Total synthesis of (R)-telomestatin

Article abstract of DOI:10.1021/ol061793i

We have achieved a total synthesis of telomestatin, and its absolute configuration was determined to be (R). Coupling of cysteine-containing trisoxazole amine and serine-containing trisoxazole carboxylic acid, followed by macrocyclization, provided a 24-m

Full text of DOI:10.1021/ol061793i

ORGANIC
LETTERS
2006
Vol. 8, No. 18
4165-4167
Total Synthesis of (R)-Telomestatin
Takayuki Doi,*,† Masahito Yoshida,† Kazuo Shin-ya,§,‡ and Takashi Takahashi*,†
Department of Applied Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguro, Tokyo 152-8552, Japan, The UniVersity of Tokyo, Institute of Molecular and
Cellular Biosciences, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan, and National
Institute of AdVanced Industrial Science and Technology (AIST), Biological
Information Research Center (BIRC), Protein Network Team, Low M.W. Chemical
Laboratory, AIST Tokyo Waterfront Bio-IT Research Building, 2-42 Aomi, Koto-ku,
Tokyo 135-0064, Japan
doit@apc.titech.ac.jp; ttak@apc.titech.ac.jp
Received July 21, 2006
ABSTRACT
We have achieved a total synthesis of telomestatin, and its absolute configuration was determined to be (R). Coupling of cysteine-containing
trisoxazole amine and serine-containing trisoxazole carboxylic acid, followed by macrocyclization, provided a 24-membered diamide. The
seventh oxazole ring was formed by a Shin’s procedure via dehydroamide. Cyclodehydration of a modified (R)-cysteine-(S-tBu) moiety using
Kelly’s method (PPh3(O)−Tf2O) with anisole furnished (R)-telomestatin, whose CD spectrum was in good agreement with that of the natural
product.
Telomestatin (1), isolated from Streptomyces anulatus 3533-
SV4, is a potent specific telomerase inhibitor (IC50 ) 5.0
nM) because it acts on a human telomere sequence to
stabilize the specific DNA structure called G-quadruplex
without affecting DNA polymerases or reverse trans-
criptases.1-3 The unique macrocyclic structure of telomestatin
consists of the macrocyclic linkage of two methyloxazoles,
five oxazoles, and one thiazoline ring. Although several
synthetic approaches to telomestatin4 and an oligo-oxazole
ring system5 have been reported, a total synthesis of
telomestatin has been reported only in a patent.6 We wish
to report a total synthesis of telomestatin (1) and its complete
stereochemical assignment.
As a thiazoline is labile for hydrolysis, the thiazoline ring
in 1 would be formed at the final stage in the synthesis. We
selected N-Boc-Cys-(S-tBu)-Thr-OMe (2) and 2,5-disubsti-
tuted oxazoles 3 and 4 as the constituent units (Figure 1).
Oxidation of the secondary alcohol in dipeptide 2 with SO3‚
† Tokyo Institute of Technology.
§ The University of Tokyo.
(4) Endoh, N.; Tsuboi, K.; Kim, R.; Yonezawa, Y.; Shin, C. Heterocycles
2003, 60, 1567-1572.
‡ National Institute of Advanced Industrial Science and Technology.
(1) Shin-ya, K.; Wierzba, K.; Matsuo, K.; Ohtani, T.; Yamada, Y.;
Furihata, K.; Hayakawa, Y.; Seto, H. J. Am. Chem. Soc. 2001, 123, 1262-
1263.
(2) (a) Kim, M.-Y.; Vankayalapati, H.; Shin-ya, K.; Wierzba, K.; Hurley,
L. H. J. Am. Chem. Soc. 2002, 124, 2098-2099. (b) Rezler, E. M.;
Seenisamy, J.; Bashyam, S.; Kim, M.-Y.; White, E.; Wilson, W. D.; Hurley,
L. H. J. Am. Chem. Soc. 2005, 127, 9439-9447.
(3) Rosu, F.; Gabelica, V.; Shin-ya, K.; De Pauw, E. Chem. Commun.
2003, 2702-2703.
(5) (a) Chattopadhyay, S. K.; Kempson, J.; McNeil, A.; Pattenden, G.;
Reader, M.; Rippon, D. E.; Waite, D. Perkin Trans. 1 2000, 2415-2428.
(b) Deeley, J.; Pattenden, G. Chem. Commun. 2005, 797-799. (c) Atkins,
J. M.; Vedejs, E. Org. Lett. 2005, 7, 3351-3354. (d) Riego, E.; Herna´ndez,
D.; Albericio, F.; AÄ lvarez, M. Synthesis 2005, 1907-1922. (e) Chatto-
padhyay, S. K.; Biswas, S.; Pal, B. K. Synthesis 2006, 1289-1294.
(6) Yamada, S.; Shigeno, K.; Kitagawa, K.; Okajima, S.; Asao, T. (Taiho
Pharmaceutical Co. Ltd., Sosei Co. Ltd.). WO 200248153; Chem. Abstr.
2002, 137, 47050.
10.1021/ol061793i CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/09/2006

clodehydration of 7 using the Burgess reagent (Et3NSO2-
NCO2Me),10 followed by treatment with BrCCl3-DBU11
afforded Cys-containing trisoxazole 8 in 75% yield.
A trisoxazole unit 11 was prepared from the coupling
between 2,5-disubstituted oxazole units 4a5a and 4b (Scheme
2). Acid cleavage of the N,O-acetonide and N-Boc groups
Scheme 2. Synthesis of Trisoxazole 11
Figure 1. Structure of telomestatin (1) and synthetic units 2-4.
in 4b, followed by condensation of the resultant amine with
acid 4a, provided bisoxazole amide 9 in 90% yield. Cyclo-
dehydration of 9 with diethylaminosulfur trifluoride (DAST)12
and oxazole formation with BrCCl3-DBU afforded tris-
oxazole 10,13 in which hydrogenolysis of the benzyl ester
furnished acid 11.
Py, followed by immediate cyclodehydration with PPh3-I2
in the presence of NEt3, provided 2,4,5-trisubstituted oxazole
5 in 73% yield (Scheme 1).7 Hydrolysis of ester 5 with LiOH,
Removal of the Boc group in 8 with 4 M HCl in dioxane
afforded the corresponding ammonium salt without affecting
the S-tBu group. Coupling of the resulting amine with acid
11 using PyBrop-DIEA provided hexaoxazole amide 12 in
85% yield (Scheme 3). Acid cleavage of the N,O-acetonide
and N-Boc groups in 12, followed by hydrolysis of the
methyl ester with LiOH, afforded the cyclization precursor.
Macrolactamization was performed using DPPA14-HOBt-
DIEA in the presence of DMAP under high dilution
conditions (3 mM) at room temperature for 3 days. Hexa-
oxazole-containing cyclic diamide 13 was isolated in 48%
overall yield after silica gel column chromatography.
It was crucial to form the seventh oxazole ring in 16 from
13. Attempts for direct cyclodehydration of 13 with DAST
or the Burgess reagent did not afford the desired oxazoline
rings except 14. Oxidation of the primary alcohol in 13,
followed by cyclodehydration, did not proceed. Therefore,
we investigated the formation of the seventh oxazole ring
Scheme 1. Synthesis of Trisoxazole 8
(10) (a) Burgess, E. M.; Penton, H. R.; Taylor, E. A. J. Org. Chem.
1973, 38, 26-31. (b) Wipf, P.; Miller, C. P. J. Org. Chem. 1993, 58, 1575-
1578. (c) Aguilar, E.; Meyers, A. I. Tetrahedron Lett. 1994, 35, 2477-
2480.
(11) Williams, D. R.; Lowder, P. D.; Gu, Y.-G.; Brooks, D. A.
Tetrahedron Lett. 1997, 38, 331-334.
followed by coupling of the resulting acid 6 with amine 34
using PyBroP8-DIEA, afforded bisoxazole amide 7.9 Cy-
(7) Wipf, P.; Miller, C. P. J. Org. Chem. 1993, 58, 3604-3606.
(8) PyBroP ) bromo-trispyrrolidino-phosphonium hexafluorophosphate.
(a) Coste, J.; Dufour, M.-N.; Pantaloni, A.; Castro, B. Tetrahedron Lett.
1990, 31, 669-672. (b) Coste, J.; Fre´rot, E.; Jouin, P.; Castro, B.
Tetrahedron Lett. 1991, 32, 1967-1970.
(9) Several condensation reagents were investigated using the corre-
sponding S-trityl derivative: PyBrop, 90%; HATU-HOAt, 78%; PyBop-
HOBt, 69%; EDCI-HOBT, 53%.
(12) (a) Burrell, G.; Evans, J. M.; Jones, G. E.; Stemp, G. Tetrahedron
Lett. 1990, 31, 3649-3652. (b) Lafargue, P.; Guenot, P.; Lellouche, J.-P.
Heterocycles 1995, 41, 947-958. (c) Phillips, A. J.; Uto, Y.; Wipf, P.;
Reno, M. J.; Williams, D. R. Org. Lett. 2000, 2, 1165-1168.
(13) Chattopadhyay et al. recently reported the synthesis of its methyl
ester. See ref 5e.
(14) DPPA ) diphenyl phosphorazidate. Shioiri, T.; Ninomiya, K.;
Yamada, S. J. Am. Chem. Soc. 1972, 94, 6203-6205.
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Org. Lett., Vol. 8, No. 18, 2006

presence of MS4A formed an R-methoxy-â-bromo cyclic
peptide (90%).15 Subsequent cyclization with K2CO3 afforded
4-methoxyoxazoline derivatives 15 as a 1:1 mixture of
diastereomers (79%).16
Scheme 3. Total Synthesis of Telomestatin (1)
Treatment of 15 with camphorsulfonic acid (5 equiv) in
toluene at 70 °C in the presence of MS5A resulted in the
elimination of MeOH leading to the desired cyclic hepta-
oxazole 16.17 Finally, thiazoline formation was carried out
by Kelly’s method18 with a modification. Cyclodehydration
and deprotection of the S-tBu group in 16 were performed
using PPh3(O)-Tf2O-anisole in CH2Cl2 at room temperature
to provide (R)-telomestatin (1).19 It was difficult, however,
to purify 1 by either normal-phase or reversed-phase HPLC.
The low isolated yield (20%) was due to the property of the
product. The spectral data (NMR, UV, CD) of the synthetic
(R)-1 were in good agreement with those of the natural
product. It was determined that the configuration of the
cysteine residue in natural telomestatin was (R).
In conclusion, we have demonstrated the total synthesis
of optically active telomestatin (1) by a convergent route:
condensation of Cys-(S-tBu)-containing trisoxazole 8 and
trisoxazole carboxylic acid 11, macrolactamization, formation
of the seventh oxazole ring, and construction of the thiazoline
ring by a modified Kelly’s method. On the basis of the
synthesis, the absolute configuration of telomestatin was
determined to be (R). Further study for the synthesis of its
analogues and their biological evaluation is underway in our
laboratories.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No.
17035026, T.D.) from MEXT. We would like to thank Prof.
Kazuo Nagasawa (Tokyo University of Agriculture and
Technology) for a fruitful discussion. We also deeply thank
Mr. Katsuhiko Kushida (Varian Technologies Japan, Ltd.),
Prof. Hisakazu Mihara, and Dr. Tsuyoshi Takahashi
(Tokyotech) for measurement of the NMR and CD spectra
of telomestatin.
Supporting Information Available: Experimental
details, NMR spectra of 12-16, and NMR, UV, and CD
spectra of 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL061793I
(15) In the absence of MS4A, the sulfide moiety was partially oxidized.
The S-trityl derivative was decomposed under these reaction conditions.
(16) Cs2CO3 is usually selected for an oxazoline formation. However,
the elimination of thiol was observed in this system.
(17) In the absence of MS5A, ring opening of the oxazoline by hydrolysis
was observed.
(18) You, S.; Razavi, H.; Kelly, J. W. Angew. Chem., Int. Ed. 2003, 42,
83-85.
(19) Without addition of anisole, the reaction was not complete.
via dehydroamide 14 according to the method reported by
Shin et al.4 Mesylation of 13 with DBU provided 14 in 92%
yield. Treatment of 14 with NBS in CH2Cl2-MeOH in the
Org. Lett., Vol. 8, No. 18, 2006
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