A convenient synthesis of 3-methyleneoxindoles: Cytotoxic metabolites of indole-3-acetic acids

Article abstract of DOI:10.1016/S0040-4039(02)00855-9

3-Methyleneoxindole is a cytotoxic metabolite of indole-3-acetic acid with potential for use in cancer therapy. This species and ring-substituted analogues are conveniently synthesised from the corresponding isatins via a Peterson olefination.

Full text of DOI:10.1016/S0040-4039(02)00855-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4671–4673
A convenient synthesis of 3-methyleneoxindoles:
cytotoxic metabolites of indole-3-acetic acids
Sharon Rossiter*
Gray Cancer Institute, PO Box 100, Mount Vernon Hospital, Northwood, Middlesex HA6 2JR, UK
Received 26 February 2002; revised 24 April 2002; accepted 2 May 2002
Abstract—3-Methyleneoxindole is a cytotoxic metabolite of indole-3-acetic acid with potential for use in cancer therapy. This
species and ring-substituted analogues are conveniently synthesised from the corresponding isatins via a Peterson olefination.
© 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
to be 2 that exhibits toxicity to mammalian cells and
causes damage to plasmid DNA;7 therefore IAA, which
is non-toxic, is potentially useful as a prodrug for
enzyme targeted cancer therapy.7,8 Our recent studies
on the oxidation of substituted IAAs8 have led to a
number of questions about the mechanism of toxicity
and the nature of the cytotoxic species in each case, and
so we required pure samples of 2 and substituted ana-
logues for further study.
3-Methyleneoxindole 2 is a product of plant peroxidase
oxidation of the naturally-occurring auxin indole-3-ace-
tic acid, IAA 1 (Scheme 1).1–3 This interesting species
does not appear to have any auxin or anti-auxin prop-
erties,4,5 but has been shown to be toxic to bacteria.6
More recent studies have shown that oxidation of IAA
with horseradish peroxidase generates a species believed
The previous literature synthesis of 2 involves the NBS
bromination and oxidation of the parent IAA to give
3-bromooxindole-3-acetic acid, followed by aqueous or
base-catalysed elimination to give 2.9 However, it is
reported that 2 was only isolated at ca. 90% purity and
is unstable in solutions above 10−4 M, hindering further
purification.9 In our hands this synthesis proved to be
unreliable for the generation of high-quality samples of
2.
CO2H
CO2H
peroxidase
- e-
N
H
N
H
1
Kornet attempted to make a number of related 3-
methyleneoxindole precursors with a view to generating
2 by elimination under cellular conditions, but did not
observe any cytotoxicity.10 Other synthetic routes to
this simple but interesting heterocycle do not appear to
have been explored. Although other 3-alkylideneoxin-
doles have been synthesised by various methods, e.g.
via a Wittig reaction of isatin11 or condensation of an
aldehyde with oxindole,12 none of these routes appear
to have been used for the synthesis of 2, and in many
cases the reaction conditions or the product purification
methods would seem to rule out their suitability. Our
preliminary investigation of the Wittig reaction between
a methylenephosphorane and isatin under mild condi-
tions was not successful.
O
N
H
2
Scheme 1. Horseradish peroxidase oxidation of IAA.
Keywords: Peterson olefination; 3-methyleneoxindole; isatin.
* Present address: The Wolfson Institute for Biomedical Research,
University College London, The Cruciform Building, Gower Street,
London WC1E 6BT, UK. Tel.: +44 20 7679 6865; fax: +44 20 7679
6799; e-mail: s.rossiter@ucl.ac.uk
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00855-9

4672
S. Rossiter / Tetrahedron Letters 43 (2002) 4671–4673
We report a simple synthesis that gives 3-methyleneox-
indole in high purity and is easily adaptable for the
synthesis of analogues corresponding to the oxidation
products of substituted indole-3-acetic acids.
Br
Br
CuBr2, EtOAc
80°C, 3-6h
O
O
R
R
N
H
N
H
6
5
MeOH, H2O
reflux, 2h
2. Results and discussion
The chosen two-step route utilises the versatile Peterson
olefination of ketones via a b-silyl alcohol intermedi-
ate.13 Reaction of isatin 3 with trimethylsilylmethyl-
magnesium chloride gave the b-silyl alcohol 4. This is
moderately stable (although aqueous workup should be
avoided), and could be purified immediately by flash
column chromatography. Pure 3-methyleneoxindole
was then conveniently obtained by stirring 4 at −78 to
0°C with boron trifluoride (diethyl etherate complex)
(Scheme 2). With careful workup, the product could be
isolated in low to moderate yields at >99% purity
(HPLC).
O
O
R
N
H
7
Scheme 3. Substituted isatins.
3. Methods
3.1. General procedure for the preparation of 3-hydroxy-
3-trimethylsilylmethyloxindoles
A number of analogues with electron-donating or elec-
tron-withdrawing groups were also synthesised (Table
1). Isatins that were not commercially available were
synthesised from the corresponding oxindole 5 using an
adaptation of Kraynack’s method.14 The 3,3-dibro-
mooxindole 6 was obtained in almost quantitative
yields by heating with copper(II) bromide,15 which we
found to be more convenient than the use of pyri-
dinium tribromide, avoiding unwanted bromination on
the benzene ring. The dibromooxindole was then
heated in aqueous methanol to give the substituted
isatin 7 (Scheme 3).
Isatin (2–5 mmol) was suspended in dry diethyl ether
(20 mL) and cooled to −78°C. Trimethylsilylmethyl-
magnesium chloride (2 equiv. of a 1 M solution in
diethyl ether) was added, with stirring. The mixture was
stirred at −78°C for 15 min, then allowed to warm to
room temperature with stirring for a further 18 h. The
reaction was quenched with methanol, and then the
entire reaction mixture was concentrated in vacuo to
give a red-brown solid. Purification by flash column
chromatography (1:1 hexanes:ethyl acetate) gave 3-
hydroxy-3-trimethylsilylmethyloxindole 4 as a white
powder, yield 63%, mp 163–164°C; lH/ppm (60 MHz,
DMSO-d6) 7.49–7.14 (4H, m, ArH), 5.99 (1H, s, OH),
1.65 (2H, s, SiCH2), 0.0 (9H, s, SiMe3); m/z 235 (M+),
220 (M+−Me), 192; found: C, 61.29; H, 7.29; N, 5.95;
C12H17NO2Si requires C, 61.24; H, 7.28; N, 5.95%.
O
HO
SiMe3
Me3SiCH2MgCl
O
O
Et2O, -78 to 20°C
N
H
N
H
3
4
3.2. General procedure for t he synthesis of 3-methylene-
oxindoles
BF3·OEt2
CH2Cl2, -78°C
3-Hydroxy-3-trimethylsilylmethyloxindole (0.5–1 mmol)
in dichloromethane (20 mL) was cooled to −78°C, and
boron trifluoride diethyl etherate (5 equiv.) was added,
with stirring. The mixture was stirred at −78°C for 2 h,
and then at 0°C for a further 1 h. The mixture was
poured into satd NaHCO3, extracted with ether (2×100
mL), the organic layers washed with NaHCO3 once
more, dried over MgSO4 and the solvent was evapo-
rated to give pure 3-methyleneoxindole 2 (99–100%
purity by HPLC) as a yellow powder, yield 38%, mp
O
N
H
2
Scheme 2. Peterson olefination of isatin.
Table 1. 3-Methyleneoxindoles prepared from isatins
Substituent
Yield, Grignard addition (%)
Yield, elimination step (%)
Mp (°C)
None
5-F
5-Cl
6-Cl
5-Me
4-Cl
63
56
47
67
55
57
38
58
49
49
36
41
244–246 (dec.)
258–260 (dec.)
210–212 (dec.)
283–285 (dec.)
244–250 (dec.)
264–268 (dec.)

S. Rossiter / Tetrahedron Letters 43 (2002) 4671–4673
4673
244–246°C dec. (lit.9 for a 90% pure sample 218°C
References
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m/z 145 (100%, M+). The UV spectrum was identical to
that of 3-methyleneoxindole generated by the enzymatic
oxidation of indole-3-acetic acid.
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4. Conclusions
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We report a simple and flexible two-step synthesis of
3-methyleneoxindoles from readily available or easily
synthesised isatins, furnishing these bioactive com-
pounds at the high purity necessary for biological stud-
ies. The reactivity of these simple heterocycles could
also be investigated further, possibly as part of a route
to more complex indoles.
Acknowledgements
This work was funded by the Association for Interna-
tional Cancer Research and the Gray Laboratory Can-
cer Research Trust.