Synthesis of the functionalized macrocyclic core of proteasome inhibitors TMC-95A and B

Article abstract of DOI:10.1002/1521-3773(20010518)40:10<1967::AID-ANIE1967>3.0.CO;2-Q

An ordered assembly of four building blocks (2-5) forms the basis of the synthesis of 1, which contains the core structure of TMC-95A and B - two potent proteasome inhibitore (see scheme; TIPS = triisopropylsilyl, Cbz = benzoxycarbonyl, Boc = tert-Boc = tert-butoxycarbonyl).

Full text of DOI:10.1002/1521-3773(20010518)40:10<1967::AID-ANIE1967>3.0.CO;2-Q

COMMUNICATIONS
TMC-95A-D (1-4)
Synthesis of the Functionalized Macrocyclic
Core of Proteasome Inhibitors TMC-95A
and B**
H
OTIPS
HN
Boc
R1
O
Macrolactamization
R2
OTIPS
HO
N
Suzuki
H
coupling
6
Songnian Lin and Samuel J. Danishefsky*
X
B
tBuO
H2N
O
O
O
HN
O
N
H
NH2
TMC-95A (1) and its diastereomers, TMC-95B, C, and D
(2 ± 4, ), are cyclic peptides recently isolated as fermentation
products of Apiospora montagnei Sacc. TC1093 derived from
O
O
O
MeO
8
NH
O
MeO
O
NH2
OMe
Peptide formation
N
Cbz
H
Cbz
N
H
5
7
R1
R2
O
8
HO
Scheme 1. Retrosynthetic plan. TIPS ˆ triisoproylsilyl, Cbz ˆ benzoxycar-
bonyl, Boc ˆ tert-butoxycarbonyl, X ˆ Br or I.
7
N
H
6
O
HN
O
N
H
O
R4
R1
R2
R3
TMC-95
1
11
O
further foresaw that a Suzuki reaction,[9] which would allow
the joining of 6 and 7, might be employed to reach macro-
lactamization precursors en route to 5. Below, we report the
encouraging realization of several important milestones
pursuant to this rough blueprint.
HO
NH2
R4
NH
A (1)
B (2)
C (3)
D (4)
H
H
OH
OH
H
CH3
H
O
14
R3
CH3
H
H
OH
OH
CH3
H
N
H
CH3
H
O
The synthesis of oxindole 6a was first attempted by using a
proposed palladium-mediated intramolecular Heck reaction
of substituted N-acryl-2,6-dibromoaniline 12 (Scheme 2).
soil samples.[1, 2] Biological studies[1] have shown that TMC-
95A inhibited the chymotrypsin-like (ChT-L), trypsin-like (T-
L), and peptidoglutamyl-hydrolyzing (PGPH) activities of the
20S proteasome[3, 4] with IC50 values of 5.4, 200, and 60 nm,
respectively. TMC-95B inhibited these activities to the same
extent as TMC-95A, while TMC-95C and D were 20 to 150
times weaker. TMC-95A has also shown cytotoxic activities
against HCT-116 (human colon carcinoma cells) and HL-60
(human promyelocytic leukemia cells) with IC50 values of 4.4
and 9.8 mm, respectively. These four diastereomers are struc-
turally characterized as novel cyclic peptides containing l-
tyrosine, l-asparagine, highly oxidized l-tryptophan, (Z)-1-
propenylamine, and 3-methyl-2-oxopentanoic acid moieties.
Although the phenyl-indole ring attachment is found in a few
natural products such as chloropeptin,[5] complestatin,[6]
diazonamide,[7] and the kistamicins,[8] the presence of an
oxindole ring in such macrocyclic systems is apparently rare.
The structural novelty of the TMC compounds and the
biological issues they raise prompted us to embark on a
program directed to their total synthesis.
O
O
a
b
OMe
OMe
O
O
N
N
Boc
O
Boc
10
9
O
N
N
Boc
O
Boc
Br
H
Cl
c
d
O
N
N
H
O
Boc
N
O
H
Br
Br
11
Scheme 2. Attempted synthesis of 7-bromooxindole 6a. a) 1. DIBAL/
toluene, 788C, 1 h; 2. methyl (triphenylphosphoranylidine)acetate,
12
6a
CH2Cl2, room temperature (RT), 88% (two steps); b) 1. LiOH, THF/
MeOH/H2O; 2. TBS-Cl, Et3N/DMAP; 3. (COCl)2, DMF (cat.); c) 2,6-
dibromoanaline, NaH, DMF/THF, 758C, 1.5 h, 44%; d) [Pd(PPh3)4] or
Pd(OAc)2, 5 ± 15%. DIBAL ˆ diisobutylaluminum hydride, TBS ˆ tert-
butyl dimethylsilyl, DMAP ˆ 4-dimethylaminopyridine.
We envisioned that the 17-membered ring could be
fashioned by macrolactamization. With the ring in place,
attention could then be directed to inclusion of the unusual
(Z)-1-propenylamide and 3-methyl-2-oxopentanoate side
chains at C-8 and C-14, respectively. The installation of
hydroxy groups at C-6 and C-7 could in principle be
conducted prior to, or after, macrocyclization (Scheme 1). We
Thus, methyl ester 9 derived from d-serine[10] was reduced
with DIBAL. This was followed by treatment with methyl
(triphenylphosphoranylidine)acetate to provide 10 in 88%
yield over two steps. Methyl ester 10 was then converted to
the corresponding acid chloride 11 under neutral conditions in
a three-step procedure as shown.[11] Acylation of 2,6-dibro-
moaniline with 11 was sluggish, and afforded 12 in only
moderate yield. Unfortunately, attempted cyclization of
dibromide 12 to 7-bromooxindole 6a provided the desired
compounds in unacceptably low yields (5 ± 15%).
Accordingly, an alternative approach was investigated
(Scheme 3). Protection of N-Boc-d-serine as shown, was
followed by reduction of the methyl ester to provide aldehyde
13 in high yield. The required 7-iodoisatin 16 was obtained in
good yield in two steps from 2-iodoaniline 14 via isonitroso-
acetanilide 15.[12] Upon heating compound 16 with hydrazine
hydrate[13] for 1 h at 1258C, and subsequent treatment with
[*] Prof. S. J. Danishefsky,[‡] Dr. S. Lin
Laboratory for Bioorganic Chemistry
Sloan-Kettering Institute for Cancer Research
1275 York Avenue, New York, NY 10021 (USA)
Fax : (‡1)212-772-8691
E-mail: s-danishefsky@ski.mskcc.org
‡
[ ] Department of Chemistry
Havemeyer Hall, Columbia University
New York, NY 10027 (USA)
[**] This work was supported by grants from the National Institutes of
Health (grant CA28824). We thank Dr. George Sukenick and Sylvi
Rusli of the MSKCC NMR Core Facility for NMR and mass spectral
analyses (NIH Grant CA08748).
Angew. Chem. Int. Ed. 2001, 40, No. 10
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4010-1967 $ 17.50+.50/0
1967

COMMUNICATIONS
O
HO
Cbz
MeO
Cbz
HO
O
HN
HN
NH2
O
1) TIPS-Cl, imidazole, 94%
a
b
HO
Boc
OMe
OMe
OMe
OH
TIPSO
Boc
H
2) DIBAL, -78 ˚C, 96%
NH
NH
O
O
L-Tyr
19
18
D-Ser(Boc)-OH
13
OH
N
O
O
B
I
CCl3(OH)2
O
O
NH2OH·HCl
H2SO4
MeO
Cbz
Cbz
d
c
HN
MeO
HN
NH2
70 ˚C, 15 min
88-98%
Na2SO4, H2O
45 ˚C, 12 h
N
H
N
O
OMe
H
OMe
I
I
I
66%
O
14
16
15
O
20
7
H
N
OTIPS
Scheme 4. Synthesis of aryl borate 7. a) 1. MeOH/SOCl2; 2. Cbz-Cl,
Boc
1) NH2NH2·H2O,
125 ˚C, 1 h
K2CO3, H2O/acetone, 96% (two steps); b) LiOH, Me2SO4, 86%; c) I2,
H
13
Ag2SO4,
MeOH,
RT, 1.5 h;
93%; d) bis(pinacolato)diboron,
N
H
O
see Table 1.
2) HCl (6 N),
60 ˚C, 2 h
[PdCl2(dppf)] ´ CH2Cl2, KOAc, DMSO, 808C, 13 h, 95%. dppf ˆ bis(di-
N
O
I
H
phenylphosphanyl)ferrocene.
I
17
81% (2 steps)
6b
Scheme 3. Synthesis of 7-iodooxindole 6b.
The appropriate components, 7-iodooxindole 6b and aryl
borate 7, were indeed joined by a Suzuki coupling.[9, 17b] After
an extensive survey of reaction conditions (catalyst loading,
temperature, and reaction time, equivalents of borate and
base) we identified a regimen that afforded coupling products
21 in 72% yield with an E/Z ratio of 2/1. The two isomers were
separable by silica gel chromatography, and the E/Z stereo-
chemistry of each was determined by NOE experiments
similar to those conducted on 6b. No epimerization of the
allylic amine position was observed in this reaction. However,
E/Z isomerization apparently did occur during the Suzuki
process. An E/Z ratio of about 2/1 was consistently obtained
starting with (E)-6b, (Z)-6b, or a mixture of the two isomers.
Conversion of (Z)-21 to (E)-21 can be affected by heating (Z)-
21 in DME at 808C for 1 day in the presence of a catalytic
amount of iodine (Scheme 5).
HCl (6n) at 608C for 2 h, 7-iodooxindole 17 was obtained in
excellent yield. Condensation of 17 and aldehyde 13 was then
carried out under several conditions as summarized in Table 1.
As seen, serviceable yields (74 ± 76%) with little or no
racemization (up to 96% ee) could be achieved in reaching
6b (Table 1, entries 3 and 4).[14] The structure of the major
component of 6b was confirmed by NOE experiments
(Figure 1) to be the E isomer.
O
1.7%
O
4%
N
H
OTIPS
H
H
6%
H
I
H
OTIPS
O
HN
N
H
O
N
O
O
H
I
H
(Z)-6b
(E)-6b
N
OTIPS
Boc
Figure 1. NOE experiments on compounds (E)-6b and (Z)-6b (CDCl3,
400 MHz).
7 (1.1 equiv)
H
[PdCl2(dppf)]·CH2Cl2, K2CO3
DME, 80 ˚C, 2 h
N
O
H
I
72% (E/Z~2/1)
The aryl borate 7, required for Suzuki cross-coupling, was
synthesized starting from l-tyrosine (Scheme 4). Methyl ester
formation with MeOH/SOCl2 followed by protection of the
amino group as a benzylcarbamate, afforded phenol 18 in
96% yield over two steps. O-methylation of 18 with Me2SO4
in the presence of LiOH ´ H2O in dry THF[15] provided 19 in
86% yield. Selective iodination of 19 at the 3-position ortho to
6b
H
N
OTIPS
Boc
OTIPS
Boc
HN
N
O
+
N
O
Cbz
H
H
Cbz
MeO
MeO
HN
HN
OMe
OMe
the methoxy group was achieved in high yield with I2/
O
O
(Z)-21
(E)-21
[16]
AgSO4
in methanol. Iodide 20 was then converted into
I2 (cat.), DME, 80 ˚C, 1d
87% (63% conv.)
aryl borate 7 in 95% yield under Miyauraꢁs conditions using
bis(pinacolato)diboron, [PdCl2(dppf)] ´ CH2Cl2, and KOAc in
DMSO for 13 h at 808C.[17]
Scheme 5. Biaryl linkage formation by Suzuki coupling. DME ˆ 1,2-
dimethoxyethane.
Table 1. Condensation of iodooxindole 17 with aldehyde 13 under different conditions.
Entry
Conditions
Yield [%]a
E/Z[b]
ee [%][c]
1
2
3
4
piperidine (cat.), MeOH or EtOH, 658C, 2 ± 3 h
piperidine (cat.), THF, RT, 17 ± 44 h
1) LDA (2.1 equiv), THF, 788C, 1 h; 2) TEA (2.5 equiv), MsCl (1.2 equiv), CH2Cl2, 60 to 308C, 2 h
1) LDA (2.0 equiv), THF, 788C, 1.5 h; 2) TEA (3 equiv), MsCl (1.5 equiv), CH2Cl2, 70 to 508C, 1.5 h
44 ± 50
55
76
2.0/1
1.7/1
1.3/1
1.2/1
0
ꢀ 10
92
74
96
1
[a] Yield of isolated product. [b] Determined by H NMR spectroscopy, two isomers are separable. [c] Determined by chiral HPLC.
1968
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4010-1968 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 10

COMMUNICATIONS
l-Asparagine was then appended to the seco framework.
Thus, hydrolysis of the a-methyl ester of 21, and conversion of
the derived acid to its N-hydroxysuccinimade ester paved the
way for amide formation with l-Asn hydrate (see compound
22). Exposure of 22 to the action of HCl (4n), followed by
cyclization of the resulting amino acid with pentafluorophenyl
diphenylphosphonate (FDPP) and DIEA in DMF, or with
EDC/HOAT in various solvents (DMF, CH2Cl2, MeCN),
however, did not provide the cyclized product. We reasoned
that the rigidity of the exo double bond at the 3-position of
oxindole ring probably tilts the amino group away from the
asparagine moiety, thereby preventing cyclization. Accord-
ingly, we decided to carry out dihydroxylation prior to
cyclization. Saponification of the methyl ester (E)-21 followed
by coupling with l-asparagine tert-butyl ester (8) as before
provided 23a in 70% yield over two steps. Treatment of 23a
with HF/pyridine afforded free alcohol 23b (Scheme 6).
primary hydroxy group gave similar results (81%, S/R ꢀ 1/1.4,
Scheme 6).
Treatment of (S)-24 with trifluoroacetic acid (TFA) in
CH2Cl2 resulted in concurrent removal of the Boc protecting
group and hydrolysis of the tert-butyl ester. The crude amino
acid was then submitted to macrolactamization using EDC/
HOATunder highly dilute conditions (4 mm) in CH2Cl2/DMF
(4/1). Cyclization progressed smoothly, providing the desired
product 5[20] in 55% yield over two steps (Scheme 7). The
HO
OH
7
HO
HO
OTIPS
OTIPS
O
8
HN
O
Boc
OtBu
O
HN
N
a, b
N
H
O
H
O
MeO
MeO
O
NH
NH2
N
O
Cbz
H
O
NH2
N
HN
H
Cbz
5 JH7,H8 = 10.4 Hz
(S)-24
Scheme 7. Macrolactamization of (S)-24. a) TFA/CH2Cl2 (4/1), RT, 2 h;
b) EDC, HOAT, DIEA, CH2Cl2/DMF (4/1, 4 mm), RT, 20 h, 55% (two
steps). DIEA ˆ N,N-diisopropylethylamine.
H
OTIPS
HN Boc
O
HO
a, b, c
O
N
H
21
large coupling constant (10.4 Hz) observed for H7 ± H8 in 5,
which is similar to those observed in TMC-95A and B (1 and
2),[2] further confirmed the configurational assignments at C6
and C7. Interestingly, treatment of (R)-24 under the same
reaction conditions did not afford any cyclization product.
In summary, the fully functionalized macrocyclic core of
proteasome inhibitors TMC-95A and B (5) has been assem-
bled. There still remain significant issues to be overcome, such
as selectivity enhancement en route to our total synthesis
goal,[21] which are currently being studied in considerable
detail.[22]
O
MeO
Boc
NH
O
NH2
Cbz
N
22
H
OR
N
H
g
d, e
O
OtBu
(E)-21
O
N
H
MeO
O
NH
O
Cbz
NH2
N
H
23a R = TIPS
23b R = H
f
Received: March 14, 2001 [Z16776]
HO
HO
OTIPS
HO
HO
HN
O
6
Boc
OtBu
OTIPS
O
[1] Y. Koguchi, J. Kohno, M. Nishio, K. Takahashi, T. Okuda, T. Ohnuki,
S. Komatsubara, J. Antibiot. 2000, 53, 105 ± 109.
[2] J. Kohno, Y. Koguchi, M. Nishio, K. Nakao, M. Kuroda, R. Shimizu, T.
Ohnuki, S. Komatsubara, J. Org. Chem. 2000, 65, 990 ± 995.
[3] A. Ciechanover, A. L. Schwartz, Proc. Natl. Acad. Sci. USA 1998, 95,
2727 ± 2730.
N
H
O
HN
6
O
Boc
+
+
O
MeO
O
N
N
H
H
N
H
NH2
25 (<5%)
O
(R)-24
HN
Cbz
(S)-24
[4] M. Orlowski, C. Michaud, Biochemistry 1989, 28, 9270 ± 9278.
[5] K. Matsuzaki, H. Ikeda, T. Ogino, A. Matsumoto, H. B. Woodruff, H.
Scheme 6. Synthesis of diols 24. a) LiOH, THF/MeOH/H2O; b) hydroxy-
succinimade, DCC, THF, 55% (two steps); c) l-Asn ´ H2O, Et3N, THF/
H2O, RT, 4 h, 70%; d) LiOH, THF/H2O, 0 8C, 1.5 h; e) H-Asn-OtBu (8),
EDC/HOAT, THF, RT, 2 h, 70% (2 steps); f) HF/Py, 84%; g) 1: OsO4/
NMO, (DHQD)2PHAL, tBuOH/H2O, RT, 1 h, 84% (S/R ꢀ 1/1.8); 2: OsO4/
NMO, (DHQ)2PHAL, tBuOH/H2O, RT, 4 h; TIPS-Cl, imidazole/DMAP,
5 h, 81% (S/R ꢀ 1/1.4). DCC ˆ 1,3-dicyclohexylcarbodiimide, EDC ˆ
1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride, HOATˆ
1-hydroxy-7-azabenzotriazole, Asn ˆ asparagine, NMO ˆ 4-methylmor-
pholine N-oxide, (DHQD)2PHAL ˆ 1,4-bis(9-O-dihydroquinidine)phthal-
azine, (DHQ)2PHAL ˆ 1,4-bis(9-O-dihydroquinine)phthalazine.
Ä
Tanaka, S. Omura, J. Antibiot. 1994, 47, 1173 ± 1174.
[6] I. Kaneko, K. Kamoshida, S. Takahashi, J. Antibiot. 1989, 42, 236 ± 241.
[7] N. Lindquist, W. Fenical, G. D. Van Duyne, J. Clardy, J. Am. Chem.
Soc. 1991, 113, 2303 ± 2304.
[8] a) N. Naruse, O. Tenmyo, S. Kobaru, M. Hatori, K. Tomita, Y.
Hamagishi, T. Oki, J. Careofokumura, J. Antibiot. 1993, 46, 1804 ±
1811; b) N. Naruse, M. Oka, M. Konishi, T. Oki, J. Antibiot. 1993, 46,
1812 ± 1818.
[9] For a review of Suzuki coupling reactions, see: N. Miyaura, A. Suzuki,
Chem. Rev. 1995, 95, 2457 ± 2483.
[10] A. McKillop, R. K. Taylor, R. J. Watson, N. Lewis, Synthesis 1994, 31 ±
33.
[11] A. Wissner, C. V. Grudzinskas, J. Org. Chem. 1978, 43, 3972 ± 3974.
[12] a) C. S. Marvel, G. S. Hiers, Org. Synth. 1941, Coll. Vol. I, 327 ± 330;
b) M. S. Newman, M. W. Logue, J. Org. Chem. 1971, 36, 1398 ± 1401;
c) V. Lisowski, M. Robba, S. Rault, J. Org. Chem. 2000, 65, 4193 ±
4194.
Dihydroxylation of 23a using OsO4/NMO in the presence of
(DHQD)2PHAL[18] at room temperature afforded the diols 24
in 84% yield (S/R ꢀ 1/1.8),[19] along with a small amount of
isatin 25 (<5%). Dihydroxylation of 23b in the presence of
(DHQ)2PHAL[18] followed by selective reprotection of the
[13] C. Crestini, R. Saladino, Synth. Commun. 1994, 24, 2835 ± 2841.
Angew. Chem. Int. Ed. 2001, 40, No. 10
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1433-7851/01/4010-1969 $ 17.50+.50/0
1969

COMMUNICATIONS
[14] Related synthetic studies on TMC-95A ± D had been reported by Ma
and Wu who employed an oxindole addition/elimination ± iodine
equilibration ± dihydroxylation approach to a portion of macrocycli-
zation precursor: D. Ma, Q. Wu, Tetrahedron Lett. 2000, 41, 9089 ±
9093.
constant between H7 and H8 is 0 Hz, indicating their syn relationship.
[20] The atropisomer shown for 5 follows from the C6 stereochemistry (see
reference [2]).
[21] While the manuscript was being processed, it has been found that
dihydroxylation of the N,O-acetonide derivative of 23b under similar
conditions to those shown in Scheme 6 followed by protecting group
adjustment provided the desired (S)-24 as the major isomer (S/R ꢀ
8/1). Details will be reported in due course. Similar results have also
been observed for 6a (see reference [14]).
[22] Note added in proof (April 12, 2001): An alternative route to the
biaryl moiety of the TMC-95 natural products by a Pd-catalyzed Stille
cross-coupling reaction of an aryl stannane tyrosine derivative and
7-iodoisatin appeared following the acceptance of this work: B. K.
Albrecht, R. M. Williams, Tetrahedron Lett. 2001, 42, 2755 ± 2757.
[15] A. Basak, M. K. Nayak, A. K. Chakraboti, Tetrahedron Lett. 1998, 39,
4883 ± 4886.
[16] W. W. Sy, Tetrahedron Lett. 1993, 34, 6223 ± 6224.
[17] a) T. Ishiyama, M. Murata, N. Miyaura, J. Org. Chem. 1995, 60, 7508 ±
7510; b) A. M. Elder, D. H. Rich, Org. Lett. 1999, 1, 1443 ± 1446.
[18] K. B. Sharpless, W. Amberg, Y. L. Bennani, G. A. Crispino, J. Hartung,
K.-S. Jeong, H.-L. Kwong, K. Morikawa, Z.-M. Wang, D. Xu, X.-L.
Zhang, J. Org. Chem. 1992, 57, 2768 ± 2771.
[19] The R configuration at C6 of (R)-24 was assigned by converting (R)-24
to its primary ± secondary diol acetonide, whereupon the coupling
1970
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Angew. Chem. Int. Ed. 2001, 40, No. 10