Synthesis of natural maleimides farinomaleins C-E and evaluation of their antifungal activity

Article abstract of DOI:10.1016/j.tetlet.2014.05.023

A practical and convenient synthesis of naturally occurring farinomaleins C-E was achieved starting from readily available ethyl 3-methyl-2-oxobutyrate and triethyl phosphonoacetate. The key steps of the sequence included a Horner-Wadsworth-Emmons condensation to obtain the precursor farinomalein A and coupling with suitable alcohols to install the chain. The synthesis of farinomalein D has been achieved starting from (R)-isopropylideneglycerol on the basis of which the S configuration was assigned to the natural compound. The antifungal activity of the synthesized compounds was evaluated against Cladosporium cladosporioides.

Full text of DOI:10.1016/j.tetlet.2014.05.023

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of natural maleimides farinomaleins C–E and
evaluation of their antifungal activity
Santosh Lahore a, Sachin T. Aiwale a, Paola Sardi b, Sabrina Dallavalle a,
⇑
a Department of Food, Environmental and Nutritional Sciences, Division of Chemistry and Molecular Biology, Università di Milano, Via Celoria 2, 20133 Milano, Italy
b Department of Food, Environmental and Nutritional Sciences, Division of Agro-Environmental Sciences, Università di Milano, Via Celoria 2, 20133 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A practical and convenient synthesis of naturally occurring farinomaleins C–E was achieved starting from
readily available ethyl 3-methyl-2-oxobutyrate and triethyl phosphonoacetate. The key steps of the
sequence included a Horner–Wadsworth–Emmons condensation to obtain the precursor farinomalein
A and coupling with suitable alcohols to install the chain. The synthesis of farinomalein D has been
achieved starting from (R)-isopropylideneglycerol on the basis of which the S configuration was assigned
to the natural compound. The antifungal activity of the synthesized compounds was evaluated against
Cladosporium cladosporioides.
Received 17 April 2014
Revised 30 April 2014
Accepted 12 May 2014
Available online xxxx
Keywords:
Farinomaleins
Maleimides
Ó 2014 Elsevier Ltd. All rights reserved.
Antifungal activity
Synthesis
Natural products
As part of our studies on the synthesis of natural compounds
endowed with antifungal activity, we have recently focused our
attention on farinomalein A, a simple maleimide isolated in 2009
from the entomopathogenic fungus Paecilomyces farinosus by Putri
et.al.1 The compound showed potent inhibition of Phytophtora
sojae, a plant pathogen that every year causes enormous damage
to soybean crops. Recently, El Amrani et al.2 have isolated three
new farinomalein derivatives, farinomaleins C–E, from an uniden-
tified endophytic fungus of the mangrove plant Avicennia marina
from Oman. We describe herein a short and convenient synthesis
of these new compounds, which are naturally occurring esters of
farinomalein A (Fig. 1).
O
O
N
OR
O
Farinomalein A (1) R = H
Farinomalein C (2) R = C4H9
O
O
N
O
OH
OH
O
Farinomalein D (3)
The first synthetic approach to farinomalein A was reported by
Miles and Yan.3 The sequence is based on the reaction of isovaler-
aldehyde and glyoxylic acid to give a c-hydroxybutenolide. Succes-
O
O
O
N
O
OH
sive oxidation to the corresponding anhydride and treatment with
b-alanine affords farinomalein A. However, the Authors themselves
state that their synthesis is difficult to scale-up,3 due to the use of
the hazardous and expensive oxidant Dess–Martin periodinane.
More recently, an alternative strategy for the preparation of
farinomalein A has been reported by us.4 The synthesis has been
developed in four steps with high yield and without using any
hazardous chemical (Scheme 1).
Farinomalein E (4)
O
Figure 1. Structures of farinomaleins.
We envisioned that farinomaleins C–E could be synthesized by
simply coupling farinomalein A, obtained from the previously
reported procedure, with suitable alcohols. Thus, farinomalein C
was obtained in 92% yield by reacting farinomalein A with 1-buta-
nol using DCC and DMAP in DCM (Scheme 1).5
Similarly, the synthesis of farinomalein D was carried out. As
⇑
Corresponding author. Tel.: +39 2 50316818; fax: +39 2 50316801.
E-mail address: sabrina.dallavalle@unimi.it (S. Dallavalle).
the absolute configuration at C-20 was not assigned by the Authors,
http://dx.doi.org/10.1016/j.tetlet.2014.05.023
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lahore, S.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.023

2
S. Lahore et al. / Tetrahedron Letters xxx (2014) xxx–xxx
OEt
OEt
O
a) LiOH, THF, rt,
8h, 96%
O
O
NaH, THF, 0°C-
50°C, 1h, 80%
OEt
O
EtO
OEt
OEt
P
O
O
O
b) TFAA, quant
O
O
7
5
6
O
8
O
N
O
O
1-butanol, DCC, DMAP
DCM, rt, overnight, 92%
H2N
OH
OC4H9
O
N
OH
O
AcOH, reflux, 1.5h, 60%
O
1 (Farinomalein A)
2 (Farinomalein C)
O
DCC, DMAP, DCM
0°C, then rt,
overnight, 77%
OH
O
9
O
O
O
O
AcOH/ H2O
N
O
OH
N
O
O
OH
O
50°C, 3h, 70%
O
O
(R)-3 (R) - Farinomalein D
10
Scheme 1. Synthesis of farinomaleins A, C and D.
we decided to synthesize both the enantiomers along with racemic
farinomalein D. Scheme 1 summarizes the synthesis of (R)-farino-
malein D ((R)-3) from (S)-9. After coupling with farinomalein A
using DCC and DMAP,6 deprotection of 10 was first attempted by
using SmCl2 in ethanol at reflux. This method allowed to obtain
the desired compound in high yield but with complete racemiza-
tion. Therefore, removal of the protecting group was carried out
in AcOH/H2O (4:1, v/v) maintaining the temperature at 50 °C for
3 h, as reported by Mori.7 (R)-Farinomalein D,8 obtained in 70%
yield, showed optical rotation opposite to that reported for the nat-
carbonyl group of b-keto ester 11 was protected using standard
methodology providing ketal 12.9 Reduction of 12 with lithium
aluminium hydride gave the primary alcohol 13 in 91% yield. In
the key transformation, treatment of b-hydroxy ketal 13 with
oxalic acid and wet silica gel according to the procedure by
Hitchcock10 provided the b-hydroxyketone 14 in high yield with-
out elimination products. Compound 14 was protected as tetrahy-
dropyranyl derivative (15)11 and this latter was condensed with
methyl (triphenylphosphoranylidene) acetate via benzoic acid
catalyzed Wittig reaction.12 A 3:2 mixture of Z and E
a,b-unsatu-
ꢀ7.2 (c 1.0, MeOH), lit.4: [
a]
+7.9 (c
rated esters 16 and 17 was produced. The mixture was treated
with methanolic HCl without separation of the stereoisomers.
Following removal of the THP group, the Z isomer 16 underwent
spontaneous lactonization to 18 and could thus be removed in a
facile chromatographic separation from the desired hydroxyester
19 (43% yield after purification). Finally, saponification with
sodium hydroxide in water, followed by acidification, gave the acid
20 in quantitative yield. Farinomalein E was then obtained by
coupling of 20 with farinomalein A in 42% yield.13
25
25
ural compound ([
0.03,MeOH)).
a]
D
D
The same reaction pathway was followed to prepare (S)-farino-
25
malein D (S)-3 starting from (R)-9 in 58% overall yield ([
a]
+7.3
D
(c 1.0, MeOH)). Racemic farinomalein D was also prepared for
structure-activity relationship studies, starting from DL-1,2-
isopropylideneglycerol.
As (S)-farinomalein D showed the same sign of specific rotation
reported for the natural compound, we may safely deduce that the
absolute configuration of natural farinomalein D is S.
After several attempts, an optimized reaction sequence for the
synthesis of farinomalein E was developed (Scheme 2). The
The antifungal activity of the synthesized natural compounds
(farinomaleins A, C–E) was investigated on Cladosporium
cladosporioides using the method of bioautography.14,15
HO
OH
Oxalic acid,
O
O
O
O
O
LiAlH4, THF
O
silica gel (230-400)
pTSA, benzene
O
O
O
OH
0°C, 1h, 91%
DCM, rt, overnight, 87%
reflux, 20h, 88%
13
12
11
O
O
pTsOH,DCM,
0°C, 1h, 80%
Ph3P=CHCO2Me
O
O
OH
O
toluene, 3 days, 42%
15
14
2M HCl,
O
MeOH,
O
+
O
O
O
OH
O
+
rt, 1h, 43%
O
O
O
19
18
O
O
16
17
O
O
O
1, EDCI.HCl,
DMAP, DCM,
O
O
NaOH, H2O, THF
rt, overnight, quant.
N
O
HO
OH
OH
rt, overnight, 42%
20
4 (Farinomalein E)
O
Scheme 2. Synthesis of farinomalein E.
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S. Lahore et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 1
structure–activity information inferred from our study could pro-
vide a basis for rational design of new structurally simple anti-
fungal agents and the starting point for their optimization.
Antifungal activity of farinomaleins
Compounds
Antifungal activitya
(lg) C. cladosporioides
1
2
3
4
21a
21b
21c
21d
5
0.5
20
20
50
50
30
50
0.1
References and notes
1. Putri, S. P.; Kinoshita, H.; Ihara, F.; Igarashi, Y.; Nihira, T. J. Nat. Prod. 2009, 72,
1544–1546.
2. El Amrani, M.; Debbab, A.; Aly, A. H.; Wray, V.; Dobretsov, S.; Muller, W. E. G.;
Lin, W.; Lai, D.; Proksh, P. Tetrahedron Lett. 2012, 53, 6721–6724.
3. Miles, W.; Yan, M. Tetrahedron Lett. 2010, 51, 1710–1712.
4. Aiwale, S. A.; Dallavalle, S.; Sardi, P. Synth. Commun. 2012, 43, 1455–1459.
5. Preparation of farinomalein C. To a stirred solution of farinomalein A (0.1 g,
0.47 mmol) in DCM (5 mL) DMAP (0.86 g, 0.70 mmol) was added and the
mixture was stirred at room temperature under nitrogen for 10 min. After
addition of DCC (0.146 g, 0.70 mmol) stirring was continued for further 30 min
at room temperature, then 1-butanol (0.053 g, 0.70 mmol) was added at 10 °C.
The reaction mixture was stirred at room temperature overnight, then it was
filtered through cotton and purified by column chromatography (ethyl acetate/
hexane 2:8) to give farinomalein C (0.117 g, 92%). Rf = 0.6 (ethyl acetate/hexane
50:50). The spectroscopic data of the compound matched with those reported
in the literature.2
Prochloraz
a
Minimum amount required for the inhibition of fungal growth on thin-layer
chromatography plates (TLC).
O
R
H
N
O
6. Preparation of 3-(3-isopropyl-2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-propionic acid
2,2-dimethyl-1,3]dioxolan-4-ylmethyl ester (10). To
farinomalein A (0.2 g, 0.94 mmol) in DCM (10 mL), DCC (0.292 g, 1.4 mmol),
DMAP (0.172 g, 1.5 mmol) and (S)-isopropylideneglycerol (0.187 g,
a stirred solution of
O
HO
9
21a R = H
1.5 mmol) were added at 0 °C. The resulting mixture was stirred at room
temperature overnight under nitrogen, then it was filtered through cotton and
purified by column chromatography (ethyl acetate/hexane 2:8) to give the title
compound (0.236 g, 77%). Rf = 0.4 (ethyl acetate/hexane 3:7). [a]D25 +1.3 (c 1.0,
MeOH). 1H NMR (300 MHz, CDCl3) d 6.22 (1H, s); 4.30 (1H, m); 4.19–4.02 (3H,
m); 3.80 (2H, t, J = 7.3 Hz); 3.73 (1H, m); 2.82 (1H, m); 2.66 (2H, t, J = 7.3 Hz);
1.42 (3H, s); 1.35 (3H, s); 1.20 (6H, d, J = 6.7 Hz). 13C NMR (300 MHz, CDCl3) d
170.4, 170.2, 170.0, 155.5, 124.2, 109.4, 73.0, 65.9, 64.7, 33.1, 32.5, 26.3, 25.4,
24.9, 20.4 (ꢁ2).
21b R = CH(CH3)2
21c R = CH2CH(CH3)2
21d R = CH2Ph
Figure 2. Structures of farinomalein A analogues.
The results confirmed that farinomalein A is endowed with
antifungal activity on this pathogen, the minimum amount
required for the inhibition of fungal growth on thin-layer chroma-
tography (TLC) plates being 5
showed a ten-fold increased potency, inhibiting the fungal growth
at 0.5 g. Farinomalein E and (R)-/(S)-/racemic farinomaleins D all
showed a lower activity (20 g), thus confirming that the stereo-
chemistry did not affect the antifungal activity (Table 1).
To depict preliminary structure–activity relationships for this
class of compounds, the results were compared to those obtained
testing a series of farinomalein A derivatives previously prepared
by our group4 (compounds 21a–d, Fig. 2).
The minimum amount of compounds 21a–d required for fungal
growth inhibition was 6 to 10-fold larger than for 1 (30–50 lg)
(Table 1).
The results clearly indicate that the introduction of different
chains to the carboxylic group, or the esterification with polar
7. Mori, K. Tetrahedron 2012, 68, 8441–8449.
8. Preparation of (R)-farinomalein D. (3) A solution of compound 10 (0.158 g) in
AcOH/H2O (4:1, v/v; 4.66 mL) was stirred at 50 °C (bath temperature) for 3 h.
The heating bath was removed, and saturated aqueous NaHCO3 (10 mL) was
added. The mixture was extracted with ethyl acetate (3 ꢁ 20 mL). The
combined organic extracts were washed with brine and dried with
anhydrous sodium sulfate. The solvent was removed under reduced pressure
and the residue was purified by flash chromatography on silica gel (ethyl
acetate) to obtain the title compound (70%). Rf = 0.35 (ethyl acetate/hexane
50:50). [a]2D5 ꢀ7.2 (c 1.0, MeOH). The spectroscopic data of the compound
matched with those reported in the literature.2
9. Nogawa, M.; Sugawara, S.; Iizuka, R.; Shimojo, M.; Ohta, H.; Hatanaka, M.;
Matsumoto, K. Tetrahedron 2006, 62, 12071–12083.
10. Hitchcock, S. R.; Perron, F.; Martin, V. A.; Albizati, K. F. Synthesis 1990, 1059–
1061.
11. White, J. D.; Carter, J. P.; Kezar, H. S. J. Org. Chem. 1982, 47, 929–932.
12. Keller-Schierlein, W.; Widmer, J.; Maurer, B. Helv. Chim. Acta 1972, 55, 198–
205.
13. Farinomalein E. To a stirred solution of farinomalein A (0.2 g, 0.94 mmol) in
DCM (3 mL) DMAP (0.115 g, 0.94 mmol) was added and the solution was
stirred at room temperature under nitrogen for 10 min. After addition of
EDCIꢂHCl (0.181 g, 0.94 mmol), the solution was stirred at room temperature
for further 30 min, then 20 (0.123 g, 0.94 mmol) dissolved in 2 mL DCM was
added at 10 °C. The reaction mixture was stirred at room temperature
overnight. Purification by column chromatography (ethyl acetate/hexane
3:7) gave farinomalein E (0.132 g, 42%). Rf = 0.5 (ethyl acetate/hexane 1:1).
The spectroscopic data of the compound matched with those reported in the
literature.2
lg. Interestingly, farinomalein C
l
l
a
moieties, was detrimental for activity. Conversely, a lipophilic
chain as in farinomalein C led to an increase of activity, comparable
with that of the reference compound prochloraz. These data con-
firm that farinomaleins can be considered promising candidates
in the field of antifungal compounds.
Current efforts are directed towards the synthesis of new ana-
logues. Notwithstanding the fact that the molecular mechanism
of action of the reported compounds remains to be defined,
14. Danelutte, A. P.; Lago, J. H. G.; Young, M. C. M.; Kato, M. J. Phytochemistry 2003,
64, 555–559.
15. Homans, A. L.; Fuchs, A. J. Chromatogr. 1970, 51, 327–329.
Please cite this article in press as: Lahore, S.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.023