One-pot high-throughput synthesis of a 160-membered library of methyl 3,5-diaryl-isoxazoline-5-carboxylate pharmacophores by a 2·2·2- component reaction

Article abstract of DOI:10.1021/co400015w

A simple and efficient methodology has been developed for the synthesis of methyl 3,5-diaryl-isoxazoline-5-carboxylates in a high-throughput fashion. This was accomplished in one-pot by a sequence of three 2-component reactions steps (2·2·2-CR), whereby compounds were obtained in overall 30-66% isolated yields. The functional group diversity was established by synthesizing a 160-membered library.

Full text of DOI:10.1021/co400015w

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Article
One-pot High Throughput Synthesis of 160-Membered
Library of Methyl 3,5-diaryl-isoxazoline-5-carboxylate
Pharmacophores by a 2•2•2-Component Reaction
Kallolmay Biswas, Arvind Kumar, and Koushik Das Sarma
ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/co400015w • Publication Date (Web): 02 Apr 2013
Downloaded from http://pubs.acs.org on April 5, 2013
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Oneꢀpot High Throughput Synthesis of 160ꢀ
Membered Library of Methyl 3,5ꢀdiarylꢀisoxazolineꢀ
5ꢀcarboxylate Pharmacophores by a 2ꢁ2ꢁ2ꢀ
Component Reaction
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Kallolmay Biswas, Arvind Kumar, Koushik Das Sarma*
Jubilant Chemsys Ltd., Bꢀ34, Sectorꢀ58, Noida, UP, Indiaꢀ201301
Table of Contents Graphic
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Oneꢀpot High Throughput Synthesis of 160ꢀ
Membered Library of Methyl 3,5ꢀdiarylꢀisoxazolineꢀ
5ꢀcarboxylate Pharmacophores by a 2ꢁ2ꢁ2ꢀ
Component Reaction
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Kallolmay Biswas, Arvind Kumar, Koushik Das Sarma*
Jubilant Chemsys Ltd., Bꢀ34, Sectorꢀ58, Noida, UP, Indiaꢀ201301
KEYWORDS: Methyl 3,5ꢀdiarylꢀisoxazolineꢀ5ꢀcarboxylate, oneꢀpot, high throughput, 1,3ꢀ
dipolar cycloaddition, 2ꢁ2ꢁ2ꢀcomponent reaction.
ABSTRACT: A simple and efficient methodology has been developed for the synthesis of
methyl 3,5ꢀdiarylꢀisoxazolineꢀ5ꢀcarboxylates in a high throughput fashion. This was
accomplished in oneꢀpot by a sequence of three 2ꢀcomponent reactions steps (2ꢁ2ꢁ2ꢀCR) whereby
compounds were obtained in overall 30ꢀ66% isolated yields. The functional group diversity was
established by synthesizing a 160ꢀmembered library.
INTRODUCTION
Isoxazolines have found extensive usage as pharmacophores in recent times. For example,
isoxazoline derivatives have been reported as calcium release activated calcium (CRAC) channel
modulators,1 macrophage migration inhibitory factor (MIF) antagonists,2 caspase inhibitors,3
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phophodiesteraseꢀ4 (PDEꢀ4) inhibitors,4 cell adhesion inhibitors,5 cystic fibrosis transmembrane
conductance regulator (CFTR) protein activators,6 factor Xa inhibitors,7 tubulin polymerization
inhibitors,8 inhibitors of matrix metalloproteinases and/or TNFꢀα converting enzyme,9 and DNA
methyltransferase 1 Inhibitors.10 Compounds containing isoxazoline moiety have been reported
as potential antimicrobial, analgesic and antiꢀstress agents.11 Isoxazolines are also useful
synthons in organic synthesis and have been used for the synthesis of isoxazoles, βꢀlactams, γꢀ
amino alcohols, and primary alcohols.12 Key intermediates in the synthesis of Elliott’s alcohol
and resmethrins, Monatin, a natural sweetener, and natural products involve isoxazoline.13
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In view of the wide ranging use of the isoxazoline moiety in organic and medicinal chemistry,
a high throughput (HT) diversity oriented synthesis of this key scaffold could be of significant
interest. We were particularly interested in a HT synthesis of 3,5ꢀdiarylꢀisoxazolineꢀ5ꢀ
carboxylate derivatives as we intended to use the carboxylate group for further derivatization.
Recently, synthesis of 72ꢀmembered isoxazolinoꢀβꢀketoamide library by a 2ꢁ3ꢀcomponent
reaction was reported.14 However, this method was not amenable to modifications required for
the synthesis of 3,5ꢀdiarylꢀisoxazolineꢀ5ꢀcarboxylate derivatives. We present here the synthesis
of 160ꢀmembered library of methyl 3,5ꢀdiarylꢀisoxazolineꢀ5ꢀcarboxylate pharmacophore by a
modified HT synthesis. This involves a oneꢀpot, sequential synthesis of an aryl oxime, a methyl
2ꢀarylacrylate, an arylnitrile oxide followed by 1,3ꢀdipolar cycloaddition between the methyl 2ꢀ
arylacrylate and the arylnitrile oxide.
RESULTS AND DISCUSSION
Synthesis of isoxazolines by 1,3ꢀdipolar cycloadditions between alkenes and a nitrile oxide is
very well known in organic synthesis.15 There are several reports of library syntheses of
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isoxazolines from alkenes and nitrile oxides in both solution phase and on solid phase.16
However, in all these examples the oxime and the nitrile oxide precursor were preꢀformed
separately before reacting with the alkene. As seen in scheme 1, the synthesis of methyl 3,5ꢀ
diarylꢀisoxazolineꢀ5ꢀcarboxylates can be considered as a composite of two sets of unrelated
reactions synthesizing methyl 2ꢀarylacrylate and arylnitrile oxide (in-situ) followed by
combining the products in the last step.
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Scheme 1. Retrosynthetic Analysis
The initial proofꢀofꢀconcept study focused on investigating the feasibility of achieving this
multicomponent reaction in oneꢀpot. Keeping in mind the scalability features and cost factors,
we preferred a solution phase approach. Individually these are all 2ꢀCR sequences. However, the
combination of four reactants in a oneꢀpot fashion can lead to undesired sideꢀproduct formation
and, consequently, lower or no isoxazoline yield. Thereby, selection of the right sequence of
addition, choice of solvent, base and additive would hold key to the success of this
transformation.
Our initial attempts centered on synthesizing the methyl 2ꢀ(2ꢀnitrophenyl)acrylate first
followed by the addition of 4ꢀfluorobenzaldehyde and hydroxyl amine in the same pot to
generate the oxime. The 1,3ꢀdipolar cycloaddition reaction could then be performed by addition
of bleach to generate the nitrile oxide in-situ.
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The oximes are normally synthesized in polar protic solvents in presence of an inorganic or
organic base.17 The synthesis of 2ꢀphenylacrylates generally uses K2CO3 as a base in presence of
a phaseꢀtransfer catalyst and some of the commonly used solvents are DMF, DMFꢀH2O and
toluene.18 We thus tried various combinations of EtOH, DMF and H2O for the oneꢀpot
methodology. In the first step, methyl 2ꢀ(2ꢀnitrophenyl)acrylate 4{1} was synthesized and 4ꢀ
fluorobenzaldehyde 1{1} and hydroxyl amine were added to the same pot after cooling the
reaction to rt (Scheme 2, sequence 1). The oxime 2{1} was formed by heating the resulting
mixture. In the final step, the nitrile oxide was generated in-situ by addition of bleach to the
chilled reaction mixture of oxime 2{1} (and alkene 4{1}) to accomplish the 1,3ꢀdipolar
cycloaddition reaction. However, the presence of desired isoxazoline 5{1,1} could not be
identified by LCMS of the reaction mixture. We reasoned that the solvent and choice of base
played a role in this oneꢀpot multicomponent transformation. Next we used toluene/EtOH as the
solvent mixture of choice. Though, satisfactory alkene 4{1} formation was observed by LCMS,
only a trace amount of product was seen by LCMS at the end of the completed sequence.
To identify the most suitable reaction condition, we further studied the individual steps as seen
in Scheme 2 (table for optimization of Stepꢀ1.1). Toluene as the solvent was found to result the
best isolated yield for both the steps. However, a small amount of product 5{1,1} (~10% isolated
yield) was obtained by using toluene as the only solvent (with 3 equiv of K2CO3 without any
further addition of base). Though it was possible to isolate the desired isoxazolines, the yield was
still unsatisfactory and formation of undesired side products was observed. Similar results were
obtained with other methyl 2ꢀarylacetates and aldehydes. On further probing, methyl 2ꢀ
arylacrylate 4 was found to be undergoing decomposition under the reaction conditions.
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As a next step, we evaluated the effect of reversal of the sequence of the reactions (Scheme 2,
sequence 2). 4ꢀFluorobenzaldehyde oxime 2{1} was synthesized first followed by the addition of
methyl 2ꢀphenylacetate and paraformaldehyde to form methyl 2ꢀ(2ꢀnitrophenyl)acrylate 4{1}.
The K2CO3 loading was reduced to ~1.5 equiv by using NaOAc as a base in the first step (oxime
2{1} formation, Scheme 2, table for optimization of Stepꢀ1.2). The final step was performed by
adding bleach to this reaction mixture as before without any further modification. Interestingly,
the corresponding isoxazoline 5{1,1} was obtained in 56% isolated yield after purification by
preparative HPLC.
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The general scheme for this multicomponent synthesis is shown in Scheme 3. To expand the
scope of this method, a diverse array of reactants was chosen (Figure 1). A total of 18
benzaldehydes were used. There was no significant effect on conversion with both electron
donating and electron withdrawing substitution on the aryl ring. The oꢀ, mꢀ or pꢀsubstitution did
not have any effect on the yield as well. The reaction was successful with isonicotinaldehyde
(19{1}) and 1ꢀnaphthaldehyde (20{1}) with yields comparable to the benzaldehydes. The
robustness of this HT synthesis was also established by using 8 methyl arylacetates. Under the
present reaction conditions a variety of substituents were found to be tolerated well. The isolated
yields were found to be in the range of 30ꢀ66% after purification of the crude products by
preparative HPLC (Table 1). Molecular weights ranged from 297 to 476 as a result of functional
group diversity.
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Scheme 2. Optimization of Reaction Sequences for the Synthesis of Methyl 3ꢀ(4ꢀfluorophenyl)ꢀ
5ꢀ(2ꢀnitrophenyl)ꢀ4,5ꢀdihydroisoxazoleꢀ5ꢀcarboxylate (5{1,1}) by a Oneꢀpot Multicomponent
(2ꢁ2ꢁ2ꢀComponent) Reaction
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Scheme 3. Optimized Conditions for Methyl 3,5ꢀdiarylꢀisoxazolineꢀ5ꢀcarboxylate Synthesis by a
Oneꢀpot Multicomponent (2ꢁ2ꢁ2ꢀComponent) Reaction
In conclusion, we have successfully synthesized a 160ꢀmembered library of methyl 3,5ꢀdiarylꢀ
isoxazolineꢀ5ꢀcarboxylates. By suitable selection of the reaction sequence, the synthesis could be
achieved in oneꢀpot through sequential 2ꢀCR steps. This 2ꢁ2ꢁ2ꢀCR variant of otherwise very
common synthesis methodology was unexplored before. A wide range of functional groups was
tolerated and the compounds were obtained in satisfactory overall yields and purity suitable for
biological activity screening.
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Figure 1. Variations on aromatic aldehydes (1) and methyl arylacetates (3) used.
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Table 1. Library of Methyl 3,5ꢀdiarylꢀisoxazolineꢀ5ꢀcarboxylates
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Entry
Aldehyde, 1
Methyl
arylacetate, 3
Methyl 3,5ꢀdiarylꢀ
isoxazolineꢀ5ꢀ
carboxylate, 5
% Yield
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1
2
{1}
{1}
{1}
{1}
{1}
{1}
{1}
{1}
{2}
{2}
{2}
{2}
{2}
{2}
{2}
{2}
{3}
{3}
{3}
{3}
{3}
{3}
{3}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
5{1,1}
5{1,2}
5{1,3}
5{1,4}
5{1,5}
5{1,6}
5{1,7}
5{1,8}
5{2,1}
5{2,2}
5{2,3}
5{2,4}
5{2,5}
5{2,6}
5{2,7}
5{2,8}
5{3,1}
5{3,2}
5{3,3}
5{3,4}
5{3,5}
5{3,6}
5{3,7}
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42
40
42
60
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40
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38
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{3}
{4}
{4}
{4}
{4}
{4}
{4}
{4}
{4}
{5}
{5}
{5}
{5}
{5}
{5}
{5}
{5}
{6}
{6}
{6}
{6}
{6}
{6}
{6}
{6}
{7}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
5{3,8}
5{4,1}
5{4,2}
5{4,3}
5{4,4}
5{4,5}
5{4,6}
5{4,7}
5{4,8}
5{5,1}
5{5,2}
5{5,3}
5{5,4}
5{5,5}
5{5,6}
5{5,7}
5{5,8}
5{6,1}
5{6,2}
5{6,3}
5{6,4}
5{6,5}
5{6,6}
5{6,7}
5{6,8}
5{7,1}
5{7,2}
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{7}
{7}
{7}
{7}
{7}
{7}
{8}
{8}
{8}
{8}
{8}
{8}
{8}
{8}
{9}
{9}
{9}
{9}
{9}
{9}
{9}
{9}
{10}
{10}
{10}
{10}
{10}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
5{7,3}
5{7,4}
5{7,5}
5{7,6}
5{7,7}
5{7,8}
5{8,1}
5{8,2}
5{8,3}
5{8,4}
5{8,5}
5{8,6}
5{8,7}
5{8,8}
5{9,1}
5{9,2}
5{9,3}
5{9,4}
5{9,5}
5{9,6}
5{9,7}
5{9,8}
5{10,1}
5{10,2}
5{10,3}
5{10,4}
5{10,5}
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{10}
{10}
{10}
{11}
{11}
{11}
{11}
{11}
{11}
{11}
{11}
{12}
{12}
{12}
{12}
{12}
{12}
{12}
{12}
{13}
{13}
{13}
{13}
{13}
{13}
{13}
{13}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
{7}
{8}
{1}
{2}
{3}
{4}
{5}
{6}
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EXPERIMENTAL PROCEDURES
General procedure for the one-pot reaction. A mixture of aromatic aldehyde (0.5 mmol),
hydroxylamine hydrochloride (0.6 mmol), and NaOAc (0.9 mmol) in toluene (2 mL) was stirred
o
at 95 C for 1 h. The reaction mixture was cooled to room temperature followed by the addition
of the ester (0.6 mmol), paraformaldehyde (1.7 mmol), potassium carbonate (0.8 mmol),
o
tetrabutylammonium iodide (0.06 mmol) to the same reaction pot and was heated at 95 C for 1
o
h. The reaction mixture was then cooled to 0 C and bleach (5 mL) was added under vigorous
stirring. It was then stirred at room temperature for an additional 0.5 h and allowed to settle. The
organic layer was separated and the residual aqueous layer was extracted with ethyl acetate (2 ×
5 mL). The combined organic layer was passed through a plug of anhyd Na2SO4 and
concentrated in a centrifugal evaporator (Genevac HTꢀ4X) to give the desired crude product.
This was purified by preparative HPLC (Column: Xbridge Prep C18, 19 × 250 mm; mobile
phase: 5 mM (NH4)2CO3: MeCN, 70/10 – 30/90; flow rate: 12 mL/min; run time: 10ꢀ20 min).
Methyl 3-(4-fluorophenyl)-4,5-dihydro-5-(2-nitrophenyl)isoxazole-5-carboxylate (5{1,1}).
1
Isolated yield: 56%. H NMR (400 MHz, CDCl3) δ 7.8.24 (d, J = 8.0 Hz, 1H), 8.08 (d, J = 8.0
Hz, 1H), 7.75 (t, J = 8.0 Hz, 1H), 7.65ꢀ7.69 (m, 2H), 7.57 (t, J = 8.4 Hz, 1H), 7.09 (t, J = 8.8 Hz,
2H), 4.78 (d, J = 17.6 Hz, 1H), 3.76 (s, 3H), 3.43 (d, J = 17.6 Hz, 1H); UPLCꢀMS (M++H):
345.01, 91% purity.
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AUTHOR INFORMATION
Corresponding Author
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*Eꢀmail: kdsarma@gmail.com
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ACKNOWLEDGMENT
We thank Dr. Sudhir Singh and Dr. Subir Basak for support. Analytical support from Ms. Richa
Jindal and Mr. Brijendra Thakur is gratefully acknowledged.
ASSOCIATED CONTENT
Supporting Information. 1H NMR spectra and UPLCꢀMS data of 45 representative compounds.
1H & 13C NMR and UPLCꢀMS traces of representative compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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