Synthesis and biological activity of 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-oxadiazoles

Article abstract of DOI:10.1016/j.bmcl.2009.07.139

Acid hydrazides were coupled with acrylic acid derivatives and cyclodehydration gave 1,3,4-oxadiazoles. Lastly, in-situ nitrile oxide formation from aryl oximes treated with sodium hypochlorite, and subsequent 1,3-dipolar cycloaddition to the exomethylene

Full text of DOI:10.1016/j.bmcl.2009.07.139

Bioorganic & Medicinal Chemistry Letters 19 (2009) 5796–5798
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological activity of 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-
oxadiazoles
Kristin A. Milinkevich a, Choong L. Yoo a, Thomas C. Sparks b, Beth A. Lorsbach b, Mark J. Kurth a,
*
a Department of Chemistry, One Shields Ave., University of California, Davis, CA 95616, USA
b Dow AgroSciences LLC, Discovery R&D, 9330 Zionsville Rd., Indianapolis, IN 46268, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Acid hydrazides were coupled with acrylic acid derivatives and cyclodehydration gave 1,3,4-oxadiazoles.
Lastly, in-situ nitrile oxide formation from aryl oximes treated with sodium hypochlorite, and subsequent
1,3-dipolar cycloaddition to the exomethylene moiety delivered 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-oxa-
diazoles. This library was evaluated in a high-throughput screen at Dow AgroSciences. Several com-
pounds were active against fungal pathogens and pest insects.
Received 26 June 2009
Revised 24 July 2009
Accepted 28 July 2009
Available online 6 August 2009
Ó 2009 Published by Elsevier Ltd.
Keywords:
1,3,4-Oxadiazole
Isoxazoline
Diheterocylce
Insecticidal activity
Small molecule library
1,3,4-Oxadiazoles are an important class of heterocyclic com-
pounds with a variety of biological activities. Substituted 1,3,4-
oxadiazoles have shown antibacterial,1 anti-inflammatory,1b anti-
fungal,1a,2 anticonvulsant and muscle relaxant,3 insecticide,4 and
anion sensing and fluorescent patterning activities.5 The wide-
spread use of 1,3,4-oxadiazoles as a scaffold in medicinal chemistry
establishes this moiety as a member of the privileged structures
class.6 Isoxazolines are also an important heterocycle; molecules
containing this moiety have been found to elicit herbicidal,7 anti-
inflammatory,8 anti-tuberculosis,9 antifungal,10 anti-influenza,11
antibacterial,12 spermicidal, and anti-HIV properties.13 As part of
collaboration with Dow AgroSciences, lead generation libraries
are designed and biologically evaluated for new chemistry as well
as product pipeline potential. Indeed, there are several reports of
combinatorial libraries as discovery tools for agrochemicals and
these have provided numerous hits and leads.14 These libraries
serve to both produce molecules with activity and, simultaneously,
provide valuable information to facilitate optimization. This cou-
pled with the broad spectrum of biological activity of 1,3,4-oxadi-
azole and isoxazoline derivatives prompted us synthesize
heterocycle 6 (Figure 1). While the core structure is known in the
literature,15 most examples display diversification only on the
isoxazoline moiety. We envisioned that tethering together the
two heterocycles with diversification at three points in the mole-
cule could lead to new properties. Due to the importance of each
heterocycle in the literature, a library based on this scaffold was
pursued. Herein we report the synthesis and biological activity of
a library with diversification of three positions in the molecule.
R3
R3
C
N O
O
N
HO
NH2
R2
O
R1
R1
N
H
O
R2
O
N
N
6
Figure 1. Retrosynthetic analysis of proposed 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-
oxadiazole library.
O
R2
HO
O
O
H
N
PPh3, Cl3CCCl3
DIEA, CH3CN
2
NH2
R1
N
H
R2
R1
N
H
E DC, DCM
O
1{1-4 }
3{1-4,1-2 }
R3
R3
N
OH
N
5
O
R1
R1
O
O
R2
R2
NaOCl, DCM
N
N
N N
4{1-4,1-2}
6{1-4,1-2,1-10}
* Corresponding author. Tel.: +1 530 752 8192; fax: +1 530 752 8995.
E-mail address: mjkurth@ucdavis.edu (M.J. Kurth).
Scheme 1. Preparation of racemic 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-oxadiazole
library; experimental details and spectral data are provided in Supplementary Data.
0960-894X/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2009.07.139

K. A. Milinkevich et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5796–5798
5797
Table 1
Table 2
Library diversity inputs
Fungicidal (PYR
& SEP) and insecticidal (BAW) activity for subset of 2-(4,5-
dihydroisoxazol-5-yl)-1,3,4-oxadiazole library.
R3
Entry
R1
R2
R3
PYR
SEP
BAW
N
O
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
C6H5
C6H5
C6H5
Benzyl
H
H
H
CH3
CH3
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
4-Pyridyl
4-Cl–C6H4
4-F–C6H4
4-NO2–C6H4
4-Cl–C6H4
2,6-Di-Cl–C6H3
4-Pyridyl
4-Cl–C6H4
2,6-Di-Cl–C6H3
4-F–C6H4
3-Cl–C6H4
2,6-Di-Cl–C6H3
C6H5
3-Cl–C6H4
4-NO2–C6H4
3-Cl–C6H4
0
0
0
0
0
30
0
30
70
0
0
10
0
0
10
0
0
17
17
17
17
17
0
50
0
0
33
50
17
0
0
50
R1
O
R2
30
30
50
40
30
40
0
50
30
50
40
30
30
50
N
N
6{ 1-4,1-2,1-10}
4-Cl–C6H4
C6H5
4-Cl–C6H4
3-Cl–C6H4
Benzyl
3-Cl–C6H4
3-Cl–C6H4
3-Cl–C6H4
3-Cl–C6H4
C6H5
R2
R3
R1
{1 }
{1}
{1}
{6 }
4-Cl-C6H4
CH3
4-NO2-C6H4
4-pyridyl
{2 } C6H5
{2 } H
{2 } 4-F-C6H4
{7 } 2,6-di-Cl-C6H3
{3 } benzyl
{4 } 3-Cl-C6H4
{3} 4-Cl-C6H4
{4 } 3-pyridyl
{8 } C6H5
{9 } 4-OCH3-C6H4
{10 } 4-CH3-C6H4
{5 } 3-Cl-C6H4
4-Cl–C6H4
4-Cl–C6H4
CH3
CH3
40
10
PYR = Pyricularia oryzae; SEP = Septoria tritici; data is percent disease control.
BAW = Spodoptera exigua (beet armyworm); data is percent mortality.
The synthesis of the target scaffold 6, as seen in Scheme 1, be-
gan with EDC-mediated coupling of hydrazide chemset 1 with ac-
rylic acid chemset 2 (Table 1).22 When methacrylic acid (R2 = CH3)
was used, formation of the product proceeded cleanly in nearly
quantitative yields. However, when acrylic acid (R2 = H) was used,
significant byproduct, presumably from the polymerization of the
acrylic acid, made isolation of the product difficult. Due to the
low yielding nature of this reaction and difficulties with purifica-
tion, only ten derivatives where R2 = H were synthesized.
There are several known methods for the synthesis of 1,3,4-oxa-
diazoles,16 but approaches based on cyclodehydration of diacyl
hydrazides are the most commonly used.1d,6,17 This approach often
employs strongly acidic reagents or dehydrating reagents that pro-
mote cyclization.1d Due to the alkene functionality present in com-
pound 3, a dehydrating reagent was chosen over strongly acidic
conditions. Typical examples of dehydrating reagents include tri-
phenylphosphine, thionyl chloride, triflic anhydride, phosphorus
pentoxide, and phosphorus oxychloride.1d These reagents are
known to result in significant byproduct formation, but due to
the sensitive nature of compound 3, this approach was selected.
The method chosen utilizes a mild triphenylphosphine and hexa-
chloroethane mediated cyclodehydration to form the 1,3,4-oxadi-
azoles.17 Although byproduct formation was observed, the
desired product can be easily isolated from the mixture by purifi-
cation via flash column chromatography. This method was em-
ployed to isolate all 1,3,4-oxadiazole intermediates in moderate
to high yields (51–96%).23
With the 1,3,4-oxadiazole exomethylene intermediate 4 in
hand, the last step to diheterocycle 6 was to introduce the isoxaz-
oline component by way of the nitrile oxide 1,3-dipolar cycloaddi-
tion. This was accomplished via biphasic treatment of
exomethylenes 4{1-4,1-2} with reagent chemset 5 (aryl oximes,
Table 1) and bleach (Huisgen’s method18) to afford the targeted
2-(4,5-dihydroisoxazol-5-yl)-1,3,4-oxadiazoles 6{1-4,1-2,1-10} in
yields ranging from 40–86%.24 The lower end yields arise from
the chlorine and pyridine diversity elements where solubility
was an issue for these compounds. It has been shown that the
5,5-disubstituted isoxazoline is the major regioisomer when react-
ing a 1,1,-disubstituted alkene with a nitrile oxide.19 Comparison of
the proton NMR isoxazoline methylene protons in 6{1-4,1-2,1-10}
to similar systems established that the 5,5-disubstituted regio-
isomer was the desired product.20,21
activity, and Pyricularia oryzae (rice blast) and Septoria tritici
(wheat leaf blotch) for fungicial activity. While none of the com-
pounds tested displayed any herbicidal activity, several show mod-
est disease control (Table 2), however the observed activity was far
below that for fungicide standards such as azoxystrobin. Four of
the sixteen compounds in the set passed the HTS insect screens
against larvae of Spodoptera exigua (beet armyworm), a 25% pass
rate. Moreover, some initial trends in the SAR were observed, as
seen in entries 8, 11, 12, and 16, which have a 4-Cl or 3-Cl phenyl
substituent on the oxadiazole and on the isoxazoline rings, sug-
gesting that chlorine substituents are important for insecticidal
activity. Unfortunately, further testing of the four compounds in
secondary assays against larvae of S. exigua and Helicoverpa zea
(corn earworm) showed no activity when compared to the insecti-
cide standard spinosad. Future efforts to increase the insect activity
will comprise expanding these early SAR trends and surveying bio-
isosteres of the oxadiazole ring, including screening 5-(thiazol-5-
yl)-4,5-dihydro-isoxazoles from a previous library.20
In summary, a route to 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-oxa-
diazoles has been developed leading to a 50-member library of
1,3,4-oxadiazoloisoxazolines. Key steps included diacyl hydrazide
formation, followed by cyclodehydration to give the 1,3,4-oxadiaz-
ole, and 1,3-dipolar cycloaddition to give the isoxazoline. These
compounds were evaluated in the insecticide HTS and exhibited
weak to moderate insecticidal activity. Early SAR trends point to
chlorine substituents as important for insecticidal activity. Future
efforts to increase the insecticidal activity will include screening
other heterocyclic rings isosteres.
Acknowledgments
This work was supported by the National Science Foundation
(CHE-0614756) and the National Institute for General Medical Sci-
ences (GM076151). NMR spectrometers used in this work were
funded in part by the National Science Foundation (CHE-0443516
and CHE-9808183). The authors would also like to thank Mr. Jeff-
ery Butler for his synthetic assistance, and Mr. Brian Waldman,
Mr. Tye Burgland, and Ms. Cathy Young for assistance with the
bioassays.
Sixteen compounds from this collection were screened broadly
for herbicidal, fungicidal, and insecticidal activity.25 This set of
compounds was screened against Helianthus annuus (sunflower)
and Digitaria sanguinalis (crabgrass) as a measure of herbicidal
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.07.139.

5798
K. A. Milinkevich et al. / Bioorg. Med. Chem. Lett. 19 (2009) 5796–5798
16.1 mmol) was added and the mixture was stirred for 10 min after which time
References and notes
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Purity was determined to be 100% by HPLC analysis.
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