C O M M U N I C A T I O N S
Scheme 3a
Scheme 4a
a Reagents and conditions: (a) pentafluorophenol, DIC, CH2Cl2; (b) 4,
i-Pr2EtN, DMF; (c) acetic anhydride, pyridine, then H2O/dioxane.
for the synthesis of Tup and Tuv and conditions for reducing 7
(Scheme 2).11 The synthesis and biological activity of analogues
will be reported in due course.
a Reagents and conditions: (a) i-Pr2EtN, CH2Cl2; (b) TESOTf, lutidine,
CH2Cl2; (c) KHMDS, THF, -45 °C, then ClCH2OCOCH2CH(CH3)2; (d)
Mep pentafluorophenyl ester, H2, Pd/C, EtOAc; (e) AcOH/THF/H2O; (f)
Me3SnOH, Cl(CH2)2Cl, 60 °C.
Acknowledgment. This work was supported by the NSF (CHE-
0446173). H.M.P. gratefully acknowledges a graduate fellowship
from Eli Lilly, and A.W.P. acknowledges an ACS Medicinal
Chemistry Fellowship sponsored by BMS. We thank Dr. Frederick
J. Hollander and Dr. Allen G. Oliver of the Berkeley CHEXray
facility for solving the X-ray crystal structures for the determination
of the absolute configurations of 4 and 9.
isobutyl carbonate was next attempted. A number of bases with
lithium, sodium, and potassium counterions in a number of solvents
were evaluated with KHMDS in THF providing the highest yield
(73%). The alkylation reaction proved to be very sensitive to sterics.
For example, in investigations of alcohol protecting groups, we
found that <10% alkylation occurred when the TES group was
replaced by a TIPS group. The azide served as an ideal masking
group not only because it prevented Ile N-alkylation but also
because it could be reduced under neutral reaction conditions that
did not result in any cleavage of the labile N,O-acetal. Pd-catalyzed
hydrogenation in the presence of the pentafluorophenyl ester of
Mep followed by silyl ether deprotection gave the tripeptide product
14 in 67-78% yield for the two steps. Under these conditions,
undesired cyclization of the amine intermediate upon the N,O-acetal
functionality was not observed. Moreover, by coupling the L-
enantiomer of Mep, we further demonstrated that 14 was not
contaminated with the undesired diastereomer. Selective cleavage
of the methyl ester without hydrolysis of the more reactive N,O-
acetal was next accomplished by employing Me3SnOH, which
Nicolaou had demonstrated to be effective for the highly selective
hydrolysis of methyl esters over more hindered ester derivatives.15
Treatment of 14 with Me3SnOH at 60 °C for 20 h resulted in <5%
cleavage of the N,O-acetal, and the desired acid 15 was obtained
in 67% yield.
Incorporation of the Tup fragment was accomplished by activa-
tion of acid 15 as the pentafluorophenyl ester followed by coupling
with amine hydrochloride 4 to give 16 in 85% overall yield (Scheme
4). Acetylation of 16 proceeded in 82% yield to provide 1 that
was identical by all spectroscopic methods with tubulysin D isolated
from natural sources.
In conclusion, the total synthesis of tubulysin D was ac-
complished in 13% overall yield over 16 steps for the longest linear
sequence and is the first synthesis reported for any member of the
tubulysin family that incorporates the essential N,O-acetal. The
synthetic route should not only allow access to all of the naturally
occurring tubulysin derivatives and truncated analogues but also
enable the synthesis of most of the stereoisomers of tubulysin D
by appropriate selection of tert-butanesulfinamide stereochemistry
Supporting Information Available: Complete experimental details
and spectral data for all compounds described (PDF, CIF). This material
References
(1) Sasse, F.; Steinmetz, H.; Ho¨fle, G.; Reichenbach, H. J. Antibiot. 2000,
53, 879-885.
(2) (a) Steinmetz, H.; Glaser, N.; Herdtweck, E.; Sasse, F.; Reichenbach, H.;
Ho¨fle, G. Angew. Chem., Int. Ed. 2004, 43, 4888-4892. (b) Sanmann,
A.; Sasse, F.; Mu¨ller, R. Chem. Biol. 2004, 11, 1071-1079.
(3) Khalil, M. W.; Sasse, F.; Lu¨nsdorf, H.; Elnakady, Y. A.; Reichenbach,
H. ChemBioChem 2006, 7, 678-683.
(4) Kaur, G.; Hollingshead, M.; Holbeck, S.; Schauer-Vukasinovic, V.;
Camalier, R. F.; Doemling, A.; Agarwal, S. Biochem. J. 2006, 396, 235-
242.
(5) For publications on the synthesis of fragments of tubulysin, see: (a) Ho¨fle,
G.; Glaser, N.; Leibold, T.; Karama, U.; Sasse, F.; Steinmetz, H. Pure
Appl. Chem. 2003, 75, 167-178. (b) Wipf, P.; Takada, T.; Rishel, M. J.
Org. Lett. 2004, 6, 4057-4060. (c) Friestad, G.; Marie´, J.-C.; Deveau,
A. M. Org. Lett. 2004, 6, 3249-3252.
(6) Do¨mling and co-workers have recently reported the total syntheses of
tubulysins U and V for which biological activity has not yet been reported.
These derivatives do not contain the essential N,O-acetal functionality.
Do¨mling, A.; Beck, B.; Eicshelberger, U.; Sakamuri, S.; Menon, S.; Chen,
Q.-Z.; Lu, Y.; Wessjohann, L. A. Angew. Chem., Int. Ed. 2006, 45, 7235-
7239.
(7) Simple N,O-acetals have been evaluated as pro-drug linkages. Iley, J.;
Moreira, R.; Calheiros, T.; Mendes, E. Pharm. Res. 1997, 14, 1634-
1639.
(8) Liu, G.; Cogan, D. A.; Owens, T. D.; Tang, T. P.; Ellman, J. A. J. Org.
Chem. 1999, 64, 1278-1284.
(9) For the asymmetric reductive coupling of chiral nitronates and â-substituted
R,â-unsaturated carbonyl compounds, see: Masson, G.; Cividino, P.; Py,
S.; Vallee, Y. Angew. Chem., Int. Ed. 2003, 42, 2265-2268.
(10) Inami, K.; Shiba, T. Bull. Chem. Soc. Jpn. 1985, 58, 352-360.
(11) Kochi, T.; Tang, T. P.; Ellman, J. A. J. Am. Chem. Soc. 2003, 125, 11276-
11282.
(12) Borg, G.; Cogan, D. A.; Ellman, J. A. Tetrahedron Lett. 1999, 40, 6709-
6712.
(13) Lundquist, J. T., IV; Pelletier, J. C. Org. Lett. 2001, 3, 781-783.
(14) Standard carbamate protecting groups would likely result in competing
alkylation of the Ile nitrogen. For a leading reference on organic azides
in synthesis, see: Brase, S.; Gil, C.; Knepper, K.; Zimmermann, V. Angew.
Chem., Int. Ed. 2005, 44, 5188-5240.
(15) Nicolaou, K. C.; Estrada, A. A.; Zak, M.; Lee, S. H.; Safina, B. S. Angew.
Chem., Int. Ed. 2005, 44, 1378-1382.
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