New highly active taxoids from 9β-Dihydrobaccatin-9,10-acetals

Article abstract of DOI:10.1016/S0960-894X(02)00069-0

To synthesize new highly active taxoids, we designed and synthesized 9β-dihydro-9,10-acetal taxoids. In vitro study of these analogues clearly showed them to be more potent than docetaxel.

Full text of DOI:10.1016/S0960-894X(02)00069-0

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1083–1086
New Highly Active Taxoids from 9ꢀ-Dihydrobaccatin-9,10-acetals
Takashi Ishiyama, Shin Iimura, Satoru Ohsuki, Kouichi Uoto,
Hirofumi Terasawa and Tsunehiko Soga*
Medicinal Chemistry Research Laboratory, Daiichi Pharmaceutical Co., Ltd, Tokyo R&D Center,
16-13 Kita-kasai 1-Chome Edogawa-ku, Tokyo 134-8630, Japan
Received 16 October 2001; accepted 25 January 2002
Abstract—To synthesize new highly active taxoids, we designed and synthesized 9b-dihydro-9,10-acetal taxoids. In vitro study of
these analogues clearly showed them to be more potent than docetaxel. # 2002 Elsevier Science Ltd. All rights reserved.
Introduction
the presence of an acid catalyst.5 In this reaction, it was
found that acetonide, 4-methoxybenzylidene, and pro-
penylidene group could be obtained in satisfactory
yields. To determine the structures of 6a–c, the 7,
13-hydroxyl groups were acetylated, and 1H NMR
spectra of 7a–c supported the structures.6
Paclitaxel (1, Taxol1)1 and docetaxel (2, Taxotere1)2
are currently considered to be two of the most impor-
tant drugs in cancer chemotherapy (Fig. 1). To synthe-
size new highly active analogues of these drugs, we
designed 9,10-acetal taxoids. Ahond et al. reported the
synthesis of 7-deoxy-9a-dihydro-9,10-isopropylidenedo-
cetaxel (3) from a mixture of the natural products
Taxine B and Isotaxine B, 9a-dihydro taxoids that were
isolated from Taxus canadenesis (Fig. 2).3 The cyto-
toxicity of 3, however, was reported to be the same as
that of docetaxel (2).
To introduce a phenylisoserine side chain to the
13-hydroxyl group of 6a–c, b-lactams (11, 12) were
reacted with 6a–c in the presence of NaHMDS. Con-
trary to our expectation, there was no selectivity
between the 7-hydroxyl group and the 13-hydroxyl
We hypothesized that the configuration of the
9-hydroxyl group is crucial and that there is a possibility
to synthesize new highly active taxoids from the
9b-dihydrobaccatin skeleton reported by Holton et al.4
Here we report 9b-dihydro-9,10-acetal taxoids, which
showed activity stronger than that of docetaxel against
several tumor cell lines.
Figure 1.
Chemical Synthesis
10-Deacetylbaccatin III (4) was reduced by using
n-Bu4NBH4 to give the key compound 10-deacetyl-9b-di-
hydrobaccatin III (5) (Scheme 1).4 To synthesize
9,10-acetal taxoids, several aldehydes and ketones,
whose structures are not shown, were reacted with 5 in
*Corresponding author. Fax: +81-3-5696-8344; e-mail: iimurgdw@
daiichipharm.co.jp
Figure 2.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00069-0

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T. Ishiyama et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1083–1086
group. The desired 9a–d were obtained in low yields.
Deprotection of 9a–d by the reported method gave
10a–d7 (Scheme 2).8
TES group was studied (Scheme 3). It was found that
13a was synthesized by using TESOTf and 2,6-di-tert-
butylpyridine in dichloromethane at ꢀ78 ꢁC in fairly
good yield (Table 1, entry 5). The following reaction
gave 10d in high yield (Scheme 4).
To improve synthetic yields of the 9,10-acetal taxoids,
selective protection of the 7-hydroxyl group of 6b by
We synthesized 9b-dihydro-30-furyldocetaxel (16) for
comparison. Although 16 was not obtained from 10a by
acidic deprotection of the acetal group, we synthesized
16 from
5
via 10-deacetyl-7,10-bis TES-9b-di-
hydrobaccatin III (14) (Scheme 5).9
Scheme 1. Reagents and conditions: (a) n-Bu4NBH4, 1:1 dioxane–
CH2Cl2, rt, 19 h (68%); (b) 2,2-dimethoxypropane or acrolein diethyl-
acetal or 4-methoxybenzaldehyde dimethylacetal, CSA, CH2Cl2,
dioxane, rt, 1 h, (57% for 6a, 45% for 6b, 25% for 6c); (c) Ac2O,
Et3N, DMAP, CH2Cl2, rt (70% for 7a, 88% for 7b, 72% for 7c).
Scheme 3.
Scheme 4. Reagents and conditions: (a) 11, NaHMDS, THF, ꢀ55 ꢁC,
0.5 h (71%); (b) HF-pyridine, pyridine, rt (78%).
Scheme 2. Reagents and conditions: (a) (1) 11 or 12, NaHMDS, THF,
ꢀ55 ꢁC, 0.5 h (15% for 8a, 19% for 9a, 19% for 8b, 19% for 9b, 8.8%
for 8c, 13% for 9c, 31% for 8d, 9.8% for 9d); (b) HF-pyridine, pyri-
dine, rt (88% for 10a, 87% for 10b, 68% for 10c, 70% for 10d).
Scheme 5. Reagents and conditions: (a) TESCl, Et3N, DMF, rt (9%);
(b) 12, NaHMDS, THF, ꢀ55 ꢁC, 0.5 h (59%); (c) HF-pyridine, pyri-
dine, rt (59%).

T. Ishiyama et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1083–1086
1085
Table 1. 7-O selective silylation of 6b
Entry
Conditions
13a (%)
13b (%)
1
2
3
4
5
TESCl (1.5 equiv), imidazole (1.8 equiv), DMF, rt
TESCl (15 equiv), imidazole (18 equiv), DMF, 65 ꢁC
TESCl (1.2 equiv), pyridine, rt
25
0
27
24
80
50
63
30
20
0
TESOTf (1.1 equiv), 2,6-lutidine (1.5 equiv), CH2Cl2, ꢀ78 ꢁC
TESOTf (1.3 equiv), 2,6-di-tert-butylpyridine (1.5 equiv), CH2Cl2, ꢀ78 ꢁC
Table 2. Cytotoxicity of 9-b-dihydro taxoidsa
References and Notes
1. Wani, M. C.; Taylor, H. L.; Wall, M. E.; Coggon, P.;
Mcphail, A. J. Am. Chem. Soc. 1971, 93, 2325.
2. Gueritte-Voegelein, F.; Guenard, D.; Lavelle, F.; Le Goff,
M.-T.; Mangatal, L.; Potier, P. J. Med. Chem. 1991, 34, 992.
3. Poujol, H.; Mourabit, A. A.; Ahond, A.; Poupat, C.;
Potier, P. Tetrahedron 1997, 53, 12575.
4. (a) Holton R. A. WO patent 15599, 1995. (b) Holton R. A.
WO Patent 20485, 1995. In this reaction, the 9-a-isomer was
not obtained.
5. There were no differences among CSA, TsOH and PPTS in
this reaction.
R1
R2, R3
Cytotoxic activity GI50 (ng/mL)b
6. Comparison of 7-, 9-, 10-, and 13-protons of 6a–c and 7a–c
supported the structures (Table 3).
PC-6
PC-12
PC-6/VCR
2
Ph
FR
FR
FR
Ph
0.408–2.55
0.331
0.743
6.26
0.365
21.0
11.7–72.7
0.235
1.30
0.605
0.328
37.3
39.6–230
1.88
1.27
9.25
4.64
422
Table 3. Chemical shifts of 7-, 9-, 10-, and 13-protons (ppm)
10a
10b
10c
10d
16
AC
PP
MB
PP
7
9
10
13
6a
7a
6b
7b
6c
7c
4.04
5.14
4.16
5.20
4.15
5.22
3.85
4.01
3.89
3.96
3.98
4.09
5.58
5.50
5.30
5.31
5.47
5.46
4.80
6.13
4.82
6.15
4.84
6.16
FR
H, H
aThe in vitro experiments were performed with three different cell
lines: PC-6, a human small cell lung cancer,10 its variant, PC-6/
VCR29–9, a vincristine-resistant cell line expressing P-glycoprotein,11
and PC-12, a human non-small cell lung cancer cell line.10 Determi-
nation of GI50 was performed by using the MTT assay.12 The cells
were exposed continuously to the test compounds for 72 h.
bGrowth inhibition of 50%: the concentration required to obtain half
of the maximal inhibition for cell growth.
7. Analytical data of 10a–d are as follows. 10a: mp
133–135 ꢁC; 1H NMR (CDCl3) d 1.08 and 1.28 (each 3H, each
s, MeÂ2), 1.41 (9H, s, tert-Bu), 1.58, 1.65, 1.67, and 1.70 (each
3H, each s, MeÂ4), 1.83–1.94 (1H, m), 2.07–2.27 (2H, m), 2.36
(3H, s, Ac), 2.29–2.47 (1H, m), 2.94 (1H, d, H-3, J=4.9 Hz),
3.83 (1H, d, H-9, J=7.3 Hz), 4.32 and 4.39 (each 1H, ABq,
H-20, H-200, J=8.7 Hz), 4.65–4.76 (2H, m), 5.10 (1H, s),
5.30–5.42 (2H, m), 5.54 (1H, d, H-10, J=7.3 Hz), 6.05 (1H, d,
H-2, J=4.9 Hz), 6.11 (1H, d, furan, J=3.5 Hz), 6.36 (1H, dd,
furan, J=3.5, J=1.4 Hz), 7.39 (1H, d, furan, J=1.4 Hz), 7.48
(2H, t, Bz, J=7.3 Hz), 7.60 (1H, t, Bz, J=7.3 Hz), 8.11 (2H, d,
Bz, J=7.3 Hz). 10b: mp 147–150 ꢁC; FAB-MS m/z 838
(M+1)+; 1H NMR (CDCl3) d 1.28, 1.62, 1.69, and 1.71 (each
3H, each s, MeÂ4), 1.41 (9H, s, tert-Bu), 2.05–2.26 (3H, m),
2.29–2.44 (1H, m), 2.35 (3H, s, Ac), 2.93 (1H, d, H-3, J=4.9
Hz), 3.89 (1H, d, H-9, J=6.8 Hz), 4.04–4.16 (1H, m, H-7),
4.32 and 4.39 (each 1H, ABq, H-20, H-200, J=8.3 Hz), 4.71
(1H, s like), 5.10 (1H, s like), 5.22 (1H, d, acetal, J=5.9 Hz),
5.27 (1H, d, H-10, J=6.8 Hz), 5.32–5.46 (2H, m), 5.46 (H, d,
CH¼CH2, J=10.8 Hz), 5.57 (1H, d, CH¼CH2, J=17.6 Hz),
5.97–6.19 (2H, m, H-13, CH¼CH2), 6.08 (1H, d, H-2, J=4.9
Hz), 6.32 (1H, d, furan, J=1.9 Hz), 6.36 (1H, dd, furan,
J=3.0, J=1.9 Hz), 7.39 (1H, d, furan, J=3.0 Hz), 7.48 (2H, t,
Bz, J=7.8 Hz), 7.60 (1H, t, Bz, J=7.8 Hz), 8.10 (2H, d, Bz,
J=7.8 Hz). 10c: mp 148–151 ꢁC; FAB-MS m/z 918 (M+1)+;
1H NMR (CDCl3) d 1.30 (3H, s, Mde), 1.42 (9H, s, tert-Bu),
1.56 (3H, s, Me), 1.76 (6H, s, MeÂ2), 2.10–2.26 (3H, m), 2.36
(3H, s, Ac), 2.31–2.48 (1H, m), 2.99 (1H, d, H-3, J=4.9 Hz),
Results (Biological Activity) and Discussion
The antitumor activities of the 9b-dihydrotaxoids (10a–d,
16) were evaluated in vitro against three cell lines, PC-6,
PC-12, and PC-6/VCR. The PC-12 and PC-6/VCR cell
lines are expressing P-glycoprotein. 9b-Dihydro-
9,10-acetal taxoids (10a–d) showed strong activities
against these cell lines (Table 2). On the contrary, the
activity of 9b-dihydrodocetaxel (16) was less potent
than docetaxel. These data clearly showed the effective-
ness of acetal groups in the 9,10-position of the taxane
skeleton and suggested the importance of the b-configur-
ation of the 9-OH group.
In conclusion, we synthesized several 9b-dihydro-
9,10-acetal taxoids and found that analogues based on
the 9,10-acetal taxane skeleton are more potent than
docetaxel. It appears that the 9b-configuration is
important for increasing the potency. Further investi-
gation of these highly active 9b-9,10-acetal taxoids will
be reported in the near future.

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T. Ishiyama et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1083–1086
3.84 (3H, s, OMe), 3.98 (1H, d, H-9, J=7.4 Hz), 4.05–4.17
(1H, m, H-7), 4.30 and 4.38 (each 1H, ABq, H-20, H-200,
J=8.3 Hz), 4.57 (1H, d, J=8.3 Hz), 4.72 (1H, d, J=3.9 Hz),
5.11 (1H, s like), 5.38 (2H, broad s), 5.43 (1H, d, J=7.4 Hz),
5.80 (1H, s, acetal), 6.07 (1H, d, H-2, J=4.9 Hz), 6.15 (1H,
broad t, H-13, J=8.0 Hz), 6.32 (1H, d, furan, J=3.8 Hz), 6.36
(1H, dd, furan, J=3.8 Hz, J=2.0 Hz), 6.93 (2H, d like aro-
matic protons of MP, J=8.8 Hz), 7.40 (1H, d, furan, J=2.0
Hz), 7.43–7.53 (4H, m), 7.60 (1H, t, Bz, J=7.3 Hz), 8.11 (2H,
d, Bz, J=7.3 Hz). 10d: mp 145–150 ꢁC; FAB-MS m/z 848
acetal, J=6.3 Hz), 5.26 (1H, d, H-10, J=6.8 Hz), 5.30 (1H,
broad d, J=9.7 Hz), 5.97–6.13 (2H, m, H-13, CH¼CH2), 6.07
(1H, d, H-2, J=4.3 Hz), 7.20–7.45 (5H, m, aromatic protons),
7.47 (2H, t, Bz, J=7.4 Hz), 7.60 (1H, t, Bz, J=7.4 Hz), 8.10
(2H, d, Bz, J=7.4 Hz).
8. Ojima, I. Acc. Chem. Res. 1995, 28, 383, and references
cited therein.
9. In step a, there was no selectivity among the 7-, 10-, and
13-hydroxyl groups of 5.
10. Mitsui, I.; Kumazawa, E.; Hirota, Y.; Aonuma, M.;
Sugimori, M.; Ohsuki, S.; Uoto, K.; Ejima, A.; Terasawa, H.;
Sato, K. Jpn. J. Cancer Res. 1995, 86, 776.
11. Joto, N.; Ishii, M.; Minami, M.; Kuga, H.; Mitsui, I.;
Tohgo, A. Int. J. Cancer 1997, 72, 680.
12. Alley, M. C.; Scudiero, D. A.; Monks, A.; Hursey, M. L.;
Czerwinski, M. J.; Fine, D. L.; Abbott, B. J.; Mayo, J. G.;
Shoemaker, R. H.; Boyd, M. R. Cancer Res. 1988, 48, 589.
1
(M+1)+; H NMR (CDCl3) d 1.26 (3H, s, Me), 1.40 (9H, s,
tert-Bu), 1.61 (6H, s, MeÂ2), 1.68 (3H, s, Me), 1.91 (1H, s,
OH), 2.00–2.36 (3H, m), 2.30 (3H, s, Ac), 2.39 (1H, dd, J=9.8,
15.2 Hz), 2.90 (1H, d, H-3, J=4.9 Hz), 3.85 (1H, d, H-9,
J=5.8 Hz), 4.05–4.15 (1H, m, H-7), 4.16 (1H, broad s), 4.32
and 4.38 (each 1H, ABq, H-20, H-200, J=8.8 Hz), 4.57 (1H, d,
H-5, J=8.3 Hz), 4.62 (1H, broad s), 5.10 (1H, s), 5.22 (1H, d,