Synthesis and biological evaluation of novel paclitaxel (Taxol) D-ring modified analogues

Article abstract of DOI:10.1021/jo982095h

The semisynthesis and biological activity of paclitaxel (Taxol) analogues in which the oxygen atom in ring D is substituted by a sulfur or a selenium atom is presented. These derivatives were synthesized and tested in order to make more transparent the role of the oxetane ring in the biological activity of paclitaxel. The sulfur derivatives were found to be less active than paclitaxel in biological assays, while the selenium derivative could not be converted to its 4-acyl analogue. The results with the sulfur analogues suggest that the oxygen atom in the oxetane ring plays an important role in the mechanism by which paclitaxel exhibits its anticancer activity.

Full text of DOI:10.1021/jo982095h

2694
J . Org. Chem. 1999, 64, 2694-2703
Syn th esis a n d Biologica l Eva lu a tion of Novel P a clita xel (Ta xol)
D-Rin g Mod ified An a logu es
A. A. Leslie Gunatilaka,† Frank D. Ramdayal, Maria H. Sarragiotto,‡ and
David G. I. Kingston*
Department of Chemistry, Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061-0212
Dan L. Sackett and Ernest Hamel
Laboratory of Drug Discovery Research and Development, Developmental Therapeutics Program,
Division of Cancer Treatment and Diagnosis, National Cancer Institute, Frederick Cancer Research and
Development Center, Frederick, Maryland 21702
Received October 19, 1998
The semisynthesis and biological activity of paclitaxel (Taxol) analogues in which the oxygen atom
in ring D is substituted by a sulfur or a selenium atom is presented. These derivatives were
synthesized and tested in order to make more transparent the role of the oxetane ring in the
biological activity of paclitaxel. The sulfur derivatives were found to be less active than paclitaxel
in biological assays, while the selenium derivative could not be converted to its 4-acyl analogue.
The results with the sulfur analogues suggest that the oxygen atom in the oxetane ring plays an
important role in the mechanism by which paclitaxel exhibits its anticancer activity.
In tr od u ction
The anticancer drug paclitaxel (Taxol, 1a ), a diterpe-
noid isolated from the bark of Taxus brevifolia, is
clinically used in the treatment of ovarian and breast
cancer.1 Since its discovery in the late 1960s,2 intensive
studies of its chemistry and structure-activity relation-
ships have been reported.3 These studies have established
that the C-13 side chain, the ester groups at C-2 and C-4,
and the oxetane ring are all essential for biological
activity,3 but the precise role of these groups in paclitax-
el’s activity remains obscure.
It is known that paclitaxel binds to microtubules,
thereby stabilizing them and disrupting the equilibrium
between tubulin and microtubules, and thus halting cell
division,4 but the conformation in which paclitaxel binds
to microtubules and the actual binding site are not known
in detail. As far as the conformation of paclitaxel is
concerned, NMR studies in aqueous solution indicate that
a “hydrophobic collapse” conformation, resulting from
hydrophobic clustering of the C-2 benzoyl, C-3′ phenyl,
and C-4 acetate groups, is probably important for provid-
ing the critical side chain conformation relevant to the
binding of paclitaxel to microtubules.5 These findings
provide at least a partial explanation for the importance
of the groups indicated but do not provide any clue to
the importance of the oxetane ring.
It is possible to devise various hypotheses for the
importance of the oxetane ring to paclitaxel’s activity.
Thus it might cause the acetoxy group to adopt an
orientation in space favorable for interaction with mi-
crotubules by increasing the rigidity of ring C (confor-
mational role) or it might be directly involved in receptor
binding by its oxygen atom through hydrogen bonding
(electronic role). Thus the effect of replacement of the
oxygen atom in the oxetane ring by another heteroatom
on the activity of paclitaxel might clarify whether the role
* To whom correspondence should be addressed. Phone: 540-231-
6570. Fax: 540-231-7702. E-mail: DKingston@vt.edu.
† Present address: Bioresources Research Facility, University of
Arizona, Tucson, AZ 85706-6800.
‡ Visiting faculty from Department of Chemistry, Universidade
Estadual de Maringa, Maringa, PR-Brazil.
(1) (a) Holmes, F. A.; Kudelka, A. P.; Karanagh, J . J .; Huber, M.
H.; Ajani, J . A.; Valero, V. In Taxane Anticancer Agents: Basic Science
and Current Status; Georg, G. I., Chen, T. T., Ojima, I.; Vyas, D. M.,
Eds.; ACS Symposium Series 583, American Chemical Society: Wash-
ington, DC, 1994; pp 31-57. (b) Arbuck, S. G.; Blaylock, B. A. In
Taxol: Science and Applications; Suffness, M., Ed.; CRC Press Inc.:
Boca Raton, FL, 1995; pp 379-415. (c) McGuire, W. P., Rowinsky, E.
K., Eds. Paclitaxel in Cancer Treatment; Marcel Dekker: New York,
Basel, Hong Kong, 1995; Vol. 8, pp 1-349.
(2) Wani, M. C.; Taylor, H. L.; Wall, M. E.; Coggon, P.; McPhail, A.
T. J . Am. Chem. Soc. 1971, 93, 2325-2327.
(3) For recent reviews: (a) Kingston, D. G. I.; Molinero, A. A.;
Rimoldi, J . M. Prog. Chem. Org. Nat. Prod. 1993, 61, 1-188. (b)
Suffness, M. Annu. Rep. Med. Chem. 1993, 28, 305-314. (c) Georg, G.
I.; Ali, S. M.; Zymunt, J .; J ayasinghe, J . R. Expert Opin. Ther. Pat.
1994, 12, 222-227. (d) Chen, S. H.; Farina, V. Paclitaxel (Taxol)
Chemistry and Structure Activity Relationships. In The Chemistry and
Pharmacology of Taxol and its Derivatives; Farina, V., Ed.; Elsevier:
New York, 1995; pp 165-223. (e) Kingston, D. G. I. Trends Biotechnol.
1994, 12, 222-227.
(5) (a) Vander Velde, D. G.; George, G. I.; Grunewald, G. L.; Gunn,
C. W.; Mitscher, L. A. J . Am. Chem. Soc. 1993, 115, 11650-11651. (b)
Dubois, J .; Guenard, D.; Gueritte-Voegelein, F.; Guerida, N.; Potier,
P.; Gillet, B.; Beloeil, J . C. Tetrahedron 1993, 49, 6533-6544. (c)
Williams, H. J .; Scott, A. I.; Dieden, R. A.; Swindell, C. S.; Chirlian, L.
E.; France, M. M.; Heerding, J . M.; Kraus, N. E. Tetrahedron 1993,
49, 6545-6560. (d) Paloma, L. G.; Guy, R. K.; Wrasidio, W.; Nicolaou,
K. C. Chem. Biol. 1994, 1, 107-112.
(4) Horwitz, S. B. Trends Pharmacol. Sci. 1992, 13, 134-136.
10.1021/jo982095h CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/30/1999

Novel Paclitaxol D-Ring Modified Analogues
J . Org. Chem., Vol. 64, No. 8, 1999 2695
Sch em e 1
of the oxetane ring is primarily conformational or pri-
marily electronic.
struction of the D-ring from a C-5R-halo-4,20-oxirane
intermediate with simultaneous introduction of the het-
eroatom (Scheme 1). The C-5R leaving group could be
introduced from the oxetane ring opening, and the 4,20-
epoxy group would be formed from cyclization of the C-4-
hydroxy group onto the appropriately functionalized C-20
hydroxyl group. Protection of C-1 and C-2 hydroxy groups
was required to avoid the formation of undesired products
in the oxetane ring opening step;8 the protection of these
groups as a cyclic carbonate was expected to facilitate
acylation of C-4 and also to create conditions for the
introduction of the benzoate functionality at C-2.9
Recently, the syntheses of the azetidine derivatives 2a
and 2b6 and 3a and 3b7 were reported. The derivatives
The synthetic approach we have developed to the
4,20-oxirane intermediate 10 is shown in Scheme 2. The
starting material, paclitaxel (1a ), was protected at the
C-2′ and C-7 positions by tert-butyldimethylsilyl and
triethysilyl groups, respectively, to give the protected
derivative 4 in 91% yield.10 Selective hydrolysis of the
C-2 benzoate and C-4 acetate groups of 4 with Triton B
afforded the triol 6 in 66% yield.10 Protection of the C-1
and C-2 hydroxy groups in 6 with carbonyldiimidazole
and imidazole11 gave the 1,2-carbonate derivative 7 in
85% yield.
2a and 2b lacked the benzoyloxy function at both the C-2
and the C-13 side chain, both of which are known to be
important for biological activity,3 and thus did not provide
an appropriate model to test the importance of the
oxetane ring. The azetidine analogues 3a and 3b were
both inactive in an in vitro cytotoxicity assay; analogue
3a did show some tubulin polymerization activity, but
this was 16 times less than that of docetaxel (1b).7 These
results were explained in terms of a specific interaction
of the heteroatom with the protein. The presence of a
nitrogen atom in the azetidine ring also posed the
problem of basicity, in that it would be protonated at
neutral pH and thus would presumably interact with
tubulin in a very different way than a neutral oxygen
atom. For these reasons, the role of the oxetane ring on
the activity of paclitaxel was still unclear, and the
synthesis of other D-ring modified analogues of paclitaxel
was necessary.
Opening of the oxetane ring in the protected derivative
7 was first attempted with trimethylsilyl chloride and
lithium iodide, but these conditions produced a mixture
of compounds. After some experimentation, it was found
that treatment of 7 at -78 °C in dichloromethane with
iodotrimethylsilane followed by a careful workup afforded
the 5R-iodo-20-hydroxy-D-seco-paclitaxel derivative 8 in
93% yield. When workup was effected at room temper-
ature, the formation of the C-2′ desilylated compound 9
was also observed. Finally, the formation of the 5R-iodo-
20-epoxy derivative 10 was achieved in 96% yield by the
reaction of 8 with trifluoromethanesulfonyl chloride and
DMAP.
Replacement of the oxygen atom with a sulfur or a
selenium atom would maintain the neutrality of the ring
while at the same time change both steric and electronic
effects in a predictable way. We thus decided to embark
on the synthesis of the 5,20-thiapaclitaxel analogue 16
and the 5,20-selenapaclitaxel analogue 24.
(8) Chen, S. H.; Huang, S.; Wei, J .; Farina, V. Tetrahedron 1993,
49, 2805-2828.
(9) (a) Holton, R. A.; Somoza, C.; Kim, H. B. Liang, F.; Biediger, R.
J .; Boatman, D.; Shindo, M.; Smith, C. C.; Kim, S.; Nadizadeh, H.;
Suzuki, Y.; Tao, C.; Vu, P.; Tang, S.; Zhang, P.; Murthi, K. K.; Gentile,
L. N.; Liu, J . H. J . Am. Chem. Soc. 1994, 116, 1597-1599. (b) Nicolaou,
K. C.; Yang, Z.; Liu, J . J .; Ueno, H.; Nantermet, P. G.; Guy, R. K.;
Claiborne, C. F.; Renaud, J .; Couladouros, E. A.; Paulvannan, K.;
Sorensen, E.; J . Nature 1994, 367, 630-634. (c) Masters, J . J .; Linj, J .
T.; Snyder, L. B.; Young, W. B.; Danishefsky, S. J . Angew. Chem., Int.
Ed. Engl. 1995, 34, 1723-1726.
Resu lts a n d Discu ssion
Ch em istr y. Our strategy for the synthesis of 5,20-
thiapaclitaxel analogues was envisaged to involve recon-
(6) Fenoglio, I.; Nano, G. M.; Vander Velde, D. G.; Appendino, G.
Tetrahedron Lett. 1996, 37, 3202-3206.
(7) Marder-Karsenti, R.; Dubois, J .; Bricard, L.; Gue´nard, D.;
Gue´ritte-Voegelein, F. J . Org. Chem. 1997, 62, 6631-6637.
(10) Chordia, M. D.; Kingston, D. G. I. J . Org. Chem. 1996, 61, 799-
801.
(11) Nicolaou, K. C.; Renaud, J .; Nantermet, P. G.; Couladouros, E.
A.; Guy, R. K.; Wrasidlo, W. J . Am. Chem. Soc. 1995, 117, 2409-2420.

2696 J . Org. Chem., Vol. 64, No. 8, 1999
Gunatilaka et al.
Sch em e 2a
a
Key: (i) TBDMS/imidazole, DMF, 60 °C, 2 h, then Et3SiCl/imiazole, 1 h, 91%; (ii) Triton B, CH2Cl2, -78 to -10 °C, TLC control, 66%;
(iii) carbonyldiimidazole, CH2Cl2, 35-40 °C, 24 h, 85%; (iv) Me3SiI, CH2Cl2, -78 °C, 45 min, 93%; (v) TfCl/DMAP, CH2Cl2, 0 °C to rt, 1
h, 96%.
Sch em e 3a
a
Key: (i) Li2S, THF, rt, 28 h, then carbonyldiimidazole/imidazole, rt, 12 h, 11 (56%), 12 (13%); (ii) LHMDS, THF, -78 °C (7 min), rt
(1 min), -78 °C (2 min), then methyl chloroformate, -78 °C (10 min), 14 (39%), 11 (41%); (iii) PhLi, THF, -78 °C, 3 min, 61%; (iv)
HF-pyridine, rt, 9 h, 76%; (v) m-CPBA, CH2Cl2, 0 °C to rt, 80 min, 61%.
Having made the key intermediate 10, we next turned
our attention to the preparation of 5,20-thiapaclitaxel
analogues; the synthetic route is summarized in Scheme
3. Treatment of the intermediate 10 with lithium sulfide
in THF at room temperature, with monitoring of the
reaction by TLC, showed that there was a complete
disappearance of starting material after 28 h. The 1H
NMR spectrum of the product obtained indicated it to
be a 1,2,4-triol different from 6. Treatment of the crude
triol reaction product with carbonyldiimidazole and imi-
dazole afforded two products after purification, namely
the sulfide 11 (56%) as the major product and the
disulfide 12 (13%) as the minor product. When a very
large excess of Li2S was used the major product from the
reaction of 10 with Li2S was the disulfide 12 (42%), and
sulfide 11 (20%) became the minor product, so either 11
or 12 could be formed as the major product by controlling
the concentration of Li2S. The mechanism for the forma-
tion of 11 presumably involves nucleophilic attack on the
primary C-20 of the 4,20-epoxy group by the sulfide ion,
followed by a ring closure step involving attack on C-5
and displacement of iodide;12 the alternative pathway of
initial attack at C-5 is less likely on steric grounds. Thus,
(12) A similar cyclization was used in Wender’s synthesis of taxol:
Wender, P. A.; Badham, N. F.; Conway, S. P.; Floreancig, P. E.; Glass,
T. E.; Houze, J . B.; Krauss, N. E.; Lee, D.; Marquess, D. G.; McGrane,
P. L.; Meng, W.; Natchus, M. G.; Shuker, A. J .; Sutton, J . C.; Taylor,
R. E. J . Am. Chem. Soc. 1997, 119, 2757-2758.

Novel Paclitaxol D-Ring Modified Analogues
J . Org. Chem., Vol. 64, No. 8, 1999 2697
Sch em e 4a
a
Key: (i) LHMDS, THF, -78 °C (7 min), rt (1 min), -78 °C (2 min), then methyl chloroformate, -78 °C (10 min), 69%; (ii) PhLi, THF,
-78 °C, 3 min, 70%; (iii) HF-pyridine, rt, 9 h, 87%.
contrary to previous reports,13 nucleophilic opening of the
4,20-epoxide is possible under the conditions employed.
The sulfide 11 could be oxidized with 3-chloroperbenzoic
acid to the sulfone (13) in 61% yield.
The sulfide analogue 16 of paclitaxel contains two
structural changes compared to paclitaxel, in that the
oxygen atom in the oxetane ring of paclitaxel has been
replaced by a sulfur atom and the C-4 acetate group has
been replaced by a methoxycarbonyl group. Hence, to get
a better approximation of the independent effect of each
of these structural changes on biological activity, we
synthesized the paclitaxel derivative 19 (Scheme 4), in
which only the C-4 acetate in paclitaxel has been replaced
by a methoxycarbonyl group. By doing this we were able
to compare the biological activity of paclitaxel with 19
and then the biological activity of 19 with the sulfide 16.
Thus, as shown in Scheme 4, treatment of the protected
1,2-carbonate derivative 7 with LHMDS and methyl
chloroformate gave 17 (69% yield) substituted at the C-4
position by a methoxycarbonyl group. Benzoylation of 17
with phenyllithium gave the 4-methoxycarbonyl-2-ben-
zoyl derivative 18 in 70% yield. Finally, removal of the
C-2′ and C-7 silyl protecting groups in 18 with HF-
pyridine gave the required product 19 in 87% yield.
We next turned our attention to the subsequent steps
of the synthesis, i.e acetylation of C-4, benzoylation of
C-2 and deprotection of C-2′ and C-7 positions. Surpris-
ingly, the C-4 hydroxy group in 11 was found to be
resistant to several acylation conditions. For example,
treatment of 11 with Ac2O/DMAP in CH2Cl2 or LHMDS/
AcCl, or the use of other acylating agents such as
cyclopropyl carbonyl chloride or N-acetylimidazole, led
only to the recovery of starting material. Attempted
acetylation under forcing conditions with acetic anhy-
dride and DMAP in toluene at 80 °C also failed to provide
the acetylated product. After many attempts, it was found
that treatment of 11 with LHMDS and methyl chloro-
formate afforded the derivative 14 in 39% yield, along
with unreacted starting material (41%). Attempts to
increase the yield by increasing the reaction time, tem-
perature, or concentration of reagents gave a mixture of
polar products. Benzoylation of the hydroxy group at C-2
in 14 was effected by reaction with phenyllithium, at -78
°C in THF,14 affording 15 in 61% yield. Hydrolysis of the
methoxycarbonyl group and formation of a complex
mixture of products was observed when workup was
carried out by standard methods. Thus a modified
workup, involving quenching of the reaction with dilute
aqueous HCl solution, was employed to avoid the hy-
drolysis of the methoxycarbonyl group. Conversion of 15
to the 5,20-thiapaclitaxel analogue 16 was achieved in
76% yield by treatment with HF-pyridine at room
temperature. The spectroscopic data for 16 were consis-
tent with its assigned structure.
The successful preparation of the 5,20-thiapaclitaxel
analogue by reconstruction of the four-membered ring
with the simultaneous introduction of a heteroatom from
intermediate 10 led us to extend this strategy to the
synthesis of the 5,20-selenapaclitaxel analogue 23 (Scheme
5).
A similar replacement of an oxygen atom for a sele-
nium atom could be achieved by reaction of key inter-
mediate 10 with lithium selenide. As reported,15 orga-
noselenium compounds can be prepared by reacting alkyl
halides with lithium selenide, the latter being available
from the reaction of Li(C2H5)3BH with gray selenium.
Indeed, treatment of a THF solution of 10 with Li2Se
prepared as above resulted in the formation of the 5,20-
selena derivative 20 in 67% yield.
Attempts to acetylate the C-4 hydroxy group of the
disulfide 12 using the methods just described were also
unsuccessful and led to the recovery of unreacted 12.
Also, attempts to react 12 with methyl chloroformate
under the conditions that were used successfully with the
sulfide, 11, were also unsuccessful and led to the recovery
of unreacted 12. Thus, it appears that the C-4 hydroxy
group of 12 is very sterically hindered and hence very
difficult to acylate.
Regrettably we were unable to isolate the desired
4-acyl analogue 21 by acylation of 20. The C-4 hydroxy
group of 20 was unreactive toward several acylating
conditions, such as Ac2O/DMAP in CH2Cl2, LHMDS/AcCl
in THF, acylimidazole/DMAP, or LHMDS and LHMDS/
cyclopropyl carbonyl chloride. Attempted acylation with
LHMDS/methyl chloroformate, which had proven to be
successful in the case of the thiapaclitaxel analogue 11,
was unsuccessful in this case and afforded a reaction
product that was tentatively identified as a mixture of
the desired analogue 21 and the ring-opened derivative
25 (Scheme 5). The presumed compound 21 was however
unstable and was converted to 25 either on attempted
(13) (a) Holton, R. Total Synthesis of Paclitaxel from Camphor. In
Taxane Anticancer Agents: Basic Science and Current Status; George,
I., Chen, T. T., Ojima, I., Vyas, D. M., Eds.; ACS Symposium Series
583, American Chemical Society: Washington, DC, 1995; pp 288-301.
(b) Margraff, R. M.; Bezard, D.; Bouzart, J . D.; Commerc¸on, A. Bioorg.
Med. Chem. Lett. 1994, 4, 233-236.
(14) Holton, R. A.; Kim, H. B.; Somoza, C.; Liang, F.; Biendiger, R.
J .; Boatman, P. D.; Shindo, M.; Smith, C. C.; Kim, S.; Nadizadeh, H.;
Suzuki, Y.; Tao, C.; Yu, P.; Tang, S.; Zhang, P.; Murthi, K. K.; Gentile,
L. N. Liu, J . H. J . Am. Chem. Soc. 1994, 116, 1599-1600.
(15) Hopkins, P. B.; Fuchs, P. L. J . Org. Chem. 1978, 43, 1204-
1208.

2698 J . Org. Chem., Vol. 64, No. 8, 1999
Gunatilaka et al.
Sch em e 5a
a
Key: (i) Li2Se, THF, -20 to 25 °C, 2 h, 67%; (ii) LHMDS, THF, -78 °C (5 min), rt (1 min), then methyl chloroformate, -78 °C (5 min),
25 (48%), 20 (14%); (iii) PhLi, THF, -78 °C, 3 min, 43%; (iv) HF-pyridine, rt, 8 h, 70.
Ta ble 1. Com p a r ison of Biologica l Activities of Com p ou n d s 16 a n d 19 w ith Th ose of P a clita xel a n d Doceta xel
effects on human cancer cell growth IC50 (nM)
enhancement of tubulin
polymerization (EC50, µM, ( SD)
Burkitt
prostate
breast
compound
condition 1a
condition 2b
lymphomac carcinomad melanomad carcinomad
ovarian carcinomad
CA46
5
3
>1000
2
PC3
4
0.9
>2500
1
LOX-IMV1
MCF-7
2
0.5
NT
0.2
1A9 1A9PTX10 1A9PTX22
Paclitaxel
Docetaxel
16
8.4 ( 0.8
5.2 ( 0.7
>50
1.5 ( 0.2
1.5 ( 0.2
13 ( 0.7
1.1 ( 0.1
6
4
2
60
10
60
20
0.9
NTe
0.6
NT
2
NT
20
NT
20
19
4.2 ( 0.1
a
Reaction mixtures (100 µL) contained 1.0 mg/mL purified bovine brain tubulin, 0.4 M monosodium glutamate (pH 6.6 in 2.5 M stock
solution with HCl), 5% (v/v) dimethyl sulfoxide as drug solvent, and varying drug concentrations. Incubation was for 20 min at room
temperature. Samples were centrifuged at 14000 rpm for 10 min in an Eppendorf model 5417C centrifuge at room temperature, and the
protein content of 20 µL of the supernatant was determined. Without drug the supernatant contained at least 95% as much protein as
the total reaction mixture. The EC50 is the drug concentration that reduced supernatant protein by 50%. The experiment was performed
b
3 times. Besides tubulin and drugs, reaction mixtures contained 0.65 M monosodium glutamate and 0.1 mM GTP. Conditions were
otherwise unchanged. c Cells were grown for 4 days in suspension culture. The IC50 is the drug concentration that reduced cell number
d
by 50%. Cells were grown for 4 days as monolayer cultures in microtiter plates. The IC50 is the drug concentration that reduced cell
protein by 50%. e Not tested.
purification or on standing in CDCl3 solution. The forma-
tion of 25 presumably occurs by intramolecular attack
of the carbomethoxy carbonyl group of 21 on C-5 with
concomitant ring-opening, followed by a prototopic shift.
A similar migration of an acetate group to C-5 with
D-ring opening was observed by treating 20 with Ac2O/
DMAP in toluene at 80 °C.
Since 20 could not be converted to 21, it was subjected
directly to reaction with phenyllithium to afford the C-2
benzoylated analogue 22 in 43% yield. Deprotection of
22 with HF-pyridine led to compound 23 in 70% yield,
and this was subjected to testing for cytotoxicity in the
HCT 116 cell line.
cancer cell lines (Table 1). We found that 16 had
negligible activity in all assays. Compound 19, however,
was clearly superior in activity to paclitaxel and even
docetaxel with purified tubulin, and 19 was also more
inhibitory than both paclitaxel and docetaxel against the
growth of most of the human cancer cell lines examined.
Initial qualitative studies with tubulin were performed
by turbidimetry, which demonstrated that 19 was more
potent than paclitaxel, while 16 was much less active.
Further work suggested that 19 would have biochemical
properties similar to those of docetaxel, and no reaction
condition was found where the slight activity of 16 could
be demonstrated in the absence of GTP in the reaction
mixture (GTP is an essential component for “normal”
microtubule assembly, i.e., without drug). We had previ-
ously developed an assay for the ready quantitative
Biologica l Act ivit ies. Compounds 16 and 19 were
examined for effects on the polymerization of purified
bovine brain tubulin and on the growth of several human

Novel Paclitaxol D-Ring Modified Analogues
J . Org. Chem., Vol. 64, No. 8, 1999 2699
comparison of paclitaxel analogues with activity superior
to that of the parent drug.16 This was drug-induced
assembly at room temperature in 0.4 M monosodium
glutamate (pH 6.6) without GTP, with the EC50 defined
as the drug concentration that reduced supernatant
tubulin concentration by 50%. Although the tubulin used
in current studies yields substantially lower EC50 values
for paclitaxel and docetaxel (Table 1; condition 1, 8.4 and
5.2 µM), compounds such as 19, which are more active
than paclitaxel, can still be readily distinguished. In this
assay, weakly active compounds such as 16 are indistin-
guishable from those devoid of activity. Although we had
previously shown that increasing the glutamate concen-
tration allowed one to identify many such weakly active
compounds,16 this was insufficient to obtain a quantita-
tive evaluation of the partial activity of 16.
We, therefore, evaluated GTP-containing reaction con-
ditions that would permit quantitative evaluation of 16
and other poorly active paclitaxel analogues. As in the
previous work, our goal was to obtain a reaction condition
in which polymer formation at room temperature (mea-
sured by residual protein in supernatants following
centrifugation at room temperature) would be negligible.
The best results were obtained when reaction mixtures
contained 0.65 M glutamate, 0.1 mM GTP, and 10 µM
tubulin. As in higher glutamate alone, in this reaction
condition compounds more active than paclitaxel yield
EC50 values that differ little from that obtained with
paclitaxel. But the partial activity of 16 could now be
quantitated, and it appears to be at least 9-fold less active
than paclitaxel.
Despite its weak activity with tubulin, compound 16
had no effect on the growth of the first two human cancer
cell lines examined (a Burkitt lymphoma and a prostate
carcinoma line), and so it was not studied further.
Compound 19, however, was highly active in these two
lines, and we extended the analysis to a melanoma line,
a breast carcinoma line, and a parental ovarian carci-
noma line (1A9) and two paclitaxel-resistant lines derived
from it. Paclitaxel resistance in these lines (PTX10 and
PTX22) was shown to be caused by two different muta-
tions in the dominant â-tubulin gene expressed in 1A9
cells.17 All cell lines were modestly more sensitive to
docetaxel than to paclitaxel, and all cells except the
prostate carcinoma line and the paclitaxel-resistant
mutants were more sensitive to 19 than to docetaxel. In
the current experiments the 22-fold relative resistance
to paclitaxel of 1A9PTX10 and 1A9PTX22 cells as com-
pared with the parental cells was reduced to about 4-5-
fold to docetaxel and about 10-12-fold to compound 19.
that substitution of the oxygen by a nitrogen also causes
a marked reduction in activity,7 but it extends it by
demonstrating that the activity loss is not due to any
basicity of the heteroatom. It can thus be concluded either
that this region of the pharmacophore of paclitaxel is very
sensitive to steric effects or that the oxetane oxygen is
acting as a hydrogen bond acceptor.
Exp er im en ta l Section
Gen er a l Meth od s. All chemicals were purchased from
Aldrich Chemical Co. with the exception of iodotrimethylsilane,
which was obtained from Fluka. Chemicals were used without
further purification, unless indicated otherwise. All anhydrous
reactions were performed in oven-dried glassware under a
positive pressure of argon. Anhydrous tetrahydrofuran (THF)
and dimethylformamide (DMF) were purchased from Aldrich
and used directly. Anhydrous CH2Cl2 was distilled from
calcium hydride. All reactions were monitored by TLC (silica
gel GF plates), visualized under short-wave UV light, and
developed with vanillin/H2SO4 reagent spray. Analytical TLC
was performed on kieselgel 60 F254 (E merck), 0.2 mm
thickness. Preparative TLC was performed on silica gel GF
plates (20 × 20 cm), 0.5 or 1.0 mm thickness, and purchased
from Analtech. Flash chromatography was carried out on silica
gel purchased from Baxter Scientific Products (S/P brand silica
gel 60 Å, 230-400 mesh). Melting points were determined on
a Kofler hot-stage apparatus and were uncorrected. All NMR
spectra were recorded in CDCl3 using solvent (δC 77.0 ppm)
or residual CHCl3 (δH 7.24 ppm) as internal standards.
1
Coupling constants were reported in hertz. H and 13C NMR
spectra were assigned primarily by comparison of the chemical
shifts and coupling constants with those of related compounds
or with the aid of 2D-NMR experiments (COSY, DEPT,
HETCOR, HMQC, and HMBC). Microanalyses were obtained
from Atlantic Microlabs, Inc., GA, and exact mass measure-
ments were performed at the Nebraska Center for Mass
Spectrometry. All solvent evaporation was done under reduced
pressure.
2′-(ter t-Bu tyld im eth ylsilyl)-7-(tr ieth ylsilyl)p a clita xel
(4). To a stirred solution of 1a (1.525 g, 1.786 mmol) in
anhydrous DMF (14.12 mL) were added imidazole (605 mg,
8.887 mmol) and tert-butyldimethylsilyl chloride (1.344 g,
8.917 mmol). The reaction mixture was stirred at 60 °C for 2
h and cooled to room temperature, and additional imidazole
(605 mg, 8.887 mmol) and triethylsilyl chloride (0.85 mL, 5.064
mmol) were added. The reaction mixture was stirred at room
temperature for 1 h and then diluted with EtOAc (200 mL).
The organic phase was washed successively with water (4 ×
60 mL) and brine (60 mL). The organic layer was dried over
anhydrous Na2SO4 and evaporated to dryness. The crude
product was purified by flash chromatography (hexanes:
EtOAc, 2:1) to give 4 (1.756 g, 91%) as a white amorphous
solid: mp 129-130 °C. The NMR spectra for 4 were identical
with those previously reported.10
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-7-
(tr ieth ylsilyl)p a clita xel (6). To a stirred solution of 4 (510
mg, 0.47 mmol) in anhydrous CH2Cl2 (22.4 mL) at -78 °C was
added benzyltrimethylammonium hydroxide (Triton B, 626 µL
of 40% w/v solution in methanol). The reaction mixture was
stirred at -78 °C for 10 min. The dry ice-acetone cooling bath
was then replaced with a dry ice-diethylene glycol bath and
the reaction mixture stirred at -10 to - 15 °C for a further 2
h. The progress of the reaction was monitored by TLC (silica
gel, 50% hexanes in EtOAc), which indicated the initial
formation of the compound 2′-(tert-butyldimethylsilyl)-2-de-
benzoyl-7-(triethylsilyl)paclitaxel10 (5) with a lower Rf than
that of the starting material (4). As the reaction progressed,
5 was converted to the required product, 2′-(tert-butyldimeth-
ylsilyl)-2-debenzoyl-4-deacetyl-7-(triethylsilyl)paclitaxel (6),
which had an Rf intermediate between those of 4 and 5. After
2 h there was no starting material remaining. The major
product was 6, as well as a very polar product at the origin of
the TLC plate. The reaction mixture was then diluted with
The selenapaclitaxel analogue 23 was evaluated for
cytotoxicity in the HCT 116 cell line. It showed no
detectable activity, consistent with the fact that 4-acyl
substitution is necessary for activity.18
Con clu sion . Substitution of a sulfur for the oxygen
of the oxetane ring in paclitaxel causes a significant
diminution of its tubulin-assembly activity and cytotox-
icity. This finding is consistent with the previous finding
(16) Lin, C. M.; J iang, Y. Q.; Chaudhary, A. G.; Rimoldi, J . M.;
Kingston, D. G. I.; Hamel, E. Cancer Chemother. Pharmacol. 1996,
38, 136-140.
(17) Giannakakou, P.; Sackett, D. L.; Kang, Y.-K.; Zhan, Z.; Buters,
J . T. M.; Fojo, T.; Poruchynsky, M. S. J . Biol. Chem. 1997, 272, 17118-
17125.
(18) Neidigh, K. A.; Gharpure, M. M.; Rimoldi, J . M.; Kingston, D.
G. I.; J iang, Y. Q.; Hamel, E. Tetrahedron Lett. 1994, 35, 6839-6842.

2700 J . Org. Chem., Vol. 64, No. 8, 1999
Gunatilaka et al.
cold CH2Cl2 (-78 °C, 60 mL) and quenched with cold 0.1 N
HCl (0 °C, 5 mL). The CH2Cl2 layer was washed successively
with with water (2 × 10 mL), 5% NaHCO3 (10 mL), H2O (5
mL), and brine (5 mL). The organic phase was dried over
anhydrous Na2SO4 and evaporated to dryness. The crude
product was purified by flash chromatography (hexanes:
EtOAc, 1:1) to give 6 (291 mg, 66%). The NMR data (1H and
13C) were identical with those reported previously.10
which time the solution became very viscous. It was kept in
the 0 °C cooling bath for a total of 8 min and then left at room
temperature for 50 min. After this period, 100 mL of EtOAc
and 15 mL water were added to the reaction flask. The organic
layer was separated from the aqueous layer, washed succes-
sively with water (2 × 15 mL) and brine (15 mL), and dried
over anhydrous Na2SO4. The organic phase was evaporated
to dryness and the crude product purified by flash chroma-
tography (EtOAc:hexanes, 3:7) to give 10 (266 mg, 96%) as a
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-7-
(tr ieth ylsilyl)p a clita xel 1,2-Ca r bon a te (7). To a solution
of 6 (935 mg, 1 mmol) in anhydrous CH2Cl2 (19 mL) were
added carbonyldiimidazole (3.523 g, 21.73 mmol) and imidazole
(218 mg, 3.20 mmol). The reaction mixture was stirred under
argon at 35-40 °C for 24 h. After this period, the reaction
mixture was diluted with 200 mL of CH2Cl2 and washed
successively with 5% HCl (5 × 60 mL), 5% NaHCO3 (2 × 60
mL), H2O (60 mL), and brine (60 mL). The organic phase was
dried over anhydrous Na2SO4 and evaporated to dryness. The
crude product was purified by flash chromatography (40%
EtOAc in hexanes) to give 7 (818 mg, 85%) as a white
1
colorless crystalline solid: mp 241-243 °C; H NMR δ -0.32
(s, 3H), -0.13 (s, 3H), 0.56 (q, 6H, J ) 7.88 Hz), 0.80 (s, 9H),
0.89 (t, 9H, J ) 7.94 Hz), 1.16 (s, 3H), 1.25 (s, 3H), 1.31 (s,
3H), 2.16 (s, 3H), 2.28 (m, 1H), 2.41 (s, 3H), 2.39 (m, 1H), 2.57
(dd, 1H, J ) 9.77, 15.87 Hz), 2.68 (dd, 1H, J ) 6.87, 15.87
Hz), 2.94 (d, 1H, J ) 4.88 Hz), 3.86 (d, 1H, J ) 4.73 Hz), 3.87
(d, 1H, J ) 4.43 Hz), 4.18 (d, 1H, J ) 5.03 Hz), 4.21 (m, 1H),
4.50 (d, 1H, J ) 1.99 Hz), 4.53 (m, 1H), 5.71 (dd, 1H, J ) 1.53,
8.85 Hz), 6.24 (t, 1H, J ) 7.79 Hz), 6.54 (s, 1H), 7.07 (d, 1H, J
) 8.69 Hz), 7.26-7.53 (m, 8H), 7.80 (d, 2H, J ) 7.02 Hz); 13C
NMR δ -5.65, -5.32, 5.12, 6.63, 12.23, 17.34, 18.17, 19.86,
20.75, 25.56, 26.30, 33.18, 39.36, 40.10, 41.29, 41.89, 55.85,
58.79, 60.24, 62.82, 69.95, 70.56, 75.67, 75.83, 80.72, 89.08,
126.85, 127.05, 127.73, 128.52, 128.66, 131.58, 131.87, 134.33,
138.57, 143.96, 152.28, 166.55, 169.08, 171.16, 201.54; HR-
FABMS calcd for C51H70INO12Si2Na [M + Na]+ m/z 1094.3379,
found 1094.3347. Anal. Calcd for C51H70INO12Si2: C, 57.13; H,
6.68; N, 1.31. Found: C, C, 57.04; H, 6.50; N, 1.27.
1
crystalline solid: mp 222-224 °C; H NMR δ -0.28 (s, 3H),
-0.11 (s, 3H), 0.59 (q, 6H, J ) 7.83 Hz), 0.82 (s, 9H), 0.92 (t,
9H, J ) 7.86 Hz), 1.16 (s, 3H), 1.18 (s, 3H), 1.65 (s, 3H), 1.98
(m, 1H), 2.16 (s, 6H), 2.47 (dd, 1H, J ) 9.77, 15.26 Hz), 2.57
(m, 1H), 2.75 (dd, 1H, J ) 4.58, 15.71 Hz), 2.99 (d, 1H, J )
6.11 Hz), 4.21 (dd, 1H, J ) 6.87, 10.53 Hz), 4.39 (d, 1H, J )
5.34 Hz), 4.53 (d, 1H, J ) 8.70 Hz), 4.53 (d, 1H, J ) 1.68 Hz),
4.59 (d, 1H, J ) 8.69 Hz), 4.94 (dd, 1H, J ) 2.29, 9.31 Hz),
5.15 (s, 1H), 5.85 (dd, 1H, J ) 1.6, 9.0 Hz), 5.89 (brt, 1H, J )
7.0 Hz), 6.43 (s, 1H), 7.26-7.54 (m, 8H), 7.75 (d, 2H, J ) 8.32
Hz); 13C NMR δ -5.88, -5.58, 5.21, 6.75, 9.92, 16.54, 18.23,
19.22, 20.81, 25.50, 26.40, 32.46, 37.92, 41.05, 48.61, 55.24,
60.07, 71.04, 72.17, 73.19, 75.14, 76.00, 80.23, 81.17, 87.51,
89.80, 126.68, 127.05, 127.97, 128.55, 128.86, 132.14, 133.41,
133.98, 139.13, 141.29, 152.69, 167.38, 169.18, 170.42, 202.32.
FABMS m/z 962 [M + H]+. Anal. Calcd for C51H71NO13Si2: C,
63.65; H, 7.44; N, 1.46. Found: C, 63.77; H, 7.61; N, 1.46.
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-7-
(tr ieth ylsilyl)-5-r-iod o-20-h yd r oxy-D-seco-p a clita xel 1,2-
Ca r bon a te (8). To a stirred solution of 7 (389 mg, 0.40 mmol)
in anhydrous CH2Cl2 (23 mL) at -78 °C was added iodotrim-
ethylsilane (220 µL, 1.62 mmol). The reaction mixture was
stirred at -78 °C for 45 min and quenched with CH2Cl2 (180
mL) precooled to -78 °C, followed by water (100 mL, 0 °C).
The organic layer was separated and washed successively with
0.05 M Na2S2O4 (2 ×100 mL), water (25 mL), and brine (25
mL). The CH2Cl2 layer was dried over anhydrous Na2SO4 and
evaporated to dryness. The crude product was purified by flash
silica gel chromatography (EtOAc:hexanes, 2:3) to give 413 mg
(93%) of 8 as a white crystalline solid: mp 153-155 °C; 1H
NMR δ -0.36 (s, 3H), -0.15 (s, 3H), 0.56 (q, 6H, J ) 7.93 Hz),
0.75 (s, 9H), 0.92 (t, 9H, J ) 7.94 Hz), 1.08 (s, 3H), 1.11 (s,
3H), 1.18 (s, 3H), 2.03 (m, 1H), 2.15 (s, 3H), 2.23 (td, 1H, J )
3.21, 15.72 Hz), 2.36 (dd, 1H, J ) 9.77, 15.56 Hz), 2.45 (d, 3H,
J ) 1.07 Hz), 3.16 (dd, 1H, J ) 5.72, 15.49 Hz), 3.54 (d, 1H, J
) 10.98 Hz), 3.59 (d, 1H, J ) 4.88 Hz), 3.99 (d, 1H, J ) 11.14
Hz), 4.26 (d, 1H, J ) 4.88 Hz), 4.50 (dd, 1H, J ) 3.74, 10.61
Hz), 4.59 (d, 1H, J ) 1.83 Hz), 5.07 (t, 1H, J ) 2.98 Hz), 5.83
(dd, 1H, J ) 1.30, 8.17 Hz), 6.11 (dd, 1H, J ) 8.17 Hz), 6.54
(s, 1H), 7.17 (d, 1H, J ) 8.24 Hz), 7.26-7.54 (m, 8H), 7.74 (d,
2H, J ) 7.02 Hz); 13C NMR δ -6.00, -5.61, 5.14, 6.75, 13.38,
17.67, 18.16, 19.70, 20.79, 25.45, 26.34, 32.49, 38.90, 41.01,
45.34, 46.52, 55.75, 60.73, 62.01, 70.61, 71.02, 73.58, 75.04,
75.43, 82.13, 89.74, 126.92, 127.06, 127.61, 128.35, 128.90,
132.01, 133.01, 133.79, 138.54, 143.22, 152.71, 167.32, 169.08,
171.18, 201.42; HRFABMS calcd for C51H72NO13Si2INa [M+Na]+
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-7-
(tr ieth ylsilyl)-5,20-d eoxy-5,20-su lfin ylp a clita xel 1,2-Ca r -
bon a te (11) a n d 2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-
4-d ea cet yl-7-(t r iet h ylsilyl)-5,20-d eoxy-5,20-d it h ia p a cli-
ta xel 1,2-Ca r bon a te (12). To a mixture of 10 (94.3 mg, 0.088
mmol) and Li2S (25 mg, 0.544 mmol) was added anhydrous
THF (7.8 mL). The resulting suspension was stirred under
argon at room temperature for 28 h (TLC control for the
disappearance of 10). The reaction mixture was diluted with
EtOAc (115 mL) and 0.05 M Na2S2O4 (25 mL). The organic
layer was separated from the aqueous phase and washed
successively with 0.05 M Na2S2O4 solution (25 mL), water (15
mL), and brine (15 mL). The organic phase was dried over
anhydrous Na2SO4 and evaporated to dryness. The major
product had a lower Rf than 10, and 13C NMR indicated that
the 1, 2-carbonate protecting group was absent in this product.
Hence, the crude product was treated with carbonyldiimidazole
in order to synthesize the 1,2-carbonate-protected compound;
i.e to the previously dried crude product in anhydrous CH2Cl2
(1.9 mL) were added carbonyldiimidazole (201 mg, 1.24 mmol)
and imidazole (56 mg, 0.82 mmol). The reaction mixture was
stirred under argon at room temperature for 12 h and worked
up as described in the preparation of 7. The crude product was
purified by preparative TLC (15% EtOAc in hexanes) to give
two products. The major product was 2′-(tert-butyldimethyl-
silyl)-2-debenzoyl-4-deacetyl-7-(triethylsilyl)-5,20-deoxy-5,20-
sulfinylpaclitaxel 1,2-carbonate (11) (48.1 mg, 56%) and the
minor product was 2′-(tert-butyldimethylsilyl)-2-debenzoyl-4-
deacetyl-7-(triethylsilyl)-5,20-deoxy-5,20-dithiapaclitaxel 1,2-
carbonate (12) (11.2 mg, 13%). Compounds 11 and 12 had very
similar Rf values and required multiple developments by
preparative TLC to separate them. With a large excess of Li2S
(103.5 mg, 2.25 mmol) and 10 (64.4 mg, 0.06 mmol) in 5.4 mL
of THF over 28 h at room temperature, the major product was
12 (25.4 mg, 42%) and the minor product was 11 (11.6 mg,
1
20%). Compound 11: H NMR δ -0.29 (s, 3H), -0.11 (s, 3H),
0.58 (q, 6H, J ) 7.78 Hz), 0.83 (s, 9H), 0.91 (t, 9H, J ) 7.94
Hz), 1.157 (s, 3H), 1.164 (s, 3H), 1.63 (s, 3H), 2.155 (s, 3H),
2.165 (s, 3H), 2.34 (m, 1H), 2.51 (m, 2H), 2.75 (d, 1H, J ) 5.34
Hz), 2.90 (dd, 1H, J ) 4.66, 15.34 Hz), 3.10 (d, 1H, J ) 11.75
Hz), 3.47 (t, 1H, J ) 9.00 Hz), 3.78 (d, 1H, J ) 11.90 Hz), 4.08
(m, 1H), 4.29 (d, 1H, J ) 5.34 Hz), 4.52 (d, 1H, J ) 1.53 Hz),
5.35 (bs, 1H), 5.87 (m, 1H), 5.93 (d, 1H, J ) 8.85 Hz), 6.41 (s,
1H), 7.27-7.52 (m, 8H), 7.78 (d, 2H, J ) 7.02 Hz); 13C NMR δ
-5.89, -5.56, 5.25, 6.76, 11.32, 16.59, 18.23, 19.20, 20.80,
25.52, 26.46, 32.48, 39.73, 40.84, 41.08, 47.79, 51.44, 55.29,
60.28, 71.14, 72.44, 75.30, 75.95, 78.93, 81.44, 89.84, 126.63,
127.07, 127.92, 128.59, 128.83, 132.07, 133.58, 133.69, 139.33,
m/z 1112.3485, found 1112.3488. Anal. Calcd for C51H72NO13
-
Si2I: C, 56.19; H, 6.66; N, 1.28. Found: C, 55.90; H, 6.62; N,
1.52.
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-7-
(tr ieth ylsilyl)-5-r-iod o-4,20-ep oxy-D-seco-p a clita xel 1,2-
Ca r bon a te (10). To a stirred solution of 8 (288 mg, 0.26 mmol)
and DMAP (425 mg, 3.48 mmol) in anhydrous CH2Cl2 at 0 °C
was added trifluoromethanesulfonyl chloride (300 µL, 2.82
mmol). The reaction mixture was stirred at 0 °C for 2 min, at

Novel Paclitaxol D-Ring Modified Analogues
J . Org. Chem., Vol. 64, No. 8, 1999 2701
141.35, 152.78, 167.31, 169.21, 170.40, 202.27; HRFABMS
calcd for C51H72NO12Si2S [M + H]+ m/z 978.4314, found
978.4290 (2.4 ppm dev). Compound 12: 1H NMR δ -0.28 (s,
3H), -0.12 (s, 3H), 0.59 (q, 6H, J ) 7.78 Hz), 0.83 (s 9H), 0.91
(t, 9H, J ) 7.94 Hz), 1.18 (s, 6H), 1.28 (s, 3H), 2.16 (s, 3H),
2.18 (d, 3H, J ) 1.22 Hz), 2.22 (m, 1H), 2.47 (m, 2H), 3.04 (dd,
1H, J ) 4.43, 15.72 Hz), 3.32 (bs, 2H), 3.37 (d, 1H, J ) 5.34
Hz), 3.38 (m, 1H), 4.20 (dd, 1H, J ) 4.50, 11.37 Hz), 4.29 (d,
1H, J ) 5.04 Hz), 4.49 (d, 1H, J ) 1.53 Hz), 4.66 (bs, 1H),
5.78 (dd, 1H, J ) 1.30, 8.47 Hz), 5.92 (dd, 1H, J ) 3.28, 10.00
Hz), 6.53 (s, 1H), 7.22 (d, 1H, J ) 8.54 Hz), 7.27-7.53 (m, 8H),
7.76 (d, 2H, J ) 6.87 Hz); 13C NMR δ -5.86, -5.58, 5.19, 6.76,
10.88, 16.73, 18.23, 19.11, 20.80, 25.52, 26.68, 32.35, 35.48,
41.31, 49.32, 49.43, 55.31, 61.69, 64.95, 71.17, 72.57, 75.16,
75.64, 81.32, 86.13, 89.71, 126.72, 127.02, 127.94, 128.55,
128.78, 131.93, 133.70, 133.76, 139.11, 141.30, 152.37, 167.10,
169.19, 170.40, 201.86; HRFABMS calcd for C51H72NO12Si2S2
[M + H]+ m/z 1010.4035, found 1010.4051 (1.6 ppm dev).
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-7-
(tr ieth ylsilyl)-5,20-d eoxy-5,20-su lfon ylp a clita xel 1,2-Ca r -
bon a te (13). To a stirred solution of 11 (12.7 mg, 0.013 mmol)
in anhydrous CH2Cl2 (250 µL) at 0 °C was added dropwise over
5 min a solution of 3-chloroperbenzoic acid (12.7 mg, 0.074
mmol) in 250 µL of anhydrous CH2Cl2. The reaction mixture
was stirred at 0 °C for 30 min. The cooling bath was removed
and the reaction mixture stirred at room temperature for 50
min. The reaction mixture was diluted with CH2Cl2 (45 mL)
and saturated sodium sulfite (15 mL). The organic layer was
separated from the aqueous phase and washed successively
with 5% NaHCO3 (2 × 17 mL), water (15 mL), and brine (15
mL). The organic phase was dried over anhydrous Na2SO4 and
evaporated to dryness. The crude product was purified by
preparative TLC (EtOAc:hexanes, 3:7) to give 13 (8 mg, 61%):
1H NMR δ -0.21 (s, 3H), -0.08 (s, 3H), 0.62 (q, 6H, J ) 7.71
Hz), 0.87 (s, 9H), 0.94 (t, 9H, J ) 7.86 Hz), 1.19 (s, 3H), 1.20
(s, 3H), 1.38 (s, 3H), 2.18 (s, 3H), 2.20 (d, 3H, J ) 1.07 Hz),
2.43 (m, 2H), 2.50 (dd, 1H, J ) 9.77, 15.72 Hz), 2.70 (dd, 1H,
J ) 4.58, 15.72 Hz), 3.34 (d, 1H, J ) 5.49 Hz), 4.22 (t, 1H, J
) 8.39 Hz), 4.28 (dd, 1H, J ) 4.28, 15.26 Hz), 4.33 (d, 1H, J )
5.49 Hz), 4.48 (m, 1H), 4.58 (d, 1H, J ) 1.53 Hz), 5.89 (d, 1H,
J ) 9.62 Hz), 5.94 (dd, 1H, J ) 3.37, 9.77 Hz), 6.24 (br s, 1H),
1H, J ) 8.02 Hz), 6.41 (s, 1H), 7.05 (d, 1H, J ) 9.31 Hz), 7.27-
7.53 (m, 8H), 7.78 (d, 2H, J ) 7.02 Hz); 13C NMR δ -5.96,
-5.28, 5.24, 6.73, 11.99, 15.34, 18.06, 20.39, 20.77, 25.48, 25.59,
32.24, 35.37, 40.35, 41.24, 41.67, 46.41, 54.88, 56.98, 59.90,
68.90, 72.10, 75.16, 75.78, 81.31, 89.31, 89.93, 126.35, 127.02,
127.62, 128.51, 128.79, 130.87, 131.78, 134.18, 138.62, 143.75,
150.86, 152.37, 166.73, 169.08, 171.47, 201.89; HRFABMS
calcd for C53H73NO14Si2SLi [M + Li]+ m/z 1042.4450, found
1042.4465 (-1.4 ppm dev).
2′-(ter t-Bu tyld im eth ylsilyl)-4-d ea cetyl-4-m eth oxyca r -
b on yl-7-(t r iet h ylsilyl)-5,20-d eoxy-5,20-su lfin ylp a clit a x-
el (15). A solution of 14 (9.0 mg, 0.009 mmol) in anhydrous
THF (160 µL) was stirred at -78 °C under argon. To the
solution of 14 was added phenyllithium (0.045 mmol, 25 µL of
a 1.8 M solution in cyclohexanes-ether). The reaction mixture
was stirred at -78 °C for 3 min and quenched with EtOAc
(25 mL). The EtOAc layer was washed successively with 5%
HCl (2 × 8 mL), 5% NaHCO3 (2 × 6 mL), water (6 mL), and
brine (10 mL). The organic layer was dried over anhydrous
Na2SO4 and evaporated to dryness. Purification of the crude
product by preparative TLC (hexanes:EtOAc, 4:1) gave 5.9 mg
(61%) of 15: 1H NMR δ -0.37 (s, 3H), -0.07 (s, 3H), 0.56 (q,
6H, J ) 7.68 Hz), 0.75 (s, 9H), 0.90 (t, 9H, J ) 7.94 Hz), 1.11
(s, 3H), 1.19 (s, 3H), 1.79 (s, 3H), 2.02-2.21 (m, 2H), 2.11 (s,
3H), 2.15 (s, 3H), 2.38 (dd, 1H, J ) 8.63, 15.65 Hz), 2.54 (m,
1H), 3.13 (d, 1H, J ) 12.06 Hz), 3.48 (d, 1H, J ) 12.20 Hz),
3.96 (d, 1H, J ) 7.17 Hz), 4.05 (dd, 1H, J ) 6.94, 10.61 Hz),
4.09 (s, 3H), 4.33 (dd, 1H, J ) 5.65, 11.30 Hz), 4.73 (d, 1H, J
) 1.83 Hz), 5.66 (d, 1H, J ) 7.33 Hz), 5.93 (d, 1H, J ) 8.32
Hz), 6.26 (t, 1H, J ) 8.07 Hz), 6.44 (s, 1H), 7.09 (d, 1H, J )
9.30 Hz), 7.26-7.57 (m, 11H), 7.76 (d, 2H, J ) 7.18 Hz), 8.12
(d, 2H, J ) 7.02 Hz); 13C NMR δ -6.06, -5.29, 5.31, 6.77, 11.85,
14.62, 18.11, 20.90, 21.06, 25.47, 26.61, 35.95, 40.03, 42.80,
42.91, 48.90, 55.13, 56.91, 58.67, 70.15, 72.36, 75.19, 75.29,
75.32, 78.88, 90.74, 126.37, 126.97, 127.58, 128.50, 128.77,
128.83, 129.59, 130.20, 131.69, 132.70, 133.64, 134.35, 138.79,
140.83, 151.42, 166.74, 167.30, 169.32, 171.36, 201.63; HR-
FABMS calcd for C59H79NO14Si2SLi [M + Li]+ m/z 1120.4920,
found 1120.4948 (-2.5 ppm dev).
4-De a ce t yl-4-m e t h oxyca r b on yl-5,20-d e oxy-5,20-su l-
fin ylp a clita xel (16). Compound 15 (7.3 mg, 0.007 mmol) was
dissolved in THF (830 µL) in a 5 mL Teflon flask. The solution
of 15 was stirred at 0 °C and a cold solution (0 °C) of hydrogen
fluoride-pyridine (60 µL) added. The reaction mixture was
stirred at room temperature for 9 h and diluted with EtOAc
(25 mL). The organic layer was washed successively with 5%
NaHCO3 (2 × 6 mL), 5% HCl (2 × 6 mL), water (6 mL), and
brine (10 mL). The organic phase was dried over anhydrous
Na2SO4 and evaporated to dryness. The crude product was
purified by preparative TLC (hexanes:EtOAc, 1:1) to give 4.4
mg (76%) of 16: 1H NMR δ 1.11 (s, 3H), 1.19 (s, 3H), 1.75 (s,
3H), 1.86 (d, 3H, J ) 1.07 Hz), 2.19 (m, 1H), 2.22 (s, 3H), 2.33
(m, 2H), 2.56 (m, 2H), 3.12 (d, 1H, J ) 11.90 Hz), 3.49 (d, 1H,
J ) 12.20 Hz), 3.58 (d, 1H, J ) 3.66 Hz), 3.90 (d, 1H, J ) 7.17
Hz), 3.93 (s, 3H), 4.01 (dd, 1H, J ) 7.17, 10.38 Hz), 4.22 (m,
1H), 4.83 (dd, 1H, J ) 2.29, 3.66 Hz), 5.66 (d, 1H, J ) 7.17
Hz), 5.94 (d, 1H, J ) 9.00 Hz), 6.16 (t, 1H, J ) 8.47 Hz), 6.24
(s, 1H), 7.03 (d, 1H, J ) 9.15 Hz), 7.31-7.59 (m, 11H), 7.76 (d,
2H, J ) 7.02 Hz), 8.12 (d, 2H, J ) 7.17 Hz); 13C NMR δ 11.25,
15.18, 20.87, 21.56, 27.02, 36.01, 36.17, 38.43, 42.77, 43.00,
47.60, 54.33, 56.37, 58.87, 71.87, 72.14, 73.37, 75.25, 75.67,
79.06, 90.87, 126.88, 127.01, 128.07, 128.71, 128.82, 128.86,
129.44, 130.18, 131.88, 132.39, 133.74, 133.83, 138.49, 142.53,
151.99, 166.64, 167.14, 171.38, 172.70, 203.46; HRFABMS
calcd for C47H51NO14SNa [M + Na]+ m/z 908.2928, found
908.2933 (-0.8 ppm dev).
6.46 (s,1H), 7.28-7.56 (m, 8H), 7.77 (d, 2H, J ) 7.02 Hz); 13
C
NMR δ -5.73, -5.54, 5.18, 6.73, 10.78, 16.50, 18.26, 19.16,
20.76, 25.55, 26.37, 28.35, 32.52, 41.14, 50.43, 54.71, 59.74,
63.00, 70.70, 71.18, 75.23, 75.68, 77.65, 80.84, 82.82, 89.98,
127.02, 127.17, 128.17, 128.64, 128.85, 132.26, 133.11, 133.45,
138.66, 141.58, 152.03, 167.15, 169.13, 170.47, 200.95; HR-
FABMS calcd for C51H72NO14Si2S [M + H]+ m/z 1010.4212,
found 1010.4206 (0.6 ppm dev).
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-4-
m et h oxyca r b on yl-7-(t r iet h ylsilyl)-5,20-d eoxy-5,20-su l-
fin ylp a clita xel 1,2-Ca r bon a te (14). To a stirred solution of
11 (23.3 mg, 0.024 mmol) in anhydrous THF (730 µL), at -78
°C under argon, was added 90 µL (0.09 mmol) of a 1.0 M of
solution of lithium bis(trimethysilyl)amide (LHMDS) in THF.
The reaction mixture was stirred at -78 °C for 7 min. The
reaction flask was removed from the -78 °C bath and stirred
at room temperature for 1 min. It was then placed in the -78
°C bath and stirred for 2 min, and freshly distilled methyl
chloroformate (20 µL, 0.26 mmol) was added. The reaction
mixture was stirred at -78 °C for 10 min and quenched with
EtOAc (60 mL). The EtOAc layer was washed successively
with 5% HCl (2 × 20 mL), 5% NaHCO3 (2 × 20 mL), water
(15 mL), and brine (20 mL). The organic layer was dried over
anhydrous Na2SO4 and evaporated to dryness. The crude
product was purified by preparative TLC (hexanes:EtOAc, 4:1)
to give unreacted 11 (9.5 mg, 41%) and 14 (9.7 mg, 39%): 1H
NMR δ -0.39 (s, 3H), -0.07 (s, 3H), 0.56 (q, 6H, J ) 7.73 Hz),
0.77 (s, 9H), 0.88 (t, 9H, J ) 7.94 Hz), 1.20 (s, 3H), 1.27 (s,
3H), 1.87 (s, 3H), 2.07 (d, 3H, J ) 1.22 Hz), 2.14 (s, 3H), 2.17
(m, 2H), 2.47 (dd, 1H, J ) 9.69, 15.64 Hz), 2.64 (m, 1H), 3.47
(d, 1H, J ) 6.11 Hz), 3.54 (d, 1H, J ) 12.82 Hz), 3.60 (d, 1H,
J ) 12.97 Hz), 3.99 (s, 3H), 4.17 (dd, 1H, J ) 5.19, 10.99 Hz),
4.31 (dd, 1H, J ) 6.18, 10.45 Hz), 4.44 (d, 1H, J ) 6.10 Hz),
4.71 (d, 1H, J ) 1.83 Hz), 5.86 (d, 1H, J ) 8.55 Hz), 6.22 (t,
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-4-
m eth oxyca r bon yl-7-(tr ieth ylsilyl)p a clita xel 1,2-Ca r bon -
a te (17). Acetylation of 7 to give 17 was done using the same
methodology as for the preparation of 14 from 11. Reaction of
7 (28.4 mg, 0.03 mmol) in anhydrous THF (906 µL) with
LHMDS (96 µL) and methyl chloroformate (24 µL) gave 20.8
mg (69%) of 17 after workup and preparative TLC (hexanes:
EtOAc), 4:1): 1H NMR δ -0.38 (s, 3H), -0.08 (s, 3H), 0.56 (q,
6H, J ) 7.58 Hz), 0.77 (s, 9H), 0.89 (t, 9H, J ) 7.94 Hz), 1.21

2702 J . Org. Chem., Vol. 64, No. 8, 1999
Gunatilaka et al.
(s, 3H), 1.28 (s, 3H), 1.75 (s, 3H), 1.94 (dd, 1H, J ) 10.76, 14.73
Hz), 2.04 (d, 3H, J ) 0.61 Hz), 2.14 (s, 3H), 2.24 (dd, 1H, J )
7.71, 15.80 Hz), 2.52 (dd, 1H, J ) 9.61, 15.72 Hz), 2.62 (m,
1H), 3.53 (d, 1H, J ) 5.65 Hz), 3.96 (s, 3H), 4.39 (dd, 1H, J )
7.1, 9.54 Hz), 4.51 (m, 2H), 4.69 (d, 1H, J ) 1.53 Hz), 4.72 (d,
1H, J ) 9.31 Hz), 5.09 (d, 1H, J ) 8.69 Hz), 5.77 (d, 1H, J )
9.46 Hz), 6.22 (t, 1H, J ) 8.17 Hz), 6.43 (s, 1H), 7.01 (d, 1H, J
) 9.31 Hz), 7.26-7.54 (m, 8H), 7.77 (d, 2H, J ) 6.86 Hz); 13C
NMR δ -5.91, -5.26, 5.16, 6.71, 10.06, 15.19, 18.06, 20.47,
20.73, 25.46, 32.09, 37.86, 41.40, 43.54, 54.96, 57.05, 59.92,
69.24, 71.49, 75.01, 75.74, 75.79, 80.98, 81.86, 83.53, 89.65,
126.36, 127.00, 127.72, 128.57, 128.78, 131.53, 131.79, 134.13,
138.41, 143.68, 151.57, 152.38, 166.82, 169.03, 171.55, 201.72;
HRFABMS calcd for C53H74NO15Si2 [M + H]+ m/z 1020.4597,
found 1020.4568 (2.8 ppm dev).
2′-(ter t-Bu tyld im eth ylsilyl)-4-d ea cetyl-4-m eth oxyca r -
bon yl-7-(tr ieth ylsilyl)p a clita xel (18). The benzoylation of
17 to give 18 was done using the same methodology as for the
preparation of 15 from 14. Benzoylation of 17 (13.5 mg, 0.013
mmol) in anhydrous THF (250 µL) with phenyllithium (38 µL)
afforded after workup and preparative TLC (hexanes:EtOAc,
3:1) 10.2 mg (70%) of 18: 1H NMR δ -0.35 (s, 3H), -0.05 (s,
3H), 0.56 (q, 6H, J ) 7.58 Hz), 0.77 (s, 9H), 0.91 (t, 9H, J )
7.93 Hz), 1.13 (s, 3H), 1.20 (s, 3H), 1.69 (s, 3H), 1.91 (m, 1H),
2.06 (s, 3H), 2.07-2.15 (m, 1H), 2.16 (s, 3H), 2.41 (dd, 1H, J )
8.70, 15.72 Hz), 2.53 (m, 1H), 3.95 (d, 1H, J ) 7.48 Hz), 4.05
(s, 3H), 4.24 (d, 1H, J ) 8.70 Hz), 4.34 (d, 1H, J ) 8.39 Hz),
4.45 (dd, 1H, J ) 6.56, 10.68 Hz), 4.69 (d, 1H, J ) 1.68 Hz),
4.98 (d, 1H, J ) 7.94 Hz), 5.70 (d, 1H, J ) 6.87 Hz), 5.79 (d,
1H, J ) 8.39 Hz), 6.25 (t, 1H, J ) 8.78 Hz), 6.45 (s, 1H), 7.08
(d, 1H, J ) 9.01 Hz), 7.27-7.58 (m, 11H), 7.74 (d, 2H, J )
7.18 Hz), 8.11 (d, 2H, J ) 7.17 Hz); 13C NMR δ -5.94, -5.23,
5.24, 6.74, 10.12, 14.47, 18.11, 20.87, 21.04, 25.47, 26.51, 35.60,
37.16, 43.20, 46.72, 55.36, 56.66, 58.30, 70.64, 72.04, 74.86,
75.07, 75.15, 76.10, 78.50, 83.29, 84.01, 126.41, 126.96, 127.73,
128.58, 128.73, 128.76, 129.22, 130.13, 131.73, 133.67, 133.69,
134.22, 138.68, 140.41, 152.33, 166.85, 167.07, 169.30, 171.42,
201.57; HRFABMS calcd for C59H80NO15Si2 [M + H]+ m/z
1098.5067, found 1098.5101 (-3.2 ppm dev).
4-Dea cetyl-4-m eth oxyca r bon ylp a clita xel (19). Depro-
tection of the C-2′ and C-7 position in 18 to give 19 was done
using the same methodology as that used to convert 15 to 16.
Deprotection of the C-7 and C-2′ positions in 18 (13.1 mg, 0.012
mmol) with HF-pyridine (100 µL) in anhydrous THF (1.5 mL)
afforded after preparative TLC (hexanes:EtOAc, 1:1) com-
pound 19 (9 mg, 87%) as an amorphous solid: 1H NMR δ 1.12
(s, 3H), 1.21 (s, 3H), 1.66 (s, 3H), 1.82 (s, 3H), 1.87 (m, 1H),
2.22 (s, 3H), 2.30 (dd, 1H, J ) 9.09, 15.49 Hz), 2.43 (dd, 1H, J
) 8.55, 15.72 Hz), 2.52 (m, 1H), 3.62 (bs, 1H), 3.80 (s, 3H),
3.84 (d, 1H, J ) 7.02 Hz), 4.20 (d, 1H, J ) 8.70 Hz), 4.33 (d,
1H, J ) 8.54 Hz), 4.36 (dd, 1H, J ) 6.72, 10.84 Hz), 4.79 (d,
1H, J ) 1.83 Hz), 4.96 (dd, 1H, J ) 1.83, 9.46 Hz), 5.69 (d,
1H, J ) 7.02 Hz), 5.82 (dd, 1H, J ) 1.99, 9.01 Hz), 6.18 (t, 1H,
J ) 8.32 Hz), 6.25 (s, 1H), 6.94 (d, 1H, J ) 9.15 Hz), 7.31-
7.60 (m, 11H), 7.72 (d, 2H, J ) 7.01 Hz), 8.14 (d, 2H, J ) 7.18
Hz); 13C NMR δ 9.63, 14.96, 20.87, 21.72, 26.87, 35.38, 35.82,
43.08, 45.87, 54.74, 56.00, 58.33, 71.99, 72.30, 73.01, 74.87,
75.59, 75.99, 78.84, 83.14, 84.09, 127.01, 127.10, 128.28,
128.69, 128.74, 128.96, 129.11, 130.23, 131.91, 133.37, 133.73,
138.42, 142.08, 153.12, 166.89, 167.04, 171.32, 172.97, 203.48;
HRFABMS calcd for C47H51NO15Li [M + Li]+ m/z 876.3419,
found 876.3402 (1.9 ppm dev).
2′-(ter t-Bu tyld im eth ylsilyl)-2-d eben zoyl-4-d ea cetyl-7-
(tr ieth ylsilyl)-5,20-d eoxy-5,20-selen a p a clita xel 1,2-Ca r -
bon a te (20). To a stirred solution of Superhydride (0.6 mL, 1
M solution in THF, 0.6 mmol) was added selenium powder
(25.3 mg, 0.32 mmol) under argon at room temperature. A
further 3.7 mg (0.05 mmol) of selenium powder was added and
the white suspension turned pale red. Sufficient Superhydride
was syringed into the solution until the latter changed to a
light pink color. The resulting suspension was stirred at room
temperature for 20 min and cooled to -20 °C, and after 5 min
a solution of epoxide 10 (7.0 mg, 0.0065 mmol) in anhydrous
THF (0.25 mL) was added. The cooling bath was removed and
stirring continued at room temperature for 2 h. The reaction
mixture was worked up by adding excess EtOAc and a few
crystals of BHT (2,6-di-tert-butyl-4-methylphenol). The result-
ing solution was filtered twice through a short column of silica
gel and the filtrate evaporated to dryness. The crude product
was purified by preparative TLC (30% hexanes in EtOAc) to
give the selenapaclitaxel analogue 20 as a white foam (4.5 mg,
67%): 1H NMR δ -0.28 (s, 3H), -0.10 (s, 3H), 0.60 (q, 6H, J
) 7.9 Hz), 0.80 (s, 9H), 0.82 (t, 9H, J ) 7.9 Hz), 1.18 (br s,
6H), 1.60 (br s, 3H), 2.16 (br s, 6H), 2.50 (m, 1H), 2.55 (m,
1H), 2.65 (d, 1H, J ) 5.0 Hz), 3.00 (d, 1H, J ) 11.0 Hz), 3.03
(m, 1H), 3.35 (t, 1H, J ) 9.2 Hz), 3.94 (d, 1H, J ) 11.0 Hz),
4.01 (m, 1H), 4.27 (d, 1H, J ) 5.0 Hz), 4.51 (d, 1H, J ) 1.7
Hz), 5.38 (s, 1H), 5.86 (m, 1H), 5.92 (dd, 1H, J ) 1.7, 8.9 Hz),
6.41 (s, 1H), 7.20-7.60 (m, 8H), 7.80 (d, 2H, J ) 8.0 Hz); 13C
NMR δ -5.8, -5.5, 5.3, 6.7, 11.6, 16.6, 18.3, 19.8, 20.8, 25.5,
26.5, 28.4, 32.4, 37.5, 41.2, 41.6, 52.5, 55.4, 60.6, 71.2, 72.7,
75.3, 75.9, 80.9, 81.5, 89.8, 126.7, 127.1, 127.9, 128.6, 128.8,
132.0, 133.7, 139.4, 141.4, 152.9, 167.3, 169.2, 170.4, 202.2;
HRFABMS calcd for C51H71NO12Si2SeNa [M + Na]+ m/z
1048.3578, found 1048.3587.
Acyla tion of Selen a p a clita xel 20. To a stirred solution
of 20 (7.0 mg, 0.007 mmol) in anhydrous THF at -78 °C under
argon was added 20 µL (0.02 mmol, 3 equiv) of a 1.0 M of
solution of LHMDS in THF. The reaction mixture was stirred
at -78 °C for 5 min. The reaction flask was removed from the
-78 °C bath and stirred at room temperature for 1 min. At
the end of this period, the reaction flask was returned to the
-78 °C bath and stirred for 2 min, and freshly distilled methyl
chloroformate (5 µL, 0.06 mmol, 9 equiv) was added. The
reaction mixture was stirred at -78 °C for 5 min and quenched
with EtOAc (60 mL). The EtOAc layer was washed succes-
sively with 5% HCl (2 × 20 mL), 5% NaHCO3 (2 × 20 mL),
water (15 mL), and brine (20 mL). The organic layer was dried
over anhydrous Na2SO4 and evaporated to dryness, and the
residue was purified by preparative TLC (hexanes:EtOAc, 4:1)
to yield a mixture of unreacted starting material (20, 1.0 mg,
14%) and 25 (3.5 mg, 48%): 1H NMR δ -0.24 (s, 3H), -0.01
(s, 3H), 0.51 (q, 6H, J ) 7.9 Hz), 0.86 (s, 9H), 0.87 (t, 9H, J )
7.9 Hz), 1.19 (s, 3H), 1.22 (s, 3H), 1.26 (s, 3H), 1.80 (m, 1H),
2.17 (s, 3H), 2.22 (s, 3H), 2.32 (m, 2H), 2.51 (m, 1H), 3.62 (s,
3H), 3.78 (d, 1H, J ) 5.2 Hz), 4.35 (m, 1H), 4.68 (d, 1H, J )
1.8 Hz), 5.24 (br t, 1H), 5.48 (br s, 1H), 5.57 (dd, 1H, J ) 1.7,
8.6 Hz), 5.92 (br s, 1H), 6.23 (m, 1H), 6.57 (s, 1H), 7.08 (d, 1H,
J ) 8.7 Hz), 7.22-7.54 (m, 8H), 7.76 (d, 2H, J ) 7.0 Hz); 13C
NMR δ -5.7, -5.2, 6.6, 10.4, 15.6, 18.2, 19.8, 20.8, 25.5, 26.2,
33.1, 35.8, 41.2, 43.6, 55.1, 55.7, 62.3, 69.5, 69.6, 74.7, 75.5,
81.8, 82.4, 89.4, 121.7, 126.8, 126.9, 127.8, 128.4, 128.7, 131.7,
136.6, 139.1, 143.0, 154.3, 166.3, 169.3, 171.3, 202.7; FABMS
calcd for C53H75NO14Si2Se [M + H]+ m/z 1083, found m/z (rel
int) 1083 [M + H]+ (7%), 1026 [M - Se + Na]+ (100%).
2′-(ter t-Bu tyld im eth ylsilyl)-4-d ea cetyl-7-(tr ieth ylsilyl)-
5,20-d eoxy-5,20-selen a p a clita xel (22). The reaction of se-
lenium compound 20 (10.0 mg, 0.01 mmol) in THF (0.2 mL)
with phenyllithium (30 µL, 0.05 mmoL, 5 equiv) at -78 °C for
3 min afforded compound 22 (5.2 mg, 43%): 1H NMR δ -0.30
(s, 3H), -0.10 (s, 3H), 0.60 (q, 6H, J ) 8.0 Hz), 0.80 (s, 9H),
0.83 (t, 9H, J ) 8.0 Hz), 1.18 (s, 3H), 1.20 (s, 3H), 1.60 (s, 3H),
2.18 (s, 3H), 2.20 (s, 3H), 2.38 (m, 1H), 2.60 (m, 2H), 2.70 (d,
1H, J ) 11.0 Hz), 3.05 (m, 1H), 3.10 (d, 1H, J ) 5.0 Hz), 3.30
(m, 1H), 3.82 (d, 1H, J ) 11.0 Hz), 3.95 (m, 1H), 4.52 (d, 1H,
J ) 1.8 Hz), 5.05 (s, 1H), 5.60 (d, 1H, J ) 5.0 Hz), 5.90 (m,
1H), 6.15 (dd, 1H, J ) 1.7, 8.9 Hz), 6.49 (s, 1H), 7.20-7.60 (m,
11H), 7.80 (d, 2H, J ) 8.0 Hz), 8.18 (d, 2H, J ) 8.0 Hz); 13C
NMR δ -5.8, -5.5, 5.3, 6.8, 10.9, 16.2, 18.3, 18.9, 20.9, 25.6,
27.3, 29.2, 35.5, 39.2, 41.3, 42.9, 54.7, 55.1, 60.4, 71.8, 72.8,
75.3, 75.4, 75.5, 77.3, 81.8, 126.9, 127.1, 128.3, 128.6, 128.8,
129.1, 130.4, 131.7, 133.3, 134.9, 135.9, 138.8, 139.6, 166.7,
167.6, 169.5, 170.3, 202.2; HRFABMS calcd for C57H77NO12
-
Si2SeNa [M + Na]+ m/z 1126.4047, found 1126.4021.
4-Deacetyl-5,20-deoxy-5,20-selen apaclitaxel (23). Depro-
tection of the selena compound 22 (7.5 mg, 0.007 mmol) with
hydrogen fluoride-pyridine (75 µL) yielded 23 (4.1 mg, 70%)
as a colorless amorphous solid: 1H NMR δ 1.20 (s, 6H), 1.60
(s, 3H), 2.00 (s, 3H), 2.12 (s, 3H), 2.40 (m, 1H), 2.50 (m, 2H),
2.67 (d, 1H, J ) 10.0 Hz), 3.14 (d, 1H, J ) 5.0 Hz), 3.19 (m,

Novel Paclitaxol D-Ring Modified Analogues
J . Org. Chem., Vol. 64, No. 8, 1999 2703
1H), 3.28 (m, 1H), 3.38 (br s, 1H), 3.84 (m, 2H), 4.54 (br s,
1H), 5.30 (br s, 1H), 5.52 (d, 1H, J ) 5.0 Hz), 5.59 (m, 1H),
6.20 (dd, 1H, J ) 1.7, 8.9 Hz), 6.35 (s, 1H), 7.10-7.60 (m, 11H),
7.80 (d, 2H, J ) 8.0 Hz), 8.20 (d, 2H, J ) 8.0 Hz); 13C NMR δ
10.5, 16.6, 19.4, 20.9, 27.6, 29.7, 31.6, 35.2, 39.1, 42.8, 53.9,
60.0, 72.8, 73.5, 75.4, 75.7, 77.5, 81.9, 127.1, 127.2, 128.2, 128.7,
128.8, 129.0, 130.3, 131.8, 133.4, 134.5, 135.6, 139.8, 140.1,
National Cancer Institute to D.G.I.K. and by a grant
from the Universidade Estadual de Maringa to M.H.S.
The assistance of Bristol-Myers Squibb Pharmaceutical
Research Institute in performing the HCT 116 cyto-
toxicity assay on compound 23 is also gratefully ac-
knowledged.
166.8, 167.4, 170.9, 172.1, 204.2; HRFABMS calcd for C45H49
-
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
compounds 8, 10, 11-20, 22, 23, and 25. This material is
available free of charge via the Internet at http://pubs.acs.org.
NO12SeNa [M + Na]+ m/z 898.2318, found 898.2333.
Ack n ow led gm en t. We are grateful for financial
support of this work by grant CA 69571 from the
J O982095H