiyaku Co., for providing authentic samples of TMC-95A/B
that were valuable for spectral comparison. We would also
like to thank Prof. Samuel J. Danishefsky (Columbia
University, Sloan-Kettering) for providing a 1H NMR
spectrum of compound 27.
a Reaction conditions: (a) TESOTf, 2,6-lutidine, CH2Cl2, DMF,
from 0 °C to rt, 12 h, ∼40%.
2,6-lutidine to afford 27 in approximately 40% isolated yield
Supporting Information Available: Complete spectro-
scopic data for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
(Scheme 5). The H NMR spectral characteristics of this
1
substance exactly matched those of the H NMR spectrum
kindly provided to us by Professor Danishefsky (see Sup-
porting Information).4a
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Org. Lett., Vol. 5, No. 2, 2003