Studies on Taxol biosynthesis: preparation of taxadiene-diol and triol derivatives by deoxygenation of taxusin

Article abstract of DOI:10.1016/j.tet.2008.04.008

The putative Taxol biosynthesis metabolites, taxa-4(20),11(12)-diene-5α,13α-diol (7), taxa-4(20),11(12)-diene-5α,9α,13α-triol (9), and taxa-4(20),11(12)-diene-5α,10β,13α-triol (10), have been prepared by Barton deoxygenation of the C-9 and C10-hydroxyl groups of protected derivatives of taxusin, a major taxoid metabolite isolated from Yew heart wood. The synthetic protocol devised is amenable for the preparation of isotopically labeled congeners that will be useful to probe further intermediate steps in the biosynthesis of Taxol.

Full text of DOI:10.1016/j.tet.2008.04.008

Tetrahedron 64 (2008) 6561–6567
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Studies on Taxol biosynthesis: preparation of taxadiene-diol and triol
derivatives by deoxygenation of taxusin
a
a
c
a,b,
*
Hui Li , Tohru Horiguchi , Rodney Croteau , Robert M. Williams
a
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
University of Colorado Cancer Center, Aurora, CO 80045, USA
Institute of Biological Chemistry, Washington State University, Pullman, WA 99164-6340, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The putative Taxol biosynthesis metabolites, taxa-4(20),11(12)-diene-5
a,13a-diol (7), taxa-4(20),11(12)-
Received 22 February 2008
Received in revised form 2 April 2008
Accepted 3 April 2008
diene-5 ,9 ,13 -triol (9), and taxa-4(20),11(12)-diene-5 ,10 ,13 -triol (10), have been prepared by
a
a
a
a
b
a
Barton deoxygenation of the C-9 and C10-hydroxyl groups of protected derivatives of taxusin, a major
taxoid metabolite isolated from Yew heart wood. The synthetic protocol devised is amenable for the
preparation of isotopically labeled congeners that will be useful to probe further intermediate steps in
the biosynthesis of Taxol.
Available online 8 April 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
elucidate a detailed understanding of the steps of Taxol bio-
synthesis and the identification of the associated genes.
The approval of Taxol (Paclitaxel, 1) as a clinically useful thera-
peutic agent against ovarian and other types of cancer continues to
stimulate interest in both the chemical synthesis and the bio-
genesis of this agent.1 Numerous approaches for the synthesis of
Previously, we have reported a combination of in vivo feeding
studies and investigations with cell-free enzyme systems, using
Taxus sp. stem tissue or suspension cultured Taxus sp. cells as
bioconversion vectors. The experimental results of these in-
vestigations have led us to surmise that the early steps of Taxol
biosynthesis proceed in sequence from the initial conversion of
geranylgeranyl pyrophosphate (2) to the parent diene 3 catalyzed
by taxadiene synthase (Scheme 1). Following this first committed
step in Taxol biosynthesis, taxadiene hydroxylase, a cytochrome
P-450-dependent enzyme, regioselectively hydroxylates 3 with
,2
3
4
the taxane framework and for Taxol itself have been reported but
the architectural complexity of the Taxol core mandates lengthy
syntheses, with low overall yields rendering chemical synthesis
approaches to this drug impractical for large-scale manufacturing
of clinical-grade material. The bark of Taxus brevifolia Nutt., the
Pacific yew tree, served as the initial source for commercial-scale
6
,7
1
production of Taxol. However, this species grows in environmen-
allylic transposition of the 4,5-double bond, to the first oxygenated
8
tally sensitive areas of the Pacific Northwest and has become an
untenable source for large-scale Taxol production. Alternative
approaches for Taxol production such as semi-synthesis from
metabolite, taxa-4(20),11(12)-diene-5
a
-ol (4). The resulting in-
termediate taxadienol 4 can be subsequently acetylated through
the action of taxadienol-O-acetyltransferase to provide the
9
10-deacetylbaccatin III, which can be isolated from the needles of
corresponding acetate 5. Studies have shown that taxa-4(20),11(12)-
the European yew Taxus baccata, a renewable resource, were sub-
sequently adopted as the current commercial method for Taxol
production.5 Additionally, it is reasonable to anticipate that Taxol
will become utilized for other types of cancer and as a consequence,
pressure on the population of yew species worldwide will increase
significantly. Alternative sources for Taxol production based on
biological systems have become an important goal and our labo-
ratories have sought to employ emerging technologies based on the
genetic manipulation of Taxus sp. cell cultures to address this
problem. In order to meet these objectives, we have sought to
diene-5
diene-5
a
a
-acetate (5) was converted to taxa-4(20),11(12)-
,10b-diol-5-acetate (6) by taxoid 10b-hydroxylase.
10–12
Experiments also demonstrated that taxa-4(20),11(12)-diene-5
(4) was converted by a 13
a
-ol
a
-hydroxylase to taxa-4(20),11(12)-diene-
12b
5a
,13
a
-diol (7, Scheme 1),
thereby introducing uncertainties as
to the precise order of the early hydroxylation and C5-acetylation
diverging from the confirmed intermediate taxadienol (4). The
complementary but opposite substrate selectivities of the 13
hydroxylase and the 10 -hydroxylase, which favors taxadien-5 -ol
and taxadien-5 -acetate as substrates, respectively, as well as
a-
b
a
a
13
feeding studies of Taxus sp. cells with the relevant intermediates,
suggest a bifurcation in the early stages of Taxol biosynthesis.
14
*
Corresponding author.
E-mail address: rmw@lamar.colostate.edu (R.M. Williams).
Based on the relative abundance of the several hundreds defined
taxoids bearing oxy-functional groups at various positions, the
0
040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.04.008

6562
H. Li et al. / Tetrahedron 64 (2008) 6561–6567
Me
Me
Me
Me
Me
Me
Me
Me
taxadiene synthase
Me
Me
taxadiene hydroxylase
O , NADPH
2
Me
Me
Me
OPP
H
H
OR
Me
3
, taxa-4(5),11(12)-diene
4, taxa-4(20),11(12)-dien-5α-ol (R = OH)
5, taxa-4(20),11(12)-dien-5α-acetate (R = OAc)
2
, geranylgeranyl pyrophosphate
HO
HO
Me
Me
Me
Me
Me
?
Me
1
0β-hydroxylase
Me
Me
5
O , NADPH
2
H
H
OAc
H
H
OAc
HO
HO
OH
6
, taxa-4(20),11(12)-diene-
8, taxa-4(20),11(12)-diene-
Me
Me
5
α,10
β
-diol-5-acetate
5
α,2
β,10
β
-triol-5-acetate
Me
HO
Me
OH
H
Me
HO
H
Me
HO
OAc
Me
Me
Me
HO
Me
Me
Me
13α
-hydroxylase
?
4
1
1, putative pentaol
O , NADPH
2
H
H
H
H
OH
OH
7
, taxa-4(20),11(12)-diene-
9, taxa-4(20),11(12)-diene-5α,9α,13
α-triol
5
α
,13α-diol
AcO
O
1
8
HO
1
9
?
Me
1
10
9
Me
BzNHO
11
Me
OH
6
Me
2
16
Me
Me
Me
7
17 8
1
3
Ph
O
Me
2
HO
1
3
4
5
OH
14
H
H
H
HO
O
OH
,10 ,13
BzO AcO
, Taxol
1
0, taxa-4(20),11(12)-diene-5
α
β
α-triol
1
Scheme 1. Possible divergence of taxadiene to lightly oxygenated taxoids and convergence to a putative pentaol.
subsequent order of hydroxylation reactions downstream appears
likely to occur at C10, followed by C13 and C9, and later C7 and C2,
the precise timing of acetylation at C5 is, however, uncertain.
Taxusinwas partiallydeacylatedbytreatment with K
2
CO
3
inTHF/
ꢀ
MeOH at 0 C for 3 days, which gave the 5,13-diol 12 predominantly
with some 9,10-diol. A small amount of the more polar taxusin tet-
raol was produced but easily separated (Scheme 2). The diol 12 was
treated with TESCl in the presence of imidazole in DMF followed by
deacetylation of the 9,10-diacetyl groups using lithium aluminum
hydride (LAH) in THF, which provided diol 13 in high yield. Exposure
of 13 to LHMDS followed by addition of carbon disulfide and methyl
iodide gave an inseparable mixture of the xanthates 14a and 14b in
approximately 1:1 ratio. The C-9 and C-10-hydroxyl residues were
successfully removed by Barton deoxygenation, respectively, to give
an inseparable mixture of 15a and 15b. Finally, removal of the O-TES
ethers by treatment with TBAF in the presence of acetic acid in THF
14
Considering numerous possible parallel sequences of oxygenation,
we have decided to investigate a series of plausible intermediates
7
downstream from 4 by way of the C13-oxygenated species that can
be converted into taxa-4(20),11(12)-diene-5
a,9a,13a-triol (9) or
taxa-4(20),11(12)-diene-5 ,10 ,13 -triol (10). In order to evaluate
a
b
a
the possible intermediacy of such species, we required access to
authentic specimens of 7, 9, and 10, since we have found that suf-
ficiently good substrates for subsequent hydroxylation reactions
are often produced in trace amounts and whose structures are very
difficult to confirm. We report herein, the semi-synthesis of 7, 9,
and 10 by radical deoxygenation of taxusin.
gave the desired triols taxa-4(20),11(12)-diene-5
a,10b,13a-triol (10)
and taxa-4(20),11(12)-diene-5 ,9 ,13 -triol (9), which were sepa-
a
a
a
2
. Results and discussion
rable by silica gel chromatography (Scheme 2).
The mixture of 15a/15b also proved useful for the synthesis of
Taxusin was selected as a starting substrate due to its structural
the corresponding taxa-4(20),11(12)-diene-5a,13a-diol. As shown
relatedness to a variety of putative pathway metabolites and its
high natural abundance in European and Japanese yew heart
woods. Previous studies from our laboratory have revealed that
taxusin can be deacetylated efficiently to the corresponding tetraol
and re-protected with modest efficiency to form a series of differ-
entially protected polyols that can be selectively deoxygenated by
classical radical deoxygenation methods with the exception of the
C13 alcohol.15 All attempts to remove the C13 alcohol result in
rearrangement of the C11(12)-bridgehead alkene to the C12(13)
alkene. Despite our failure to deoxygenate taxusin at C-13, this
substrate proved to be useful for making lightly oxygenated taxoids
with oxygenation at C13.
in Scheme 3, 15a/15b was subjected to treatment with LHMDS in
THF followed by the addition of carbon disulfide and methyl iodide
to give a mixture of the xanthates 16a/16b, which could be sepa-
rated on column but was used as a mixture in the following step.
Barton deoxygenation of 16a/16b yielded the protected diol 17 in
79% yield. Final treatment of 17 with HF/pyridine complex afforded
taxa-4(20),11(12)-diene-5a,13a-diol (7) (Scheme 3).
While this route provided the first authentic specimens of 9 and
10, it proved to be capricious due to the lability of the O-TES ethers.
As such, a more robust, second-generation synthesis was devised as
shown in Scheme
4 where O-diethyl-iso-propylsilyl ethers
(ODEIPS) were employed.

H. Li et al. / Tetrahedron 64 (2008) 6561–6567
6563
AcO
AcO
OAc
Me
OAc
Me
Me
AcO
Me
K CO
2
3
Me
Me
Me
Me
HO
THF/MeOH (1:1)
o
0
C (80%)
H
H
H
H
taxusin
OH
R1
OAc
12
HO
OH
Me
R
2
1
2
. TESCl, Im., DMF
. LAH, THF
LHMDS, THF
Me
TESO
Me
TESO
Me
Me
Me
Me
Me
CS ; MeI (47%)
2
9
0% (two steps)
H
H
H
H
OTES
OTES
14a, R1 = OH, R2 = OCS Me
13
2
1
4b, R1 = OCS Me, R = OH
2 2
R1
R
Me
TESO
2
Me
Bu SnH, AIBN
TBAF, AcOH
THF
3
Me
Me
tol.; Δ (86%)
H
H
OTES
1
5a, R = OH, R = H
1 2
15b, R = H, R = OH
1
2
HO
OH
Me
Me
Me
HO
Me
Me
Me
Me
+
HO
Me
H
H
H
OH
H
OH
9
(38%)
10 (51%)
Scheme 2. Synthesis of taxa-4(20),11(12)-diene-5a,10b,13a-triol (10) and taxa-4(20),11(12)-diene-5a,9a,13a-triol (9).
R1
R1
R2
The synthesis of these substrates with deuterium labeling is
enabled through the use of tri-n-butyltin deuteride in the radical
deoxygenation step.
R2
Me
Me
TESO
Me
Me
Me
Me
LHMDS, THF
Me
Me
TESO
CS ; MeI
2
H
H
H
H
3. Experimental
OTES
OTES
1
5a, R = OH, R2 = H
16a, R = OCS Me, R = H (57%)
1
1
2
2
15b, R = H, R2 = OH
16b, R = H, R = OCS Me (28%)
3.1. 5,13-Dideacetyltaxusin (12)
1
1
2
2
Me
TESO
Me
Me
Me
To a solution of taxusin (120 mg, 0.24 mmol) in THF/MeOH
Bu SnH, AIBN
3
HF-py
4%
ꢀ
(
4 mL, 1:1) was added K
mixture was stirred for 3 days at 0 C, after which time EtOAc
2
CO
3
(49 mg, 0.36 mmol) at 0 C. The
tol.; Δ (79%)
ꢀ
8
H
H
17
OTES
4
(20 mL) and satd NH Cl (aq) (5 mL) were added to the mixture. The
Me
Me
organic layer was separated and washed with brine. The organic
layer was concentrated and purified by chromatography (silica gel,
Me
1
HO
Me
hexanes/EtOAc 1:1), yielding 12 (80 mg, 80%). H NMR (300 MHz,
CDCl
.69 (s, 1H), 4.40–4.24 (br s, 1H), 4.28 (s, 1H), 3.32 (d, J¼8.4 Hz, 1H),
3.24 (d, J¼4.5 Hz, 1H), 2.90–2.70 (comp, 2H), 2.20 (d, J¼1.2 Hz, 3H),
.02 (s, 3H), 1.98 (s, 3H), 1.94–1.54 (comp, 7H), 1.49 (s, 3H), 1.24–1.16
3
)
d
6.05 (d, J¼10.2 Hz, 1H), 5.69 (d, J¼10.2 Hz, 1H), 5.04 (s, 1H),
H
H
OH
4
7
2
Scheme 3. Synthesis of taxa-4(20),11(12)-diene-5
a,13a-diol.
13
3
(m, 1H), 0.88 (s, 3H), 0.68 (s, 3H); C NMR (75 MHz, CDCl ) d 170.5,
1
3
3
70.1, 152.3, 141.7, 134.8, 110.7, 77.7, 74.3, 73.6, 68.5, 43.4, 39.5, 38.6,
6.3, 35.8, 32.8, 30.5, 27.2, 26.6, 25.9, 21.2, 20.9, 17.1, 16.7; IR (NaCl)
In addition, we found that conversion of taxusin into the cor-
responding tetraol followed by protection of the 9,10-diol unit as its
cyclic carbonate proved advantageous and reproducible.
ꢁ1
450 (br), 2944, 1724, 1242 cm ; HRMS calcd for C24
H O
35 6
þ
(M ꢁH) 419.2434, found 419.2422.
Taxusin was converted into tetraol 18 by global reductive
removal of the acetates with LAH. Cyclic carbonate formation
followed by protection of the C5 and C13 hydroxyl groups as
O-DEIPS ethers proceeded in 62% overall yield. Reductive re-
moval of the cyclic carbonate followed by xanthate formation
gave the inseparable xanthates 22a/22b, which were directly
subjected to Barton deoxygenation conditions. Final removal of
the silyl ethers with tetra-n-butylammonium fluoride (TBAF)
produced 9 and 10 (77% and 17% yields, respectively). Similarly,
3.2. 5 ,13
a
a-Bis(triethylsilyloxy)-taxa-4(20),11(12)-diene-
9a
,10b-diol (13)
To the solution of 12 (1.32 g, 3.13 mmol) and imidazole (534 mg,
7.85 mmol) in DMF (5 mL), TESCl (991 mg, 6.57 mmol) was added
ꢀ
at 0 C. The mixture was stirred for 24 h at rt. The mixture was
diluted by diethyl ether (40 mL), washed with water, NH
and brine, and dried over Na SO , yielding a crude mixture, which
4
Cl (aq),
2
4
2
3a and 23b were converted to 7 in good overall yield as shown
was used in the next reaction without further purification. Thus,
a solution of the crude diacetate prepared above and LAH (238 mg,
in Scheme 5.

6564
H. Li et al. / Tetrahedron 64 (2008) 6561–6567
AcO
H
HO
OAc
Me
OH
Me
AcO
Me
Me
LAH
CDI
Me
Me
Me
Me
HO
toluene
THF
96%
70%
H
H
H
OAc
OH
O
taxusin
18
O
O
O
O
Me
O
Me
LAH
DEIPSCl, imid.
DMF
Me
Me
Me
Me
Me
Me
THF
100%
HO
DEIPSO
89%
H
H
H
H
OH
ODEIPS
19
20
HO
R1
OH
Me
R
2
Me
Me
KHMDS; CS ; MeI
Me
2
Me
Me
Me
THF
DEIPSO
Me
DEIPSO
5
0%
H
H
H
H
ODEIPS
22a, R = OH, R2 = OCS Me
ODEIPS
Me
2
1
1
2
2
2b, R1 = OCS Me, R = OH
2
2
R1
R2
Me
Bu SnH, AIBN
Me
Me
TBAF
DMF
3
DEIPSO
tol.; Δ (85%)
H
H
ODEIPS
2
3a, R1 = OH, R2 = H
23b, R1 = H, R2 = OH
HO
Me
OH
Me
Me
Me
Me
Me
Me
+
HO
HO
Me
H
H
H
OH
H
OH
9
(77%)
10 (17%)
Scheme 4. Cyclic carbonate route to 9 and 10.
S
MeS
OH
Me
Me
O
Me
Me
Me
KHMDS; CS2; MeI
Me
DEIPSO
DEIPSO
Me
Me
THF
91%
H
H
ODEIPS
H
H
ODEIPS
Me
2
3b
24b
Me
DEIPSO
Me
Me
TBAF
THF
Bu SnH, AIBN
3
Me
Me
Me
Me
tol.; Δ (74%)
HO
83%
H
H
H
ODEIPS
H
OH
25
7
Scheme 5. Conversion of 23b to 7.
6
.26 mmol) in THF (10 mL) were stirred at rt for 15 h. The mixture
d
155.4,140.0,135.5,109.4, 79.6, 74.7, 73.0, 69.4, 43.1, 42.0, 39.7, 36.7,
ꢀ
was diluted by THF/H
2
O (3:1, 10 mL) at 0 C. The mixture was fil-
35.9, 32.0, 31.6, 29.0, 28.5, 25.6, 18.4, 14.9, 7.2, 5.1, 4.7.
tered and the filtrate was diluted with EtOAc. The mixture was
washed by NH Cl (aq) and brine, dried over Na SO , and condensed
in vacuo, yielding diol 13 (1.60 g, 90%). H NMR (400 MHz, CDCl
4
2
4
3.3. Taxa-4(20),11(12)-diene-5a,10b,13a-triol (10)
1
3
)
d
4.91 (s, 1H), 4.80 (d, J¼9.8 Hz, 1H), 4.82–4.75 (m, 1H), 4.58 (s, 1H),
3.3.1. Procedure A
To the solution of 13 (137 mg, 0.24 mmol) in THF (3 mL), LHMDS
(0.25 mmol, 1 M THF solution) was added at 0 C. After stirring for
4
2
1
.15 (t, J¼2.6 Hz, 1H), 4.12 (d, J¼9.8 Hz, 1H), 2.98 (d, J¼5.6 Hz, 1H),
.36 (td, J¼9.2, 14.0 Hz, 1H), 2.48–2.22 (br s, 2H), 1.99 (s, 3H), 1.76–
ꢀ
.50 (comp, 7H), 1.50 (s, 3H), 1.17 (s, 3H), 1.18–1.10 (m, 1H), 1.2–0.88
5 min, CS
2
(0.1 mL) was added to the mixture. The mixture was
L, 36 mg, 0.25 mmol) was
13
(
comp, 21H), 0.70–0.55 (comp, 12H); C NMR (100 MHz, CDCl
3
)
stirred for 30 min at rt and then MeI (16 m

H. Li et al. / Tetrahedron 64 (2008) 6561–6567
6565
added. The mixture was stirred for another 14 h at rt. The mixture
was diluted with CH Cl (20 mL) and washed with water. The
organic layer was dried over Na SO and concentrated in vacuo. The
residue was purified by preparative TLC (hexanes/EtOAc 30:1),
yielding an inseparable mixture of 14a and 14b (75 mg, 47%).
To a solution of 14a and 14b (75 mg, 0.11 mmol) obtained above
3.6. 5a,13a-Bis(triethylsilyloxy)-taxa-4(20),11(12)-diene (17)
2
2
2
4
To a solution of the mixture of 16a and 16b (61 mg, 0.095 mmol)
obtained above in toluene (2 mL), AIBN (cat.) and n-Bu SnH (52 L,
3
m
0.19 mmol) were added. The mixture was stirred at reflux for 3 h.
The mixture was concentrated and purified by chromatography
1
in toluene (2 mL), AIBN (cat.) and n-Bu
3
SnH (63
mL, 0.23 mmol)
(silica gel, heptane), yielding 17 (40 mg, 79%). H NMR (400 MHz,
were added. The mixture was stirred at reflux for 3 h. The mixture
was concentrated and purified by flash chromatography (silica gel,
hexanes/EtOAc 11:1), yielding an inseparable mixture of 15a and
benzne-d
6
)
d
4.96–4.78 (m, 1H), 4.64 (s, 1H), 4.184 (t, J¼3.0 Hz, 1H),
3.33 (m, 1H), 2.90 (dt, J¼3.6, 12.8 Hz, 1H), 2.44 (td, J¼9.2, 13.6 Hz,
1H), 2.41 (dt, J¼4.8, 13.2 Hz, 1H), 2.14 (s, 3H), 2.15–2.00 (comp, 2H),
1.80–1.60 (comp, 5H), 1.38 (s, 3H), 1.44–1.34 (m, 1H), 1.30–1.20 (m,
1H), 1.16 (s, 3H), 1.14–1.00 (comp, 18H), 1.00–0.90 (comp, 2H), 0.80–
15b (54 mg, 86%).
To a solution of 15a and 15b (67 mg, 0.12 mmol) in THF were
added TBAF (0.26 mmol,1 M THF solution) and AcOH (1 drop). After
13
6
0.62 (comp, 15H); C NMR (100 MHz, benzne-d ) 156.8, 137.0,
9
h, additional TBAF (0.52 mmol) was added and the mixture was
stirred for 72 h. The mixture was diluted with EtOAc and washed
with NH Cl (aq) and brine. The organic layer was concentrated and
purified by preparative TLC (silica gel, hexanes/EtOAc 1:1, then
CHCl /MeOH 20:1), yielding triol 10 (20 mg, 51%) and triol 9 (15 mg,
133.8, 108.7, 75.0, 70.1, 42.6, 41.7, 40.8, 40.5, 37.0, 36.5, 35.0, 32.9,
31.1, 30.1, 28.2, 26.3, 23.3,15.3, 7.8, 7.7, 5.8, 5.4; IR (NaCl): 2951, 2361,
ꢁ1
4
1457, 1239, 1118, 1062, 1007, 886, 838, 739 cm
3.7. Taxa-4(20),11(12)-diene-5 ,13 -diol (7)
.7.1. Procedure A
.
3
a
a
3
8%).
3
3
.3.2. Procedure B
A solution of 23a and 23b (280 mg, 0.485 mmol) and TBAF
To a solution of 17 (25 mg, 0.047 mmol) in pyridine (2 mL) was
added HF/pyridine (0.2 mL, 70% solution in pyridine) at 0 C. The
mixture was stirred for 3 h at rt, diluted with EtOAc (30 mL), and
ꢀ
ꢀ
(
651 mg, 2.50 mmol) in DMF (30 mL) was stirred at 55 C for 12 h.
The mixture was diluted with EtOAc (75 mL), and washed with H
2
O
washed with saturated CuSO
4
4
, water, NH Cl (aq), and brine. The
(
3ꢂ50 mL) and brine (50 mL). The organic layer was concentrated
organic layer was dried over Na
residue was purified by chromatography (silica gel, CHCl /MeOH
3
2
SO and concentrated in vacuo. The
4
and purified by flash chromatography (silica gel, eluted with hex-
anes/EtOAc 3:2 then 1:1), yielding triol 9 (120 mg, 77%) and triol 10
9:1), yielding 7 (12 mg, 84%).
1
(
27 mg,17%). H NMR (300 MHz, CDCl
3
)
d
5.15 (dd, J¼5.7,11.7 Hz,1H),
4
.97 (s,1H), 4.64 (s,1H), 4.38–4.22 (comp, 2H), 3.22 (s,1H), 2.88–2.60
comp, 3H), 2.18 (dd, J¼11.7, 14.4 Hz, 1H), 2.12–2.00 (m, 1H), 2.01 (d,
J¼1.2 Hz, 3H), 1.72–1.52 (comp, 7H), 1.48 (s, 3H), 1.38–1.10 (comp,
3.7.2. Procedure B
To a solution of 25 (60 mg, 0.107 mmol) in THF (10 mL) were
added TBAF (140 mg, 0.535 mmol). The mixture was stirred at 55 C
overnight and concentrated and purified by flash chromatography
(silica gel, hexanes/EtOAc 3:2), yielding diol 7 (27 mg, 83%). H NMR
3
(300 MHz, CDCl ) d 4.96 (s, 1H), 4.63 (s, 1H), 4.38–4.24 (br s, 1H),
(
ꢀ
13
2
1
3
1
H), 0.93 (s, 3H), 0.66 (s, 3H); C NMR (100 MHz, CD
42.5, 136.9, 109.6, 75.2, 69.1, 68.7, 48.4, 41.5, 39.8, 39.7, 37.6, 35.9,
4.1, 34.0, 32.3, 25.9, 22.5, 21.5, 17.2; IR (KBr) 3349 (br), 2927, 1652,
3
CN) 156.2,
1
ꢁ
1
þ
456, 1004, 894 cm ; HRMS (M þNa) calcd for C20
H
32
O
3
Na
4.27 (t, J¼2.6 Hz, 1H), 3.51 (br s, 1H), 2.85 (dd, J¼5.4, 13.2 Hz, 1H),
3
43.2244, found 343.2240.
2.83–2.70 (comp, 2H), 2.23 (dt, J¼5.7, 12.9 Hz, 1H), 2.18–2.09 (br s,
2H), 1.95 (s, 3H), 2.00–1.90 (m, 1H), 1.82–1.60 (comp, 5H), 1.32
(
(
1
2
1
s, 3H), 1.28–1.16 (comp, 2H), 1.00 (ddd, J¼2.4, 4.5, 13.2 Hz, 1H), 0.88
3
.4. Taxa-4(20),11(12)-diene-5a a,13a-triol (9)
,9
1H NMR (300 MHz, CDCl
) d 5.03 (s, 1H), 4.69 (s, 1H), 4.40–4.30
3
13
3
s, 3H), 0.60 (s, 3H); C NMR (75 MHz, CDCl ) d 153.7, 139.5, 132.5,
09.5, 74.7, 68.7, 40.4, 39.6, 39.24, 39.18, 36.2, 35.8, 32.4, 32.1, 31.4,
7.4, 25.4, 24.9, 22.2, 16.3; IR (NaCl): 3359 (br), 2921, 1439,
(
(
br s, 1H), 4.32 (s, 1H), 4.14–4.02 (m, 1H), 3.55 (s, 1H), 2.96–2.70
ꢁ
1
þ
008 cm ; HRMS calcd for C20
H
32
O
2
Na (M þNa) 327.2294, found
comp, 3H), 2.54 (dt, J¼3.8, 13.2 Hz, 1H), 2.30–2.15 (br s, 1H), 2.00 (s,
3
27.2292.
3
1
H), 1.93 (dt, J¼5.1, 13.2 Hz, 1H), 1.80–1.60 (comp, 5H), 1.55–1.45 (m,
H), 1.38–1.32 (m, 1H), 1.33 (s, 3H), 1.20 (dd, J¼3.9, 15.3 Hz, 1H), 0.92
3
.8. Cyclic carbonate (19)
s, 3H), 0.83 (s, 3H); 1 C NMR (75 MHz, acetone-d
3
)
d
156.4, 138.2,
(
6
1
3
36.3, 109.6, 76.8, 75.9, 69.6, 46.7, 40.9, 40.6, 38.1, 37.6, 37.1, 32.8,
A solution of tetraol 18 (6.41 g, 19.1 mmol) and CDI (5.0 g,
0.8 mmol) in toluene (300 mL) was heated at 70 C overnight.
2.5, 28.4, 26.6, 26.3, 19.0, 17.6; IR (NaCl) 3350 (br), 2941, 1448,
ꢀ
3
ꢁ
1
þ
1101 cm ; HRMS calcd for C20
32
H O
3
Na (M þNa) 343.2244, found
After cooling to rt, the mixture was concentrated and the residue
was purified by flash chromatography (silica, 1:1 EtOAc/hexanes) to
3
43.2240.
1
give 19 as a white powder (4.80 g, 70%). H NMR (300 MHz, ace-
3
.5. 5
a
,13
a
-Bis(triethylsilyloxy)-10
b
-S-methyl
tone-d
6
)
d
5.79 (d, J¼12.0 Hz, 1H), 5.09 (s, 1H), 4.95 (d, J¼12.0 Hz,
dithiocarboxytaxa-4(20),11(12)-diene (16a) and 5
di(triethylsilyloxy)-9 -S-methyl dithiocarboxytaxa-4
20),11(12)-diene-10 -ol (16b)
a
,13a
-
1H), 4.72 (s, 1H), 4.45 (app dt, J¼4.5, 9.8 Hz, 1H), 4.31 (d, J¼2.4 Hz,
1H), 4.17 (d, J¼2.1 Hz,1H), 3.76 (d, J¼9.6 Hz,1H), 3.16–3.10 (br s,1H),
2.84 (ddd, J¼9.0, 10.2, 15.3 Hz, 1H), 2.18 (d, J¼1.2 Hz, 3H), 1.99 (dd,
J¼4.5, 13.5 Hz, 1H), 1.86–1.80 (comp, 5H), 1.46 (s, 3H), 1.48–1.40 (m,
a
(
b
13
To the solution of the mixture of 15a and 15b (62 mg, 0.11 mmol)
1H), 1.18 (d, J¼4.8, 15.3 Hz, 1H), 1.03 (s, 3H), 0.92 (s, 3H); C NMR
in THF (2 mL), LHMDS (0.12 mL, 1 M THF solution) was added at
(75 MHz, acetone-d
6
) d 155.7, 154.0, 150.5, 133.7, 110.8, 88.2, 81.1,
ꢀ
0
C. After stirring for 10 min, CS
2
(0.1 mL) was added to the mix-
L,
.12 mmol) was added. The mixture was stirred for 3 h at rt. The
mixture was diluted with CH Cl (20 mL) and washed with water.
The organic layer was dried over Na SO and concentrated in vacuo.
The residue was purified by PTLC (hexanes/EtOAc 100:1), yielding
6a (41 mg, 57%) and 16b (20 mg, 28%). These were used for the
next step without further purification.
75.1, 69.3, 41.5, 41.4, 40.2, 37.6, 36.8, 33.9, 31.7, 28.8, 27.1, 26.3, 18.3,
17.8; IR (NaCl): 3407 (br), 2944, 2877, 1801, 1190, 1016 cm ; HRMS
calcd for C21
ꢁ
1
ture. The mixture was stirred for 2 h at rt and then MeI (7
m
þ
0
H
30
O
5
Na (M þNa) 385.1991, found 385.1982.
2
2
2
4
3.9. Cyclic carbonate (20)
1
A solution of diol 19 (475 mg, 1.31 mmol), imidazole (700 mg,
10.2 mmol), and DEIPSCl (0.63 mL, 0.34 mmol) in DMF (7 mL) was

6566
H. Li et al. / Tetrahedron 64 (2008) 6561–6567
stirred at rt for 40 h. The mixture was diluted with EtOAc (25 mL),
and washed with H
O (3ꢂ25 mL) and brine (25 mL). The organic
layer was dried over Na SO . After filtration, the filtrate was con-
centrated and the residue was purified by flash chromatography
2872, 1645, 1466, 1240, 1078 cmꢁ1; HRMS calcd for C36
(M ꢁH) 665.3914, found 665.3905.
65
H O
3
2
S Si
2
þ
2
2
4
3.12. 5a,13a-Bis(diethylisopropylsilyloxy)-4(20),11(12)-
(
(
5
(
1
silica, 1:20 then 1:10 EtOAc/hexanes) to give 20 as a clear liquid
diene (25)
1
726 mg, 89%). H NMR (300 MHz, CDCl
.06 (d, J¼12.0 Hz, 1H), 4.96 (s, 1H), 4.84 (app t, J¼9.0 Hz, 1H), 4.60
s, 1H), 4.18 (t, J¼2.3 Hz, 1H), 2.70–2.63 (s, 1H), 2.41 (td, J¼9.3,
4.1 Hz, 1H), 2.03 (d, J¼0.6 Hz, 3H), 1.88–1.73 (comp, 2H), 1.67–1.55
comp, 4H), 1.50 (s, 3H), 1.53–1.44 (m, 1H), 1.18 (s, 3H), 1.10–0.83
3
)
d
5.58 (d, J¼12.0 Hz, 1H),
A
solution of 24b (160 mg, 0.24 mmol), AIBN (20 mg,
0
.12 mmol), and Bu
3
SnH (140 mg, 127
m
L, 0.48 mmol) in toluene
ꢀ
(15 mL) was heated at 115 C for 3 h. The mixture was concentrated
(
(
and the residue purified by flash chromatography (silica gel, hex-
anes then 1:100 EtOAc/hexanes) to give 25 as clear oil (100 mg,
13
3
comp, 30H), 0.70–0.54 (comp, 8H); C NMR (75 MHz, CDCl )
d
155.1,152.2,149.2,129.2,110.1, 87.1, 79.6, 77.0, 74.1, 69.4, 42.0, 39.6,
9.4, 35.7, 35.4, 32.4, 30.9, 28.2, 28.0, 25.3, 17.6, 17.5, 17.4, 15.4, 13.0,
1
7
4
4%). H NMR (300 MHz, CDCl
3
)
d
4.81 (s, 1H), 4.75 (t, J¼8.9 Hz, 1H),
3
.55 (s, 1H), 4.14 (t, J¼3.5 Hz, 1H), 3.06 (d, J¼6.3 Hz, 1H), 2.76 (ddd,
J¼5.4, 12.6, 14.1 Hz, 1H), 2.33 (td, J¼9.3, 13.8 Hz, 1H), 2.16 (dt, J¼4.1,
3.2 Hz, 1H), 2.10–1.90 (comp, 2H), 1.89 (s, 3H), 1.80–1.40 (comp,
H), 1.37 (s, 3H), 1.33–1.12 (comp, 3H), 1.05 (s, 3H), 1.02–0.84 (comp,
5H), 0.65 (s, 3H), 0.74–0.62 (comp, 8H); C NMR (75 MHz, CDCl )
3
156.4, 136.4, 133.3, 107.8, 73.9, 69.3, 42.0, 41.2, 39.9, 39.7, 36.8,
5.7, 34.8, 32.1, 30.7, 30.1, 27.8, 25.4, 22.7, 17.7, 17.6, 17.5, 14.6, 13.1,
1
2.8, 7.20, 7.18, 7.15, 7.10, 3.7, 3.5, 3.4, 3.1; IR (NaCl): 2944, 2877,
ꢁ1
þ
1809, 1463, 1184 cm ; HRMS calcd for C35
H
62
O
5
Si
2
Na (M þNa)
1
6
41.4034, found 641.4050.
6
2
d
13
3
.10. 5
a,13a
-Bis(diethylisopropylsilyloxy)-taxa-4(20),11(12)-
3
diene-9
a
,10
b-diol (21)
1
2.9, 7.33, 7.30, 7.27, 7.2, 3.9, 3.7, 3.5, 3.3; IR (NaCl): 2941, 1458,
ꢁ1
þ
1016 cm ; HRMS calcd for C34
H
63
O
2
Si
2
(M ꢁH) 559.4365, found
A mixture of the cyclic carbonate 20 (1.5 g, 2.42 mmol) and
LiAlH (184 mg, 4.85 mmol) in THF (150 mL) was stirred at rt for 6 h
whereupon EtOAc (10 mL) was added slowly to quench excess
LiAlH . The mixture was partitioned between EtOAc (80 mL), H
40 mL), and brine (40 mL). The aqueous layer was extracted with
EtOAc (2ꢂ80 mL) and the combined organic layers were washed
with brine (150 mL). The organic layer was dried over Na SO and
559.4367.
4
4
2
O
Acknowledgements
(
We thank the National Institutes of Health (Grant CA 70375 to
R.M.W., and Grant CA 55254 to R.C.) for financial support. We thank
the Yamada Science Foundation for Fellowship Support (to T.H.).
We are grateful to the Japanese Society for the Promotion of Science
2
4
filtered. The filtrate was concentrated and the residue was purified
by flash chromatography (silica gel, 1:7 then 1:3 EtOAc/hexanes) to
1
give 21 as a white solid (1.44 g, 100%). H NMR (300 MHz, CDCl
3
)
(JSPS) for Fellowship Support (to T.H.).
d
4.89 (s, 1H), 4.80 (t, J¼1.2 Hz, 1H), 4.76 (d, J¼9.9 Hz, 1H), 4.57 (s,
1
2
1
2
d
H), 4.15 (s, 1H), 4.10 (d, J¼9.9 Hz, 1H), 2.90 (d, J¼5.4 Hz, 1H), 2.50–
.20 (br s, 2H), 2.33 (td, J¼9.3, 13.2 Hz, 1H), 1.97 (d, J¼1.2 Hz, 3H),
References and notes
.72–1.52 (comp, 7H), 1.47 (s, 3H), 1.15 (s, 3H), 1.04–0.84 (comp,
1. (a) Suffness, M. Taxane Anticancer Agents: Basic Science and Current Status;
13
7H), 0.84 (s, 3H), 0.70–0.54 (comp, 8H); C NMR (75 MHz, CDCl
3
)
Georg, G. I., Chen, T. T., Ojima, I., Vyas, D. M., Eds.; American Chemical Society:
Washington, DC, 1995; pp 1–17; (b) Suffness, M.; Wall, M. E. Taxol: Science and
Applications; Suffness, M., Ed.; CRC: Boca Raton, FL, 1995; pp 3–25; (c) Holmes,
F. A.; Kudelka, A. P.; Kavanagh, J. J.; Huber, M. H.; Ajani, J. A.; Valero, V. Taxane
Anticancer Agents: Basic Science and Current Status; Georg, G. I., Chen, T. T.,
Ojima, I., Vyas, D. M., Eds.; American Chemical Society: Washington, DC, 1995;
pp 31–57; (d) Golspiel, B. R. Pharmacotherapy 1997, 17, 110S–125S.
155.1,139.7,135.3,109.6, 79.4, 74.7, 72.7, 69.3, 42.9, 42.0, 39.4, 36.6,
5.5, 31.9, 31.4, 29.0, 28.5, 25.5, 18.3, 17.6, 17.54, 17.52, 17.49, 14.5,
3
1
3.1, 12.9, 7.24, 7.21, 7.17, 3.8, 3.6, 3.5, 3.2; IR (NaCl): 3402 (br), 2944,
ꢁ
1
þ
1463, 1072, 1014 cm ; HRMS calcd for C34
H
64
O
4
Si
2
Na (M þNa)
615.4241, found 615.4238.
Ò
2
. Paclitaxel is the generic name for Taxol , a registered trademark of Bristol–Myers
Squibb; because of its greater familiarity, the term ‘‘Taxol’’ is used throughout.
. For a review, see: Swindell, C. S. Org. Prep. Proced. Int. 1991, 23, 465–543.
. (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. S.; Liu, J. H. J. Am. Chem. Soc. 1994, 116,
3
4
3
.11. 5
a,13a-Bis(diethylisopropylsilyloxy)-9a-S-methyl
dithiocarboxytaxa-4(20),11(12)-diene (24b)
1
597–1598; (b) Holton, R. A.; Somoza, C.; Kim, H. B.; Liang, F.; Biediger, R. J.;
A solution of KHMDS (1.18 mL, 0.5 M in toluene, 0.59 mmol) was
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. S.; Liu, J. H. J. Am. Chem. Soc.
1994, 116, 1599–1600; (c) Nicolaou, K. C.; Nantermet, P. G.; Ueno, H.; Guy, R. K.;
Coulandouros, E. A.; Sorensen, E. J. J. Am. Chem. Soc. 1995, 117, 624–633; (d)
Nicolaou, K. C.; Liu, J. J.; Yangh, H.; Claiborne, C. F.; Hwang, C. K.; Nakada, M.;
Nantermet, P. G.; Ueno, H.; Guy, R. K.; Sorensen, E. J. J. Am. Chem. Soc. 1995, 117,
ꢀ
added to 23b (200 mg, 0.347 mmol) in THF (20 mL) at ꢁ10 C. The
ꢀ
mixture was stirred at 0 C for 10 min and then at rt for 20 min,
whereupon carbon disulfide (36
mL, 0.58 mmol) was added and the
solution was stirred for another 30 min. MeI (36
mL, 0.54 mmol)
6
34–644; (e) Nicolaou, K. C.; Liu, J. J.; Yangh, H.; Claiborne, C. F.; Renaud, J.;
Nantermet, P. G.; Guy, R. K.; Shibayama, K. J. Am. Chem. Soc. 1995, 117, 645–652;
f) Nicolaou, K. C.; Ueno, H.; Liu, J. J.; Yangh, H.; Renaud, J.; Nantermet, P. G.;
was then added and the mixture was allowed to stir overnight. The
mixture was then partitioned between EtOAc (25 mL) and 1% AcOH
in water (25 mL), and the organic layer separated. The organic layer
(
Paulvannan, K.; Chadha, R. J. Am. Chem. Soc. 1995, 117, 653–659; (g) Danishefsky,
S. J.; Masters, J. J.; Young, W. B.; Link, J. T.; Snyder, L. B.; Magee, T. B.; Jung, D. K.;
Isaacs, R. C. A.; Bornman, W. G.; Alaimo, C. A.; Coburn, C. A.; DiGrandi, M. J.
J. Am. Chem. Soc. 1996, 118, 2843–2859; (h) Wender, P. A.; Badham, N. F.;
Conway, S. P.; Floreancig, P. E.; Glass, T. E.; Granicher, C.; Houze, J. B.;
Janichen, J.; Lee, D.; Marquess, D. G.; McGrane, P. L.; Meng, W.; Mucciaro, T. P.;
Muhlebach, M.; Natchus, M. G.; Paulsen, H.; Rawlins, D. B.; Satkofsky, J.; Shuker,
A. J.; Sutton, J. C.; Taylor, R. E.; Tomooka, K. I. J. Am. Chem. Soc. 1997, 119, 2755–
2756; (i) 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,
2 4
was washed with brine (25 mL) and dried over Na SO . After fil-
tration, the filtrate was concentrated and the residue was purified
by flash chromatography (silica gel, hexanes then 1:100 EtOAc/
1
hexanes) to give 24b as a clear oil (210 mg, 91%). H NMR (300 MHz,
CDCl
3
)
d
6.20 (dd, J¼5.7, 11.7 Hz, 1H), 4.92 (s, 1H), 4.80 (app t,
J¼9.0 Hz, 1H), 4.61 (s, 1H), 4.18 (s, 1H), 3.19 (d, J¼5.1 Hz, 1H), 2.89 (t,
J¼12.9 Hz, 1H), 2.56 (s, 3H), 2.55–2.48 (br s, 1H), 2.32 (td, J¼9.0,
1
4.1 Hz, 1H), 2.20–2.05 (m, 1H), 1.98 (s, 3H), 1.80–1.55 (comp, 6H),
2757–2758; (j) Mukaiyama, T.; Shiina, I.; Iwadare, H.; Saitoh, M.; Nishimura, T.;
1
0
d
.63 (s, 3H), 1.17–1.08 (m, 1H), 1.11 (s, 3H), 1.05–0.90 (comp, 26H),
Ohkawa, N.; Sakoh, H.; Nishimura, K.; Tani, Y.; Hasegawa, M.; Yamada, K.;
Saitoh, K. Chem.dEur. J. 1999, 5, 121–161; (k) Kusama, H.; Hara, R.; Kawahara,
S.; Nishimori, T.; Kashima, H.; Nakamura, N.; Morihira, K.; Kuwajima, I. J. Am.
Chem. Soc. 2000, 122, 3811–3820.
13
3
.71 (s, 3H), 0.70–0.60 (comp, 8H); C NMR (75 MHz, CDCl )
215.6, 154.2, 136.3, 132.1, 109.9, 90.7, 74.4, 60.4, 45.2, 41.3, 39.6,
6.8, 35.2, 31.2, 30.7, 30.5, 28.9, 28.5, 26.7, 19.0, 18.3, 17.7, 17.6, 17.55,
3
5
. Nicolaou, K. C.; Dai, W.-M.; Guy, R. K. Angew. Chem., Int. Ed. Engl. 1994, 33,
17.53, 14.7, 13.1, 12.9, 7.29, 7.25, 7.2, 3.8, 3.6, 3.5, 3.2; IR (NaCl): 2945,
15–44.

H. Li et al. / Tetrahedron 64 (2008) 6561–6567
6567
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