The asymmetric total synthesis of (+)-cytotrienin A, an ansamycin-type anticancer drug

Article abstract of DOI:10.1002/anie.200802079

(Chemical Equation Presented) A star-studded lineup: (+)-Cytotrienin A was the target of an asymmetric total synthesis featuring an enantioselective aldol reaction, α-aminoxylation, deoxygenation, and a ring-closing metathesis to form the 21-membered macrolactam. This first total synthesis confirms the relative and absolute confguration of the molecule.

Full text of DOI:10.1002/anie.200802079

Angewandte
Chemie
DOI: 10.1002/anie.200802079
Natural Product Synthesis
The Asymmetric Total Synthesis of (+)-Cytotrienin A, an Ansamycin-
Type Anticancer Drug**
Yujiro Hayashi,* Mitsuru Shoji, Hayato Ishikawa, Junichiro Yamaguchi, Tomohiro Tamura,
Hiroki Imai, Yosuke Nishigaya, Kenichi Takabe, Hideaki Kakeya, and Hiroyuki Osada
Cytotrienin A (1) is a microbial antitumor secondary metab-
olite that was isolated from the fermentation broth of
Streptomyces sp. RK95-74 from soil.[1] It possesses an
accomplished the total synthesis of other members of this
class of natural products, including trienomycins A and F,[6]
mycotrienin A,[7] and thiazinotrienomycin E.[8] Although the
macrocyclic core of cytotrienin A has been synthesized in its
protected form by Panek et al.[9] and Kirschning et al.,[10] the
total synthesis of cytotrienin A, with the side chain attached,
has not been reported. The relative and absolute stereochem-
istry has not been confirmed, but has been assigned based on
analogous mycotrienin natural products. Herein we report the
first total synthesis of the naturally occurring enantiomer of
cytotrienin A, which confirms its relative and absolute
stereochemistry.
We envisioned installing the side chain midway through
the synthesis and constructing the triene unit at a late stage by
ring-closing metathesis (RCM).[11] We reasoned that intro-
duction of the bulky side chain after formation of the
macrocyclic core would be difficult, and also, a long sequence
of reactions after the construction of the labile triene unit
would be avoided. Other noteworthy features of our
approach are the use of novel organocatalyzed and proline-
mediated enantioselective reactions, both of which have been
developed by our research group.[12] Specifically, we planned
to form two (C11 and C12) of the three contiguous chiral
centers with an aldol reaction by using an organocatalyst, and
to control the configuration at C3 by using proline-mediated
a-aminoxylation.
E,E,E-triene motif within a 21-membered cyclic lactam,
which also contains four chiral centers. These are common
structural features of the ansamycin class of natural products,
which include the mycotrienins (or ansatrienins),[2] trienomy-
cins,[2c,3] thiazinotrienomycins,[4] and trierixin.[5] Cytotrie-
nin A, with its unusual aminocyclopropane carboxylic acid
side chain, exhibits potent apoptosis-inducing activity on HL-
60 cells with an ED50 value of 7.7 nm. To facilitate elucidation
of its mechanism of action, the development of a method for
the total synthesis and derivatization of cytotrienin A is highly
desirable. The research groups of Smith and Panek have
The synthesis started with an organocatalyzed aldol
reaction which was found to be problematic. The original
procedure[13] which used proline was not practical for large-
scale synthesis owing to the excess amount of furfural
required (10 equivalents), low yield, and low diastereoselec-
tivity [Eq. (1)]. After some experimentation, diol 2 was
obtained in good yield and with good d.e. when the reaction
was conducted without solvent using surfactant-proline con-
[*] Prof. Dr. Y. Hayashi, Dr. M. Shoji,[+] Dr. H. Ishikawa, J. Yamaguchi,
T. Tamura, H. Imai, Y. Nishigaya, K. Takabe
Department of Industrial Chemistry, Faculty of Engineering
Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-
8601 (Japan)
Fax: (+81)3-5261-4631
E-mail: hayashi@ci.kagu.tus.ac.jp
Homepage: http://www.ci.kagu.tus.ac.jp/lab/org-chem1/
Prof. Dr. H. Kakeya
Graduate School of Pharmaceutical Sciences
Kyoto University, Sakyo-ku, Kyoto-city, Kyoto 606-8501 (Japan)
Prof. Dr. H. Osada
Antibiotics Laboratory
RIKEN Discovery Research Institute
Wako, Saitama 351-0198 (Japan)
[+] Present address:
Department of Chemistry, Graduate School of Science
Tohoku University, Sendai 980-8578 (Japan)
[**] This work was supported by a Grant-in-Aid for Creative Scientific
Research from The Ministry of Education, Culture, Sports, Science,
and Technology (MEXT).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802079.
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Scheme 1. Reagents and conditions: a) 33, 48C, 48 h; b) NaBH4, MeOH, 08C, 1 h (77%, 96% ee, anti:syn 6.2:1); c) p-MeOPhCH(OMe)2, PPTS,
benzene, 808C, 1 h (64%, >99% ee after recrystallization); d) DIBAL-H, Et2O, À788C to À108C, 128 h [80% (92% brsm)]; e) SO3·py, DMSO,
Et3N, CH2Cl2, 08C, 45 min (quant.); f) 34, tBuLi, THF, À788C, 1 h; Me2Zn, 08C, 20 min; then 5 at À788C; À358C, 3 h (79%); g) TIPSOTf, 2,6-
lutidine, CH2Cl2, 08C, 23 h (99%); h) O2, Rose Bengal, EtCN, hn, À788C, 8 h; Me2S, À208C, 15 h; DABCO, À208C, 2 h; i) NaBH4, CeCl3·7H2O,
EtOH, 08C, 20 min (81% from 7); j) TrCl, Et3N, CH2Cl2, 238C, 3 h (93%); k) 1H-benzotriazole-1-carbaldehyde, DMAP, CH2Cl2, 238C, 4 h (quant.);
l) [Pd2(dba)3]·CHCl3, nBu3P, HCO2NH4, 1,4-dioxane, 238C, 67 h (76%); m) DDQ, CH2Cl2/pH 7 phosphate buffer, 08C, 4 h (96%); n) 35, DMAP,
Et3N, 08C, 20 min (98%); o) py(HF)x, THF, 238C, 17 h (84%); p) I2, Ph3P, imidazole, benzene, 238C, 30 min; q) 16, LHMDS, THF, À908C,
40 min; then 15 at À908C; À658C, 18 h (78% from 14); r) (Boc)2O, DMAP, CH2Cl2, 238C, 30 min (96%); s) 1,3-propanedithiol, Et3N, MeOH,
238C, 18 h (87%); t) 1-cyclohexenecarboxylic acid, EDCI, DMAP, CH2Cl2, 238C, 21 h (78%); u) pyrrolidine, CH2Cl2, 238C, 5 h; v) NaBH4, EtOH,
508C, 21 h (57% from 20); w) AllocCl, Et3N, CH2Cl2, 238C, 40 min; x) TsOH·H2O, MeOH, 238C, 1 h (94% from 22); y) MnO2, CH2Cl2, 238C,
40 min; z) [Ph3P+CH3]IÀ, tBuOK, THF, 08C, 30 min (74% from 24); aa) 46% HF, MeCN, 238C, 16 h [68% (91% brsm)]; ab) TESOTf, iPr2EtN,
CH2Cl2, 238C, 30 min (99%); ac) NaBH4, S8, THF, 508C, 2.5 h; ad) 42, BOP-Cl, iPr2EtN, toluene, 238C, 8 h; K2CO3, MeOH, 238C, 10 min (79%
from 28); ae) MnO2, CH2Cl2, 238C, 5 min; NaBH4, MeOH, 08C; af) 4-triethylsiloxy-3-penten-2-one, DMF, 238C to 508C, 9 h (77% from 30);
ag) Grubbs I catalyst (40 mol%), CH2Cl2, 238C, 71 h [39% (51% brsm)]; ah) Amberlyst 15, THF/H2O (10:1), 238C, 47 h (95%). Definitions of
acronyms given in reference [28].
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jugated catalyst 33.[14] This catalyst was developed by us for
aldol reactions in the presence of water. As this reaction
proceeds without solvent, scale-up and purification are
straightforward. Diol 2 was treated with p-anisaldehyde
dimethyl acetal in the presence of PPTS to provide 3, which
was isolated in diastereomerically and optically pure form
(> 99% ee) after recrystallization (and without the need for
column chromatography; Scheme 1).
Carboxylic acid 42 was synthesized as shown in Scheme 2.
Proline-mediated a-aminoxylation[24] of aldehyde 36 pro-
ceeded efficiently to provide 37. Under Horner–Emmons
reaction conditions, a crude sample of 37 was converted into
Reduction of 3 with DIBAL-H gave primary alcohol 4 in
80% yield (92% yield based on recovered starting material
(brsm)). Alcohol 4 was oxidized to aldehyde 5 quantitatively.
The reaction of 5 with vinyl zincate,[15] prepared from vinyl
iodide 34 with tBuLi and Me2Zn, proceeded in a highly
diastereoselective manner to afford 6 as a single isomer in
79% yield. Notably, other vinyl metals gave low diastereo-
selectivities.[16] The secondary hydroxy group of 6 was
protected with the TIPS group. The furan ring was cleaved
by oxidation with O2 under irradiation conditions in the
presence of Rose Bengal.[17] Subsequent cis/trans isomeriza-
tion using DABCO, followed by Luche reduction[18] gave diol
8 as a mixture of diastereomers at C10 in 81% yield (over 3
steps). The primary hydroxy group of 8 was protected as the
trityl ether. The free hydroxy group of 9 was converted into
formate ester 10, which was removed by reduction using a
Scheme 2. Reagents and conditions: a) nitrosobenzene, l-proline,
MeCN, À208C, 24 h; b) triethyl phosphonoacetate, NaH, THF, 238C,
45 min; c) CuSO4, MeOH, 08C, 46 h (46% from 36, 98% ee); d) MeI,
NaH, DMF, 08C, 1 h (94%); e) DIBAL-H, CH2Cl2, À788C to À408C,
2 h; f) MnO2, CH2Cl2, 238C, 2 h; g) [Ph3P+CH3]IÀ, tBuOK, THF, 08C,
15 min (66% from 40); h) py(HF)x, MeCN, 08C, 1.5 h; i) SO3·py,
DMSO, Et3N, CH2Cl2, 08C, 50 min; j) NaClO2, NaH2PO4·2H2O, 2-
methyl-2-butene, tBuOH/H2O (3:1), 238C, 1 h (56% from 41).
palladium–PBu3 complex with the protocol developed by
[19]
Tsuji and co-workers
to provide 11 as a single isomer
without positional or E/Z isomerization. Removal of the
PMB group followed by reaction with acid chloride 35[20] gave
ester 13. Selective removal of the TIPS group gave primary
alcohol 14, which was transformed into iodide 15 with PPh3
and I2. Coupling of fragment 15 and sulfone 16 was success-
fully performed by the lithiation of hydroxysulfone 16 with
LHMDS, followed by alkylation using 15 to afford 17 in 78%
yield (over 2 steps). After protection of the phenol of 17 as its
Boc derivative, the azide moiety was reduced to an amine
with 1,3-propanedithiol,[21] and the amide bond with cyclo-
hexenyl carboxylic acid was constructed to provide 20 in good
yield. This completed installation of the side chain.
Carrying out desulfonylation without affecting the nitro
group was difficult. After experimentation, a novel method
was developed which consisted of removal of the Boc group
with pyrrolidine[22] followed by treatment of phenol 21 with
NaBH4. This method provided 22 in 57% yield (over 2 steps)
through a retro-Michael reaction with SO2Ph, probably
involving o-quinonemethide, followed by reduction with
NaBH4. The phenol was protected as its Alloc derivative
and removal of the Tr group gave alcohol 24 in 94% yield
(over 2 steps). Oxidation of 24 with MnO2, followed by a
Wittig reaction gave diene 26 in 74% yield (over 2 steps). As
we could not remove the TIPS group after construction of the
triene moiety, this protecting group was replaced with the
easily removable TES group at this stage. Treatment with HF
provided 27 in 91% yield (brsm), then reaction with TESOTf
alcohol 39 by treatment with CuSO4 in MeOH giving 46%
yield (over 3 steps) with 98% ee. Williamson ether synthesis
gave 40 in 94% yield. Diene 41 was synthesized by a three-
step procedure: reduction with DIBAL-H, oxidation with
=
MnO2, and a Wittig reaction (Ph3P CH2). Carboxylic acid 42
was constructed by removal of the TBS group, oxidation with
SO3·pyridine,[25] and subsequent oxidation by the method of
Pinnick and co-workers.[26]
All that remained to complete the synthesis was the
crucial ring formation. The protecting group of the phenol
was converted from methyl to the more easily removable TES
group through an oxidation/reduction sequence: 1) oxidation
to the quinone with MnO2, 2) reduction to hydroquinone 31
with NaBH4, 3) immediate protection of 31 with 4-triethylsi-
loxy-3-penten-2-one[27] (this was the best silylating reagent in
this particular case as low yields were obtained with other
reagents because of the facile oxidation of hydroquinone 31 to
quinone by adventitious O2). Next RCM methodology, which
had been used by Panek and co-workers in the synthesis of the
core lactam of cytotrienin, was employed.[9] This reaction
proceeded slowly when catalyzed by the first-generation
Grubbs catalyst to afford triene in 39% yield along with
recovered starting material 32 (23%), and therefore, a good
conversion (51% brsm) was obtained. Removal of the TES
group with Amberlyst 15 gave (+)-cytotrienin A (1) in 95%
yield. Synthetic cytotrienin A exhibited spectroscopic proper-
ties identical to those of the natural product[1] (1H NMR and
IR spectroscopy, Rf value, optical rotation, and HPLC
analysis) which confirms the absolute stereochemistry.
In summary, the first asymmetric total synthesis of (+)-
cytotrienin A has been achieved, and its absolute configu-
ration has been confirmed. There are several noteworthy
features to this total synthesis: a practical diastereo- and
afforded 28 quantitatively. The nitro group was reduced with
[23]
NaBH2S3
and was accompanied by removal the Alloc
group to provide 29. The amine 29 was treated with carboxylic
acid 42 (vide infra) in the presence of BOP-Cl to afford 30 in
79% yield (over 2 steps).
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reaction without positional or E/Z isomerization (from 10 to
11), desulfonylation using NaBH4 (from 21 to 22), control of
the absolute configuration at C3 by proline-mediated a-
aminoxylation, and RCM for the formation of the 21-
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Received: May 2, 2008
Published online: July 21, 2008
Keywords: aldol reaction · asymmetric synthesis ·
.
organocatalysis · ring-closing metathesis · total synthesis
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acetone,
DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone,
DIBAL-H = diisobutylaluminum hydride, DMAP = 4-dimethy-
laminopyridine, DMF = N,N-dimethylformamide, DMSO =
dimethyl sulfoxide, EDCI = 1-(3-dimethylaminopropyl)-3-ethyl-
carbodiimide hydrochloride, LHMDS = lithium hexamethyldi-
silazaide, PPTS = pyridinium p-toluenesulfonate, py = pyridine,
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isopropylsilyl, Tr= trityl, Ts = 4-toluenesulfonyl.
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