Total synthesis of tryprostatins a and b

Article abstract of DOI:10.1002/anie.201004963

A reasonable approach to the radical: The establishment of reliable conditions for the radical-mediated construction of indoles enabled the highly efficient synthesis of tryprostatins A and B. Use of the radical initiator 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) has allowed to carry out the radical cyclization at just 30°C, thereby suppressing the formation of by-products. Copyright

Full text of DOI:10.1002/anie.201004963

Communications
DOI: 10.1002/anie.201004963
Natural Products Synthesis
Total Synthesis of Tryprostatins A and B**
Takayuki Yamakawa, Eiji Ideue, Jun Shimokawa, and Tohru Fukuyama*
Multiple-drug resistance toward cell-cycle inhibitors fre-
quently becomes an obstacle in cancer chemotherapy. One
strategy for circumventing this problem is to develop
antimitotic agents that operate by a new mode of action.
Two such compounds, tryprostatins A and B, were isolated in
1995 from the fermentation broth of Aspergillus fumigatus
BM939 by Osada and co-workers.[1] Tryprostatin A holds
great promise, because it selectively arrests the cell cycle at
the mitotic phase in tsFT210 cells.[2] Interesting biological
activity combined with an apparently simple structure has
stimulated the synthetic community, and several total syn-
theses have already been reported.[3,4]
Our research group has long been involved with the
radical-mediated construction of a variety of indole cores and
the application of these methods to natural product syn-
thesis.[5,6] We envisioned that our method could also be
Scheme 1. Retrosynthetic analysis. Boc=tert-butoxycarbonyl.
applied to the synthesis of the 2,3-disubstituted indole moiety
of the tryprostatins. The interesting structural features as well
as the therapeutic potential of tryprostatin A analogues
indicated in a recent structure–activity-relationship study[4e]
prompted us to launch a program toward the synthesis of
tryprostatins.
We reasoned that radical-mediated cyclization and sub-
sequent palladium-mediated coupling with a prenyl-group
donor would enable facile construction of the 2,3-disubsti-
tuted indole core structure 3 (Scheme 1). The requisite ortho-
alkenyl isocyanide 4 would be prepared from the alkyne 5,
which in turn could be accessed by Sonogashira coupling of
the aromatic iodide 6 with the terminal alkyne 7. To enable
synthesis of the target compounds with high enantiomeric
purity, the alkyne 7 derived from the Garner aldehyde (8)[7]
was employed as a latent amino acid unit to be incorporated
into the diketopiperazine.
Initially, we focused our efforts on the synthesis of the
isocyanide 12 as a radical-cyclization precursor (Scheme 2).
The alkyne 7 was prepared from 8 according to the known
protocol[8] with a slight modification. After the Sonogashira
coupling between 7 and 2-iodoformanilide (9), partial reduc-
tion of the triple bond was examined. Whereas the use of the
Lindlar catalyst, Pd/C, nickel boride,[9] or diimide[10] resulted
in no reaction or overreduction, treatment with Zn/LiCuBr2
Scheme 2. Synthesis of the ortho-alkenyl isocyanide 12: a) CBr4, PPh3,
Et3N, CH2Cl2, À60–08C, 88%; b) EtMgBr, THF, 08C, 98%; c) 9, CuI,
[PdCl2(PPh3)2], Et3N, room temperature, 97%; d) Zn, BrCH2CH2Br,
CuBr, LiBr, THF, CF3CH2OH, 708C, 99%; e) triphosgene, pyridine,
CH2Cl2, 08C, 87%.
in ethanol[11] gave the desired product 11 along with the
corresponding amine. The use of 2,2,2-trifluoroethanol[12] as
the solvent suppressed the undesired solvolysis and improved
the yield of 11 to 99%. Subsequent dehydration with
bis(trichloromethyl) carbonate (triphosgene) gave the
ortho-alkenyl isocyanide 12 and thus set the stage for a
radical-mediated cyclization.
When isocyanide 12 thus obtained was subjected to our
previously established radical-cyclization conditions,[5a] the
desired 5-exo adduct 13 was obtained in moderate yield after
acidic treatment with silica gel along with a considerable
amount of products 14 and 15 of a 6-endo cyclization–
cleavage process (Scheme 3). We were aware of the tendency
of this imidoyl-radical cyclization to give a mixture of the
5-exo and 6-endo products if the intermediate radical of the
[*] T. Yamakawa, E. Ideue, J. Shimokawa, Prof. Dr. T. Fukuyama
Graduate School of Pharmaceutical Sciences, University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)358-028-694
E-mail: fukuyama@mol.f.u-tokyo.ac.jp
[**] Financial support for this research was provided by Grants-in-Aid
(21790009 and 20002004) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004963.
9262
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When this method was applied to the radical cyclization of
the isocyanide 12, complete selectivity was observed for the
cyclization of the imidoyl radical to give the 2-stannylindole
21 (Scheme 4). Although we previously reported a one-pot
Stille-type coupling reaction between aromatic or vinylic
halides and 2-stannylindoles generated in situ for the
Scheme 3. Troublesome radical-mediated cyclization: a) nBu3SnH
(2.0 equiv), AIBN (15 mol%), MeCN, 1008C; silica gel, room temper-
ature. AIBN=azobisisobutyronitrile.
5-exo cyclization is not stabilized by the neighboring sub-
stituent.[5] Although we showed in our previous studies that
the 5-exo cyclization is dominant under conditions of kinetic
control,[5d] generally applicable conditions for the formation
of 2-stannylindoles had yet to be determined. In an attempt to
reduce the reaction temperature, we employed 2,2’-azobis(4-
methoxy-2,4-dimethylvaleronitrile) (V-70, 20) as a radical
initiator with a lower decomposition temperature.[13] To assess
the potential of V-70, we chose substrates without radical-
stabilizing substituents. When subjected to the reported
conditions (AIBN, 1008C), substrates 16 and 17 afforded a
mixture of the 5-exo adduct 18 and the 6-endo adduct 19, and
cyclization of the trans isomer 17 proved to be more difficult
than that of the cis isomer 16 (Table 1, entries 1 and 4).[14]
However, the use of V-70 at 308C virtually suppressed the
formation of the 6-endo adduct: after acidic treatment, indole
18 was obtained in good yield as the dominant product,
regardless of the configuration of the substrate (Table 1,
entries 3 and 6). Thus, we established reliable conditions for
imidoyl-radical-mediated indole synthesis.
Scheme 4. Total synthesis of tryprostatin B: a) nBu3SnH (3.0 equiv),
V-70 (20, 20 mol%), toluene, 308C; b) 22, LiCl (3.0 equiv), [Pd2(dba)3]
(10 mol%), AsPh3 (40 mol%), DMF, 808C, 73%; c) Boc2O, DMAP,
MeCN, room temperature, 99%; d) TFA/THF/H2O (4:2:1), 08C, 88%;
e) TEMPO, PhI(OAc)2, MeCN, phosphate buffer (pH 6.8), room tem-
perature, 91%; f) 26, HATU, iPr2NEt, CH2Cl2, room temperature, 88%;
g) NMP, reflux, 89%. dba=dibenzylideneacetone, DMAP=4-dimethyl-
aminopyridine, DMF=N,N-dimethylformamide, HATU=O-(7-azaben-
zotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluorophosphate,
TEMPO=2,2,6,6-tetramethylpiperidin-1-oxyl, TFA=trifluoroacetic acid.
Table 1: Low-temperature radical cyclization.
preparation of 2,3-disubstituted indoles,[5a] no such coupling
reactions with allylic systems have been reported. When
[Pd(PPh3)4] was used as the catalyst, the desired product was
obtained in only 29% yield with prenyl acetate (22) as the
coupling partner. Extensive investigations revealed that a
combination of [Pd2(dba)3], triphenylarsine, and lithium
chloride gave the best results and afforded the desired
product 23 in 82% yield in a one-pot process from the
isocyanide 12.
Once construction of the indole moiety was complete, 23
was converted into the corresponding amino acid 25 in a
three-step sequence consisting of protection of the indole
with a Boc group, hydrolysis of the acetonide, and oxidation
of the resulting alcohol 24 to the carboxylic acid 25 with
TEMPO. After condensation with l-proline methyl ester
Entry Substrate Initiator Solvent T [8C] 18 [%][d] 19 [%][d]
1[a]
2[b]
3[c]
4[a]
5[b]
6[c]
16
16
16
17
17
17
AIBN
AIBN
V-70
AIBN
AIBN
V-70
MeCN
toluene 100
toluene
MeCN
toluene 100
toluene 30
100
72
64
84
51
48
77
18
23
2
33
34
6
30
100
[a] See reference [5a] for the reaction conditions. [b] Reaction conditions:
nBu3SnH (2.0 equiv), AIBN (15 mol%), toluene (0.2m), 1 h, 1008C; 1m
aqueous HCl, room temperature. [c] Reaction conditions: nBu3SnH
(2.0 equiv), V-70 (20, 15 mol%), toluene (0.2m), 3 h, 308C; 1m aqueous
HCl, room temperature. [d] Yield of the isolated product.
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Communications
Scheme 5. Total synthesis of tryprostatin A: a) HCO2H, Ac2O, CH2Cl2, 08C, 98%; b) 7, CuI, [PdCl2(PPh3)2], Et3N/THF, room temperature, 98%;
c) Zn, BrCH2CH2Br, CuBr, LiBr, THF, CF3CH2OH, 708C, 94%; d) triphosgene, pyridine, CH2Cl2, 08C, 85%; e) nBu3SnH (3.0 equiv), V-70
(20 mol%), toluene, 308C; f) 22, LiCl (3.0 equiv), [Pd2(dba)3] (10 mol%), AsPh3 (40 mol%), DMF, 808C, 80%; g) Boc2O, DMAP, MeCN, room
temperature, 92%; h) TFA/THF/H2O (4:2:1), 08C, 96%; i) TEMPO, PhI(OAc)2, phosphate buffer (pH 6.8), MeCN, 08C, 91%; j) 26, HATU,
iPr2NEt, CH2Cl2, room temperature, 77%; k) NMP, reflux, 78%.
b) C.-B. Cui, H. Kakeya, G. Okada, R. Onose, H. Osada, J.
(26), the remaining transformations involved the removal of
Antibiot. 1996, 49, 527 – 533; c) C.-B. Cui, H. Kakeya, H. Osada,
J. Antibiot. 1996, 49, 534 – 540.
the two Boc groups in 27 and cyclization to construct a
diketopiperazine core structure. However, the conventional
method for removing Boc groups under acidic conditions
caused substantial decomposition of the substrate. Thus, we
attempted thermal removal of the Boc groups.[15] After
extensive optimization, this transformation was carried out
by heating the substrate under reflux in N-methylpyrrolidi-
none (NMP). Under these conditions, spontaneous cycliza-
tion occurred to give tryprostatin B (2) in 89% yield. Thus,
tryprostatin B was synthesized in 11 steps from 8 in 33%
overall yield on a half-gram scale.
Having established a generally applicable protocol for the
preparation of 2-stannylindoles with V-70, we undertook the
total synthesis of tryprostatin A (1) to showcase the synthetic
utility of this approach (Scheme 5). The synthesis commenced
with the preparation of 28 from 4-methoxy-2-nitroaniline in a
two-step, slightly modified sequence, including the Sand-
meyer reaction and iron-mediated reduction.[4c,16] By follow-
ing the route established for tryprostatin B, we synthesized
tryprostatin A in 30% overall yield from 28 on a half-gram
scale.
In summary, optimization of the radical-mediated indole
synthesis by the use of V-70 made it possible to access
2-stannyl 3-substituted indoles with substituents at C3 that
cannot effectively stabilize the radical intermediate. In
combination with a subsequent palladium-mediated coupling
reaction at C2, the radical cyclization enables the preparation
of a variety of 2,3-disubstituted indoles, such as tryprosta-
tins A and B. Further investigations into the synthesis and
biological activity of tryprostatin analogues are under way.
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Received: August 9, 2010
Published online: October 28, 2010
Keywords: alkaloids · heterocycles · natural products ·
.
radical reactions · total synthesis
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