Total synthesis of N14-desacetoxytubulysin H

Article abstract of DOI:10.1021/ol070415q

Equation presented The N¹⁴-desacetoxy analogue of tubulysin H was prepared in 20 steps and 2.1% overall yield. Our strategy features a thiazole anion addition to assemble the tubuvaline residue at the C(10)-C(11) bond, as well as acylations at

Full text of DOI:10.1021/ol070415q

ORGANIC
LETTERS
2007
Vol. 9, No. 8
1605-1607
Total Synthesis of
N14-Desacetoxytubulysin H
Peter Wipf* and Zhiyong Wang
Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
pwipf+@pitt.edu
Received February 19, 2007
ABSTRACT
The N14-desacetoxy analogue of tubulysin H was prepared in 20 steps and 2.1% overall yield. Our strategy features a thiazole anion addition
to assemble the tubuvaline residue at the C(10)−
C(11) bond, as well as acylations at N5, N14, and N17. This iterative coupling approach, as well
as the removal of the labile N,O-acetal at N14, enables the synthesis of analogues for detailed studies of structure
family of potent tubulin disrupters.
−activity relationships in this
Tubulysins are part of a family of antimitotic agents isolated
in 1996 from the myxobacterial strains Archangium gephyra
and Angiococcus disciformis (Figure 1).1 They possess potent
cell growth inhibitory activity that exceeds that of epothilones,
vinblastine, and taxol. Therefore, they represent attractive
leads for the development of new anticancer agents. Due to
the limited amount and structural diversity generated in the
fermentation process, a total synthesis of tubulysins is highly
desirable. Structurally, these compounds are related to several
short peptide derivatives, including the marine natural
products dolastatins2 and the anticancer drug LU 103793.3
A dipeptide segment comprised of hydrophobic N-terminal
amino acids, N-methylpipecolic acid (Mep), and isoleucine
(Ile) is followed by the thiazole-containing residue tubuvaline
(Tuv), and terminated by tubuphenylalanine (Tup) or tubu-
tyrosine (Tut). The unusual N,O-acetal-containing tubulysins
A-I have IC50 values of 0.3-7 ng/mL against the human
(1) (a) Sasse, F.; Steinmetz, H.; Heil, J.; Ho¨fle, G.; Reichenbach, H. J.
Antibiot. 2000, 53, 879. (b) Steinmetz, H.; Glaser, N.; Herdtweck, E.; Sasse,
F.; Reichenbach, H.; Ho¨fle, G. Angew. Chem., Int. Ed. 2004, 43, 4888.
(2) (a) Pettit, G. R. Pure Appl. Chem. 1994, 66, 2271. (b) Kerbrat, P.;
Dieras, V.; Pavlidis, N.; Ravaud, A.; Wanders, J.; Fumoleau, P. Eur. J.
Cancer 2003, 39, 317.
(3) (a) de Arruda, M.; Cocchiaro, C. A.; Nelson, C. M.; Grinnell, C.
M.; Janssen, B.; Haupt, A.; Barlozzari, D. Cancer Res. 1995, 55, 3085. (b)
Marks, R. S.; Graham, D. L.; Sloan, J. A.; Hillman, S.; Fishkoff, S.; Krook,
J. E.; Okuno, S. H.; Mailliard, J. A.; Fitch, T. R.; Addo, F. Am. J. Clin.
Oncol. 2003, 26, 336.
(4) (a) Do¨mling, A.; Beck, B.; Eichelberger, U.; Sakamuri, S.; Menon,
S.; Chen, Q.-Z.; Lu, Y.; Wessjohann, L. A. Angew. Chem., Int. Ed. 2006,
45, 7235. (b) Peltier, H. M.; McMahon, J. P.; Patterson, A. W.; Ellman, J.
A. J. Am. Chem. Soc. 2006, 128, 16018. (c) Sani, M.; Fossati, G.; Huegenot,
F.; Zanda, M. Angew. Chem., Int. Ed. 2007. Accepted for publication. For
other synthetic approaches, see: (d) Ho¨fle, G.; Glaser, N.; Leibold, T.;
Karama, U.; Sasse, F.; Steinmetz, H. Pure Appl. Chem. 2003, 75, 167. (e)
Friestad, G. K.; Marie´, J.-C.; Deveau, A. M. Org. Lett. 2004, 6, 3249. (f)
Wipf, P.; Takada, T.; Rishel, M. J. Org. Lett. 2004, 6, 4057. For a review,
see: (g) Neri, D.; Fossati, G.; Zanda, M. ChemMedChem 2006, 1, 175.
Figure 1. Structures of tubulysins. A-I are reported in the Ho¨fle
et al. isolation papers,1 and U, V, and Z appear in the work of
Do¨mling et al.4a
10.1021/ol070415q CCC: $37.00
© 2007 American Chemical Society
Published on Web 03/17/2007

cervix carcinoma, multidrug resistant cell line KB-V1.1b
Similar to dolastatin, hemiasterlin, and HTI-286,5a tubulysins
inhibit vinblastine binding to tubulin, disrupt tubulin polym-
erization, and are likely to bind at the peptide site of the
vinca domain of â-tubulin.5b,c
also more attractive for structure-activity studies involving
replacements of the thiazole ring, we decided to pursue this
approach, which was mainly precedented in the use of
2-thiazolyllithium reagents as aldehyde synthons for chain
extensions by Dondoni et al.6e-f
Previously, we prepared an advanced Tuv-Tup derivative,4f
but were unable to efficiently remove the Cbz-protecting
group by various methods including hydrogenation over Pd/
C.6g
Because of their distinct structure and high potency,
tubulysins have attracted considerable synthetic interest.4
Very recently Do¨mling and co-workers synthesized stereo-
isomers of tubulysins U and V using a three-component
condensation approach,4a Ellman and co-workers prepared
tubulysin D featuring tert-butanesulfinamide chemistry,4b and
Zanda and co-workers reported a scalable synthesis of
tubulysins U and V.4c We hypothesized that the conversion
of the labile N,O-acetal functionality in tubulysins A-I to
a simple N-alkyl group would largely conserve the excep-
tional activity but increase the bioavailability of these
analogues. As a proof of principle for this theory, we
embarked on a synthesis of N14-desacetoxytubulysin H (1).
Retrosynthetically, we initially considered disconnecting 1
at all amide bonds and constructing the thiazole ring through
a cyclodehydration process (Figure 2).4f,6a-d However, dis-
A Boc-protecting strategy was thus desired, and we
attempted to construct the syn-alcohol 9b by the addition of
the thiazole anion from 8 to aldehyde 7 (Scheme 1). The
Scheme 1. Synthesis of N-Boc-N-methyl Tubuvaline 11
Figure 2. Alternative retrosynthetic analysis and building blocks
for N14-desacetoxytubulysin H (1) based on carbon-heteroatom
bond formation (pathway A) or carbon-carbon bond formation
(pathway B).
requisite aldehyde 7 was prepared from Cbz-Val-OH 3 by
chain homologation to ester 4, reduction, N-methylation, and
alcohol f aldehyde oxidation protocols. The thiazole 87a was
generated in multigram quantities from thiourea in 30% yield
connection of the C(10)-C(11) bond offered a more
convergent fragment assembly strategy. Since pathway B was
(6) (a) Wipf, P.; Venkatraman, S. Synlett 1997, 1. (b) Wipf, P.; Reeves,
J. T.; Balachandran, R.; Day, B. W. J. Med. Chem. 2002, 45, 1901. (c)
Wipf, P.; Venkatraman, S. J. Org. Chem. 1996, 61, 8004. (d) Wipf, P.;
Aslan, D. C.; Southwick, E. C.; Lazo, J. S. Bioorg. Med. Chem. Lett. 2001,
11, 313. (e) Dondoni, A.; Fantin, G.; Fogagnolo, M.; Medici, A.; Pedrini,
P. J. Org. Chem. 1988, 53, 1748. (f) Dondoni, A.; Marra, A. Chem. ReV.
2004, 104, 2557. (g) Wipf, P.; Rishel, M. J. Unpublished results.
(7) (a) Silberg, A.; Ursu, A. ReV. Roum. Chim. 1965, 10, 897. (b) Kelly,
T. R.; Jagoe, C. T.; Gu, Z. Tetrahedron Lett. 1991, 32, 4263.
(5) (a) Huryn, D. M.; Wipf, P. In Cancer Drug Design and DiscoVery;
Neidle, S., Ed.; Academic Press: San Diego, in press. (b) Khalil, M. W.;
Sasse, F.; Luensdorf, H.; Elnakady, Y. A.; Reichenbach, H. ChemBioChem
2006, 7, 678. (c) Kaur, G.; Hollingshead, M.; Holbeck, S.; Schauer-
Vukasinovic, V.; Camalier, R. F.; Do¨mling, A.; Agarwal, S. Biochem. J.
2006, 396, 235.
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Org. Lett., Vol. 9, No. 8, 2007

over a four-step sequence. Our initial attempt to generate
the thiazole anion in the form of 2-(trimethylsilyl)thiazole6e
was unsuccessful, and the corresponding lithium anion7b
required carefully controlled reaction conditions. Alterna-
tively, the thiazole Grignard reagent8 generated by exchange
with sec-butylmagnesium chloride led to a clean 1,2-addition
to aldehyde 7, forming the separable epimers 9a and 9b in
a 1:2 ratio in 60% yield. The configuration at the newly
formed stereogenic carbon was determined by a double
derivatization method with R-methoxyphenylacetic acid
(MPA).9a The desired major isomer 9b was acylated,
deprotected, and oxidized by a two-step sequence9b-d to give
Tuv derivative 11.
to further utilize 13 in the synthesis. Therefore, a double
protection of the amino group was necessary. Alcohol 14
was smoothly oxidized and allylated to give ester 15, and
the subsequent N-deprotection with trifluoroacetic acid and
coupling with the mixed anhydride of 11 afforded dipeptide
16. Removal of the Boc-group from the methylated N-
terminus of 16 followed by condensation with either Boc-
Ile-OH or Fmoc-Ile-OH, using various coupling agents,
including DEPBT,10 BEP,11 HATU,12 TBTU,13 PyBop,14 and
BOP-Cl,15 always resulted in incomplete conversion and low
yield due to the congested steric environment and the reduced
reactivity of the N-methylated amine.16 After extensive
experimentation, we found that the acyl fluoride Fmoc-Ile-
F17 was superior for this difficult coupling, and 17 was
obtained in 80% yield. Removal of the Fmoc-protective
group and coupling with the pentafluorophenyl ester of
Mep4b gave the desired tetrapeptide intermediate. Finally,
the allyl ester was efficiently removed by Pd(PPh3)4 in the
presence of dimedone18 as an allyl cation scavenger to give
target molecule 1 after HPLC purification in 44% overall
yield for the three steps.
The synthesis of the Tup derivative 14 started from
diastereomerically pure 12 (Scheme 2), which had previously
Scheme 2. Synthesis of 1
In summary, we have developed an asymmetric total
synthesis of N14-desacetoxytubulysin H in 20 steps and 2.1%
yield for the longest linear sequence. The combination of
readily available R-amino acids as building blocks and a
convergent strategy should allow for the synthesis of a range
of stereoisomers and structurally modified tubulysins. Our
disconnection of the tubulysin scaffold at the C(10)-C(11)
bond is unique among the previously reported synthetic
approaches.4 The biological evaluation of 1 and additional
SAR studies will be reported in due course.
Acknowledgment. This work has been generously sup-
ported by a grant from the National Cancer Institute
(CA78039). We thank Prof. M. Zanda (Istituto di Chimica
del Riconoscimento Molecolare, Milano, Italy) for a preprint
of his tubulysin synthesis.
Supporting Information Available: Experimental pro-
cedures and spectral data for all new compounds, including
copies of 1H and 13C NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL070415Q
(9) (a) Seco, J. M.; Quinoa, E.; Riguera, R. Tetrahedron: Asymmetry
2001, 12, 2915. (b) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48,
4155. (c) Bal, B. S.; Childers, W. E., Jr.; Pinnick, H. W. Tetrahedron 1981,
37, 2091. (d) Ashworth, P.; Broadbelt, B.; Jankowski, P.; Kocienski, P.;
Pimm, A.; Bell, R. Synthesis 1995, 199.
(10) Li, H.; Jiang, X.; Ye, Y.-H.; Fan, C.; Romoff, T.; Goodman, M.
Org. Lett. 1999, 1, 91.
(11) Li, P.; Xu, J.-C. Tetrahedron 2000, 56, 8119.
(12) Carpino, L. A.; El-Faham, A.; Albericio, F. Tetrahedron Lett. 1994,
35, 2279.
(13) Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D. Tetrahedron
Lett. 1989, 30, 1927.
(14) Coste, J.; Le-Nguyen, D.; Castro, B. Tetrahedron Lett. 1990, 31,
205.
(15) Tung, R. D.; Rich, D. H. J. Am. Chem. Soc. 1985, 107, 4342.
(16) Humphrey, J. M.; Chamberlin, A. R. Chem. ReV. 1997, 97, 2243.
(17) Carpino, L. A.; Sadat-Aalaee, D.; Chao, H. G.; DeSelms, R. H. J.
Am. Chem. Soc. 1990, 112, 9651.
(18) Kunz, H.; Unverzagt, C. Angew. Chem., Int. Ed. Engl. 1984, 23,
436.
been prepared during our synthesis of tubulysin segments.4f
When 12 was deprotected with TBAF and oxidized, the
N-Boc-pyrrolidinone 13 was obtained, and we were unable
(8) (a) Spiess, A.; Heckmann, G.; Bach, T. Synlett 2004, 131. (b) Abarbri,
M.; Thibonnet, J.; Berillon, L.; Dehmel, F.; Rottla¨nder, M.; Knochel, P. J.
Org. Chem. 2000, 65, 4618.
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