Design, synthesis and cytotoxicity of 7-deoxy aryl discodermolide analogues

Article abstract of DOI:10.1016/j.bmcl.2004.01.102

A series of 7-deoxy discodermolide analogues in which the lactone fragment 'C' was replaced by aryl substituents were designed, synthesized, and evaluated for cytotoxicity.

Full text of DOI:10.1016/j.bmcl.2004.01.102

Bioorganic & Medicinal Chemistry Letters 14 (2004) 2335–2338
Design, synthesis and cytotoxicity of 7-deoxy aryl discodermolide
analogues
Mark A. Burlingame,a,* Simon J. Shaw,a Kurt F. Sundermann,a Dan Zhang,a
Joseph Petryka,a Esteban Mendoza,a Fenghua Liu,a David C. Myles,a
Matthew J. LaMarche,b Tomoyasu Hirose,b B. Scott Freezeb and Amos B. Smith, IIIb,*
aDepartment of Chemistry, Kosan Biosciences Inc., 3832 Bay Center Place, Hayward, CA 94545, USA
bDepartment of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA
Received 19 December 2003; revised 28 January 2004; accepted 28 January 2004
Abstract—A series of 7-deoxy discodermolide analogues in which the lactone fragment ÔCÕ was replaced by aryl substituents were
designed, synthesized, and evaluated for cytotoxicity.
Ó 2004 Elsevier Ltd. All rights reserved.
Discodermolide (+)-1 (Fig. 1), a polyketide natural
product isolated from the marine sponge Discodermia
dissoluta,1 has potent growth inhibitory activity against
human tumor cell lines.2;3 The mode of action, similar to
that of paclitaxel, involves the assembly and stabiliza-
tion of microtubules, leading to mitotic arrest and,
ultimately, to cell death.2;3 Attempts to acquire practi-
cally useful quantities of (+)-discodermolide, either by
cultivation of the producing sponge, or via harvesting of
the organism in the wild have thus far failed; therefore
totally synthetic (+)-1 was necessary to permit human
clinical trials.4;5
A number of groups have reported total syntheses of
(+)-discodermolide (1).6 In addition, a large number of
analogues have also been disclosed,6c;g;7 some of which
are significantly truncated yet retain considerable cyto-
toxic activity. Although the majority of these com-
pounds do not show the level of cell growth inhibitory
potency required to support further preclinical investi-
gation, the potential benefits of more synthetically
accessible compounds continues to fuel significant
research efforts.
B
The previously reported (+)-2,3-dehydrodiscodermolide
analogue 27a (Fig. 2) displays in vitro cytotoxicity
O
O
OH
OH
A
NH2
HO
O
HO
O
O
OH
OH
(+)-2
IC50 = 0.002 µM
NH2
C
O
(+)-Discodermolide
HO
O
(+)-Discodermolide
IC50 = 0.004 µM
(1)
Figure 1.
O
(+)-2,3-Dehydrodiscodermolide
(2)
Keywords: Discodermolide.
* Corresponding authors. Tel.: +1-510-7315210; fax: +1-510-7328401;
e-mail: burlingame@kosan.com
Figure 2.
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.01.102

2336
M. A. Burlingame et al. / Bioorg. Med. Chem. Lett. 14(2004) 2335–2338
activity superior to that of the parent discodermolide
when evaluated against the A549 human lung carcinoma
cell line. Based on this data we reasoned that the ÔCÕ
region of (+)-discodermolide (Fig. 1) could be further
simplified while preserving significant cell growth
inhibitory activity. Our investigation began with the
Wittig union of the previously reported salt (+)-36f and
hydrocinnamaldehyde (4a), followed by oxidative
removal of the PMB ether, carbamate installation, and
global deprotection, yielding the considerably simplified
derivative (+)-6a (Scheme 1, R1@R2 ¼ H) in which the
entire C-region has been replaced with a phenylethylene
substituent. Pleasingly, this analogue retained consid-
erable cytotoxic potency in four human tumor cell lines
(Table 1), despite the removal of five stereogenic centers.
followed by oxidation of the primary alcohol with the
Dess–Martin periodinane9 provided the aldehyde 8.
Installation of the diene, according to the procedure of
Yamamoto and co-workers10 gave the triene 9. Oxida-
tive removal of the PMB protecting groups, followed by
selective oxidation of the primary alcohol using TEMPO
with iodobenzene diacetate as the co-oxidant furnished
our desired aldehyde 10. Wittig coupling of the phos-
phonium salts 17a–j (3–5 equiv) gave the aryl alcohols
11a–j in excellent yields. Treatment of the latter, in turn,
with trichloroacetylisocyanate11 and methanolic potas-
sium carbonate provided the corresponding carbamates.
Removal of the tert-butyldimethylsilyl protecting
groups (TBS) with methanolic HCl gave the final aryl
discodermolide analogues 12a–j.
Reincorporation of oxygenation as in 6b proved to
augment activity relative to the unsubstituted case.
Comparison of 6b to the parent (+)-discodermolide
suggests that the phenol hydroxyl could mimic either the
endocyclic oxygen or the C(3) hydroxyl group, both of
which have been implicated in intramolecular hydrogen
bonding interactions in the natural product.8 Further
oxygenation as in 6c provided no additional increase in
cytotoxicity.
The requisite phosphonium salts 17a–h (Scheme 3) were
prepared starting from the commercially available (Al-
drich) Grignard reagent solutions in THF or ether (13a–
h). Treatment of the solutions directly with allylbromide
led to the allyl adducts 14a–h. Hydroboration of the
olefin followed by a peroxide work-up then gave the aryl
propanols 15a–h. Iodination of the propanols was
accomplished using iodine, triphenylphosphine, and
imidazole. Subsequent displacement of the primary
iodides 16a–j with triphenylphosphine in toluene at
100 °C completed the construction of the phosphonium
salts 17a–j.
Seeking to explore further the effect of derivatization of
the aromatic substituent, we designed a small library of
congeners incorporating a range of functionalized phe-
nyl rings as well as two isosteric thiophenes. Here we
surmised that a reversal of the Wittig coupling partners
would obviate the necessity for ultrahigh pressure in the
formation of the salt (+)-3,6f providing a more efficient
manifold for the production of analogues.
Aryl propanols 15i and 15j were synthesized as is shown
in Scheme 4. The hydroxyl group of 3-hydroxy-2-
methylbenzoic acid 18 was protected as the TBS ether
employing the corresponding silyl triflate and triethyl-
amine. The free acid was then treated with trimethylsi-
lyldiazomethane to provide the methyl ester, which was
reduced to the corresponding benzylic alcohol using
diisobutylaluminum hydride. Catalytic oxidation of the
Hence, beginning with the known intermediate (+)-76f
(Scheme 2), regioselective reduction of the PMP acetal,
O
O
OH
OH
PMBO
OTBS
OTBS
NaHMDS, THF,
-20 -> -10
1. DDQ, H2O
C
˚
NH2
O
2. a. Cl3CCONCO
b. Al2O3
PMBO
OTBS
OTBS
PPh3I
3. 6N HCl, MeOH
R1
R1
R1
(+) - 3
4a-c
R2
6a-c
R2
5a-c
R2
4a, 5a R1 = R2 = H
4b, 5b R1 = H, R2 = -OTBS
4c, 5c R1 = R2 = -OTBS
Scheme 1.
Table 1. Cytotoxicity observed for analogues 6a–c
Cytotoxicity IC50 (lM)
R1
R2
MCF-7
MCF-7/ADR
A549
SKOV-3
6a
6b
6c
H
H
0.48
0.13
0.16
0.56
0.39
0.51
0.19
0.31
0.35
0.04
0.04
H
OH
OH
OH
>1.0

M. A. Burlingame et al. / Bioorg. Med. Chem. Lett. 14(2004) 2335–2338
2337
1. DIBAL
2. Dess-Martin
periodinane
O
Ph2P
Ti(OiPr)4Li
CH3I
PMBO
OTBS
OTBS
O
O
OTBS
OTBS
PMBO
OTBS
OTBS
OPMB
PMP
8
OPMB
OPMB
9
(+)-7
TEMPO,
I(OAc)2
1. Cl3CCONCO
K2CO3, MeOH
2. 6N HCl, MeOH
17a-j, n-BuLi
OH OTBS
OTBS
OH OTBS
OTBS
H
O
10
11a-j
Ar
O
O
OH
OH
NH2
12a-j
Ar
Scheme 2.
9-BBN,
H2O2, NaOH
1. TBSOTf, Et3N
2. TMSCHN2
3. DIBAL-H
4. TPAP
MgBr
O
Br
O
O
Ar
OH
Ar
Ar
PPh3
MeO
15a-j
HO
TBSO
13a-h
14a-h
OH
H
I2, PPh3,
PPh3, 80 oC,
Toluene
18
19
imidazole
Ar =
Ar
I
Ar
PPh3I
16a-j
17a-j
O
1. H2, Pd/C
2. DIBAL-H
TBSO
TBSO
OMe
OH
20
15i
F
O
steps
a
e
b
c
g
d
h
HO
TBSO
F
F
Cl
OMe
S
OH
OH
21
15j
Scheme 4.
S
PhO
f
F
Table 2. Cytotoxicity observed for analogues 12a–j
Cytotoxicity IC50 (lM)
OH
OH
i
j
Ar
MCF-7
MCF-7/ADR
A549
SKOV-3
12a
12b
12c
12d
12e
12f
12g
12h
12i
0.37
0.40
0.33
0.60
1.0
0.50
0.65
0.50
2.0
0.65
0.58
0.60
1.0
0.40
0.38
0.37
0.36
1.0
Scheme 3.
alcohol using TPAP12 and N-methylmorpholine oxide
gave benzaldehyde 19. Exposure of the aldehyde to
(carbomethoxymethylene)triphenylphosphorane led to
unsaturated methyl ester 20, which was reduced in two
steps to give the desired aryl propanol 15i. The related
alcohol 15j was synthesized in an analogous fashion
from 3-hydroxy-4-methylbenzoic acid 21.
4.0
4.0
4.0
4.0
0.85
0.37
4.0
0.44
0.37
10.0
0.27
0.41
0.62
10.0
1.0
0.79
10.0
0.91
4.0
0.49
0.77
12j
4.0
The analogues 12a–j were assayed against four human
tumor cell lines (Table 2), including MCF-7/ADR, an
adriamycin and paclitaxel resistant cell line, which over
expresses the MDR P-glycoprotein. The majority of the
analogues display sub-micromolar activity against the

2338
M. A. Burlingame et al. / Bioorg. Med. Chem. Lett. 14(2004) 2335–2338
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resistant line (6a,b, 12a–c,g). The most potent com-
pounds display a range of meta substituents (OH, F, Cl,
OMe) while maintaining similar activities. The methyl
substituted thiophene 12g also displayed very good
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as demonstrated by the difluoro analogue 12d, and the
unsubstituted thiophene 12f. Substitution at the para
position also appears to be quite sensitive (cf. 12a vs 12e,
and 12i vs 12j). Clearly, opportunities exist in further
exploration of the meta position; other heterocycles may
also permit further enhancements of cytotoxic activity.
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