Studies on taxol biosynthesis. Preparation of 5α-acetoxytaxa-4(20),11-dien-2α,10β-diol derivatives by deoxygenation of a taxadiene tetra-acetate obtained from Japanese yew

Article abstract of DOI:10.1016/S0040-4020(02)01450-3

The putative metabolite, 5α-acetoxytaxa-4(20),11-dien-2α,10β-diol (7), which is a promising candidate as a biosynthetic pathway triol in taxol biosynthesis, has been prepared by Barton deoxygenation of the C-14-hydroxyl group of a differentially protected derivative of natural 2α,5α,10β-triacetoxy-14β-(2-methyl)-butyryloxytaxa-4 (20),11-diene (8), a major taxoid metabolite isolated from Japanese 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/S0040-4020(02)01450-3

Tetrahedron 59 (2003) 267–273
Studies on taxol biosynthesis. Preparation of 5a-acetoxytaxa-
4(20),11-dien-2a,10b-diol derivatives by deoxygenation of a
taxadiene tetra-acetate obtained from Japanese yew
Tohru Horiguchi,a Christopher D. Rithner,a Rodney Croteaub and Robert M. Williamsa
,
*
aDepartment of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
bInstitute of Biological Chemistry, Washington State University, Pullman, WA 99164, USA
Received 31 July 2002; accepted 4 September 2002
Abstract—The putative metabolite, 5a-acetoxytaxa-4(20),11-dien-2a,10b-diol (7), which is a promising candidate as a biosynthetic
pathway triol in taxol biosynthesis, has been prepared by Barton deoxygenation of the C-14-hydroxyl group of a differentially protected
derivative of natural 2a,5a,10b-triacetoxy-14b-(2-methyl)-butyryloxytaxa-4(20),11-diene (8), a major taxoid metabolite isolated from
Japanese 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. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
we have sought to elucidate a detailed understanding of the
steps of taxol biosynthesis and the identification of the
associated genes.
The approval of taxol (Paclitaxel, 1) as a therapeutic agent
against ovarian and other types of cancer has continued to
elicit interest in both the chemical synthesis and bio-
synthesis of this agent.1,2 A diverse retinue of approaches
for the synthesis of the taxane framework3 and for taxol
itself4 have evolved but the complexity of the fascinating
structure of taxol mandates lengthy syntheses, that result in
low overall yields rendering totally synthetic approaches to
this potent antitumor agent impractical for large-scale
manufacturing of clinical-grade material. The Pacific yew,
Taxus brevifolia Nutt., the initial source for commercial
scale production of taxol, grows in environmentally
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 10-deacetylbaccatin III, that can be isolated
from the needles of the European yew Taxus baccata, a
renewable resource, have been adopted as the current
commercial method for taxol production.5 However, it is
expected that taxol will become utilized for other types of
cancer and as a consequence, pressure on the yew
population worldwide would increase significantly. Alterna-
tive biological methods for taxol production has become an
important goal and our laboratories have sought to employ
emerging technologies based on the genetic manipulation of
Taxus sp. cell cultures to address this problem. To this end,
We have previously reported a combination of in vivo
feeding studies and investigations with cell-free enzyme
systems, using yew stem tissue or suspension cultured Taxus
sp. cells as bioconversion vectors. These investigations have
revealed 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).6,7 Following this first
committed step in taxol biosynthesis, taxadiene hydroxyl-
ase, a cytochrome P-450-dependent enzyme, regio-
selectively hydroxylates 3 with allylic transposition, to the
first oxygenated metabolite, taxa-4(20),11-diene-5a-ol (4).8
The third step appears to involve taxadienol-O-acetyltrans-
ferase, that subsequently acylates 4 to provide the acetate 5,
which has proven to be a superior substrate for downstream
hydroxylation reactions.9
After the formation of 5, we have observed that the
downstream hydroxylation reactions enter a very complex
matrix and the elucidation of a single linear path to taxol has
proven extremely challenging. This appears to be due to the
softening of substrate specificity by several of the remaining
hydroxylating enzymes that can accept 5 as a suitable
substrate in vitro or presumably, in vivo. We have deployed
two major approaches to identify the genes and associated
intermediates from 5 to taxol. In the first approach, we
have obtained a set of related full-length cytochrome
P-450 clones by the method of differential display of
Keywords: deoxygenation; taxadiene; taxol mandates.
*
Corresponding author. Tel.: þ1-9704916747; fax: þ1-9704913944;
e-mail: rmw@chem.colostate.edu
0040–4020/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01450-3

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T. Horiguchi et al. / Tetrahedron 59 (2003) 267–273
presently only capable of conveniently generating sub-
milligram quantities of a given metabolite, which has
proven insufficient for the substrate requirements of down-
stream bioconversion experiments. We have thus devoted
considerable effort to devising totally synthetic and semi-
synthetic methods to prepare these lightly oxygenated
taxoids, such as compound 7. Herein, we report a method
to semi-synthetically prepare 7 and its derivatives from
2a,5a,10b-triacetoxy-14b-(2-methyl)-butyryloxytaxa-
4(20),11-diene (8), a component of Japanese Yew heart
wood.16 This method is easily applicable for the preparation
of multi-milligram quantities of stable- and/or radio-
isotopomers of 7.
2. Results and discussion
The starting substrate, 10b-diethylisopropylsilyloxy-
4(20),11-taxadiene-2a,5a,14b-triol (9), was prepared from
2a,5a,10b-triacetoxy-14b-(2-methyl)-butyryloxytaxa-
4(20),11-diene (8), following the method we have pre-
viously reported in good yield.14 Although the reaction of
triol 9 with carbon disulfide and methyl iodide resulted in
the selective formation of the 2a-methyldithiocarboxyl
ester,14 the reaction of triol 9 with phenylchlorothiono
carbonate under similar conditions gave the 2a-phenoxy-
thiocarbonyl ester 10 as a minor product (12%) along with
the desired 14b-phenoxythiocarbonyl ester 11 (36%) and
the diacylated material 12 (22%) (Scheme 2).
Scheme 1. Early steps in taxol biosynthesis.
mRNA-reverse transcription-PCR, followed by traditional
library screening. Clones are selected based on sequence
homology to other plant cytochrome P-450s and used to
individually transform Saccharomyces cerevisiae and the
transformed yeast clones are then screened for hydroxylase
activity with several synthetic, labeled taxoids as substrates.
In one instance, we identified a clone that produced
5a-acetoxytaxa-4(20),11-dien-10b-ol (6) employing taxa-
4(20),11-dien-5a-acetate (5) as a substrate.10 This material
has been characterized by 1H NMR and mass spectroscopy
and has been shown to incorporate into taxol in vivo.
All three compounds proved useful and as described in
detail below, served as substrates from which the known
biogenetic intermediate 6, putative metabolite 7, the free
2a,5a,10b-triol 15 and 5a,10b,14b-triol 19 were prepared.
These four substances are valuable as authentic standards to
facilitate the identification of metabolites bioconverted from
4, 5 and 6.6a,10,12
In a second approach, we have examined microsomal
bioconversion of synthetic substrate 5 with induced Taxus
cuspidata. microsomes to more polar products. This
approach however, yielded several diol mono-acetates
including 5a-acetoxytaxa-4(20),11-dien-2a-ol11 and 5a-
acetoxytaxa-4(20),11-dien-13a-ol along with other polyols
tentatively identified as triols, tetraols, and pentaols by
GC–MS.12 Comparable preliminary bioconversion studies
with microsomes from uninduced Taxus canadensis cells,
have also shown the conversion of 5, primarily to a more
polar metabolite tentatively identified by LC–MS as a
pentaol monoacetate.6a Based on the existing map of
naturally co-occurring taxoids, the subsequent order of
hydroxylation downstream from 6 appears likely to occur at
C2 and C9, then C13.13 Based on this supposition, a very
likely intermediate downstream from 6 is 5a-acetoxy
taxadien-2a,10b-diol (7) or 5a-acetoxy taxadien-9a (or
b),10b-diol.
Due to the very low yield of the intermediate metabolites
that may be obtained from natural sources, we have relied
heavily on synthetic, tritium-labeled taxadienes 3,15 4,8 58
and 614 as substrates from which in vivo and in vitro
bioconversion strategies have been utilized to identify
lightly oxygenated taxoids downstream of these substances.
It must be stressed that, bioconversion of synthetic 6 is
Scheme 2.

T. Horiguchi et al. / Tetrahedron 59 (2003) 267–273
269
presence of imidazole in DMF resulted in quantitative
conversion to regioisomer 10. This presumably occurred by
transposition of the 14-phenoxythiocarbonyl group to the
2-position catalyzed by imidazole. Based on the failure of
this approach, direct acetylation of diol 14 was examined.
(Table 1).
Acetylation of 14 with AcCl in THF containing DMAP
(Table 1, entry 1) or with acetic acid in the presence of DCC
(Table 1, entry 2) gave an inseparable mixture of the C-2-
monoacetate 16 and the C-5-monoacetate 17. Alternatively,
acetylation of 14 with AcCl or AcBr utilizing LHMDS in
THF at 2188C, provided the desired C5-monoacetate 17 as
the sole product in low yields (Table 1, entries 3 and 4).
Although the net conversion of 14 to 17 was poor, recovery
of the starting material was reasonably high and the reaction
was clean enough to render this the most practical approach.
Scheme 3.
Although removal of the O-DEIPS group from compound
14 or from 5a-acetoxy-10b-diethylisopropylsilyloxytaxa-
4(20),11-diene with TBAF in THF proceeded efficiently in
24 h, subjecting the structurally related mixture of 16 and 17
to these conditions required 14 days, and the diol mono-
acetate products proved inseparable. We found that the
DEIPS group of 17 was more easily removed by treatment
with HF–pyridine complex for 4 h at room temperature to
provide 7 in 94% isolated yield (Scheme 5).
Treatment of 2a-phenoxythiocarbonylester 10 under
similar conditions gave the desired C2-deoxygention
product 18 in 52% yield. (Scheme 6). Removal of
Scheme 4.
Table 1.
Entry
Conditions
Product (%)
Recovered 14 (%)
1
2
3
4
(1.2 equiv.) AcCl, (1.3 equiv.) DMAP, THF, 08C to rt
16þ17 (17%; 1:1 mixture)
16þ17 (22%; 3:1 mixture)
17 (8%)
83
68
65
74
(1.2 equiv.) AcOH, (2 equiv.) DMAP, (2 equiv.) DCC, (2 equiv.) HOBt, THF, rt
(1.05 equiv.) AcCl, (1.1 equiv.) LHMDS, THF, 2188C to rt
(1.05 equiv.) AcBr, (1.1 equiv.) LHMDS, THF, 2188C to rt
17 (12%)
Treatment of 2a,14b-di-phenoxythiocarbonyl ester 12 with
tri-n-butyl tin hydride in the presence of AIBN in toluene at
100–1208C, provided 13 in 52% yield (Scheme 3). We have
previously described the conversion of 13 into 5a-acetoxy
taxa-4(20),11-dien-10b-ol 6, which has been identified as an
early stage intermediate.14
the O-DEIPS group from 18 was accomplished with
HF–pyridine complex for 4 h at room temperature, furnish-
ing triol 19 in 92% yield.
The structures of compounds 7, 15, and 19 were fully
characterized by 1D (1H and 13C) and 2D (HH, HSQC, and
HMBC) NMR analysis.
Treatment of 14-phenoxythiocarbonylester 11 under similar
conditions gave the desired C14-deoxygention product 14 in
52% yield. (Scheme 4). Removal of the O-DEIPS group
from 14 was accomplished with TBAF in acetic acid-THF,
furnishing triol 15 in 92% yield. (Scheme 4).
In order to regioselectively introduce the acetyl moiety onto
the C5-hydroxy group, selective and differential protection
of diol 11 was examined. Treatment of 11 with TESCl in the
Scheme 5.

270
T. Horiguchi et al. / Tetrahedron 59 (2003) 267–273
[CH3CH2]2SiCH[CH3]2), 0.88 (s, 3H, Me-19), 0.67–0.58
(m, 4H, [CH3CH2]2SiCH[CH3]2); 13C NMR (100 MHz,
CDCl3) 193.6, 153.6, 148.0, 138.7, 131.6, 129.7, 126.8,
122.3,113.8, 84.3, 76.8, 68.2, 67.7, 63.0, 48.7, 41.9, 41.2,
40.3, 38.0, 33.3, 31.8, 31.3, 26.2, 23.0, 21.2, 17.6, 17.6,
13.2, 7.4, 7.3, 4.1; IR (NaCl) 3420, 2952, 1490, 1457, 1293,
1200, 1047, 1013, 688 cm21; HR-FABMS calcd for
C34H52O5SiSNa (MþNa) 623.3202, found 623.3226.
3.1.2. Compound 11
[a]2D0¼þ39 (c 0.11, CHCl3); 1H NMR (400 MHz, CDCl3) d
7.42 (dd, J¼7.9, 8.1 Hz, 2H, Arom.), 7.29 (m, 1H, Arom.),
7.12 (m, 2H, Arom.), 5.49 (dd, J¼5.1, 9.4 Hz, 1H, H-14),
5.46 (s, 1H, H-20), 5.17 (s, 1H, H-20), 5.05 (dd, 1H, J¼5.8,
11.1 Hz, H-10), 4.13 (m, 1H, H-2), 4.21 (br.s, 1H, H-5), 3.04
(d, J¼5.8 Hz, 1H, H-3), 2.85 (d, J¼10.2 Hz, 1H, OH-2),
2.78 (dd, J¼9.4, 18.5 Hz, 1H, H-13), 2.64 (dd, J¼4.9,
18.1 Hz, 1H, H-13), 2.22 (dd, J¼4.9, 18.1 Hz, 1H, H-9),
2.12 (br.s, 1H, H-1), 2.00 (m, 1H, H-7), 1.99 (s, 3H, Me-18),
1.73 (m, 2H, H-6), 1.66 (s, 3H, Me-16), 1.49 (dd, J¼5.8,
15.1 Hz, 1H, H-9), 1.19 (s, 3H, Me-17), 1.08 (m, 1H, H-7),
1.04–0.88 (m, 13H, [CH3CH2]2SiCH[CH3]2), 0.85 (s, 3H,
Me-19), 0.68 –0.58 (m, 4H, [CH3CH2]2SiCH[CH3]2); 13C
NMR (100 MHz, CDCl3) d 194.5, 153.4, 149.3, 139.4,
129.8, 129.7, 126.8, 122.1, 114.8, 82.6, 76.7, 69.8, 67.7,
63.0, 60.6, 48.8, 42.1, 40.4, 37.9, 33.6, 31.9, 31.5, 25.8,
22.6, 21.1, 17.6, 17.6, 14.4, 13.2, 7.4, 7.3, 4.1; IR (NaCl)
3404, 2951, 1592, 1490, 1456, 1343, 1279, 1192, 1056,
Scheme 6.
In summary, 5a-acetoxytaxadien-2a,10b-diol (7), the
corresponding triol 15, and 5a,10b,14b-triol 19 have been
conveniently prepared from the readily available
2a,5a,10b-triacetoxy-14b-(2-methyl)-butyryloxytaxa-
4(20),11-diene 8, one of the main taxoid components in
Japanese yew heart wood. The synthetic methodology
described here is readily amenable for the preparation of
isotopically labeled congeners that are required for our
ongoing biosynthetic studies. Utilization of these new
lightly oxygenated taxoids is presently being explored to
probe downstream steps in the biosynthesis of taxol and will
be reported on in due course.
688 cm21
;
HR-FABMS calcd for C34H51O4SiS
(Mþ2H2Oþ1) 583.3277, found 583.3267.
3.1.3. Compound 12
3. Experimental
[a]2D0¼þ36 (c 0.19, CHCl3); 1H NMR (400 MHz, CDCl3) d
7.70 (m, 4H, Arom.), 7.22 (m, 2H, Arom.), 7.04 (d, 4H,
Arom.), 5.91 (dd, J¼1.6, 6.4 Hz, 1H, H-2), 5.53 (dd, J¼4.7,
9.4 Hz, 1H, H-14), 5.14 (s, 1H, H-20), 5.06 (dd, J¼5.8,
11.3 Hz, 1H, H-10), 4.82 (s, 1H, H-20), 4.19 (br.s, 1H, H-5),
3.40 (d, J¼6.4 Hz, 1H, 1H, H-3), 3.01 (dd, J¼9.2, 18.8 Hz,
1H, H-13), 2.51 (m, 1H, H-13), 2.33 (dd, J¼11.3, 15.1 Hz,
1H, H-9), 2.27 (br.s, 1H, H-1), 2.05 (m, 1H, H-7), 1.98 (s,
3H, Me-18), 1.74 (s, 3H, Me-16), 1.70 (m, 2H, H-6), 1.54
(m, 1H, H-9), 1.18 (s, 3H, Me-17), 1.08 (m, 1H, H-7),
1.00–0.88 (m, 13H, [CH3CH2]2SiCH[CH3]2), 0.86 (s, 3H,
Me-19), 0.66–0.56 (m, 4H, [CH3CH2]2SiCH[CH3]2); 13C
NMR (100 MHz, CDCl3) d 194.0, 193.7, 153.6, 147.2,
139.2, 130.7, 129.7, 129.7, 126.7, 122.2, 122.2, 114.3, 83.1.
81.5, 76.6, 67.7, 58.3, 48.7, 41.2, 40.3, 38.5, 37.8, 33.4,
32.1, 30.9, 25.9, 23.0, 21.1, 17.6, 17.6, 13.2, 7.4, 7.3, 4.1; IR
(NaCl): 3447, 2953, 1593, 1490, 1457, 1437, 1419, 1288,
1098, 1012, 954, 926, 882, 869, 823, 759, 726, 689,
668 cm21; HR-FABMS calcd for C41H56O6SiS2Na (MþNa)
759.3185, found 759.3178.
3.1. Preparation of phenoxythiocarbonylesters 10–12
To a solution of 9 (18.8 mg, 0.040 mmol) in THF (2 mL),
LHMDS (60 mL as 1 M THF solution) was added at 2108C.
After being stirred for 5 min, PhOCSCl (7 mL, 0.052 mmol)
was added and the mixture was stirred for 2 h at rt. The
reaction mixture was diluted with EtOAc (15 mL) and
washed with brine. The organic layer was dried over
Na2SO4, and concentrated in vacuo. The residue was
purified by chromatography (silica gel, eluted with CHCl3/
MeOH 20:1), yielding 10 (3.0 mg, 12%) and recovered 9
12%. The more polar fraction was additionally purified by
chromatography (silica gel, eluted with hexane/EtOAc 6:1)
to give 11 (8.6 mg, 36%), and 12 (6.5 mg, 22%).
3.1.1. Compound 10
[a]2D0¼þ98 (c 0.13, CHCl3); 1H NMR (400 MHz, CDCl3) d
7.43 (m, 2H, Arom.), 7.30 (m, 1H, Arom.), 7.12 (dd, J¼0.3,
8.3 Hz, 1H, Arom.), 5.92 (dd, J¼2.1, 6.4 Hz, 1H, H-2), 5.18
(s, 1H, H-20), 5.07 (dd, J¼5.8, 11.3 Hz, 1H, H-10), 4.82 (s,
1H, H-20), 4.22 (dd, J¼2.6, 2.7 Hz, 1H, H-5), 4.15 (m, 1H,
H-14), 3.39 (d, J¼6.2 Hz, 1H, H-3), 2.64 (dd, J¼9.2,
18.3 Hz, 1H, H-13), 2.47 (dd, J¼5.1, 18.3 Hz, 1H, H-13),
2.36 (dd, J¼7.0, 14.9 Hz, 1H, H-9), 2.06 (dt, J¼6.8,
12.6 Hz, 1H, H-7), 1.98 (s, 3H, Me-18), 1.94 (d, J¼1.9 Hz,
1H, H-1), 1.70 (m, 2H, H-6), 1.74 (s, 3H, Me-16), 1.55 (dd,
J¼5.5, 15.1 Hz, 1H, H-9), 1.25 (s, 3H, Me-17), 1.10 (ddd,
J¼2.1, 4.3, 13.4 Hz, 1H, H-7), 1.03–0.90 (m, 13H,
3.1.4. 10b-Diethylisopropylsilyloxytaxa-4(20),11-dien-
5a-ol (13)
To a solution of 12 (4.4 mg, 0.0056 mmol) in toluene
(2 mL), AIBN (cat.) and nBu3SnH (16 mL, 0.056 mmol)
were added. The mixture was stirred at reflux temperature
for 3 h. The mixture was condensed and purified by
chromatography (silica gel, eluted with hexane/EtOAc

T. Horiguchi et al. / Tetrahedron 59 (2003) 267–273
271
9:1), yielding crude 13 (1.4 mg, 53%). All pertinent
spectroscopic and analytical data is reported in Ref. 14.
LHMDS (0.10 mmol, 100 mL as 1 M THF solution) was
added at 2188C. After being stirred for 5 min, acetyl
bromide (7 mL, 0.098 mmol) was added and the mixture
was stirred for 2 h at rt. The mixture was diluted with EtOAc
(15 mL) and washed with aqueous NaHCO3, aqueous
NH4Cl, and brine. The organic layer was dried over
Na2SO4, and concentrated in vacuo. The residue was
purified by chromatography (silica gel, eluted with hexane/
EtOAc 4:1–3:1), yielding 17 (5.1 mg, 12%) and recovered
14 (29.5 mg, 74%). Employing the same procedure using
AcCl and diol 14 (28.4 mg, 0.063 mmol) gave 17 (2.4 mg,
8%) and recovered 14 (65%). Data for 17: [a]2D0¼þ47 (c
0.51, CHCl3); 1H NMR (400 MHz, CDCl3) d 5.45 (dd,
J¼1.2, 1.7 Hz, 1H, H-20), 5.32 (d, J¼1.3 Hz, 1H, H-20),
5.23 (dd, J¼2.6, 3.0 Hz, 1H, H-5), 4.99 (dd, J¼5.8, 11.3 Hz,
1H, H-10), 4.06 (d, J¼4.7 Hz, 1H, H-2), 2.96 (d, J¼6.4 Hz,
1H, H-3), 2.43 (m, 1H, H-13), 2.28 (dd, J¼11.3, 14.9 Hz,
1H, H-9), 2.12 (s, 3H, MeCO),1.96–1.80 (m, 4H, H-1, H-7,
H-13, and H-14) 1.91 (s, 3H, Me-18), 1.75 (m, 2H, H-6),
1.60 (m, 1H, OH), 1.59 (s, 3H, Me-16), 1.50 (dd, J¼5.3,
14.7 Hz, 1H, H-7), 1.50 (m, 1H, H-14), 1.15 (ddd, J¼3.2,
3.6, 13.0 Hz, 1H, H-7), 1.12 (s, 3H, Me-17), 1.02–0.92 (m,
13H, [CH3CH2]2SiCH[CH3]2), 0.87(s, 3H, Me-19), 0.66–
0.56 (m, 4H, [CH3CH2]2SiCH[CH3]2); 13C NMR
(100 MHz, CDCl3) d 170.19 (Ac), 145.4, 138.3, 131.3,
117.4 (C20), 79.0 (C5), 70.6 (C2), 67.9 (C10), 55.2 (C1),
48.7 (C9), 43.4 (C3), 40.0, 37.2, 34.3 (C7), 32.3 (Me), 30.2
(C13), 29.6 (C6), 25.5 (Me), 23.1 (Me), 22.2 (Ac), 21.3
(Me), 18.2 (C14), 17.7 (DEIPS), 13.3 (DEIPS), 7.0
(DEIPS), 4.2 (DEIPS); IR (NaCl): 3502, 2951, 1739,
1635, 1464, 1368, 1239, 1198, 1105, 1049, 1015, 957,
881, 827, 726, 668, 417 cm21; HR-FABMS (Mþ2H) calcd
for C29H49O4Si 489.3400, found 489.3385.
3.1.5. Barton deoxygenation of 11: 10b-diethyliso-
propylsilyloxy taxa-4(20),11-dien-2a, 5a-diol (14)
To a solution of 11 (17.3 mg, 0.029 mmol) in toluene
(2 mL), AIBN (cat.) and nBu3SnH (78 mL, 0.29 mmol)
were added. The mixture was stirred at reflux temperature
for 3 h. The mixture was condensed and purified by
chromatography (silica gel, eluted with hexane/EtOAc
6:1–5:1 hexane/EtOAc and fianlly EtoAc only), yielding
1
crude 14 (6.8 mg, 52%): [a]2D0¼þ32 (c 0.68, CHCl3); H
NMR (400 MHz, CDCl3) d 5.26 (dd, J¼1.5, 1.7 Hz, 1H,
H-20), 5.13 (d, J¼1.7 Hz, 1H, H-20), 4.98 (dd, J¼5.5,
11.3 Hz, 1H, H-10), 4.17 (d, J¼2.1 Hz, 1H, H-5), 4.02 (t,
J¼6.4 Hz, 1H, H-2), 3.20 (d, J¼6.2 Hz, 1H, H-3), 2.32
(ddd, J¼4.7, 11.3, 17.9 Hz, 1H, H-13), 2.19 (dd, J¼11.5,
14.9 Hz, 1H, H-9), 2.06–1.86 (m, 3H, H-7, H-13, and
H-14), 1.86 (s, 3H, Me-18), 1.78–1.56 (m, 4H, H-1, H-6,
and OH-2), 1.54 (s, 3H, Me-16), 1.52–1.36 (m, 3H, H-9,
H-14, and OH-5), 1.08 (s, 3H, Me-17), 1.02 (dt, 1H, J¼14.3,
4.1 Hz, H-7), 1.00–0.82 (m, 13H, [CH3CH2]2SiCH[CH3]2),
0.81 (s, 3H, Me-19), 0.64–0.44 (m, 4H, [CH3CH2]2-
SiCH[CH3]2); 13C NMR (100 MHz, CDCl3) 150.5, 137.5,
132.2, 114.1, 76.8, 70.6, 68.0, 55.2, 48.7, 41.4, 40.4, 37.2,
33.6, 32.4, 32.0, 30.0, 25.5, 22.9, 21.3, 18.2, 17.7, 17.6,
13.2, 7.4, 7.4, 4.1; IR (KBr):3369, 2930, 2361, 1635, 1457,
1386, 1281, 1232, 1053, 1011, 915, 827, 723 cm21
;
HR-FABMS calcd for C27H47O3Si (Mþ2H) 447.3294,
found 447.3288.
3.1.6. Deprotection of DEIPS group of compound 14 to
compound 15. Taxa-4(20),11-dien-2a, 5a,10b-triol
3.1.8. 5a-Acetoxytaxa-4(20),11-dien-2a,10b-diol (7)
To a solution of 14 (5.4 mg, 0.012 mmol) in THF were
added TBAF (0.06 mmol, 60 mL as 1 M THF solution) and
AcOH (one drop). The mixture was stirred for 24 h. The
mixture was diluted with EtOAc and washed with aqueous
NH4Cl and brine. The organic layer was condensed and
purified by chromatography (silica gel, eluted with CHCl3/
MeOH 20:1), yielding crude 15 (3.5 mg, 92%): [a]2D0¼þ19
(c 0.17, EtOAc); 1H NMR (300 MHz, CDCl3) d 5.32 (s, 1H,
H-20), 5.19 (s, 1H, H-20), 5.12 (dd, J¼5.5, 11.7 Hz, 1H,
H-10), 4.23 (t, J¼2.8 Hz, 1H, H-5), 4.06 (br.s, 1H, H-2),
3.22 (d, J¼6.2 Hz, 1H, H-3), 2.50–2.30 (m, 2H, H-13 and
OH), 2.23 (dd, J¼11.9, 14.5 Hz, 1H, H-9), 2.11–1.95 (m,
4H, H-1, H-7, H-13, and H-14), 1.92 (s, 3H, Me-18), 1.78–
1.68 (m, 2H, H-6), 1.60 (s, 3H, Me-16), 1.52 (m, 1H, H-9),
1.50 (m, 1H, H-14), 1.15 (s, 3H, Me-17), 1.10 (ddd, J¼2.1,
4.7, 13.4 Hz, 1H, H-7), 0.87 (s, 3H, Me-19); 13C NMR
(100 MHz, CDCl3) d 150.2 (C4), 136.6 (C12), 135.5 (C11),
114.3 (C20), 76.7 (C5), 70.6 (C2), 68.1 (C10), 55.3 (C1), 47.2
(C9), 41.3 (C3), 40.5 (C8), 37.2 (C15), 33.6 (C7), 32.7 (C17),
32.0(C6), 30.2 (C13), 25.7 (C16), 22.8 (C19), 21.5 (C18), 18.2
(C14); IR (KBr) 3406, 2925, 1653, 1631, 1445, 1380, 1261,
1097, 1026, 925, 802, 681, 468 cm21: HR-EIMS (MþNa)
calcd for C20H32O3Na 343.2249, found 343.2239.
To a solution of 17 (4.3 mg, 0.088 mmol) in pyridine (2 mL)
was added HF-Py (0.2 mL, 70% solution in pyridine) at 08C.
The mixture was stirred for 4 h at room temperature and was
diluted with EtOAc (30 mL) and washed with saturated
CuSO4, water, aqueous NH4Cl, and brine. The organic layer
was dried over Na2SO4 and concentrated in vacuo. The
residue was purified by chromatography (silica gel, eluted
with hexane/EtOAc 2:1), yielding 7 (3.0 mg, 94%):
1
[a]2D0¼þ42 (c 0.30, CHCl3); H NMR (400 MHz, CDCl3)
d, 5.43 (dd, J¼1.5, 1.7 Hz, 1H, H-20), 5.31 (d, J¼1.7 Hz,
1H, H-20), 5.21 (dd, J¼2.8, 3.0 Hz, 1H, H-5), 5.06 (dd,
J¼5.5, 11.7 Hz, 1H, H-10), 4.03 (br.s, 1H, H-2), 2.91 (d,
J¼6.2 Hz, 1H, H-3), 2.47 (m, 1H, H-13), 2.26 (dd, J¼11.7,
14.7 Hz, 1H, H-9), 2.08 (s, 3H, Ac), 2.00 (m, 2H, H-13 and
H-14), 1.98 (m, 1H, H-1), 1.94 (m, 1H, H-7), 1.91 (s, 3H,
Me-18), 1.80 (m, 2H, H-6), 1.63 (m, 1H, H-9), 1.60 (m, 1H,
OH), 1.57 (s, 3H, Me-16), 1.50 (m, 1H, H-14), 1.19 (s, 3H,
Me-17), 1.16 (ddd, J¼3.2, 3.4, 13.0 Hz, 1H, H-7), 0.86
(s, 3H, Me-19); IR (NaCl) 3420, 2985, 2931, 1734, 1456,
1447, 1369, 1242, 1198, 1074, 1019, 958, 943, 755,
667 cm21 13C NMR (100 MHz, CDCl3) d 170.1(Ac),
;
145.1, 137.4, 134.4, 117.5 (C20), 78.9 (C5), 70.5 (C2), 67.9
(C10), 55.3 (C1), 47.2 (C9), 43.3 (C3), 40.1 (C8), 37.2
(C15), 34.3 (C17), 32.55 (C17), 30.4 (C13), 29.7 (C6), 25.7
(C16), 23.0 (C19), 22.1 (Ac), 21.5 (C18), 18.2 (C14); HR-
FABMS (MþNa) calcd for C22H34O4Na 385.2355, found
385.2365.
3.1.7. 5a-Acetoxy-10b-diethylisopropylsilyloxy taxa-
4(20),11-dien-2a-ol (17)
To a solution of 14 (40 mg, 0.089 mmol) in THF (4 mL),

272
T. Horiguchi et al. / Tetrahedron 59 (2003) 267–273
3.1.9. Barton deoxygenation of 10:10b-diethyliso-
propylsilyloxy-taxa-4(20),11-dien-5a,14b -diol (18)
mass measurement of compound 15. We thank Professor
Takayuki Oritani (Laboratory of Applied Bioorganic
Chemistry, Division of Life Science, Graduate School of
Agricultural Science, Tohoku University, Japan) and Hida
Ichii Ittoubori Kyoudou Kumiai of The Engraving Crafts-
man Association, Hida, Japan for the generous gift of the
Japanese yew heart wood. We thank the Yamada Science
Foundation for fellowship support (to T. H.).
To a solution of 10 (99.1 mg, 0.0165 mmol) in toluene
(4 mL), AIBN (cat.) and nBu3SnH (0.44 mL, 1.65 mmol)
were added. The mixture was stirred at reflux temperature
for 3 h. The mixture was condensed and purified by
chromatography (silica gel, eluted with CHCl3/MeOH
20:1), yielding crude 18 (44 mg, 59%): [a]2D0¼þ88 (c
0.57, CHCl3); 1H NMR (400 MHz, CDCl3) d 5.08 (dd,
J¼5.5, 11.5 Hz, 1H, H-10), 4.97 (s, 1H, H-20), 4.67 (s, 1H,
H-20), 4.24 (dd, J¼2.8, 3.0 Hz, 1H, H-5), 3.75 (dd, J¼5.3,
9.2 Hz, 1H, H-14), 2.86 (d, J¼4.3 Hz, 1H, H-3), 2.52 (dd,
J¼9.2, 18.3 Hz, 1H, H-13), 2.40 (ddd, J¼1.1, 4.3, 18.5 Hz,
1H, H-9), 2.26 (dd, J¼11.5, 14.7 Hz, 1H, H-9), 2.07 (dt,
J¼6.2, 12.8 Hz, 1H, H-7), 1.91 (d, J¼1.1 Hz, 3H, Me-18),
1.75–1.63 (5H, m, H-1, H-2, and H-6), 1.56 (s, 3H, Me-27),
1.47 (dd, J¼5.5, 14.9 Hz, 1H, H-9), 1.18 (s, 3H, Me-16),
1.08 (ddd, J¼2.1, 4.5, 13.0 Hz, 1H, H-7), 1.02–0.9 (m, 13H,
[CH3CH2]2SiCH[CH3]2), 0.68–0.58 (m, 4H, [CH3CH2]2-
SiCH[CH3]2), 0.65 (s, 3H, Me-19); 13C NMR (100 MHz,
CDCl3) d 155.3, 140.7, 130.8, 109.7, 74.5, 72.0, 68.0, 55.9,
49.3, 42.6, 39.7, 38.8, 35.4, 33.5, 31.7, 30.2, 27.0, 26.2,
21.9, 21.2, 17.7, 17.6, 13.3, 7.4, 7.4, 4.1; IR (NaCl) 3374,
2954, 1457, 1380, 1232, 1057, 926, 826, 729, 668; HR-
FABMS (MþNa) calcd for C27H48O3SiNa 471.3270, found
471.3275.
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was added HF-Py (0.2 mL, 70% solution in pyridine) at 08C.
The mixture was stirred for 4 h at room temperature and was
diluted with EtOAc (30 mL) and washed with saturated
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[a]2D0¼þ105 (c 0.24, MeOH); 1H NMR (400 MHz, CDCl3)
d 4.96 (dd, J¼5.5, 11.7 Hz, 1H, H-10), 4.79 (s, 1H, H-20),
4.56 (t, J¼1.3 Hz, 1H, H-20), 3.88 (dd, J¼3.2, 3.6 Hz, 1H,
H-5), 3.61 (dd, J¼6.8, 7.0 Hz, 1H, H-14), 2.81 (m, 1H, H-3),
2.41 (d, J¼7.7 Hz, 2H, H-13), 2.22 (dd, J¼11.7, 14.5 Hz,
1H, H-9), 2.03 (ddd, J¼5.8, 12.8, 13.0 Hz, 1H, H-7), 1.85 (s,
3H, Me-18), 1.65 (s, 3H, Me-16), 1.61–1.49 (m, 5H, H-1,
H-2, and H-6), 1.46 (dd, J¼5.5, 14.5 Hz, 1H, H-9), 1.34
(s, 3H, Me-17), 0.96–0.80 (m, 3H, 10-OH, 14-OH, and
H-7), 0.61 (s, 3H, Me-19), 0.58 (m, 1H, 5-OH); 13C
NMR (100 MHz, CDCl3) d 156.4, 140.7, 133.2, 108.9, 74.5,
71.7, 68.0, 56.5, 48.5, 43.1, 40.1, 39.2, 35.6, 33.9, 32.6,
30.9, 27.4, 26.8, 22.2, 21.5; IR (KBr) 3362, 2924, 1646,
1445, 1377, 1262, 995, 948, 896, 757 cm21; HR-FABMS
(MþNa) calcd for C20H32O3Na 434.2249, found
434.2248.
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 Dr John Greaves (Department of
Chemistry, University of California, Urvine) for exact

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