Improved synthesis of farinomalein and its analogs

Article abstract of DOI:10.1080/00397911.2011.640969

A practical and convenient synthesis of the fungicidal natural compound farinomalein is described starting from readily available ethyl 3-methyl-2-oxobutyrate and triethyl phosphonoacetate, employing a Horner-Wadsworth-Emmons condensation as the key step. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2011.640969

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Improved Synthesis of Farinomalein and
Its Analogs
Sachin T. Aiwale a , Paola Sardi a & Sabrina Dallavalle a
a Department of Food, Environmental and Nutritional Sciences,
Università degli Studi di Milano, Milano, Italy
Accepted author version posted online: 14 Mar 2012.Version of
record first published: 05 Mar 2013.
To cite this article: Sachin T. Aiwale , Paola Sardi & Sabrina Dallavalle (2013): Improved Synthesis
of Farinomalein and Its Analogs, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 43:10, 1455-1459
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Synthetic Communications1, 43: 1455–1459, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.640969
IMPROVED SYNTHESIS OF FARINOMALEIN AND
ITS ANALOGS
Sachin T. Aiwale, Paola Sardi, and Sabrina Dallavalle
Department of Food, Environmental and Nutritional Sciences, Universita
degli Studi di Milano, Milano, Italy
`
GRAPHICAL ABSTRACT
Abstract A practical and convenient synthesis of the fungicidal natural compound farinoma-
lein is described starting from readily available ethyl 3-methyl-2-oxobutyrate and
triethyl phosphonoacetate, employing a Horner–Wadsworth–Emmons condensation as the
key step.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Antifungal; farinomalein analogs; natural compound; synthesis
INTRODUCTION
Farinomalein 1 is a relatively structurally simple maleimide isolated in
2009 from the entomopathogenic fungus Paecilomyces farinosus, by Nihira et al.[1]
Farinomalein shows potent inhibition of Phytophthora sojae, a plant pathogen that
every year causes enormous damage to soybean crops.[2]
Intrigued by the interesting biological activity and because of our ongoing
interest in the synthesis of antifungal natural products, we developed a practical
synthesis of farinomalein, which may, in principle, have value in the large-scale
preparation of the natural compound and various analogs.
Recently, a three-step synthesis of farinomalein has been reported by Miles and
Yan.[3] The sequence is based on the reaction of isovaleraldehyde and glyoxylic acid
to give a c-hydroxybutenolide. Successive oxidation to the corresponding anhydride
and treatment with b-alanine affords farinomalein. However, the authors state that
Received October 27, 2011.
Address correspondence to Sachin T. Aiwale, Department of Food, Environmental and
`
Nutritional Sciences, Universita degli Studi di Milano, via Celoria 2, 20133, Milano, Italy. E-mail:
sachin.aiwale@unimi.it
1455

1456
S. T. AIWALE, P. SARDI, AND S. DALLAVALLE
their synthesis is difficult to scaleup,[3] because of the use of hazardous and expensive
oxidant Dess–Martin periodinane. Herein we report a different synthetic approach
to farinomalein, developed in four steps with good yield and without using any
hazardous chemical. A Horner–Wadsworth–Emmons condensation is the key step
of the synthetic pathway.
RESULTS AND DISCUSSION
As outlined in Scheme 1, commercially available ethyl 3-methyl-2-oxobutyrate 2
was reacted with triethylphosphonoacetate 3 in the Horner–Wadsworth–Emmons
conditions[4] to give the diester 4[5] in 80% yield as a faint yellow oil. Hydrolysis of
the diester using LiOH gave in 96% yield the cis-diacid 5,[6] which was then stirred
with trifluoroacetic anhydride (TFAA) at room temperature to obtain the anhydride
6[7] in quantitative yield. Finally, the treatment of the anhydride with b-alanine in
acetic acid[4,8] provided farinomalein 1 in 60% yield. The procedure was scaled up
to 10-g batches. The methodology developed is flexible and allows ready preparation
of different analogs to support future structure–activity relationship studies.
The first series of farinomalein analogs was prepared as depicted in Scheme 2.
The common intermediate 6 was reacted with different commercially available ami-
noacids such as L-leucine, L-phenylalanine, L-valine, and glycine to obtain analogs
with different alkyl chains on the maleimide skeleton.
Scheme 1. Reaction pathway to farinomalein.
Scheme 2. Reaction pathway to farinomalein analogs.

IMPROVED SYNTHESIS OF FARINOMALEIN
1457
Table 1. Antifungal activity of farinomalein and analogs
Antifungal activity (mg)a
Compound
C. cladosporioides
1
7a
5
50
50
30
50
0.1
7b
7c
7d
Prochloraz
aMinimum amount required for the inhibition of fungal
growth on thin-layer chromatography plates (TLC).
As the activity of Farinomalein against Phytophthora sojae is already documen-
ted in the literature, we decided to assess the activity of 1 and analogs against Clados-
porium cladosporioides, a fungal plant pathogen that affects wheat, using the method
of bioautography[9,10] (Table 1). The bioassay results confirmed that farinomalein is
endowed with antifungal activity on this and likely on other plant pathogens, with the
minimum amount required for the inhibition of fungal growth on thin-layer chroma-
tography (TLC) plates being 5 mg. Moreover, the results clearly indicate that the
introduction of different chains was detrimental for activity. In fact, the minimum
amount of compounds 7a–d required for fungal growth inhibition was 6 to 10-fold
more than for 1 (30–50 mg).
EXPERIMENTAL
All reagents and solvents were reagent grade or were purified by standard
methods before use. Melting points were determined in open capillaries. NMR
spectra were recorded with a Bruker AMX 300 spectrometer. Elemental analyses
were recorded with a Carlo Erba EA 1108 instrument. Solvents were routinely
distilled prior to use; anhydrous tetrahydrofuran (THF) and ether (Et2O) were
obtained by distillation from sodium–benzophenone ketyl and dry dichloromethane
was obtained by distillation from phosphorus pentoxide. All reactions requiring
anhydrous conditions were performed under a positive nitrogen flow, and all glass-
ware was oven dried and=or flame dried. Isolation and purification of the com-
pounds were performed by flash column chromatography on silica gel 60 (230–400
mesh). Analytical TLC was conducted on TLC plates (silica gel 60 F254, aluminum
foil) visualized by ultraviolet light and spraying with phosphomolybdic acid and
p-anisaldehyde.
For complete experimental details please see the Supplementary Materials,
available online.
3-(3-Isopropyl-2.5-dihydro-pyrrol-1-yl)-propionic Acid (1)
b-Alanine (0.063 g, 7.1 mmol) was added to the stirred solution of 6 (0.100 g
7.1 mmol) in AcOH (0.5 ml) at room temperature. The reaction mixture was refluxed
for 2 h and then cooled to 60 ꢀC, and the solvent was evaporated under vacuum.

1458
S. T. AIWALE, P. SARDI, AND S. DALLAVALLE
EtOAc (10 ml) was added to the crude product, and the organic layer was washed
with 2 N HCl (5 ml) and brine, dried over Na2SO4, and concentrated. Purification
by flash column chromatography afforded 1 as a white solid (0.089 g, 60%). Mp
1
76 ꢀC, Rf ¼ 0.36 (diethyl ether = hexane 50:50). H NMR (300 MHz, CDCl3) d:
6.25 (1H, d, J ¼ 1.8 Hz), 3.80 (2H, t, J ¼ 7.0 Hz), 2.80–2.95 (1H, m), 2.70 (2H, t,
J ¼ 7.0 Hz), 1.21 (6H, d, J ¼ 6.9 Hz). 1H and 13C NMR (CD3OD) completely
matched those reported in the isolation paper.[1] Anal. calcd. for C10H13NO4: C,
56.86; H, 6.20; N, 6.63. Found: C, 56.98; H, 6.10; N, 6.51.
The procedure described previously was used to prepare compounds 7a–d.
(3-Isopropyl-2,5-dioxo-2,5-dihydropyrrol-1-yl)acetic Acid (7a)
1
Oil. Yield 57%. Rf ¼ 0.4 (diethyl ether = hexane 50:50). H NMR (300 MHz,
CDCl3) d: 7.80 (1H, brs), 6.30 (1H, s), 4.28 (2H, s), 2.75–2.95 (1H, m), 1.20 (6H,
d, J ¼ 7.0 Hz); 13C NMR (75 MHz, CDCl3): d: 172.5, 170.3, 169.9, 156.5, 125.0,
38.3, 26.0, 20.8 (ꢁ 2) Anal. calcd. for C9H11NO4: C, 54.82; H, 5.62; N, 7.10. Found:
C, 55.01; H, 5.41; N, 7.01.
2-(3-Isopropyl-2,5-dioxo-2,5-dihydropyrrol-1-yl)-3-methylbutyric
Acid (7b)
1
Oil. Yield 41%. Rf ¼ 0.5 (diethyl ether = hexane 50:50). H NMR (300 MHz,
CDCl3) d: 9.00 (1H, brs), 6.25 (1H, s), 4.40 (1H, d, J ¼ 8.4 Hz), 2.80–2.95 (1H, m),
2.60–2.75 (1H, m), 1.20 (6H, d, J ¼ 7.0 Hz), 1.10 (3H, d, J ¼ 7.0 Hz), 0.85 (3H, d,
J ¼ 7.0 Hz). 13C NMR (75 MHz, CDCl3): d: 174.3, 170.6, 170.3, 156.0, 124.7, 57.6,
28.4, 25.9, 20.8 (ꢁ 3), 19.5. Anal. calcd. for C12H17NO4: C, 60.24; H, 7.16; N,
5.85. Found: C, 60.66; H, 7.00; N, 5.57.
2-(3-Isopropyl-2,5-dioxo-2,5-dihydropyrrol-1-yl)-4-methylpentanoic
Acid (7c)
1
Oil. Yield 53%. Rf ¼ 0.3 (diethyl ether = hexane 50:50). H NMR (300 MHz,
CDCl3) d: 6.28 (1H, s), 4.75 (1H, dd, J ¼ 4.6, 11.3 Hz), 2.80–2.95 (1H, m), 2.20–
2.32 (1H, m), 1.80–1.95 (1H, m), 1.35-1.50 (1H, m), 1.20 (6H, d, J ¼ 7.0 Hz), 0.90
(6H, d, J ¼ 7.0 Hz). 13C NMR (75 MHz, CDCl3): d: 175.9, 170.6, 170.3, 156.1,
124.9, 50.3, 37.0, 25.9, 25.2, 23.1, 21.0, 20.8 (ꢁ 2). Anal. calcd. for C13H19NO4: C,
61.64; H, 7.56; N, 5.53. Found: C, 61.39; H, 7.87; N, 5.31.
2-(3-Isopropyl-2,5-dioxo-2,5-dihydropyrrol-1-yl)-3-phenylpropionic
Acid (7d)
1
Oil. Yield 64%. Rf ¼ 0.6 (diethyl ether = hexane 50:50). H NMR (300 MHz,
CDCl3) d: 7.10–7.30 (5H, m), 6.12 (1H, s), 4.98 (1H, dd, J ¼ 6.3, 10.5 Hz), 3.40–
3.55 (2H, m), 2.68–2.80 (1H, m), 1.18 (3H, d, J ¼ 7.0 Hz), 1.09 (3H, d, J ¼ 7.0 Hz).
13C NMR (75 MHz, CDCl3) d: 174.5, 170.2, 169.9, 155.9, 136.5, 129.0 (ꢁ 2), 128.7
(ꢁ 2), 127.0, 124.5, 52.9, 34.4, 25.8, 20.7 (ꢁ 2) Anal. calcd. for C16H17NO4: C,
66.89; H, 5.96; N, 4.88. Found: C, 66.55; H, 6.12; N, 4.65.

IMPROVED SYNTHESIS OF FARINOMALEIN
1459
ACKNOWLEDGMENT
The authors gratefully acknowledge the University of Milan (FIRST funds) for
financial support.
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