Telescoped enolate arylation/HWE procedure for the preparation of 3-alkenyl-oxindoles: The first synthesis of soulieotine

Article abstract of DOI:10.1002/ejoc.200900294

A. telescoped sequence involving palladium-catalysed intramolecular enolate arylation followed by an in situ HWE olefination has been developed to provide rapid access to 3-alkenyl-oxindoles. This "one-pot" process, which is greatly accelerated by microwa

Full text of DOI:10.1002/ejoc.200900294

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200900294
Telescoped Enolate Arylation/HWE Procedure for the Preparation of
3-Alkenyl-Oxindoles: The First Synthesis of Soulieotine
Alessia Millemaggi,[a] Alexis Perry,[a] Adrian C. Whitwood,[a] and Richard J. K. Taylor*[a]
Keywords: Horner–Wadsworth–Emmons olefination / Phosphonates / Enolate arylation / Palladium catalysis / Oxindoles /
Telescoped reactions
A telescoped sequence involving palladium-catalysed intra-
molecular enolate arylation followed by an in situ HWE ole-
fination has been developed to provide rapid access to 3-alk-
enyl-oxindoles. This “one-pot” process, which is greatly ac-
celerated by microwave irradiation, proceeds with low load-
ings of palladium(II) acetate (0.2–1.0 mol-%), and has been
used to prepare a range of adducts derived from aromatic,
heteroaromatic and aliphatic aldehydes as well as formalde-
hyde. In addition, further elaboration of the formaldehyde
adducts are described. The applicability of the process has
been established by carrying out the first synthesis of Soulie-
otine, a constituent of a traditional Chinese medicine.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
3-Alkenyl-oxindoles form the cornerstone of a range of
medicinally and biologically important compounds as well
as a number of natural products.[1] Illustrative examples are
shown in Figure 1. In terms of pharmaceuticals, mention
should be made of the pyrrolylidene oxindoles Semaxanib
(SU5416) (1)[2] and Sunitinib (SU11248) (2),[3] both of
which display antiangiogenic and anticancer activity, and
Tenidap (3), a potent inhibitor of cyclooxygenase, used for
the treatment of rheumatoid arthritis and osteo-arthritis.[4]
With respect to oxindole-containing natural products, Soul-
ieotine (4) and Ammosamide B (5) are representative exam-
ples. Soulieotine (4) was isolated from Souliea vaginata
which is employed as an antiinflammatory analgesic in
traditional Chinese medicine.[5] Ammosamide B (5) was re-
cently obtained from the marine-derived Streptomyces
strain CNR-698[6] and shown to possess cell cycle modula-
tion properties by myosin inhibition.[7] Furthermore, 3-alk-
enyl-oxindoles are versatile precursors for further synthetic
manipulation and have been employed widely in natural
product total synthesis[8] and for the preparation of novel
“unnatural” adducts.[9,10]
Figure 1. Representative 3-alkenyl-oxindoles.
3-alkenyl-oxindoles 8, as shown in Scheme 1. Thus, the ami-
dophosphonates 6 would be subjected to palladium-cata-
lysed intramolecular enolate arylation to give the cyclised
intermediates 7; subsequent addition of an aldehyde in the
presence of base should initiate Horner–Wadsworth–
Emmons (HWE) olefination to generate the target alkenyl-
oxindoles 8 in a regiocontrolled manner.
Palladium-catalysed amide enolate arylation approaches
to “simple” oxindoles are precedented,[12–14] although the
published approaches seem to require relatively high load-
ings of palladium and the use of bulky, electron-rich phos-
phane, N-heterocyclic carbene or relatively expensive BI-
NAP or dppf ligands. Our aim was to successfully ac-
complish the telescoped enolate arylation/HWE sequence
As part of a programme to develop efficient and practical
tandem/telescoped procedures to heterocyclic systems,[11]
we became interested in bioactive 3-alkenyl-oxindoles such
as those shown in Figure 1. On the basis of our recent de-
velopment of a telescoped Michael addition/HWE ole-
fination sequence leading to α-methylene-γ-butyrolac-
tones,[11b] we designed a telescoped, one-pot procedure for
the streamlined conversion of acylated halo-anilines 6 into
[a] Department of Chemistry, University of York,
Heslington, York YO10 5DD, United Kingdom
E-mail: rjkt1@york.ac.uk
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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A. Millemaggi, A. Perry, A. C. Whitwood, R. J. K. Taylor
SHORT COMMUNICATION
sponding bromide 6b gave a similar yield (Entry ii) but the
arylation step was considerably slower; the corresponding
aryl chloride 6c did not undergo coupling under these con-
ditions (Entry iii). Although successful with 6a and 6b, the
duration of both arylation and olefination processes was
unacceptable (2–24 h and then 24 –76 h in THF at reflux).
Variations in solvent, base and catalyst were therefore ex-
plored to accelerate this telescoped sequence but without
any major improvement [although it was found that potas-
sium 3,7-dimethyl-3-octylate (KDMO), ca. 3  in heptanes,
provided a more reliable and convenient source of base].
Gratifyingly, the use of microwave irradiation resulted in
a dramatic reduction in reaction times (Entries iv–vii).
Treatment of aryl bromide 6b with 5 mol-% Pd(PPh3)4 and
KDMO followed by microwave heating gave complete aryl-
ation in just 10 min and HWE olefination with benzalde-
hyde was complete in 30 min producing adduct 8a in 78%
isolated yield (Entry iv). Reducing the Pd(PPh3)4 loading
from 5 mol-% to 1 mol-% had no deleterious effect on time
or yield (Entry v), although the aryl chloride 6c still reacted
sluggishly under these conditions. (Entry vi).
Scheme 1.
shown in Scheme 1 using commercially available reagents
and low catalyst loadings. To this end, preliminary studies
were carried out using the 2-haloaniline derivatives 6a–c
(readily prepared from commercially available diethyl phos-
phonoacetic acid and the corresponding 2-haloanilines),
potassium tert-butoxide and benzaldehyde as shown in
Table 1. In the first attempt, using iodide 6a, KOtBu and
5 mol-% Pd(PPh3)4, followed by addition of benzaldehyde
after TLC analysis indicated that the arylation had gone to
Given the greater availability and stability, and lower
cost, of aryl bromides compared to aryl iodides, we decided
to optimise the bromide procedure in terms of the palla-
dium catalyst used to mediate enolate arylation. Success
completion, we were delighted to obtain the desired styr- was found with the use of 1 mol-% palladium(II) acetate
enyl-oxindole 8a in 81% overall yield (Entry i). The corre- (Entry vii) which produced adduct 8a in 87% yield. Not
Table 1. Preliminary studies on the conversion of anilides 1 into 3-styrenyl-oxindole 8a.[a]
Entry
i
Anilide
Conditions
Heat (∆) or MW
Yield 8a (%)[b]
6a
(i) Pd(PPh3)4 (5 mol-%), KOtBu, 2 h
(ii) 70 h
(i) Pd(PPh3)4 (5 mol-%), KDMO, 24 h
(ii) 24 h
∆
81
ii
6b
6c
6b
6b
6c
6b
∆
78[c]
0
iii
iv
v
Pd(PPh3)4 (5 mol-%), KDMO, 4 d
∆
(i) Pd(PPh3)4 (5 mol-%), KDMO, 10 min
(ii) 30 min
(i) Pd(PPh3)4 (1 mol-%), KDMO, 10 min
(ii) 30 min
(i) Pd(PPh3)4 (5 mol-%), KDMO, 10 min
(ii) 30 min
(i) Pd(OAc)2 (1 mol-%), KDMO, 10 min
(ii) 30 min
MW
MW
MW
MW
78[d]
82
vi
vii
22
87[e,f, g]
[a] All reactions were carried out as follows unless otherwise noted: 6 (ca. 0.3 mmol) in THF (3 mL), base (2.5 equiv.) then PhCHO
(5 equiv.). Reactions carried out in THF at reflux or under MW irradiation at 100 °C (CEM Discover, 50 W max.). [b] E/Z ratio 5:1. [c]
Identical yield using KOtBu (no product with LHMDS). [d] In the absence of the palladium catalyst no arylation was observed. [e]
Repeating this reaction exactly but with 0.2 mol-% Pd(OAc)2 gave a 46% yield of 8a. [f] Repeating this reaction in THF at reflux (no
MW) gave a 65% yield of 8a after 46 h. [g] With PhCHO (1.2 equiv.), the HWE reaction required 1.5 h giving 8a in 73% yield.
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Synthesis of Soulieotine
only is Pd(OAc)2 relatively inexpensive, it is easy to handle, Table 2. Variation of the aldehyde in the enolate arylation/HWE
sequence.[a]
oxidatively stable and has an excellent “shelf-life”. It should
also be emphasised that this coupling proceeded without
the need for phosphane additives. It was also established
that the catalyst loading could be reduced to as little as
0.2 mol-% Pd(OAc)2. In the majority of the reactions, an
excess of benzaldehyde (5 equiv.) was employed but it was
found that the sequence could be performed with a stoi-
chiometric quantity, although a longer HWE reaction time
was required. In general, where the aldehyde is of low value,
the use of 1 mol-% Pd(OAc)2 and 5 equiv. of aldehyde
seems to be optimum.
Following on from the optimisation studies, we went on
to investigate the scope of the enolate arylation/HWE se-
quence using aryl bromide 6b in terms of the aldehyde com-
ponent (Table 2).
As can be seen, in addition to benzaldehyde used in the
optimisation study, 4-nitro- and 4-bromobenzaldehyde re-
acted in the expected manner giving the corresponding ad-
ducts 8b and 8c in moderate to good yields (Entries i and
ii). Heteroaromatic aldehydes were also successful (Entries
iii and iv); thus, pyridine-3-carboxaldehyde gave adduct 8d
in 82% yield, and 2-formylbenzofuran gave the novel ad-
duct 8e in 59% yield as a single stereoisomer (tentatively
assigned as the E-isomer shown on the basis of NOESY
studies). Finally, the use of enolisable aliphatic aldehydes
and ketones was investigated; under the standard condi-
tions, only acetaldehyde gave the expected product 8f, albeit
in low yield (Entry v). It should be noted that all of the
reactions in Table 2 were carried out using the conditions
optimised for benzaldehyde; it may well be that additional
optimisation for each substrate would improve yields.
In addition to the aldehydes shown in Table 2, trapping
with formaldehyde was also explored (Scheme 2). It is
known that the expected intermediate 8g is unstable and
has to be trapped in situ.[10] By using formaldehyde in the
palladium-catalysed intramolecular enolate arylation/HWE
sequence and then adding sodium borohydride, we were
able to isolate the 3-methyl-oxindole 9[15] in excellent yield
over the three step, one-pot process. In a similar manner,
enone 8g could be intercepted with lithium dibutylcuprate
to generate the corresponding 3-pentyl-oxindole 10.[16] To
establish the generality of this one-pot elaboration se-
quence, the benzaldehyde adduct 8a was reacted in situ with
lithium dibutylcuprate followed by iodomethane; this four
step sequence (enolate arylation/HWE/conjugate addition/
alkylation) gave the 3,3Ј-dialkylated oxindole 11 in reason-
able, unoptimised yield.
Finally, our attention turned to a simple natural product
[a] All reactions were carried out using 6b (ca. 0.3 mmol) in THF
target with which to validate the utility of the enolate aryl-
ation/HWE sequence. Soulieotine (4) was isolated from the
rhizomes of Souliea vaginata, a plant employed as an anti-
inflammatory analgesic in traditional Chinese medicine.[5]
As Soulieotine (4) has not yet been prepared by total syn-
thesis, and as there is some doubt about the alkene stereo-
(3 mL), Pd(OAc)2 (1 mol-%), KDMO in heptanes (2.5 equiv.) for
10 min then RCHO (5 equiv.) for 30 min; both steps carried out
under MW irradiation at 100 °C (CEM Discover, 50 W max.). [b]
No adducts obtained using pivaldehyde and cyclohexanone.
chemistry (see later), we embarked upon its preparation. To able protecting group. We therefore commenced with the
construct an oxindole such as 4, in which the nitrogen is readily prepared amine 12[17] and prepared the PMB-pro-
unsubstituted, it was necessary to employ an easily remov- tected cyclisation precursor 13 as shown in Scheme 3. Ap-
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A. Millemaggi, A. Perry, A. C. Whitwood, R. J. K. Taylor
SHORT COMMUNICATION
Scheme 2.
Scheme 3.
plication of the enolate arylation/HWE sequence to sub-
strate 13 using 3-methylbutenal as the trapping agent gave
the expected adduct 14 in 42% unoptimised yield as an iso-
meric mixture of alkenes. It should be noted that the enol-
ate arylation was unsuccessful using Pd(OAc)2, presumably
due to palladium(II)-mediated PMB deprotection, but pro-
ceeded with tetrakis(triphenylphosphane)palladium (this ef-
fect was also observed with other N-PMB analogues). TFA
deprotection then produced a mixture of E-4 and Z-4 (2:1)
which were separated by chromatography (although they
readily interconvert; the 2:1 ratio is re-established after one
hour in CHCl3 at room temp.). The major isomer was es-
tablished as E-4 by NOE studies and by comparison of
NMR spectroscopic data with related systems.[18] The data
for E-4 corresponded well with those published[5] for Souli-
eotine, removing any uncertainty concerning the stereo-
chemistry of the exocyclic alkene. (In the publication out-
lining the isolation of Soulieotine 4,[5] the natural product
was described as the E-isomer but the Z-isomer was drawn
in error.) Finally, unambiguous proof of the structure of E-
4 was established by X-ray crystallography (Figure 2).
Figure 2. X-ray structure of E-4 depicted using ORTEP-3.
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Synthesis of Soulieotine
CCDC-723953 (for E-4) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
Summary
A
telescoped palladium-catalysed enolate arylation/ charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
HWE olefination sequence has been developed to provide
rapid access to 3-alkenyl-oxindoles from readily available
bromoanilines. This “one-pot” process is greatly accelerated
by microwave irradiation, and proceeds with low loadings
of palladium(II) acetate (0.2–1.0 mol-%). In terms of the
HWE process, aromatic, heteroaromatic and aliphatic alde-
hydes have all been successsfully employed. In addition,
formaldehyde can be utilised in the HWE reaction and the
resulting α-methylene lactam further elaborated in situ.
Soulieotine, isolated from the rhizomes of Souliea vaginata,
has been prepared to illustrate the potential of this tele-
scoped process and structural clarification has been pro-
vided [Soulieotine is predominantly (E)-6-methoxy-3-(3Ј-
methyl-2Ј-butenylidene)-2-indolinone, E-4].
Supporting Information (see also the footnote on the first page of
this article): Full characterisation data for compounds 6b, 6c, 11,
13, 14, E-4 and Z-4.
Acknowledgments
We thank the Engineering and Physical Sciences Research Council
(EPSRC) for funding (EP/029841/1; to A. M. and A. P.). We are
also grateful to Dr. T. Dransfield (University of York, mass spec-
trometry) for technical assistance. BASF are thanked for a kind
gift of KDMO in heptanes.
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Experimental Section
(E)-3-(Benzofuran-2Ј-ylmethylidene)-1-methyl-2-indolinone (8e): A
solution of aryl bromide 6b (110 mg, 0.30 mmol), KDMO (2.7 
in heptanes, 0.28 mL, 0.75 mmol), and Pd(OAc)2 (0.7 mg,
0.003 mmol) in THF (3 mL) under Ar was heated under MW irra-
diation at 100 °C (CEM Discover, 50 W max.) for 10 min. After
this time 2-benzofurancarboxaldehyde (220 mg, 1.51 mmol) was
added in one portion and the resulting solution was heated under
MW irradiation at 100 °C for 30 min. The reaction mixture was
treated with satd. NH4Cl (3 mL) and the aqueous layer was ex-
tracted with EtOAc (3ϫ3 mL). The combined organic layers were
dried with Na2SO4, filtered, and concentrated in vacuo. Purifica-
tion by flash chromatography (SiO2, petroleum ether/EtOAc, 95:5),
gave 8e (49 mg, 59%) as a yellow solid, m.p. 173–174 °C (from
CH2Cl2/pentane); Rf = 0.2 (petroleum ether/EtOAc, 80:20).
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IR (neat): ν = 1702, 1624, 1601, 1467, 1450, 1377, 1291, 1256, 1145,
˜
1
1096, 1024, 740 cm–1. H NMR (400 MHz, CDCl3): δ = 8.65 (d, J
= 7.5 Hz, 1 H, 4-H), 7.66–7.60 (m, 2 H, 4Ј-H, 6Ј-H), 7.56 (s, 1 H,
3Ј-H), 7.43 (ddd, J = 10.0, 7.5, 1.0 Hz, 1 H, 5Ј-H), 7.36–7.27 (m, 2
H, 7Ј-H, 6-H), 7.18 (s, 1 H, CH=), 7.14 (ddd, J = 7.5, 7.5, 1.0 Hz,
1 H, 5-H), 6.83 (d, J = 7.5 Hz, 1 H, 7-H), 3.28 (s, 3 H, N-Me)
ppm. 13C NMR (100 MHz, CDCl3): δ = 169.3 (C-2), 156.5 (C-7aЈ),
153.1 (C-2Ј), 144.7 (C-3a), 130.2 (C-7Ј), 128.5 (C-3), 127.2 (C-5Ј),
125.9 (C-4), 125.1 (C-3aЈ), 123.9 (C-6), 122.5 (C-4Ј), 122.2 (C-5),
121.4 (C-7a), 120.8 (C-3Ј), 115.7 (CH=), 111.7 (C-6Ј), 108.0 (C-7),
25.9 (N-Me) ppm. MS (ESI): m/z (%) = 298 (100) [M + Na]+.
HRMS (ESI): found 298.0833. C18H13NNaO2 calcd. 298.0838 (δ
=1.9 ppm error).
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SHORT COMMUNICATION
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Received: March 19, 2009
Published Online: May 5, 2009
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