Communications
Epoxidation of both boundary conformers I and II would
In summary, we have used molecular modeling and NMR
be expected to occur exo to the macrolide ring, thus leading to
epoxides 4 and 5, respectively. Given Curtin–Hammett
considerations,[15] it seems unlikely that a small increase in
the population of conformer type II in (E)-9,10-dehydro-
dEpoB (2) could, in itself, account for the shift to a selectiv-
ity[10] in the epoxidation reaction. It is more likely that the
introduction of 9,10-unsaturation lowers the barrier of
interconversion of I and II, with the latter being the more
reactive conformer.
spectroscopic analysis to account for the preferred (a)
ꢀ
stereochemistry of epoxidation of the C12 C13 double
bond of 2. Through this epoxidation, we have synthesized
the most active epothilone known, 4, which demonstrated
remarkable in vivo efficacy in xenografts. The arguments used
to account for the stereochemistry of epoxidation of 2, closely
coordinated modeling, and NMR spectroscopic analysis serve
to rationalize the remarkable potency-enhancing effect of the
(E)-9,10 double bond in this series. We also account for the
potency enhancement of the naturally configured b-epoxides
(3 and 4) and lack of enhancement from the corresponding
non-natural a-epoxides (5 and 6). While these studies are
focused on the epothilones, they point to modalities in drug
discovery of wider generality.
Computational studies in conjunction with NMR spectro-
scopic analysis prompt the notion that the incorporation
ꢀ
unsaturation at C9 C10 leads to an increase in the flexibility
of the C11–C14 region and has a substantial impact on the
mean conformation of the polypropionate region of 2.
Analysis of the relevant 1HNMR coupling constants also
points to a significant difference in the H6–H7 dihedral angle
of dEpoB (1) (J6,7 = < 1.0 Hz) and 9,10-dehydro-dEpoB (2)
(J6,7 = 6.5 Hz). Significantly, the coupling constants in this
region of the spectrum of 2 suggest an increase in conforma-
tional population in the direction observed in both the X-ray
crystal structure of EpoB[5b] and in its solution NMR spectrum
(Figure 3). Although we cannot be certain of the overall
conformation of the macrolide while bound to tubulin, it
Received: March 14, 2003 [Z51407]
Keywords: antitumor agents · conformational analysis ·
.
natural products · structure–activity relationships
[1] For reviews of epothilone chemistry and biology, see: a) K. C.
Nicolaou, A. RitzØn, K. Namoto, Chem. Commun. 2001, 1523;
b) K. H. Altmann, M. Wartmann, T. O'Reilly, Biochim. BioPhys.
Acta 2000, 1470, M79; c) C. R. Harris, S. J. Danishefsky, J. Org.
Chem. 1999, 64, 8434; d) K. C. Nicolaou, F. Roschangar, D.
Vourloumis, Angew. Chem. 1998, 110, 2014; Angew. Chem. Int.
Ed. 1998, 37, 2120.
[2] For recent examples of syntheses of novel epothilone analogues,
see: a) A. Regueiro-Ren, K. Leavitt, S.-H. Kim, G. Höfle, M.
Kiffe, J. Z. Gougoutas, J. D. DiMarco, F. Y. F. Lee, C. R. Fair-
child, B. H. Long, G. D. Vite, Org. Lett. 2002, 4, 3815; b) R. E.
Taylor, Y. Chen, A. Beatty, D. C. Myles, Y. Zhou, J. Am. Chem.
Soc. 2003, 125, 27; c) S. C. Sinha, J. Sun, Angew. Chem. 2002, 114,
1439; Angew. Chem. Int. Ed. 2002, 41, 1381; d) K. C. Nicolaou,
A. RitzØn, K. Namoto, R. M. Buey, J. F. Diaz, J. M. Andreu, M.
Wartmann, K. H. Altmann, A. O'Brate, P. Giannakakou,
Tetrahedron 2002, 58, 6413; e) J. W. Bode, E. M. Carreira, J.
Org. Chem. 2001, 66, 6410.
[3] a) C. R. Harris, S. D. Kuduk, A. Balog, K. Savin, P. W. Glunz,
S. J. Danishefsky, J. Am. Chem. Soc. 1999, 121, 7050; b) D. Meng,
P. Bertinato, A. Balog, D.-S. Su, T. Kamenecka, E. J. Sorensen,
S. J. Danishefsky, J. Am. Chem. Soc. 1997, 119, 10073.
[4] For more information about clinical trials of dEpoB, visit:
[5] a) G. Höfle, N. Bedorf, K. Gerth, H. Reichenbach, (GBF), DE-B
4138042, 1993; b) G. Höfle, N. Bedorf, H. Steinmetz, D.
Schomburg, K. Gerth, H. Reichenbach, Angew. Chem. 1996,
108, 1671; Angew. Chem. Int. Ed. Engl. 1996, 35, 1567; c) K.
Gerth, N. Bedorf, G. Höfle, H. Irschik, H. Reichenbach, J.
Antibiot. 1996, 49, 560.
[6] D.-S. Su, A. Balog, D. Meng, P. Bertinato, S. J. Danishefsky, Y.-H.
Zheng, T.-C. Chou, L. He, S. B. Horwitz, Angew. Chem. 1997,
109, 2178; Angew. Chem. Int. Ed. Engl. 1997, 36, 2093.
[7] T.-C. Chou, X. G. Zhang, X. G.; C. R. Harris, S. D. Kuduk, A.
Balog, K. A. Savin, J. R. Bertino, S. J. Danishefsky, Proc. Natl.
Acad. Sci. USA 1998, 95, 9642.
Figure 3. Preferred conformation of EpoB and 9,10-dehydro-dEpoB.
ꢀ
should be noted that the trans geometry of C9 C10 rigidifies
the C8-C9-C10-C11 torsion angle to 1808 and stabilizes the
position of the C25 methyl group through a minimization of
A1,3 strain (Figure 3). These findings lead to the suggestion
that the notable increase in biological activity of 2 relative to 1
ꢀ
reflects the impact of the double bond at C9 C10 on the
conformation of the polypropionate region.
Furthermore, comparison of the H6–H7 coupling con-
stants (4 (3.8 Hz), 5 (1.8 Hz), 3 (4.2 Hz), 6 (< 1.0 Hz))
suggests that the long-range conformational guidance pro-
vided by the epoxides is in the same direction as that favored
by the (E)-9,10 double bond of 2. Moreover, the NMR
spectroscopic data show that the long-range conformational
guidance depends on the stereochemistry of the epoxides.
Only the naturally configured b-epoxides (3 and 4) modulate
the polypropionate region to populate the highly bioactive
conformation. In contrast, long-range modulation by the non-
natural a-epoxide series (5 and 6) is sharply attentuated. This
finding nicely accounts for the higher cytotoxic activity in the
natural EpoB series (3 and 4) relative to the non-natural a-
epoxide series (5 and 6).
[8] a) A. Rivkin, F. Yoshimura, A. E. Gabarda, T.-C. Chou, H.
Dong, W. P. Tong, S. J. Danishefsky, J. Am. Chem. Soc. 2003, 125,
2899; b) for the interfacing of these findings with earlier claims,
see: J. D. White, R. G. Carter, K. F. Sundermann, M. Wartmann,
J. Am. Chem. Soc. 2001, 123, 5407, and references in [8a].
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ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2003, 42, 2518 – 2521