Convenient modular construction of medicinally important 5-acylamino-4,5-dihydroisoxazoles featuring four elements of diversity

Article abstract of DOI:10.1016/j.tetlet.2016.08.059

An efficient modular approach toward medicinally important 5-acylamino-4,5-isoxazolines via the 1,3-dipolar cycloaddition reaction of nitrile oxides and MCR-derived enamide building blocks is described. This approach results in isoxazolines containing four elements of diversity utilizing two practically simple synthetic operations.

Full text of DOI:10.1016/j.tetlet.2016.08.059

Accepted Manuscript
Convenient modular construction of medicinally important 5-acylamino-4,5-
dihydroisoxazoles featuring four elements of diversity
Alexandra Kulyashova, Mikhail Krasavin
PII:
S0040-4039(16)31086-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.08.059
TETL 48030
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 July 2016
14 August 2016
18 August 2016
Please cite this article as: Kulyashova, A., Krasavin, M., Convenient modular construction of medicinally important
5-acylamino-4,5-dihydroisoxazoles featuring four elements of diversity, Tetrahedron Letters (2016), doi: http://
dx.doi.org/10.1016/j.tetlet.2016.08.059
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Convenient modular construction of medicinally important 5-
acylamino-4,5-dihydroisoxazoles featuring four elements of diversity
Alexandra Kulyashova and Mikhail Krasavin*
Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034 Russia
* Corresponding author; phone: + 7 931 3617872, fax: +7 812 428 6939.
E-mail: m.krasavin@spbu.ru
Abstract: An efficient modular approach toward medicinally important 5-acylamino-4,5-
isoxazolines via the 1,3-dipolar cycloaddition reaction of nitrile oxides and MCR-derived
enamide building blocks is described. This approach results in isoxazolines containing four
elements of diversity utilizing two practically simple synthetic operations.
Keywords: enamide, nitrile oxide, 1,3-dipolar cycloaddition, modular synthesis,
multicomponent reactions, isoxazolines, privileged structures.
Synthetic methods that deliver, within a number of convergent steps, medicinally
important scaffolds while allowing for full and independent control over the peripheral moieties,
are of particular importance to medicinal chemistry research.1 Such methods enable the time-
and cost-efficient synthesis of lead compound analogs which is a pre-requisite to understanding
structure-activity relationships2 as well as offering a clear advantage over traditional linear
sequences which typically involve scaffold decoration with appendages in the last step.
Multicomponent chemistry provides a uniquely suitable toolbox for the development of
such methods. Indeed, the nature of the molecular scaffold is determined by the specific
multicomponent reaction (MCR) as well as various skeleton-defining post-MCR modifications,
while the periphery – by a specific selection of reagents for the synthetic array. Prominent MCRs
such as the Ugi3 or Castagnoli-Cushman4 reactions have a large enough scope to permit a wide
range of side-chain variations. Strategies based on sequential MCRs introduce even more
diversity elements which can be altered to fine-tune the compounds’ desired biological activity
and suppress side-effects.5

Isoxazolines certainly deserve to be considered as privileged cores6 for drug design,
taking into account the multitude of biological targets, including tyrosine phosphatase 1B,7
macrophage migration inhibitory factor (MIF),8 fatty acid amide hydrolase (FAAH)9 and M1
muscarinic acetylcholine receptors,10 which are perturbed by isoxazoline-based compounds.
Replacement of the carbohydrate portion of nucleosides with an isoxazoline core delivered
nucleoside analogs that inhibited viral reverse trascriptase.11-12 Furthermore, isoxazolines13-14 are
amide bond bioisosteres and, therefore, are useful in peptidomimetic design.15
One of the most popular ways to construct the isoxazoline core is the 1,3-dipolar
cycloaddition of nitrile oxides with a carbon-carbon double bond.16 The cycloaddition is
particularly facile for electron-rich olefins such as enamides, which can be prepared by a range
of methods.17 Notably, enamides 1 were discovered to be principal by-products in -lactam
syntheses via ketene-imine [2+2]-cycloaddition reactions involving -C-H imines. Presumably,
this process occurred via the reaction of imines with an acyl chloride from which a ketene is
generated (Scheme 1A).18-21 Recently, we observed a similar formation of enamides 2 when
attempting a Castagnoli-Cushman reaction of -C-H imines with dicarboxylic acid anhydrides22
which are also strong acylating agents (Scheme 1B). Upon reviewing the literature, it became
apparent that the reaction of acyl chlorides with -C-H imines in the presence of a tertiary amine
HCl scavenger has gained prominence as a three-component synthesis of enamides from
aldehyde precursors, with the interim formation of the respective imine intermediates.23-27
Scheme 1. Formation of enamides 1 and 2 in reactions involving -C-H imines and the use of
acylating agents.

To our surprise, however, literature regarding the construction of isoxazoles via 1,3-
dipolar cycloaddition mostly described the use of readily available N-vinylpyrrolidone8, 10, 28 or
N-vinylphthalimide,8 except for an isolated report describing the use of a cyclic enamide
synthon29 and a recent example involving enamides akin to 1, generated via a base-promoted
isomerization of N-allylamides.30 Thus, it appeared that a modular approach (Fig. 1) to
constructing isoxazolines from enamide building blocks with three independently variable
elements of diversity has not been investigated in connection with said three-component
approach. Herein, we report recent results from our laboratories that fill this void.
Figure 1. Modular approach to constructing 5-acylaminoisoxazolines from MCR-derived
enamides envisioned and realized in present study.
Five exemplary enamides 3a-e were prepared31 from their respective aldehydes and
primary amines via the intermediate formation of an imine intermediate using MgSO4 as a
dehydrating agent. The crude imine was treated with acyl chlorides in the presence of
triethylamine (MgSO4 was added to suppress possible enamide hydrolysis by adventitious
1
water). Formation of 3a-e was complete within 2-18 hours according to the H NMR
spectroscopic analysis of reaction mixture aliquots (Scheme 2).
Scheme 2. Three-component synthesis of enamides 3a-e.
Table 1. Enamides 3a-e prepared via a three-component approach.
Compound
R1
R2
R3
Isolated yield 3
(%)
42
H
Bn
Bn
Ph
Ph
3a
3b
Me
39

H
H
H
4-MeC6H4CH2
MeO(CH2)3
3-PyCH2
4-O2NC6H4
4-MeC6H4
Ph
48
46
41
3c
3d
3e
1
While the crude enamides were at least 85% pure by H NMR spectroscopy, attempted
use of this material in subsequent cycloaddition reactions unexpectedly delivered complex
mixture of products. Therefore, enamides 3a-e were purified by chromatography using eluents
containing triethylamine to neutralize the slightly acidic silica gel (ESI). Despite this precaution,
the isolated yields of 3a-e were somewhat modest (Table 1).
Nitrile oxides were generated in situ, from a set of known aldoximes 4 upon treatment
with N-chlorosuccinimide and subsequent Et3N-promoted elimination of HCl, in the presence of
enamides 3 (Scheme 3).32
Scheme 3. Preparation of 5-acylamino-4,5-isoxazoles 5a-l via 1,3-dipolar cycloaddition of nitrile
oxides (generated in situ from oximes 4a-e) and enamides 3a-e.
Table 2. 5-Acylamino-4,5-isoxazolines 5a-l.
Compound
R1
R2
R3
R4
Isolated yield 5
(%)
44
65
74
44
23
69
75
40
49
70
H
H
Bn
Bn
Ph
Et
5a
5b
5c
5d
5e
5f
Ph
4-MeC6H4
4-MeOC6H4
2-thienyl
4-MeC6H4
Et
H
Bn
Ph
Ph
H
Bn
Me
H
Bn
Ph
4-MeC6H4CH2
4-MeC6H4CH2
MeO(CH2)3
3-PyCH2
3-PyCH2
4-O2NC6H4
4-O2NC6H4
4-MeC6H4
Ph
H
4-MeOC6H4
Et
5g
5h
5i
H
H
4-MeC6H4
4-MeOC6H4
H
Ph
5j

H
H
3-PyCH2
3-PyCH2
Ph
Ph
3-(PhO)C6H4
2-thienyl
52
35
5k
5l
Continuing the reaction for 2-18 h (depending on the substitution pattern in 3 and 4, ESI)
led to the formation of a single major product and a number of minor by-products.
Chromatographic isolation of the major product provided the desired isoxazolines 5a-l in modest
1
13
to good yields (Table 2). The identity of these compounds was established by H and C NMR
spectroscopy as well as high-resolution mass-spectrometry data (ESI). It should be noted that,
owing to the concerted nature of the 1,3-dipolar cycloaddition process, compound 5e was
obtained as a single cis-isomer.
In summary, we have described an efficient modular approach toward medicinally
important 5-acylamino-4,5-isoxazolines employing the 1,3-dipolar cycloaddition reaction of
nitrile oxides with MCR-derived enamide building blocks, resulting in the target compounds
containing four elements of diversity, utilizing two practically simple synthetic operations. This
finding will facilitate the medicinal chemistry optimization of bioactive chemical series based on
the 5-acylamino-4,5-isoxazoline scaffolds as it permits the independent variation of each
peripheral group to explore structure-activity relationships.
Acknowledgement
We gratefully acknowledge support from the Russian Scientific Fund (Project Grant 14-50-
00069). NMR and mass spectrometry studies were performed at the Research Centre for
Magnetic Resonance, the Centre for Chemical Analysis and Materials Research of Research park
of Saint Petersburg State University.
Supplementary data
The supplementary data containing NMR spectra of the reaction products is available on
http://www.sciencedirect.com.
References and notes

1.
2.
3.
Brown, D. G.; Boström, J. J. Med. Chem. 2016, 59, 4443-4458.
Jorgensen, W. L. Acc. Chem. Res. 2009, 41, 724-733.
Ivachtchenko, A. V.; Ivanenkov, Ya. A.; Kysil, V. M.; Krasavin, M.; Ilyin, A. P. Russ.
Chem. Rev. 2010, 79, 787-817.
4.
5.
6.
Krasavin, M.; Dar'in, D. Tetrahedron Lett. 2016, 57, 1635-1640.
Hann, M. M.; Keseru, G. M. Nat. Rev. Drug Discov. 2012, 11, 355-365.
Welsch, M. E.; Snyder, S. A.; Stockwell, B. R. Curr. Opin. Chem. Biol. 2010, 14, 347-
361.
7.
8.
Ahmad, G.; Mishra, P. K.; Gupta, P.; Yadav, P. P.; Tiwari, P.; Tamrakar, A. K.;
Srivastava, A. K.; Maurya, R. Bioorg. Med. Chem. Lett. 2006, 16, 2139-2143.
Balachandran, A.; Gadekar, P. K.; Parkale, S.; Yadav, V. N.; Kamath, D.; Ramaswamy,
S.; Sharma, S.; Vishwakarma, R. A.; Dagia, N. M. Bioorg. Med. Chem. Lett. 2011, 21,
1508-1511.
9.
Dung, M. H.; Pasteris, R. J. PCT Int. Appl. WO2011072207A1; Chem. Abstr. 2011, 155,
41003.
10.
11.
12.
13.
14.
Huang, M.; Suk, D.-H.; Cho, N.-C.; Bhattarai, D.; Kang, S. B.; Kim, Y.; Pae, A. N.;
Rhim, H.; Keum, G. Bioorg. Med. Chem. Lett. 2015, 25, 1546-1551.
Xiang, Y.; Chen, J.; Schinazi, R. F.; Zhao, K. Bioorg. Med. Chem. Lett. 1996, 6, 1051-
1054.
Romeo, G.; Iannazzo, D.; Piperno, A.; Romeo, R.; Saglimbeni, M.; Chiacchio, M. A.;
Balestrieri, E.; Macchi, B.; Mastino, A. Bioorg. Med. Chem. 2006, 14, 3818-3824.
Lee, J. Y.; Chung, Y. J.; Bae, Y.-S.; Ryu, S. H.; Kim, B. H. J. Chem. Soc., Perkin Trans.
1 1998, 359-366.
Xiao, D.; Zhu, X.; Yu, Y.; Shao, N.; Wu, J.; McCormick, K. D.; Dhondi, P.; Qin, J.;
Mazzola, R.; Tang, H.; Rao, A.; Siliphaivanh, P.; Qiu, H.; Yang, X.; Rivelli, M.; Garlisi,
C. G.; Eckel, S; Mukhopadhyay, G.; Correll, C.; Rindgen, D.; Aslanian, R.; Palani, A.
Bioorg. Med. Chem. Lett. 2014, 24, 1615-1620.
15.
16.
17.
18.
Giannis, A.; Rubsam, F. Adv. Drug Res. 1997, 29, 1-78.
Frump, J. A. Chem. Rev. 1971, 71, 483-505.
Bernadat, G.; Masson, G. Synlett 2014, 25, 2842-2867.
Burgemeister, T.; Dannhardt, G.; Mach-Bindl, M.; Noeth, H. Chem.-Ztg. 1988, 112, 93-
98.
19.
Wagle, D. R.; Garai, C.; Chiang, J.; Monteleone, M. G.; Kurys, B. E.; Strohmeyer, T. W.;
Hegde, V. R.; Manhas, M. S.; Bose, A. K. J. Org. Chem. 1988, 53, 4227-4236.
D’hooghe, M.; Dejaegher, Y.; De Kimpe, N. Tetrahedron 2008, 64, 4575-4584.
Mollet, K.; D’hooghe, M.; De Kimpe, N. Tetrahedron 2012, 68, 10787-10793.
Dar'in, D.; Bakulina, O.; Chizhova, M.; Krasavin, M. Org. Lett. 2015, 17, 3930-3933.
Cook, G. R.; Barta, N. S.; Stille, J. R. J. Org. Chem. 1992, 57, 461-467.
Padwa, A.; Heidelbaugh, T. M.; Kuethe, J. T.; McClure, M. S.; Wang, Q. J. Org. Chem.
2002, 67, 5928-5937.
20.
21.
22.
23.
24.
25.
26.
Lesniak, S.; Pasternak, B. Synth. Commun. 2002, 32, 875-880.
Dekeukeleire, S.; D’hooghe, M.; Müller, C.; Vogt, D.; De Kimpe, N. New J. Chem. 2010,
34, 1079-1083.
27.
28.
Shinkevich, E.; Deblander, J.; Matthijg, S.; Jacobs, J.; De Kimpe, N.; Tehrani, K. A. Org.
Biomol. Chem. 2011, 9, 538-548.
Gong, Y.; Wang, Y.; Zhao, W.-T.; Tang, X.-Y. J. Chem. Res. 2013, 37, 499-502.

29.
30.
31.
De Almeida, V. M.; dos Santos, R. J.; da Silva Goes, A. J.; de Lima, J. G.; Correia, C. R.
D.; de Faria, A. R. Tetrahedron Lett. 2009, 50, 684-687.
Bujak, P.; Krompiec, S.; Malarz, J.; Krompiec, M.; Filapek, M.; Danikiewicz, W.; Kania,
M.; Gebarowska, K.; Grudzka, I. Tetrahedron 2010, 66, 5972-5981.
General procedure for the preparation of compounds 3a-e: A mixture of amine (10.0
mmol) and MgSO4 (1.0 g) in CH2Cl2 (100 mL) was stirred at 0 °C for 5 min. Aldehyde
(20.0 mmol) was added and stirring continued for 2–18 h with occasional monitoring of
the reaction progress by 1H NMR. Upon reaction completion, MgSO4 was filtered off and
the filtrate concentrated under reduced pressure to afford the crude imine. This was
dissolved in CH2Cl2 (100 mL) or toluene (100 mL), then Et3N (10.0 mmol) and MgSO4
(1.0 g) were added and the mixture cooled to 0 °C. After stirring briefly (5 min), the acyl
chloride (5.00-20.0 mmol, ESI) was added, the reaction allowed to reach ambient
temperature and stirring continued for 2 h. The resulting precipitate was filtered off and
the filtrate concentrated under reduced pressure. The residue was purified by column
chromatography on silica using an appropriate mixture of petroleum ether/ethyl
acetate/TEA (ESI) as eluent to afford the desired enamides 3a-e.
32.
General procedure for the preparation of compounds 5a-l: NCS (220 mg, 1.65 mmol or
300 mg, 2.25 mmol, ESI) was added in one portion to a solution of the aldoxime (1.10
mmol or 1.50 mmol, ESI) in toluene or DCM (15 mL) at r. t. and the reaction mixture
stirred for 2–18 h with occasional monitoring of the reaction progress by TLC. The
succinimide precipitate was filtered off and a solution of enamide (1 mmol) in toluene or
DCM (15 mL) was added over 5 min at 0 °C. A solution of Et3N (1.50 mmol) in toluene
(0.8 mL) was then added dropwise and the mixture stirred at r. t. for 2–18 h. The resulting
precipitate was filtered off and the filtrate concentrated under reduced pressure. The
residue was purified by column chromatography on silica using an appropriate mixture of
petroleum ether/ethyl acetate as eluent to afford isoxazolines 5a-l.

Convenient modular construction of medicinally important 5-
acylamino-4,5-dihydroisoxazoles featuring four elements of diversity
Alexandra Kulyashova and Mikhail Krasavin*

Highlights



5-acylaminoisoxazolines with four diversity elements were prepared
Enamide substrates were prepared via a multicomponent approach
Method is particularly useful for medicinal chemistry optimization