On the introduction of a trifluoromethyl substituent in the epothilone setting: chemical issues related to ring forming olefin metathesis and earliest biological findings.

Article abstract of DOI:10.1021/ol0268283

The disclosure herein describes the synthesis of 10,11-dehydro-13,14-desoxy-27-trifluoro-[17]epothilone B via a stereoselective ring-closing metathesis and provides early biological evaluation data pertinent to this compound. [reaction: see text]

Full text of DOI:10.1021/ol0268283

ORGANIC
LETTERS
2002
Vol. 4, No. 23
4081-4084
On the Introduction of a Trifluoromethyl
Substituent in the Epothilone Setting:
Chemical Issues Related to Ring
Forming Olefin Metathesis and Earliest
Biological Findings
Alexey Rivkin,† Kaustav Biswas,† Ting-Chao Chou,‡ and
,†,§
Samuel J. Danishefsky*
Laboratories for Bioorganic Chemistry, Sloan-Kettering Institute for Cancer Research,
1275 York AVenue, New York, New York 10021, and Department of Chemistry,
Columbia UniVersity, HaVemeyer Hall, New York, New York 10027
s-danishefsky@ski.mskcc.org
Received August 30, 2002
ABSTRACT
The disclosure herein describes the synthesis of 10,11-dehydro-13,14-desoxy-27-trifluoro-[17]epothilone B via a stereoselective ring-closing
metathesis and provides early biological evaluation data pertinent to this compound.
In the past five years, the epothilones have emerged as
potential new anticancer agents.1 Human clinical trials
seeking to assess issues of toxicity, optimal dosage, and likely
efficacy of several epothilones as drugs are well underway.2
For instance, 12,13-desoxyepothilone B, initially developed
in our laboratory via total synthesis, is now undergoing
human clinical trials.3 Given the massive interest in
epothilones, it is not surprising that there has been a
worldwide effort to synthesize new analogues, and to
establish their SAR with a view to identifying and developing
later generation agents for clinical evaluation.4 Given the
important role of fluorine substitution in enhancing phar-
macokinetics and chemotheraputic indices of many medicinal
agents,5 it was natural to evaluate this type of structural
perturbation in the epothilone series. We initially targeted
† Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for
Cancer Research.
‡ Preclinical Pharmacology Core Facility, Sloan-Kettering Institute for
Cancer Research.
(4) (a) Florsheimer, A.; Altmann, K. H. Expert Opin. Ther. Patents 2001,
11, 951. (b) Chou, T. C.; Zhang, X. G.; Harris, C. R.; Kuduk, S. D.; Balog,
A.; Savin, K. A.; Bertino, J. R.; Danishefky, S. J. Proc. Natl. Acad. Sci.
U.S.A. 1998, 95, 15798. (c) Chou, T. C.; Zhang, X. G.; Balog, A.; Su, D.;
Meng, D. F.; Savin, K.; Bertino, J. R.; Danishefsky, S. J. Proc. Natl. Acad.
Sci. U.S.A. 1998, 95, 9642.
(5) (a) Ojima, I.; Inoue, T.; Chakravarty, S. J. Fluorine Chem. 1999,
97, 3. (b) Newman, R. A.; Yang, J.; Finlay, M. R. V.; Cabral, F.;
Vourloumis, D.; Stephens, L. C.; Troncoso, P.; Wu, X.; Logothetis, C. J.;
Nicolaou, K. C.; Navone, N. M. Cancer Chemother. Pharmacol. 2001, 48,
319.
§ Columbia University.
(1) For extensive reviews in this field see: (a) Harris, C. R.; Danishefsky,
S. J. J. Org. Chem. 1999, 64, 8434. (b) Nicolaou, K. C.; Roschangar, F.;
Vourloumis, D. Angew. Chem., Int. Ed. 1998, 37, 2015.
(2) Chou, T. C.; O’Connor, O. A.; Tong, W. P.; Guan, Y.; Zhang, Z.-
G.; Stachel, S. J.; Lee, C.; Danishefsky, S. J. Proc. Natl. Acad. Sci. U.S.A.
2001, 98, 8113.
(3) For more information about clinical trials of dEpoB, visit: www.ko-
san.com.
10.1021/ol0268283 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/17/2002

Scheme 2a
Figure 1. Selected epothilone analogues.
compound 2, which is seen to correspond to a 26-trifluoro-
epothilone congener for synthesis and biological evaluation.
To reach compound 2, we sought to take advantage of a
highly convergent route recently reported from our laboratory
for the synthesis of epothilone 490 (6, dehydrodesoxyEpoB)
en route to dEpoB (1, Scheme 1).6 In that synthesis, we
Scheme 1
a Key: (a) LHMDS, -78 °C, 78%; (b) (i) HOAc:THF:H2O (3:
1:1); (ii) CH3ONHCH3, AlMe3; (iii) TESCl, imidazole, DMF, 79%
overall; (c) (i) vinyltributyltin, Pd2(dba)3, DMF, 80 °C, 3 h; (ii)
MeMgBr, 0 °C, 40% overall; (d) (i) n-BuLi, THF, -78 °C, 30
min; (ii) 12, -78 °C to room temperature, 81%; (iii) HOAc:THF:
H2O (3:1:1), 76% overall; (e) TMSI, CH2Cl2, 0 °C, 92%.
and high diastereoselectivity (>25:1 de). Compound 9 was
advanced in three steps to 10 as shown. Attempts to
accomplish addition of methylmagnesium bromide to the
Weinreb amide linkage of 10 failed to provide 11. The
breakdown of this reaction was attributed to the presence of
the iodoalkene linkage. However, we could accomplish our
goal by changing the order of these two C-C bond-forming
steps. Thus, reaction of 10 with vinyltributyltin under Stille
conditions could then be followed by addition of methyl
Grignard reagent to give the desired ketone 11. Condensation
of ketone 11 with phosphine oxide 12, followed by depro-
tection of the triethylsilyl ether, afforded fragment 13 in good
yield. Esterification of the resulting 13 with C1-C10 acid
fragment 146 provided the desired 15, in 75% yield (Scheme
2).
introduced a flanking vinyl group to compound 4 via a
stereospecific Stille coupling of a vinyl iodide precursor 3
with tri-n-butylvinylstannane. Ring closing metathesis fol-
lowed by deprotection led to 6, which was then transformed
to dEpoB (1) via a regioselective diimide reduction.
Attention was first directed to the synthesis of 15 (Scheme
2). Alkylation of the previously reported lithium enolate of
77 with iodide 8 (synthesized from the known alcohol 168
using TMSI in methylene chloride) afforded 9 in 78% yield
Unfortunately, attempts to carry out the ring-closing
metathesis reaction9 of 15 using the second generation
Grubbs catalyst in methylene chloride led primarily to
(6) (a) Biswas, K.; Lin, H.; Njardarson, J. T.; Chappell, M. D.; Chou, T.
C.; Guan, Y.; Tong, W. P.; He, L.; Horwitz, S. B.; Danishefsky, S. J. J.
Am. Chem. Soc. 2002, 124, 9825. (b) Rivkin, A.; Njardarson, J. T.;
Biswas, K.; Chou, T. C.; Danishefsky, S. J. J. Org. Chem. 2002, 67, 7737.
(7) Chappell, M. D.; Stachel, S. J.; Lee, C. B.; Danishefsky, S. J. Org.
Lett. 2000, 2, 1633.
(9) Reviews: (a) Grubbs, R. H.; Miller, S. J.; Fu, G. C. Acc. Chem. Res.
1995, 28, 446. (b) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34,
18. (c) Alkene Metathesis in Organic Chemistry; Fu¨rstner, A., Ed.;
Springer: Berlin, Germany, 1998. (d) Fu¨rstner, A. Angew. Chem., Int. Ed.
2000, 39, 3012. (e) Schrock, R. R. Top. Organomet. Chem. 1998, 1, 1.
(8) Prie´, G.; Thibonnet, J.; Abarbri, M.; Ducheˆne, A.; Parrain, J. Synlett
1998, 839.
4082
Org. Lett., Vol. 4, No. 23, 2002

apparentdimerization of the starting material (eq 1).10 Given
Scheme 3a
the fact that the RCM works quite well in the related setting
of 5 f 6, we naturally attributed the failure in the case of
15 to the presence of the trifluoromethyl group in a vicinal
relationship to the proposed RCM reaction center (see
asterisk in 15).
It was conjectured that the detrimental impact of the
resident 26-trifluoro substituent on the desired reaction might
be alleviated by adding a carbon spacer between the RCM
reaction center (see asterisk in 18) and the trifluoromethyl
group. Accordingly, we undertook a synthesis of 19 (eq 2)
via the ring-closing metathesis of 18, which would present
the trifluoromethyl group in the context of a 17-membered
ring containing a skipped (1,4)diene.
a Key: (a) (i) Allyltributyltin, AIBN, benzene, 80 °C, 3 h, 74%;
(ii) MeMgBr, 0 °C, 69%; (b) (i) 12, n-BuLi, THF, -78 °C, 30
min; (ii) 20, -78 °C to room temperature; (iii) HOAc:THF:H2O
(3:1:1), 83% overall.
reductive cleavage of the trichloroethoxycarbonyl protecting
group with zinc and acetic acid, followed by deprotection
of the TES ether with HF-pyridine, afforded the desired 19
containing a trifluoromethyl function at C12, albeit in the
context of the 17-membered-ring series.
Scheme 4
The synthesis program directed to 19 commenced with
the preparation of compound 21, which corresponds to the
“O-alkyl sector” of our proposed RCM substrate (Scheme
3). We began with allylation of 10, this time under radical
reaction conditions as shown.11 This conversion was followed
by reaction of the alkylated product with methylmagnesium
bromide, thus affording the required ketone 20. Condensation
of this compound with phosphine oxide 12 followed by
deprotection of the triethylsilyl ether function provided 21
in good yield.
Esterification of 21 with the C1-C10 acid fragment 14
provided the proposed RCM precursor 18 in 75% yield
(Scheme 4). Happily in this case, the ring-closing metathesis
reaction of 18 could be accomplished using the second
generation Grubbs catalyst in methylene chloride. As in the
case of the conversion of 5 f 6, the reaction provided
exclusively the trans isomer 22 in 57% yield.6 Finally,
Synthetic 19 was evaluated as to its cytotoxic activity. As
shown in Table 1, direct comparison of the previously
reported [17]ddEpoB (23) with 27-F3-[17]ddEpoB (19)
indicated that the new perfluorinated compound possessed
a comparably high cytotoxic potency.12 Though the trifluoro-
methyl isoteric substitution had little effect on the gross
(10) Dimerization occurred at 10,11-olefin, while the CF3-substituted
diene did not react at all.
(11) (a) Keck, G. E.; Yates, J. B. J. Am. Chem. Soc. 1982, 104, 5829.
(b) Review: Curran, D. P. Synthesis 1988, Part 1, p 417; Part 2, p 489.
Org. Lett., Vol. 4, No. 23, 2002
4083

We are pursuing new departures directed to the incorporation
of trifluoromethyl substituents in various epothilone settings.
Table 1. In Vitro Cytotoxicities (IC50) with Tumor Cell
Lines14
Acknowledgment. This research was supported by the
National Institutes of Health (grant numbers: CA-28824
(S.J.D.) and CA-08748 (T.-C.C.). Postdoctoral Fellowship
support is gratefully acknowledged by A.R. (NIH Cancer
Pharmacology Training Grant, T32-CA62948) and K.B.
(U.S. Army Breast Cancer Research Program, DAMD-17-
98-1-8155). The authors wish to thank Dr. George Sukenick
(NMR Core Facility, Sloan-Kettering Institute, and CA-
08748) for mass spectral and NMR analysis.
CCRF-CEM
CCRF-CEM/VBL
compd
(IC50 (µM))
(IC50 (µM))
27-Tri-F-[17]ddEpoB (19)
[17]ddEpoB (23)
[16]ddEpoB (6)
0.068
0.040
0.025
0.191
0.126
0.091
cytotoxic activity, preliminary data from metabolic degrada-
tion studies in mouse plasma showed 19 to be notably more
stable than is the parent 23.13 Since pharmokinetic issues
are likely to be critical in the actual use of any epothilone
agent as a drug, we take this finding to be quite encouraging.
Note Added after ASAP: In Scheme 4, reaction 18 to 22,
bonds were added between the two chlorides and the
ruthenium catalyst; the corrected version was posted on
October 18, 2002.
(12) Compound 23 ([17]ddEpoB) was prepared in a fashion similar to
compound 19 as described in ref 6b.
(13) Exposure of epothilones 19 and 23 to nude mouse and human plasma
led to degradation of 23 within 30 min, while epothilone 19 remained mostly
intact (see ref 2 for details) of this type of measurement. Specifics for 19
and 23 will be reported in due course.
(14) XTT assay following 72 h inhibition. CCRF-CEM is a human T-cell
acute lymphoblastic leukemia cell line. The CCRF-CEM/VBL100 cell line
overexpresses P-glycoprotein and displays a multidrug resistance phenotype
to MDR-associated oncolytics.
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL0268283
4084
Org. Lett., Vol. 4, No. 23, 2002