Total synthesis of incednam, the aglycon of incednine

Article abstract of DOI:10.1021/ol102400c

The first total synthesis of incednam (1), the aglycon of antibiotic incednine (2), is described. Incednine has been reported to exhibit significant inhibitory activity against the antiapoptotic oncoproteins Bcl-2 and Bcl-xL. The synthesis of 1 commenced

Full text of DOI:10.1021/ol102400c

ORGANIC
LETTERS
2010
Vol. 12, No. 21
5068-5071
Total Synthesis of Incednam, the
Aglycon of Incednine
Takashi Ohtani,† Shinya Tsukamoto,† Hiroshi Kanda,† Kensuke Misawa,†
Yoshifumi Urakawa,† Takahiro Fujimaki,‡ Masaya Imoto,‡ Yoshikazu Takahashi,§
Daisuke Takahashi,† and Kazunobu Toshima*,†
Department of Applied Chemistry and Department of Biosciences and Informatics,
Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku,
Yokohama 223-8522, Japan, and Institute of Microbial Chemistry, 3-14-23 Kamiosaki,
Shinagawa-ku, Tokyo 141-0021, Japan
toshima@applc.keio.ac.jp
Received October 5, 2010
ABSTRACT
The first total synthesis of incednam (1), the aglycon of antibiotic incednine (2), is described. Incednine has been reported to exhibit significant
inhibitory activity against the antiapoptotic oncoproteins Bcl-2 and Bcl-xL. The synthesis of 1 commenced with the preparation of the C1-C13
subunit 3 and the C14-C23 subunit 4. The construction of the novel 24-membered macrocycle was achieved by the application of a Stille
coupling between 3 and 4, followed by macrolactamization.
Incednam (1) is the aglycon of the 24-membered macrolac-
tam glycoside antibiotic incednine (2), which was isolated
from Streptomyses sp. in 2008.1 It was demonstrated that 2
exhibits significant inhibitory activity against the antiapop-
totic oncoproteins Bcl-2 and Bcl-xL, with a mode of action
distinctly different from those of other compounds that inhibit
the binding capacity of Bcl-xL to the pro-apoptotic protein
Bax. In addition, it is known that these proteins are
overexpressed in many cancer cells, resulting in the expan-
sion of a transformed population and the advancement of
the multidrug-resistant stage.2-4 Therefore, 2 is now expected
to be a compound in the development of novel antitumor
drugs. Furthermore, 2 is likely to be a useful tool for the
further study of Bcl-2 and Bcl-xL functions. The identifica-
tion of its target protein could provide insight into the
antiapoptotic mechanism of the Bcl-2 family proteins. From
a structural perspective, 1 and 2 contain unique salient
features: an R-methoxy-R,ꢀ-unsaturated amide moiety and
two independent conjugated polyene systems embedded in
the 24-membered macrolactam ring. As a result of the nature
of the highly conjugated polyene subunits, 1 and 2 are light-
and acid-sensitive. Although 1 was also isolated from
Streptomyses sp.,1 its semisynthesis from 2 has not been
realized, in part, because of the inherent chemical instabilities
mentioned above. Furthermore, the stereochemical config-
uration at C23 was postulated on the basis of computational
modeling studies, thus the configuration has not been
conclusively defined. Because of its important biological
† Department of Applied Chemistry.
‡ Department of Biosciences and Informatics.
§ Institute of Microbial Chemistry.
(1) Futamura, Y.; Sawa, R.; Umezawa, Y.; Igarashi, M.; Nakamura, H.;
Hasagawa, K.; Yamasaki, M.; Tashiro, E.; Takahashi, Y.; Akamatsu, Y.;
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.
10.1021/ol102400c  2010 American Chemical Society
Published on Web 10/12/2010

activity and novel molecular architecture, 1 and 2 were
considered to be prime targets for chemical synthesis. Herein
we report the first total synthesis of 1 leading to the
unambiguous stereochemical assignment of the configuration
at C23.
Scheme 1. Synthesis of the C1-C13 Subunit 3
Figure 1. Retrosynthetic analysis of incednam (1).
The retrosynthetic analysis of 1 is depicted in Figure 1.
The convergent strategy implemented toward the construction
of the novel 24-membered macrocycle is based on the
coupling of two domains: the C1-C13 subunit 3 containing
the vinyl iodide moiety and the C14-C23 subunit 4
containing the vinyl stannane moiety. This union could be
effected by the application of a Stille coupling and subse-
quent macrolactamization. Our group recently reported an
asymmetric synthesis of 3,5 employing a Sharpless asym-
metric epoxidation as the key step. However, in this sequence
an optical resolution involving the stoichiometric use of an
optical resolution agent was required to provide the enan-
tiomerically pure subunit 3. To circumvent this issue, we
first developed a direct synthetic route toward the stere-
ochemically pure subunit 3, which was facilitated by an
Evans aldol addition thereby establishing the C10 and C11
stereocenters.
and LDA proceeded smoothly to give the desired Evans anti
product 8 in 86% yield with high diastereoselectivity (dr
)10:1). Subsequent cleavage of the chiral auxiliary in 8 with
NaBH4 provided the diol 9 in 82% yield. The primary alcohol
was selectively protected as the pivaloyl ester, and the benzyl
ether was cleaved using BCl3·Me2S8 without affecting the
pendant olefin. The resulting diol was subsequently protected
using TESOTf and 2,6-lutidine to give the silyl ether 10 (74%
overall yield). The disubstituted olefin was oxidatively
cleaved with ozone to afford the aldehyde 11, which was
reduced with NaBH4 to provide the primary alcohol. Protec-
tion of the resulting primary alcohol as the PMB ether using
PMB trichloroacetimidate and Sc(OTf)3,9 provided 12 (86%
overall yield from 10). Cleavage of the pivaloyl ester in 12
using MeMgI,10 without migration of the silyl groups,
followed by Swern oxidation gave the aldehyde 13.
Horner-Wadsworth-Emmons olefination of 13 with the
phosphonate ester 1411 provided the R,ꢀ,γ,δ-unsaturated ester
The synthesis of the pentaenoate subunit 3, corresponding
to C1-C13 in 1, is summarized in Scheme 1. The known
1,3-diol 96 was prepared from methyl L-lactate (5). Benzy-
lation of 5 and subsequent coupling with the chiral auxiliary
67 provided the oxazolidinone derivative 7. Following
Kobayashi’s procedure, the titanium-mediated Evans aldol
addition of 7 with trans-crotonaldehyde using Ti(OiPr)3Cl
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Tetrahedron Lett. 2009, 50, 2270.
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Org. Lett., Vol. 12, No. 21, 2010
5069

15 in high yield with high stereoselectivity (83% overall yield
from 12 with E:Z ) >95:5). The building block 15 was an
intermediate in our previous studies and was converted into
the C1-C13 subunit 3 in 8 steps in high overall yield.5
Next, the synthesis of the C14-C23 subunit 4 was established
starting from the commercially available alcohol 16 as shown
in Scheme 2. The one-pot Swern oxidation-Wittig olefination12
the allylic bromide 22 without olefin isomerization; 22 was
readily converted into the phosphonium salt 23 (67% overall
yield from 21). The Wittig olefination of 23 with the known
aldehyde 2514 (prepared from ethyl 2-butynoate (24) in 5 steps)
was examined under several conditions. As a result, it was found
that the desired E-configured tetraene 4 could be obtained in
32% yield by using NaH and DMSO in THF15 at 0 °C. Because
4 is extremely prone to oxidation and/or photochemical
decomposition, it was used in the next step immediately after
isolation. Considering the inherent convergency of the chemical
union of 23 and 25, the modest yield obtained in the olefination
reaction is well justified. This strategy leads to the direct
construction of the labile tetraene 4, which contains the vinyl
stannane moiety necessary for the subsequent Stille coupling.
Completion of the synthesis of 1 is summarized in Scheme
3. Conditions for the Stille coupling16 of 3 with 4 was
Scheme 2. Synthesis of the C14-C23 Subunit 4
Scheme 3. Total Synthesis of Incednam (1)
of 16 gave the R,ꢀ-unsaturated ester 17 in 98% yield with
complete stereoselectivity. After the reduction of 17 with
DIBAL, the resulting allylic alcohol was protected as the
TBDPS ether in 98% overall yield. Deprotection of the
acetonide in 17 under acidic conditions and subsequent selective
tosylation of the resulting primary alcohol using Bu2SnO,
p-TsCl and Et3N13 afforded 19. The intramolecular epoxide
formation using NaOMe, followed by the selective ring opening
of the resulting epoxide with LiAlH4, led to 20 (60% overall
yield from 18). The secondary alcohol 20 was converted into
the N-substituted phthalimide 21 with stereochemical inversion
by the Mitsunobu reaction using phthalimide, DIAD, and PPh3
in 76% yield. Deprotection of the TBDPS ether in 21 with
HF·pyridine and subsequent bromination with PBr3 provided
rigorously explored. After much experimentation, it was
found that Corey’s protocol17 using Pd(0), LiCl, and CuCl
gave the best result, providing the desired coupling product
26 in 74% yield. Removal of the phthaloyl group in 26
using ethylenediamine provided the amine 27 in 76%
(14) (a) Betzer, J. F.; Delaloge, F.; Muller, B.; Panqrazi, A.; Pruner, J.
J. Org. Chem. 1997, 62, 7768. (b) Michels, D. T.; Uhee, J. U.; Vanderwal,
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M. P.; Rajappa, M. Org. Lett. 1999, 1, 447.
5070
Org. Lett., Vol. 12, No. 21, 2010

yield. Saponification of the methyl ester in 27 using
TMSOK,18 followed by treatment using reversed phase
column chromatography, provided the potassium carboxy-
late 28. Without further purification, macrolactamization
of 28 utilizing DMT-MM19 in MeOH was conducted.
Gratifyingly, it was found that the macrolactamization
proceeded smoothly to give the desired cyclic compound
29 in 27% overall yield from 27. Deprotection of the TES
groups in 29 was effected with TBAF and AcOH to furnish
incednam (1). Data for an analytical sample of the
synthetic incednam obtained by 1H NMR, 13C NMR,
HRMS (ESI-TOF), and optical rotation matched those
obtained for an authentic sample.1 These results clearly
confirm not only the complete synthesis of 1, but also
serve to conclusively establish the stereochemical con-
figuration at C23 (S configuration), which was previously
posited purely on the basis of molecular modeling.
In conclusion, we have developed a convergent synthetic
route to incednam (1), which is the aglycon of the 24-
membered macrolactam glycoside antibiotic incednine (2).
Furthermore, this synthesis serves to unambiguously define
the stereochemical configuration at C23 in 1. Additional
studies with respect to the total synthesis of incednine (2)
from 1 are currently underway in our laboratory.
Acknowledgment. With great respect, we dedicate this
work to Professor Kuniaki Tatsuta on the occasion of his
70th birthday. This research was supported in part by the
21st Century COE Program “Keio Life-Conjugated Chem-
istry”, High-Tech Research Center Project for Private
Universities: Matching Fund Subsidy, 2006-2011, and JSPS
Fellow 22·5820 from the Ministry of Education, Culture,
Sports, Science and Technology of Japan (MEXT).
Supporting Information Available: Experimental pro-
cedures and compound characterizations. This material is
available free of charge via the Internet at http://pubs.acs.org.
(18) Laganis, E. D.; Chenard, B. L. Tetrahedron Lett. 1984, 25, 5831.
(19) Kunishima, M.; Kawachi, C.; Hioki, K.; Terao, K.; Tani, S.
Tetrahedron 2001, 57, 1551.
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