A concise synthesis of maleic anhydride and maleimide natural products found in Antrodia camphorata

Article abstract of DOI:10.1016/j.tetlet.2007.01.169

Recently, the natural products maleic anhydride 1 and maleimides 2 and 3 have been isolated from the mycelium of Antrodia Camphorata, each displaying activity in LLC cell lines. We describe a concise synthesis of each of these natural compounds utilizing C-C cross-coupling methodology, namely, the Negishi and Suzuki reactions. This efficient and high yielding synthesis is convergent lending itself to production of medicinal analogues.

Full text of DOI:10.1016/j.tetlet.2007.01.169

Tetrahedron Letters 48 (2007) 2241–2244
A concise synthesis of maleic anhydride and maleimide
natural products found in Antrodia camphorata
Scott G. Stewart,* Marta E. Polomska and Rou Wei Lim
The School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, WA 6009, Australia
Received 22 December 2006; revised 25 January 2007; accepted 31 January 2007
Available online 4 February 2007
Abstract—Recently, the natural products maleic anhydride 1 and maleimides 2 and 3 have been isolated from the mycelium of
Antrodia Camphorata, each displaying activity in LLC cell lines. We describe a concise synthesis of each of these natural compounds
utilizing C–C cross-coupling methodology, namely, the Negishi and Suzuki reactions. This efficient and high yielding synthesis is
convergent lending itself to production of medicinal analogues.
Ó 2007 Elsevier Ltd. All rights reserved.
Antrodia camphorata is a parasitic fungus which grows
on the inner heartwood of the native and endangered
Taiwanese tree Cinnamomun kanehirai. This highly
sought after fungus is used in traditional Chinese tribal
medicine for the treatment of hypertension, liver cancer,
drug intoxication and inflammation and can retail at ca.
AUD 19,000 per kg.1–6 Previous studies indicate that the
A. camphorata mycelium extract consists of many
compounds such as steroids, triterpenoids, sesquiterpene
lactones, diterpenes and polysaccharides5 some of which
are in the early stages of investigation for activity
against the growth of tumour cell lines.1–5,7 Recently,
a new class of compounds (Fig. 1), bearing either maleic
anhydride 1 or maleimide 2 and 3 carbocyclic cores, has
also been isolated and these compounds were found to
be active against Lewis lung carcinoma (LLC) cell lines.1
However, as yet, there is no additional biological evalu-
ation and little is known about their mode of action.
Natural product maleic anhydrides and maleimides have
attracted the interest of several groups exploring their
biological activity.8,9 In conjunction with these studies,
there have been various synthetic approaches targeting
compounds bearing various R groups at positions C3
and C4.10–12 More recently, the above natural products
1, 2 and 3 have been prepared by the group of Argade13
through a seven-step linear based approach. Alterna-
tively, we envisaged a more convergent connection of
the alkyl and aryl fragments through Negishi14 and
Suzuki15 cross-coupling protocols. In this manner, it
was intended that analogues could be easily prepared
by simply altering the respective coupling partners.
R
O
N1
O
O
O
O
4
3
4
3
The synthesis of natural product 1 began with 3,4-
dichloromaleic anhydride (4, Aldrich) (Scheme 1).16 As
cross-coupling reactions involving 3,4-halogenated
maleic anhydrides have proven to be difficult in this lab-
oratory a protected maleimide was utilised. Thus, con-
densation of maleic anhydride 4 using benzylamine in
acetic acid afforded the benzyl protected maleimide 5
in 81% yield. Negishi cross-coupling of compound 5
with isobutylzinc bromide, Pd2(dba)3 (10 mol %) and
PPh3 (20 mol %) in THF at room temperature (optimum
conditions), furnished 60% of only mono alkylated
maleimide 6.16 Bearing in mind that this transformation
O
O
2 R = H
3 R = OH
1
Figure 1. Maleic anhydride and maleimide natural products of
Antrodia camphorata.
Keywords: Antrodia camphorata; Palladium catalysis; Negishi and
Suzuki cross-coupling.
Corresponding author. Tel.: +61 8 6488 3180; fax: +61 8 6488 1005;
e-mail: sgs@cyllene.uwa.edu.au
*
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.01.169

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S. G. Stewart et al. / Tetrahedron Letters 48 (2007) 2241–2244
Ph
Ph
O
N
O
O
N
i
O
O
ii
O
O
Cl
Cl
Cl
Cl
Cl
5
6
4
Ph
B(OH)2
O
N
O
O
O
O
iv
iii
O
7
1
8
O
O
i
Scheme 1. Reagents and conditions: (i) BnNH2, AcOH, 50 °C, 16 h, 81%; (ii) Pd2(dba)3 (10 mol %), PPh3 (20 mol %), BuZnBr, THF, 20 °C, 16 h,
60%; (iii) 7, Pd2(dba)3 (10 mol %), HP(t-Bu)3BF4 (20 mol %), Cy2NMe, dioxane, 20 °C, 3 h, 76%; (iv) KOH, THF/MeOH (1:2), 78 °C, 12 h then HCl
(2 M), 20 °C, 63%.
also falls into the category of palladium mediated conju-
gate substitution reactions,17 we decided to try a wide
range of catalysts in order to improve this yield. Unfor-
tunately, using other palladium catalysts, that is,
PdCl2(dppf) (11% yield) or even nickel based catalysts,
that is, Ni(PPh3)2Cl2 (24% yield), were either lower
yielding or failed to produce any of the desired product.
as described previously commercially available bromo-
maleic anhydride 11 was converted to the N-benzyl
derivative 12 in 83% yield. The corresponding metal
mediated conjugate substitution reaction using the Negi-
shi protocol provided alkyl maleimide 13 in 49% yield.
Surprisingly, under various catalytic conditions this con-
version was consistently lower than when using the chlo-
rinated derivative 5. Bromination of 13 using bromine
and a catalytic amount of aluminium tribromide affor-
ded vinyl bromide 14 in reasonable yield.8 Attachment
of the aryl side chain was slightly lower yielding (56%)
when treating the brominated product 14 under the same
Suzuki cross-coupling conditions as described previously
to give compound 8. Overall the described alterations
illustrated in Scheme 3 compared to the earlier pathway
(Scheme 1) were disadvantaged because of the additional
bromination step and lower yields associated with the
initial attachment of the alkyl side chain at position C3.
With chloromaleimide 6 in hand, we examined the Suzu-
ki cross-coupling reaction in order to attach the aryl ring
fragment. Boronic acid 7, required for this transforma-
tion, was prepared (Scheme 2) by initially treating the
commercially available 4-bromophenol 9 with K2CO3
and 3,3-dimethylallyl bromide to provide ether 10 in
77% yield. In a modified procedure to that reported by
Selles,11 halogen metal exchange of 10 using n-BuLi fol-
`
lowed by treatment with trimethyl borate afforded the
respective boronic ester. This latter compound was
hydrolysed (HCl 2 M) to provide boronic acid 7 (45%
from 9). Suzuki cross-coupling of chloromaleimide 6
with boronic acid 7 using Pd2(dba)3 and in situ gener-
ated P(t-Bu)3 afforded the highly fluorescent aryl malei-
mide 8 in 76% yield.18–20 Treatment of compound 8 with
KOH in THF/MeOH at reflux followed by acidic work-
up afforded the desired natural product maleic anhy-
dride 1 in 63% yield. The spectral data for compound
1 matched those reported previously.1,13
The synthesis of the natural product maleimide 2
(Scheme 4) was complete on treatment of anhydride 1
with urea to afford 2 in 60% yield. Similarly, the treat-
ment of 1 with N-hydroxylamine hydrochloride gave 3
in 79% yield.12,21 The spectral data corresponding to
both maleimides were consistent with those reported in
the literature.1,13
The aforementioned synthetic sequence is currently
being used as a template for the synthesis of a series of
derivatives for biological evaluation as liver cancer
agents. These derivatives which contain various attach-
ments to both the maleimide and maleic anhydride core
are being provided by simply changing the transmetala-
tion partners in both the Negishi and Suzuki cross-cou-
pling reactions. Interestingly, if the Suzuki reaction is
carried out on compound 5 a series of bis-arylated com-
pounds such as 15 are obtained and none of the mono-
substituted product.8 Compound 15 is one of a library
of compounds currently being screened within our
laboratories.
In order to compare the relative reactivity of the chloro-
substituted maleimides with their brominated counter-
parts, a second pathway (Scheme 3) using bromomaleic
anhydride 11 was devised. Under the same conditions
Br
B(OH)2
Br
i
ii
O
O
HO
10
7
9
Scheme 2. Reagents and conditions: (i) K2CO3, 3,3-dimethylallyl
bromide, acetone, 56 °C, 16 h, 77%; (ii) (a) n-BuLi, THF, ꢀ78 °C,
20 min then B(OMe)3, 1.5 h, ꢀ78!20 °C; (b) NH4Cl (aq), 20 °C, 0.5 h,
59%.
In summary, we have developed a novel and concise
synthesis of three natural products found in A. campho-

S. G. Stewart et al. / Tetrahedron Letters 48 (2007) 2241–2244
2243
Ph
Ph
Ph
N
N
O
N
iii
O
O
O
O
O
O
O
i
O
O
ii
3
Br
Br
Br
14
11
12
13
Ph
B(OH)2
O
N
O
O
O
v
iv
O
7
8
1
O
O
i
Scheme 3. Reagents and conditions: (i) BnNH2, AcOH, 50 °C, 16 h, 83%; (ii) Ni(PPh3)2Cl2 (5 mol %), BuZnBr, THF, 20 °C, 4 h, 49%; (iii) Br2,
AlBr3, CH2Cl2, 1 h, 0!20 °C, 55%; (iv) 7, Pd2(dba)3 (10 mol %), HP(t-Bu)3BF4 (20 mol %), Cy2NMe, dioxane, 20 °C, 3 h, 56%; (v) KOH, THF/
MeOH (1:2), 78 °C, 12 h then HCl (2 M), 20 °C, 63%.
R
H
O
N
N
O
O
O
O
O
O
i to 2
ii to 3
O
O
O
O
2 R = H
3 R = OH
1
15
Scheme 4. Reagents and conditions: (i) Urea, 140 °C, 4 h, 60%; (ii) NH2OHÆHCl, pyridine, 100 °C, 12 h, 79%.
3. Hattori, M.; Sheu, C.-C. US Patent & Trademark Office,
2005, US, 7109232, CAN 143:284829.
4. (a) Liu, J. J.; Huang, T. S.; Hsu, M.-L.; Chen, C. C.; Lin,
W. S.; Lu, F. J.; Chang, W.-H. Toxicol. Appl. Pharm.
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Chem. Technol. Biotechnol. 2005, 80, 216.
5. Chen, C.-C.; Shiao, Y.-J.; Lin, R.-D.; Shao, Y.-Y.; Lai,
M.-N.; Lin, C.-C.; Ng, L.-T.; Kuo, Y.-H. J. Nat. Prod.
2006, 69, 689, and references cited therein.
6. Hseu, Y.-C.; Wu, F.-Y.; Wu, J.-J.; Chen, J.-Y.; Chang,
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rata. Compound 1 was synthesised in four steps in 23%
overall yield while the maleimide natural products 2 and
3 were prepared in five steps and 14% and 18% overall
yields, respectively. We expect this synthesis utilising
Negishi and Suzuki reaction protocol to be highly ame-
nable to the production of a range of similar analogues
bearing this basic structure. Currently work is underway
to produce such compounds and develop a structure–
activity relationship (SAR) study for further biological
evaluation.
7. Yang, H.-L.; Chen, C.-S.; Chang, W.-H.; Lu, F.-J.; Lai,
Y.-C.; Chen, C.-C.; Hseu, T.-H.; Kuo, C.-T.; Hseu, Y.-C.
Cancer Lett. 2006, 231, 215.
Acknowledgements
8. Dubernet, M.; Caubert, V.; Guillard, J.; Viaud-Massuard,
M.-C. Tetrahedron 2005, 61, 4585.
9. Wang, J.; Soundarajan, N.; Liu, N.; Zimmermann, K.;
Naidu, B. N. Tetrahedron Lett. 2005, 46, 907.
The authors would like to thank the Ada Bartholomew
Medical Research Trust for generous financial support.
Additionally the authors are indebted to A/Professor
Dieter Wege for the supply of chemicals and glassware.
Both UWA Singapore-Australian Exchange Program
and Singapore Polytechnic are gratefully acknowledged
for the travel scholarship for R.W.L.
10. Easwar, S.; Argade, N. P. Synthesis 2006, 831.
`
11. Selles, P. Org. Lett. 2005, 7, 605.
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15. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
16. Preparation of 1-benzyl-3-chloro-4-isobutylpyrrole-2,5-
dione (6): Triphenylphosphine (100 mg, 0.38 mmol) was
added in one portion to a magnetically stirred solution of
Pd2(dba)3 (178 mg, 0.19 mmol) and the solution degassed
and stirred at rt for 0.5 h. The ensuing dark brown mixture
was treated with isobutylzinc bromide (3.9 mL, 0.5 M in
THF) followed by 3,4-dichloromaleimide 5 (500 mg,
References and notes
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S. G. Stewart et al. / Tetrahedron Letters 48 (2007) 2241–2244
1.95 mmol) in one portion. The resulting mixture was
borate (24 mg, 82.0 lmol) in dioxane (400 lL) maintained
at 20 °C under an atmosphere of argon. The resulting
mixture was stirred for 1 h, before being treated dropwise
with chloromaleimide 6 (100 mg, 0.36 mmol) in dioxane
(200 lL) followed by boronic acid 7 (84 mg, 0.40 mmol).
Stirring was continued for 14 h at 22 °C, before silica gel
(ꢁ150 mg) was added and the suspension concentrated
under reduced pressure. Subjection of this material to flash
chromatography (EtOAc/hexane 1:19) afforded the
desired product 8 (110 mg, 76%) as a fluorescent yellow
oil. Rf = 0.6 (hexane/EtOAc, 9:1); mmax (NaCl)/cmꢀ1 2959,
stirred at ambient temperature for 16 h before being
treated with silica (ca. 300 mg) and the suspension
concentrated under reduced pressure. Subjection of this
material to chromatography (EtOAc/hexane 1:19) affor-
ded dione 6 (326 mg, 60%) as a colourless oil. Rf = 0.3
(EtOAc/hexane, 1:9); mmax (NaCl)/cmꢀ1 2959, 1716, 1434,
1399, 1351, 1100, 1069, 741; 1H NMR (300 MHz, CDCl3)
d 0.96 (d, J = 7.3 Hz, 6H), 2.10 (sep, J = 7.0 Hz, 1H), 2.36
(d, J = 7.7 Hz, 2H), 4.69 (s, 2H), 7.26–7.36 (m, 5H); 13C
NMR (75 MHz; CDCl3): d 22.6, 27.9, 32.7, 42.0, 127.9,
128.4, 128.6, 134.1, 135.9, 140.6, 164.8, 168.7.
1
1703, 1605, 1511, 1434, 1402, 1246, 1179, 996; H NMR
17. Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; Wiley: Interscience, New
York, 2002; Vol. 1, Part III.2.15.
18. Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295.
19. Tietze, L. F.; Stewart, S. G.; Polomska, M. E. Eur. J. Org.
Chem. 2005, 1752.
(300 MHz, CDCl3) d 0.89 (d, J = 6.7 Hz, 6H), 1.76 (s, 3H),
1.81 (s, 3H), 2.06 (sep, J = 7.0 Hz, 1H), 2.51 (d, J =
7.2 Hz, 2H), 4.55 (d, J = 6.6 Hz, 2H), 4.72 (s, 2H), 5.54
(brt, J = 6.6 Hz, 1H), 6.98 (d, J = 8.5 Hz, 2H), 7.23–7.41
(m, 5H), 7.52 (d, J = 8.5 Hz, 2H) 13C NMR (75 MHz;
CDCl3): d 18.1, 22.7, 25.8, 28.0, 32.8, 41.6, 64.7, 114.6,
119.1, 121.3, 127.1, 127.6, 128.4, 128.5, 130.8, 136.6, 138.0,
138.6, 159.8, 171.0, 171.8; MS (EI, 70 eV): m/z (%) = 403.5
(M+Å, 10), 335 (100), 293 (25), 215 (33), 131 (58).
21. Shah, K. R.; Blanton, C. D. J. Org. Chem. 1982, 47,
502.
20. Preparation of 1-benzyl-3-isobutyl-4-[4-(3-methylbutenyl-
oxy)phenyl]pyrrole-2,5-dione (8):
N,N-Dicyclohexyl-
methylamine (174 lL, 0.80 mmol) was added to a degassed
and magnetically stirred solution of Pd2(dba)3 (36 mg,
40.0 lmol) and tri-tert-butylphosphonium tetrafluoro-