Stereocontrolled synthesis of the northern part of potent proteasome inhibitor TMC-95A.

Article abstract of DOI:10.1021/ol016303v

[reaction: see text]. A protected version of the northern part of TMC-95A, a potent and selective proteasome inhibitor, was synthesized with full stereochemical control. Highlights of this synthesis include (i) a (Z)-selective Mizoroki-Heck reaction to construct the oxyindole portion, (ii) a diastereoselective epoxidation, (iii) a 6-endo selective epoxide opening by Boc carbonyl group to establish the stereochemistry of C6, and (iv) a 1,3-elimination reaction of the L-allo-threonine derivative under Mitsunobu conditions to afford the (Z)-1-propenylamine.

Full text of DOI:10.1021/ol016303v

ORGANIC
LETTERS
2001
Vol. 3, No. 18
2863-2865
Stereocontrolled Synthesis of the
Northern Part of Potent Proteasome
Inhibitor TMC-95A
Masayuki Inoue, Hidetomo Furuyama, Hayato Sakazaki, and Masahiro Hirama*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, and
CREST, Japan Science and Technology Corporation (JST), Sendai 980-8578, Japan
hirama@ykbsc.chem.tohoku.ac.jp
Received June 20, 2001
ABSTRACT
A protected version of the northern part of TMC-95A, a potent and selective proteasome inhibitor, was synthesized with full stereochemical
control. Highlights of this synthesis include (i) a (Z)-selective Mizoroki−Heck reaction to construct the oxyindole portion, (ii) a diastereoselective
epoxidation, (iii) a 6-endo selective epoxide opening by Boc carbonyl group to establish the stereochemistry of C6, and (iv) a 1,3-elimination
reaction of the L-allo-threonine derivative under Mitsunobu conditions to afford the (Z)-1-propenylamine.
TMC-95A (1, Scheme 1) has been isolated from the
fermentation broth of Apiospora montagnei Sacc. TC 1093,
which was derived from soil samples.1 Despite the structural
dissimilarity between 1 and other known inhibitors such as
lactacystin,2 epoxomicin,3 other TMC series,4 and peptide-
derived molecules,5 studies have shown that 1 can
specifically inhibit 20S proteasome,1b the catalytic core of
proteasome.6 Thus, 1 may provide a unique insight into
Scheme 1. Plan for Synthesis of TMC-95A
(1) (a) Kohno, J.; Koguchi, Y.; Nishio, M.; Nakao, K.; Kuroda, M.;
Shimizu, R.; Ohnuki, T.; Komatsubara, S. J. Org. Chem. 2000, 65, 990.
(b) Koguchi, Y.; Kohno, J.; Nishio, M.; Takahashi, K.; Okuda, T.; Ohnuki,
T.; Komatsubara, S. J. Antibiot. 2000, 53, 105.
(2) (a) Omura, S.; Fujimoto, T.; Otoguro, K.; Matsuzaki, K.; Moriguchi,
R.; Tanaka, H.; Sasaki, Y. J. Antibiot. 1991, 44, 113. (b) Fenteany, G.;
Schreiber, S. L. J. Biol. Chem. 1998, 273, 8545 and references therein.
(3) (a) Hanada, M.; Sugawara, K.; Kaneta, K.; Toda, S.; Nishiyama, Y.;
Tomita, K.; Yamamoto, H.; Konishi, M.; Oki, T. J. Antibiot. 1992, 45,
1746. (b) Groll, M.; Kim, K. B.; Kairies, N.; Huber, R.; Crews, C. M. J.
Am. Chem. Soc. 2000, 122, 1237 and references therein.
(4) TMC-86A, B and TMC-96: (a) Koguchi, Y.; Kohno, J.; Suzuki, S.-
i.; Nishio, M.; Takahashi, K.; Ohnuki, T.; Komatsubara, S. J. Antibiot. 1999,
52, 1069. (b) Koguchi, Y.; Kohno, J.; Suzuki, S.-i.; Nishio, M.; Takahashi,
K.; Ohnuki, T.; Komatsubara, S. J. Antibiot. 2000, 53, 63. TMC-89A and
B: (c) Koguchi, Y.; Nishio, M.; Suzuki, S.-i.; Takahashi, K.; Ohnuki, T.;
Komatsubara, S. J. Antibiot. 2000, 53, 967.
(5) For a recent review on proteasome inhibitors, see: Myung, J.; Kim,
K. B.; Crews, C. M. Med. Res. ReV. 2001, 21, 245.
(6) For reviews on the biology of proteasomes, see: (a) Tanaka, K. J.
Biochem. 1998, 123, 195. (b) Ciechanover, A. EMBO J. 1998, 17, 7151.
10.1021/ol016303v CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/11/2001

understanding the detailed biological function of protea-
somes. Furthermore, proteasomes play important roles in the
activation of NF-κB7 and the processing of histocompatibility
complex (MHC) class I ligands,8 both of which are involved
in mounting inflammatory immune responses, thus suggest-
ing the use of TMC-95A and its analogues as potential
antiinflammatory agents for treating autoimmune diseases.
Notable structural features of 1 are the axially chiral biaryl
group, the highly oxidized tryptophan, and the (Z)-1-
propenylamine in the macrocyclic matrix. The biological
importance, as well as the unique architecture, prompted us
to undertake the total synthesis of 1.9 Here we report the
stereocontrolled synthesis of a protected version of the
northern part of 1.
Scheme 2a
As outlined in Scheme 1, the structure of 1 can be
retrosynthetically divided into two fragments that could be
reassembled to the macrocyclic structure by a peptide bond
formation and a Pd(0)-catalyzed biaryl coupling.9a-c,10 The
northern part can be further dissected so that the (Z)-1-
propenylamine portion could be prepared by the decarboxy-
lative 1,3-elimination of carboxylate 2. The oxidized trypto-
phan moiety could be synthesized by the oxidation of 3,
which in turn could be prepared by an intramolecular
Mizoroki-Heck reaction of dibromide 4.11
As shown in Scheme 2, DIBAL reduction followed by
Wittig reaction of a known ester 5, which was derived from
D-serine,12 afforded R,â-unsaturated ester 6 in 71% overall
yield.13 Amidation of the ester 6 with 2,6-dibromoaniline in
the presence of Me3Al resulted in 7 (59% yield),14 followed
by protection of the N22-amide with a Boc group to produce
8 in 88% yield. After numerous conditions were screened,
intramolecular Mizoroki-Heck reaction of 8 was reliably
and reproducibly carried out at room temperature,15 employ-
ing “ligandless” conditions16 that involved a catalytic amount
of Pd2(dba)3 in the presence of Et3N, to afford the trisub-
stituted olefin 9 in 86% yield (Z:E > 20:1).17
a (a) DIBAL, toluene, -78 °C; (b) Ph3PdCH2CO2Me, CH2Cl2,
71% (2 steps); (c) 2,6-dibromoaniline, Me3Al, toluene; then 6, 0
°C to rt, 59%; (d) (Boc)2O, Et3N, DMAP, CH2Cl2, rt, 88%; (e)
Pd2(dba)3‚CHCl3 (0.2 equiv), Et3N, THF-NMP (1:1), rt, 86%; (f)
DMDO, CH2Cl2, rt; (g) BF3‚Et2O (1.5 equiv), CH2Cl2, -78 to 0
°C, 87% (2 steps); (h) (+)-MTPACl, Et3N, DMAP, CH2Cl2, rt,
98%; (i) BzCl, Et3N, DMAP, CH2Cl2, rt, 83%.
(7) Palombella, V. J.; Rando, O. J.; Goldberg, A. L. Maniatis, T. Cell
1994, 78, 773.
(8) Rock, K. L.; Gramm, C.; Rothstein, L.; Clark, K.; Stein, R.; Dick,
L.; Hwang, D.; Goldberg, A. L. Cell 1994, 78, 761.
(9) For synthetic studies on TMC-95 from other laboratories, see: (a)
Lin, S.; Danishefsky, S. J. Angew. Chem., Int. Ed. 2001, 40, 1967. (b)
Albrecht, B. K.; Williams, R. M. Tetrahedron Lett. 2001, 42, 2755. (c)
Ma, D.; Wu, Q. Tetrahedron Lett. 2001, 42, 5279. (d) Ma, D.; Wu, Q.
Tetrahedron Lett. 2000, 41, 9089.
(10) For recent reviews on biaryl synthesis, see: (a) Lloyd-Williams,
P.; Giralt, E. Chem. Soc. ReV. 2001, 30, 145. (b) Stanforth, S. P. Tetrahedron
1998, 54, 263.
(11) For excellent reviews on Mizoroki-Heck reactions, see: (a) Link,
J. T.; Overman, L. E. Intramolecular Heck Reactions in Natural Product
Chemistry. In Metal-catalyzed Cross-coupling Reactions; Diedrich, F.,
Stang, P. J., Eds.; Wiley-VCH Verlag GmbH: Weihheim, 1998; pp 231-
269. (b) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009.
(c) de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33,
2379.
(12) (a) Garner, P.; Park, J. M. J. Org. Chem. 1987, 52, 2361. (b) Garner,
P.; Park, J. M. Org. Synth. 1992, 70, 18.
(13) The numbering of compounds in this paper corresponds to that of
TMC-95A.
As shown in Scheme 3, the (Z)-selectivity of this facile
intramolecular cyclization can be explained by the syn-
insertion of the alkyl palladium into (E)-olefin (15 f 16
and/or 17), followed by bond rotation (16 f 18 and/or 17
f 19), and syn-elimination of the C6-hydride to provide
predominantly (Z)-oxyindole 9 (Scheme 3). The bulky Boc
group protecting the N22-amide of 9 can explain the lack of
further Pd(0)-insertion into the C1-bromide of 9, which
would cause undesirable side reactions, and maintain the
catalytic cycle.18
Having secured the route to the oxyindole 9, the R,â-
unsaturated olefin was oxidized using 3,3-dimethyldioxirane
(DMDO),19 to yield epoxide 10 as a single diastereomer
(Scheme 2). Interestingly, we discovered that activation of
(14) (a) Lipton, M. F.; Basha, A.; Weinreb, S. M.Org. Synth. 1980, 59,
49. (b) Overman, L. E.; Paone, D. V.; Stearns, B. A. J. Am. Chem. Soc.
1999, 121, 7702.
(15) Reaction temperature is crucial for the regioselectivity of this
cyclization. For instance, the E/Z ratio of 9 was decreased to 3/1 when the
reaction was conducted at 50 °C.
(17) While this paper was in preparation, we noticed that Lin and
Danishefsky (see ref 9a) planned a similar route to this involving Heck
cyclization of a closely related substrate.
(18) Macor, J. E.; Ogilvie, R. J.; Wythes, M. J. Tetrahedron Lett. 1996,
37, 4289.
(19) Danishefsky, S. J.; Bilodeau, M. T. Angew. Chem., Int. Ed. Engl.
1996, 35, 1380.
(16) Madin, A.; Overman, L. E. Tetrahedron Lett. 1992, 33, 4859.
2864
Org. Lett., Vol. 3, No. 18, 2001

Scheme 3. Proposed Mechanism of (Z)-Selective
Mizoroki-Heck Reaction
Scheme 4a
the epoxide of 10 with BF3‚Et2O selectively provided a six-
membered carbamate 12 with a concomitant loss of the tert-
butyl group in 87% yield (2 steps).20 The observed selectivity
was reasoned that, because of the electronic assistance of
the aromatic group, the C6-O bond of 11 would be more
susceptible to cleavage than the C7-O bond. The stereo-
structure of 12 was unambiguously assigned using the
coupling constant (JH7,H8 ) 10.0 Hz) and the observed NOE
between H4 and H7. The results indicate that not only did
the epoxidation selectively occur from the opposite side of
the C8-NRBoc group of 9, but the resulting epoxide in 10
was opened via an SN2 pathway. The enantiomeric excess
of 12 was calculated to be 94% after derivatization to its
(+)-MTPA ester 13. For further synthetic manipulations, the
secondary alcohol of 12 was protected as its benzoate ester
to furnish 14 in 83% yield.
Before building the (Z)-1-propenylamine moiety, depro-
tection and oxidation of the C25-hydroxyl were necessary
(Scheme 4). Thus, treatment of 14 in the presence of a
catalytic amount of TfOH in CF3CH2OH simultaneously
removed the acetonide and Boc groups and generated the
desired alcohol 20 in 64% yield.21 Oxidation of the alcohol
20 using catalytic CrO3 and H5IO6 as a reoxidant provided
carboxylic acid 21,22 which was then coupled with L-allo-
threonine methyl ester 22 by the action of EDC and HOBt,
resulting in amide 23 (60%, 2 steps). Subsequent saponifica-
tion of methyl ester 23 led to R-alkoxy carboxylic acid 24.
Upon treatment of 24 with Mitsunobu conditions (Ph3P,
a (a) TfOH (0.5 equiv), CF3CH2OH, 0 °C to rt, 64%; (b) CrO3
(0.1 equiv), H5IO6, wet MeCN, 0 °C; (c) 22 (1.5 equiv), EDC,
HOBt, NMM, DMF, 0 °C to rt, 60% (2 steps); (d) LiOH‚H2O,
THF-H2O (1:1), 0 °C; (e) DEAD, PPh3, THF, -78 °C to rt, 40%
(2 steps).
DEAD), the decarboxylative 1,3-elimination proceeded
smoothly,23 presumably through 25, to yield the protected
northern part of TMC-95A (26) in 40% yield (2 steps).24
In conclusion, we have attained a highly stereocontrolled
synthesis of 26 through (i) (Z)-selective Mizoroki-Heck
reaction of dibromide 8 to construct the oxyindole portion,
(ii) a diastereoselective epoxidation procedure (9 f 10), (iii)
a 6-endo selective epoxide ring-opening reaction to establish
the stereochemistry of C6 stereocenter (10 f 12), and (iv)
a mild 1,3-elimination reaction of L-allo-threonine to build
the (Z)-1-propenylamine (24 f 26). Further studies directed
toward the total synthesis of TMC-95A, including derivati-
zation of 26 to a suitable form for coupling with a southern
part, are currently underway in our laboratory.
OL016303V
(23) Pansare, S. V.; Vederas, J. C. J. Org. Chem. 1989, 54, 2311.
(24) Physical data of 26: [R]25D +86.0 (c 0.62, CHCl3); IR (film) 3280,
1
1741, 1672, 1621, 1264, 1107, 708 cm-1; H NMR (500 MHz, CDCl3) δ
1.08 (3H, dd, J ) 7.5, 1.5 Hz, H29), 4.72 (1H, dq, J ) 9.0, 7.5 Hz, H28),
5.31 (1H, d, J ) 9.5 Hz, H8), 6.02 (1H, d, J ) 9.5 Hz, H7), 6.43 (1H, ddd,
J ) 10.5, 9.0, 1.5 Hz, H27), 6.96 (1H, s, amide-NH), 6.99 (1H, dd, J )
8.5, 7.5 Hz, H3), 7.36 (1H, d J ) 8.5 Hz, H4), 7.44 (2H, dd, J ) 8.0, 7.5
Hz, Bz), 7.50 (1H, d, J ) 7.5 Hz, H2), 7.59 (1H, t, J ) 7.5 Hz, Bz), 7.93
(2H, d, J ) 7.5 Hz, Bz), 8.83 (1H, d, J ) 10.5 Hz, amide-NH), 9.57 (1H,
s, amide-NH); 13C NMR (125 MHz, CDCl3) δ 10.7, 52.5, 66.5, 80.7, 103.9,
109.9, 120.8, 124.6, 125.1, 125.2, 127.8, 128.8, 130.0, 134.3, 134.6, 141.2,
151.4, 163.9, 165.2, 173.6; MALDI-TOF MS calcd for C22H19BrN3O6 [M
+ H+] 500.0457, found 500.0433.
(20) For a related approach for forming a six-membered carbamate,
see: Urabe, H.; Aoyama, Y.; Sato, F. Tetrahedron 1992, 48, 5639.
(21) (a) Kobayashi, S.; Reddy, R. S.; Sugiura, Y.; Sasaki, D.; Miyagawa,
N.; Hirama, M. J. Am. Chem. Soc. 2001, 123, 2887. (b) Holcombe, J. L.;
Livinghouse, T. J. Org. Chem. 1986, 51, 111.
(22) Zhao, M.; Li, J.; Song, Z.; Desmond, R.; Tschaen, D. M.; Grabowski,
E. J. J.; Reider, P. J. Tetrahedron Lett. 1998, 39, 5323.
Org. Lett., Vol. 3, No. 18, 2001
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