12,13-Isobaccatin III. Taxane enol esters (12,13-isotaxanes)

Full text of DOI:10.1021/ja953702a

J. Am. Chem. Soc. 1996, 118, 919-920
919
12,13-Isobaccatin III. Taxane Enol Esters
(12,13-Isotaxanes)
Robert C. Kelly,* Nancy A. Wicnienski, Ilse Gebhard,
Samuel J. Qualls, Fusen Han, Paul J. Dobrowolski,
Eldon G. Nidy, and Roy A. Johnson*
Upjohn Laboratories, The Upjohn Company
Kalamazoo, Michigan 49001
ReceiVed NoVember 3, 1995
The diterpenoid nucleus of the antitumor agent taxol1 (1) has
shown a propensity for unusual and often unexpected chemical
transformations.2 Contraction of rings A and B,3 oxetane
opening,3,4 epimerization at C-7,5 and cyclopropane formation6,7
are among the many fascinating transformations of the taxol
molecule reported to date. We report another unexpected result
observed during chemical modification of baccatin III (2) which
has allowed us to develop a new series of taxol analogs having
potent antitumor activity.
When shaken with aqueous acid and dichloromethane, the
unknown was converted to a second new material, 6, whose
molecular weight was higher by 32 mass units than that
calculated for ketone 4. When allowed to react with m-
chloroperbenzoic acid under a nitrogen atmosphere, the un-
known was slowly converted in modest yield (14%) to a third
new compound, 7. Reduction of 6 with tetra-n-butylammonium
borohydride gave a crystalline triol whose structure was shown
by X-ray crystallography to be that of the 11,12-dihydro-12â-
hydroxybaccatin III derivative 8. The triol 8 also was obtained
from 7 by reduction with sodium borohydride/cerium(III)
chloride.11
We interpret the preceding observations in terms of an enol
structure for 5 as follows. The 13C olefinic signals given above
for 5 are comparable to those reported for various simple enols,
e.g., 2-methylprop-1-en-1-ol12 (δ 136.2 for CR and 105.9 for
Câ), propen-2-ol13 (δ 156.8 for CR and 95.3 for Câ), or
1-cyclohexenol14 (δ 150.5 for CR). Conversion of 5 to ketone
4 upon chromatography is consistent with the presence of an
enol in 5. The addition of 32 mass units during workup suggests
the addition of molecular oxygen and the formation of a
hydroperoxide (as in 6), a reaction of enols having precedent
in the literature.15 Finally, the X-ray structure of the 1,12,13-
triol 8, obtained by reduction of the hydroperoxide in 6 to a
hydroxyl group, clearly defines the configuration of the hydro-
peroxide as 12â and indicates that the olefinic bond of 5 must
be between carbons 12 and 13. Together, these observations
are consistent with assignment of an enolic structure to 5, a
compound to which we have given the name16 12,13-isobaccatin
III-7-O-TES. This new example of a stable unconjugated enol
When attempting to reduce 13-ketobaccatin III-7-O-TES8 (3)
with zinc in acetic acid, we did not obtain the anticipated
11,12-dihydro-13-ketobaccatin III-7-O-TES (4).9,10 Instead,
following filtration and evaporation of the reaction solvent, we
obtained a new compound, 5, which was found by combustion
analysis to be isomeric with 4. The 13C NMR spectrum of this
new compound clearly lacked a signal for the C-13 carbonyl
required for structure 4 and had new signals for olefinic carbons
at δ 146.0 and 102.4. Further clues to the structure of this
unknown compound were provided by its reactivity. Upon
direct chromatography over silica gel, the compound was
transformed into ketone 4 as deduced from spectral properties.
(1) Wani, M. C.; Taylor, H. L.; Wall, M. E.; Coggon, P.; McPhail, A.
T. J. Am. Chem. Soc. 1971, 93, 2325. Taxol has been registered as a
trademark by Bristol-Myers Squibb; the generic name for taxol is paclitaxel.
(2) Cf.: Taxane Anticancer Agents: Basic Science and Current Status;
Georg, G. I., Chen, T. C., Ojima, I., Vyas, D. M., Eds.; ACS Symposium
Series 583; American Chemical Society: Washington, DC, 1995.
(3) Samaranayake, G.; Magri, N. F.; Jitrangsri, C.; Kingston, D. G. I. J.
Org. Chem. 1991, 56, 5114.
(4) Gue´ritte-Voegelein, F.; Gue´nard, D.; Potier, P. J. Nat. Prod. 1987,
50, 9.
(5) Chaudhary, A. G.; Rimoldi, J. M.; Kingston, D. G. I. J. Org. Chem.
1993, 58, 3798.
(6) (a) Chen, S.-H.; Huang, S.; Wei, J.; Farina, V. J. Org. Chem. 1993,
58, 4520. (b) Chen, S.-H.; Huang, S.; Farina, V. Tetrahedron Lett. 1994,
35, 41. (c) Klein, L. L.; Maring, C. J.; Li, L.; Yeung, C. M.; Thomas, S.
A.; Grampovnik, D. J.; Plattner, J. J.; Henry, R. F. J. Org. Chem. 1994, 59,
2370. (d) Bouchard, H.; Pulicani, J.-P.; Vuilhorgne, M.; Bourzat, J.-D.;
Commerc¸on, A. Tetrahedron Lett. 1994, 35, 9713.
(7) Johnson, R. A.; Nidy, E. G.; Dobrowolski, P. J.; Gebhard, I.; Qualls,
S. J.; Wicnienski, N. A.; Kelly, R. C. Tetrahedron Lett. 1994, 35, 7893.
(8) 13-Ketobaccatin III was first described in ref 1.
(9) Zinc reductions of 13-ketobaccatins, including 10-deacetyl-13-
ketobaccatin III, under various conditions have been reported: Marder, R.;
Dubois, J.; Gue´nard, D.; Gue´ritte-Voegelein, F.; Potier, P. Tetrahedron 1995,
51, 1985.
(11) Luche, J. L. J. Am. Chem. Soc. 1978, 100, 2226.
(12) Capon, B.; Siddhanta, A. K. J. Org. Chem. 1984, 49, 255.
(13) Sojka, S. A.; Poranski, C. F., Jr.; Moniz, W. B. J. Magn. Reson.
1976, 23, 417.
(14) Moniz, W. B.; Sojka, S. A.; Poranski, C. F., Jr.; Birkle, D. L. J.
Am. Chem. Soc. 1978, 100, 7940.
(10) 10-Deacetyl-11,12-dihydrobaccatin III-7-O-TES is described in ref
9, as well as in the following: Appendino, G.; Jakupovic, J.; Cravotto, G.;
Enriu`, R.; Varese, M.; Bombardelli, E. Tetrahedron Lett. 1995, 36, 3233.
For the X-ray crystallographic structure, see: Chiaroni, A.; Riche, C.;
Marder, R.; Dubois, J.; Gue´nard, D.; Gue´ritte-Voegelein, F. Acta Crystallogr.
1995, C51, 2050.
(15) (a) Attenburrow, J.; Connett, J. E.; Graham, W.; Oughton, J. F.;
Ritchie, A. C.; Wilkinson, P. A. J. Chem. Soc. 1961, 4547. (b) Enslin, P.
R. Tetrahedron 1971, 27, 1909.
(16) While we recognize that a more appropriate trivial nomenclature
for this type of structure may be 11,12-dihydro-12,13-dehydrobaccatin III,
we prefer and suggest the shortened nomenclature used in this manuscript.
0002-7863/96/1518-0919$12.00/0 © 1996 American Chemical Society

920 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Communications to the Editor
may be added to the small list of such enols previously reported
in the literature.17
protecting group of 13 was removed with Et3N‚(HF)3, giving
14. The 10-acetyl group of 14 was removed with hydrazine,7,21
giving 15, whose 1H NMR spectrum matches the chemical shifts
reported by Marder and co-workers9 for this compound. The
Enol 5 did not react with acetic anhydride but was success-
fully acylated with the taxotere18 side chain precursor 97,19 in
the presence of dicyclohexylcarbodiimide and 4-(dimethyl-
amino)pyridine in CH2Cl2-toluene, giving 10 (89%, two
diastereoisomers). We chose to acylate with the tert-butyloxy-
carbonyl (BOC) side chain of taxotere (11) rather than the
benzamide side chain of taxol (1) because, in our experience,7
and as reported by others,2 taxotere side chain analogs are
consistently more potent in antitumor assays when compared
with the taxol analog. Removal of the protecting groups from
assignments of stereochemistry at C-12 and C-13 in 13-15 are
thereby confirmed. Coupling of 13 with the taxol side chain
precursor, (4S,5R)-N-(benzoyl)-2-(2,4-dimethoxyphenyl)-4-
phenyl-5-oxazolidinecarboxylic acid, followed by removal of
the protecting groups, gave 11,12-dihydrotaxol (16). The
deprotection was attempted first in MeOH-HCl, which gener-
ated an ortho ester (17, structure not shown) with the C-1 and
C-2 oxygen atoms. Hydrolysis of 17 with aqueous acid gave
the desired 16. Likewise, coupling of 11,12-dihydrobaccatin
III-7-O-TES with BOC side chain precursor 9 and deprotection
of the coupled product gave 18. Finally, coupling of the 11,-
12-dihydro-12â-hydroxybaccatin III derivative 8 with 9 and
deprotection of the coupled product gave the 11,12-dihydro-
12â-hydroxytaxotere analog 19.
Pharmacological comparison of 12 and 19 with taxotere (11)
and 10-acetyltaxotere (20) gave IC50 values of 4.6, 38, 4, and
3.5 nM, respectively, for inhibition of the mouse L1210
leukemic cell assay.22 The IC50 for taxol analog 16 was >0.1
mM in this assay, while for taxol the IC50 is 17 nM. These in
Vitro assay results show that the potency of the isotaxol analog
12 is approximately the same as that of “normal” analogs 11
and 20, whereas the potency of the 11,12-dihydro analogs 16
and 19 is reduced in comparison to the analogous 11,12-olefinic
compounds.23 The approximate equivalent potency of 12 versus
that of taxotere (11) was confirmed by results from the A2780
human ovarian carcinoma cell growth inhibition assay24 (12
inhibits with an IC50 of 0.93 nM vs 1.8 ( 0.4 nM for 11).
10 was achieved by stirring with 4:1 HOAc-H2O at room
temperature for 4 days and gave, after chromatographic
purification, the new analog 12 (40%). Evidence that the enol
ester has been formed is seen in the spectral data for 12; for
example, the 13C NMR spectrum has signals at δ 143.3 and
121.9 comparable to signals at 144.2 and 124.3 reported20 for
the enol acetate of taxuspine D and are consistent with the enolic
ester double bond. A signal at δ 2.76 in the 1H NMR is
characteristic for the C-11 proton in this compound. It is
noteworthy that throughout these chemical manipulations, the
enol ester functionality remained intact.
Supporting Information Available: Experimental procedures and
analytical and spectroscopic data for 4-8, 10, 12, 14-19; X-ray
crystallographic data with tables of thermal and positional parameters,
bond lengths, and bond angles as well as ORTEP drawings (single
molecule and stereo representations) for 8 (21 pages). This material
is contained in many libraries on microfiche, immediately follows this
article in the microfilm version of the journal, can be ordered from the
ACS, and can be downloaded from the Internet; see any current
masthead page for ordering information and Internet access instructions.
With the 11,12-dihydro intermediates 4 and 8 in hand, we
were able to prepare 11,12-dihydrotaxol analogs. Reduction
of 11,12-dihydro-13-ketobaccatin III-7-O-TES (4) with NaBH4/
CeCl3 gave 11,12-dihydrobaccatin III-7-O-TES (13). The TES
(17) (a) Hart, H.; Rappoport, Z.; Biali, E. The Chemistry of Enols;
Rappoport, Z., Ed.; Wiley: New York, 1990; p 481. (b) Bergens, S. H.;
Bosnich, B. J. Am. Chem. Soc. 1991, 113, 958. (c) Vonwiller, S. C.; Warner,
J. A.; Mann, S. T.; Haynes, R. K. J. Am. Chem. Soc. 1995, 117, 11098. (d)
Two groups have reported without comment the formation of a stable enol
as a byproduct of taxol total synthesis: Kende, A. S.; Johnson, S.; Sanfilippo,
P.; Hodges, J. C.; Jungheim, L. N. J. Am. Chem. Soc. 1986, 108, 3513.
Nicolaou, K. C.; Liu, J.-J.; Yang, Z.; Ueno, H.; Sorensen, E. J.; Claiborne,
C. F.; Guy, R. K.; Hwang, C.-K.; Nakada, M.; Nanterment, P. G. J. Am.
Chem. Soc. 1995, 117, 634.
(18) Mangatal, L.; Adeline, M.-T.; Gue´nard, D.; Gue´ritte-Voegelein, F.;
Potier, P. Tetrahedron 1989, 45, 4177. Taxotere is a registered trademark
of Rhoˆne-Poulenc Rorer; the generic name for taxotere is docetaxel.
(19) Didier, E.; Fouque, E.; Taillepied, I.; Commerc¸on, A. Tetrahedron
Lett. 1994, 35, 2349.
(20) Kobayashi, J.; Hosoyama, H.; Shigemori, H.; Koiso, Y.; Iwasaki,
S. Experientia 1995, 51, 592.
JA953702A
(21) Datta, A.; Hepperle, M.; Georg, G. I. J. Org. Chem. 1995, 60, 761.
(22) Cf.: Li, L. H.; et al. Cancer Res. 1992, 52, 4904. We thank Joan
W. Culp for the data included here.
(23) The results described here were presented at the 210th American
Chemical Society National Meeting, Chicago IL, August 20-24, 1995,
Abstract Medi 175.
(24) The A2780 human ovarian carcinoma cell growth inhibition assay
(Lopes, N. M., McGovren, J. P., unpublished results) is based on the cell
line described in the following: Perez, R. P.; O’Dwyer, P. J.; Handel, L.
M.; Ozols, R. F.; Hamilton, T. C. Int. J. Cancer 1991, 48, 265. We thank
Narima Lopes and Patrick McGovren for permission to publish these results.