Synthesis of (+)-vinblastine and its analogues

Article abstract of DOI:10.1021/ol702040y

(Chemical Equation Presented) A synthetic route to vinblastine and its analogues with an ethynyl group, which features a stereoselective coupling of an 11-membered key intermediate with vindoline, is described. Transformations of the alkynyl moiety including a partial reduction as well as a Sonogashira coupling furnished a variety of analogues.

Full text of DOI:10.1021/ol702040y

ORGANIC
LETTERS
2007
Vol. 9, No. 23
4737-4740
Synthesis of (
Analogues†
+)-Vinblastine and Its
Tohru Miyazaki, Satoshi Yokoshima, Siro Simizu, Hiroyuki Osada,
Hidetoshi Tokuyama,‡ and Tohru Fukuyama*
Graduate School of Pharmaceutical Sciences, UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan, and Antibiotics Laboratory, DiscoVery Research
Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
fukuyama@mol.f.u-tokyo.ac.jp
Received August 20, 2007
ABSTRACT
A synthetic route to vinblastine and its analogues with an ethynyl group, which features a stereoselective coupling of an 11-membered key
intermediate with vindoline, is described. Transformations of the alkynyl moiety including a partial reduction as well as a Sonogashira coupling
furnished a variety of analogues.
Vinblastine (1), isolated from Catharanthus roseus,1 is
known to inhibit the self-assembly of tubulin into microtubles
and to prevent cell mitosis. These bioactivities have made
vinblastine an effective cancer chemotherapeutic agent
especially for treating leukemia and lymphoma patients.2 The
recent disclosure of the X-ray structure of a vinblastine-
tubulin complex would help reveal the molecular mechanism
of vinblastine and hence would make it possible to develop
novel antitumor drugs by structure-based drug design
(SBDD).3 A variety of vinblastine derivatives have been
prepared to date through total syntheses of vinblastine4 as
well as syntheses of its analogues,5 which have revealed that
the antitumor activity of vinblastine is highly affected by
the substituents at C-4′. Following completion of a total
synthesis of vinblastine in 2002,6 we decided to synthesize
a variety of its analogues based on our synthetic route, hoping
to find a more effective antitumor agent. Herein we disclose
a novel synthetic route to vinblastine and its analogues that
is amenable to synthesis of a range of C-4′ derivatives.
To attain the maximum flexibility for modification at the
C-4′ position of vinblastine, we opted to introduce an ethynyl
group in place of the ethyl group. This would allow Pd-
mediated transformations at a later stage of the synthesis,
enabling us to synthesize a variety of analogues with minimal
effort. The retrosynthetic analysis of the analogue 3 is
* Address correspondence to this author at the University of Tokyo.
† Dedicated to the memory of Professor Yoshihiko Ito (1937-2006).
‡ Current address: Graduate School of Pharmaceutical Sciences, Tohoku
University, Aramaki, Aoba, Sendai 980-8578, Japan.
(1) (a) Noble, R. L.; Beer, C. T.; Cutts, J. H. Ann. N.Y. Acad. Sci. 1958,
76, 882. (b) Svoboda, G. H.; Neuss, N.; Gorman, M. J. Am. Pharm. Assoc.,
Sci. Ed. 1959, 48, 659.
(2) Rowinsky, E. K.; Donehower, R. C. Cancer Chemotherapy and
Biotherapy; Chbner, B. A., Longo, D. L., Eds.; Lippincott-Raven Publish-
ers: Philadelphia, PA, 1996; p 263.
(3) Gigant, B.; Wang, C.; Ravellli, R. B. G.; Roussi, F.; Steinmetz, M.
O.; Curmi, P. A.; Sobel, A.; Knossow, M. Nature 2005, 435, 519.
Figure 1. Structures of vinblastine and vindoline.
10.1021/ol702040y CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/13/2007

Scheme 1
illustrated in Scheme 1. In our previous synthesis of
the ketone of 14 proceeded with excellent diastereoselectivity
(98:2) to give the optically pure diol 15 in 87% yield (2
steps), after removal of the BOM group and recrystallization.
Oxidation of the secondary alcohol followed by Baeyer-
Villiger oxidation of the resulting ketone furnished cleanly
the bicyclic lactone 16 in 78% yield (2 steps). Lactone 17
was prepared from 16 in 86% overall yield by way of a four-
step sequence involving ethanolysis of the lactone, NaBH4
reduction of the resulting lactol, acid-catalyzed lactonization,
and protection of the diol as the TES ethers.
vinblastine, we performed a stereoselective coupling of
vindoline (2) with the 11-membered upper segment, which
was prepared by means of a radical cyclization of o-
alkenylthioanilides7 as well as a macrocyclization of ni-
trobenzenesulfonamide.8 According to this retrosynthesis,
analogue 3 could be obtained from the key intermediate 4,
which in turn would be derived from lactone 6 via thioamide
5. Lactone 6 could be synthesized from cis-fused bicyclic
lactone 7 in a simple operation. Crucial addition of an ethynyl
group to ketone 8 is likely to occur from the less hindered,
convex face. Finally, ketone 8 would be derived from the
readily available carboxylic acid 9.
With the requisite lactone 17 in hand, we next turned our
attention to the construction of the indole core using a radical
Our synthesis commenced with the preparation of the
known optically active malonate 10 (96% ee) according to
Trost’s procedure9 (Scheme 2). Malonate 10 was converted
into carboxylic acid 9 in 77% yield via Krapcho decar-
balkoxylation10 followed by base hydrolysis. Oxidative
lactonization of 9 catalyzed by H2WO4 afforded a lactone
(71%),11 which was protected as its BOM ether (93%).
DIBAL reduction of the lactone 11 followed by Wittig olefin-
ation afforded enol ether 12, which was cyclized under weak-
ly acidic conditions to give dihydropyran 13 in 80% for the
three steps. Hydroboration of 13 followed by oxidative
workup afforded an alcohol, which was subjected to TPAP
oxidation12 to give the requisite ketone 14 in 80% yield (2
steps). As expected, addition of lithium TIPS acetylide to
Scheme 2
(4) (a) Mangency, P.; Adriamialisoa, R. Z.; Langlois, N.; Langlois, Y.;
Potier, P. J. Am. Chem. Soc. 1979, 101, 2243. (b) Kutney, J. P.; Choi, L.
S. L.; Nakano, J.; Tsukamoto, H.; McHugh, M.; Boulet, C. A. Heterocycles
1988, 27, 1845. (c) Kuehne, M. E.; Matson, P. A.; Bornmann, W. G. J.
Org. Chem. 1991, 56, 513. (d) Magnus, P.; Mendoza, J. S.; Stamford, A.;
Ladlow, M.; Willis, P. J. Am. Chem. Soc. 1992, 114, 10232.
(5) For a review on medicinal chemistry of vinblastine, see: Antitumor
Bisindole Alkaloids from Catharanthus roseus (L.). In The Alkaloids; Brossi,
A., Suffness, M., Eds.; Academic Press Inc.; San Diego, CA, 1990; Vol.
37, Chapters 3 and 4, pp 133-204.
(6) Yokoshima, S.; Ueda, T.; Kobayashi, S.; Sato, A.; Kuboyama, T.;
Tokuyama, H.; Fukuyama, T. J. Am. Chem. Soc. 2002, 124, 2137.
(7) Tokuyama, H.; Yamashita, T.; Reding, M. T.; Kaburagi, Y.;
Fukuyama, T. J. Am. Chem. Soc. 1999, 121, 3791.
(8) (a) Kan, T.; Fukuyama, T. Chem. Commun. 2004, 353. (b) Kan, T.;
Fukuyama, T. J. Synth. Org. Chem., Jpn. 2001, 59, 779.
(9) Trost, B. M.; Bunt, R. C. Angew. Chem., Int. Ed. Engl. 1996, 35, 99.
(10) Krapcho, A. P. Synthesis 1982, 805.
(11) Ishii, Y.; Nakano, T.; Inoue, K. WO0206262.
(12) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Synthesis
1994, 639.
4738
Org. Lett., Vol. 9, No. 23, 2007

Scheme 3
Scheme 4
cyclization of o-alkenylthioanilide (Scheme 3).7 Addition of
the enolate derived from lactone 17 to isothiocyanate 18 was
effectively performed with use of KHMDS as a base in
THF-HMPA (5:1) to give thioanilide 19 in 88% yield as an
inseparable diastereomeric mixture at C-18′. The crucial rad-
ical cyclization of 19 proceeded smoothly even in the pres-
ence of the acetylene moiety to afford indole 20 in 77% yield.
Extensive investigations to construct the 11-membered
ring13 led us to the conclusion that cyclization of 27 by means
of the Mitsunobu reaction14 would be the most effective
route. Thus, 20 was converted into the macrocyclization
precursor 27 as shown in Scheme 4. Opening the lactone
followed by a sequence of protecting group manipulations
gave diol 23 in 82% overall yield, which was converted into
epoxide 24 via a selective tosylation of the primary alcohol.15
Regioselective opening of the epoxide with sodium azide
furnished 25. Since preliminary studies of the macrocycliza-
tion under Mitsunobu conditions revealed that the R isomer
at C-18′ of 25a suffered from substantial elimination to give
the undesired dehydrated product 28, the mixture of the
epimers at C-18′ was separated at this stage by silica gel
column chromatography. The undesired R isomer 25a thus
obtained was subjected to epimerization with DBU to give
a 1:1 mixture of the R and â epimers. Iteration of the
separation-epimerization procedure four times furnished the
desired â isomer 25b in 74% combined yield from 23.
Reduction of the azide of 25b16 and subsequent nosylation
with p-NsCl afforded 26, which was converted into the
cyclization precursor 27 through a series of protecting group
manipulations in 56% overall yield.
The crucial Mitsunobu cyclization was successfully per-
formed by treatment of 27 with TMAD and n-Bu3P in
toluene,17 giving the desired 11-membered cyclic compound
29 (Scheme 5). Global cleavage of the TMS, MOM, and Boc
Scheme 5
(13) The attempted macrocyclization of the nosyl amide and the epoxide
in 21, according to the strategy employed in our previous total synthesis,
resulted in a low-yield formation of the desired macrocyclic compound 22
along with a substantial amount of a byproduct. While the structure of the
byproduct could not be determined, its 1H NMR suggested a decomposition
of the Ns moiety.
groups, introduction of a Ts group at the primary alcohol,
and a subsequent protection of the tertiary alcohol furnished
the key intermediate 30 in 81% yield for the four steps.
With use of the key intermediate 30, the critical coupling
reaction with the lower half, (-)-vindoline, was performed
(14) Mitsunobu, O. Synthesis 1981, 1.
(15) Yoshida, Y.; Shimonishi, K.; Sakakura, Y.; Okada, S.; Aso, N.;
Tanabe, Y. Synthesis 1999, 1633.
(16) Bartra, M.; Romea, P.; Urp´ı, F.; Vilarrasa, J. Tetrahedron 1990,
46, 587.
Org. Lett., Vol. 9, No. 23, 2007
4739

Scheme 6
Scheme 7
of vinblastine, the ethynyl analogue 35 slightly decreased
the cell growth inhibitory activity. The analogues 37a-c with
the arylacetylene moieties lost the activity.
Table 1. Cell Growth Inhibitory Activity of the Vinblastine
Analogues against K562 Cell Line
(Scheme 6). Chlorination of 30 with t-BuOCl afforded
chloroindolenine 31, which was immediately treated with
TFA in the presence of (-)-vindoline (2) to provide the
desired coupling product 33 in 84% yield (2 steps) as the
single detectable product. This coupling reaction is believed
to proceed via the iminium intermediate 32. Removal of the
p-Ns group followed by heating at 60 °C in the presence of
sodium bicarbonate provided vinblastine analogue 34 in 77%
yield (2 steps) with an ethynyl moiety at C-4′. To confirm
the structure including the absolute configuration, 34 was
converted into (+)-vinblastine (1) in 89% yield via a conven-
tional two-step process. The physical data of the synthetic
product were identical with those of natural (+)-vinblastine.
With the requisite vinblastine analogue 34 possessing the
ethynyl moiety at C-4′ in hand, we next investigated the
synthesis of vinblastine analogues by manipulation of the
ethynyl moiety (Scheme 7). Partial reduction of 34 followed
by desilylation provided vinyl analogue 36. Sonogashira
coupling of 34 afforded a variety of analogues. For example,
coupling with p-nitroiodobenzene proceeded under conven-
tional conditions to give 37a. While other aryl iodide such
as p-methoxyiodobenzene or 2-iodopyridine gave a consider-
able amount of the acetylene dimer of 34, use of P(2-furyl)3
as the ligand instead of PdCl2(PPh3)2 effectively suppressed
the undesired dimerization to give 37b and 37c, resepectively,
in good yields.
compd
IC50 (nM)
35
36
37a
37b
37c
27
0.89
>10000
3900
6600
vinblastine (1)
0.76
In conclusion, we have successfully developed a versatile
synthetic route to vinblastine and its analogues, which is
amenable to the preparation of a range of analogues at C-4′.
The further bioassay of the analogues is currently underway
and will be reported in due course.
Acknowledgment. This work was financially supported
in part by Grant-in-Aid (15109001, 16073205, and 17689003)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan, and PRESTO-JST. T.M. is a research
fellow of the Japan Society for the Promotion of Science
(JSPS). We would like to thank Harumi Ichimiya for
technical assistance. We are indebted to Dr. Kunisuke Izawa
of Ajinomoto Co., Inc., for an ample supply of (-)-vindoline.
Supporting Information Available: Experimental details
and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
Using the analogues thus obtained, cell growth assay with
the K562 cell line was conducted (Table 1).18 While the IC50-
value of the vinyl analogue 36 was almost the same as that
OL702040Y
(18) The effects of drugs on cell growth were measured by the WST-8
reagent assay. Ishida, K.; Wierzba, M. K.; Teruya, T.; Simizu, S.; Osada,
H. Chem. Biol. 2004, 11, 367.
(17) Tsunoda, T.; Otsuka, J.; Yamamiya, Y.; Itoˆ, S. Chem. Lett. 1994,
539.
4740
Org. Lett., Vol. 9, No. 23, 2007