An efficient synthesis of highly functionalized 4H-pyrano[3,2- d ]isoxazoles via isocyanide-based three-component reaction

Article abstract of DOI:10.1055/s-0029-1220072

We present a novel, convenient, and efficient method for synthesizing polysubstituted 4H-pyrano[3,2-d]isoxazoles based on a three-component reaction. The zwitterions generated from the reaction of isocyanides and dialkyl acetylenedicarboxylates are reacted with 3-phenylisoxazol-4-(5H)-one to produce the title compounds in good yield.

Full text of DOI:10.1055/s-0029-1220072

LETTER
1477
An Efficient Synthesis of Highly Functionalized 4H-Pyrano[3,2-d]isoxazoles
via Isocyanide-Based Three-Component Reaction
H
ighly Functional
b
d
4
H
-pyra
b
no[3,2-
d
]iso
a
xazoles s Ali Esmaeili,*a Rahele Hosseinabadi,a Azizollah Habibib
a
Chemistry Department, University of Birjand, P.O. Box 97175/615, Birjand, Iran
Fax +98(561)2502009; E-mail: aa_esmaeili@yahoo.com
b
Faculty of Chemistry, University of Tarbiatmoallem, 15719-14911 Tehran, Iran
Received 23 March 2010
Recent methods for the synthesis of pyranoisoxazoles in-
clude cycloaddition between alkynes and nitrile ox-
ides,20,21 1,3-dipolar cycloaddition of acetonitrile oxide to
an a,b-unsaturated enone,22 intramolecular 1,3-dipolar cy-
Abstract: We present a novel, convenient, and efficient method for
synthesizing polysubstituted 4H-pyrano[3,2-d]isoxazoles based on
a three-component reaction. The zwitterions generated from the
reaction of isocyanides and dialkyl acetylenedicarboxylates are
reacted with 3-phenylisoxazol-4-(5H)-one to produce the title com- cloaddition of nitrones or nitrile oxides23 and treatment of
pounds in good yield.
2-aryl-substituted 1-nitro-3-oxa-6-heptynes with BuLi
followed by treatment with acetic anhydride.24 Due to our
Key words: 4H-pyrano[3,2-d]isoxazole, isocyanides, 3-phenyl-
isoxazol-4-(5H)-one, three-component reactions, acetylenic esters
interest in heterocyclic compounds with potential biolog-
ical activity,25,26 we herein describe a novel synthesis of
highly substituted 4H-pyrano[3,2-d]isoxazoles 4, by the
reaction of an isocyanide 1 with a dialkyl acetylenedicar-
boxylate (DAAD, 2), and 3-phenylisoxazol-4-(5H)-one
(3)27 via three-component reaction in good yields
(Table 1).
Multicomponent reactions (MCR),1,2 followed by post-
condensation modifications via several ring-forming reac-
tions, are attractive due to their simple experimental
procedures, one-pot character, and synthetic efficiency.
Devising reactions that form several bonds in one step is
an important area in modern organic synthesis.3 Of pivotal
importance in this area are the isocyanide-based MCR
such as the versatile Ugi and Passerini reactions.4,5 Isocy-
anide-based MCR, leading to interesting heterocyclic
scaffolds, are particularly useful for the construction of di-
verse chemical libraries of druglike molecules.2a
Table 1 Synthesis of 4H-Pyrano[3,2-d]isoxazole Derivatives
CO2R2
CO2R2
+
Ph
N
CO2R2
Ph
MeCN
reflux
R1
N
C: +
O
N
O
O
CO2R2
O
NH
R1
1
2
3
4
One class of hetereocycles, the isoxazoles, has received
considerable attention owing to their wide range of appli-
cations, including use as organic electrolytes in nonaque-
ous batteries, photographic emulsions, and fiber dyes.6,7
Furthermore, they are valuable synthetic intermediates
due to the ease of reductive cleavage and ring transforma-
tions.8,9 Interest in pyrano-fused isoxazole derivatives
stems from the presence of these heterocyclic systems bi-
ologically active compounds.10 Consequently, there is an
ongoing interest in the synthesis of pyrano isoxazoles.
Entry
R1
R2
Product
4a
Yield (%)a
1
2
3
4
5
6
7
8
c-Hex
Me
Et
82
79
77
61
57
81
80
62
c-Hex
c-Hex
c-Hex
c-Hex
t-Bu
4b
i-PrOH
4c
neopentyl
t-Bu
4d
4e
Me
4f
One of the earliest approaches to the synthesis of these
compounds was reported by Marcus et al. in 1967, react-
ing diketene with oximes and 1,4-diazabicyclo[2,2,2]-
octane to produce pyrano[3,4-d]isoxazol-4-one.11 In con-
tinuation of the work of Marcus, Chantegrel, and co-
workers used pyrano[4,3-b]pyrans with an excess of hy-
droxylamine to synthesize pyranoisoxazoles.12 Pyra-
no[3,4-c]isoxazoles and related compounds have also
been prepared by addition of unsaturated alkoxide anions
to simple nitroolefines followed by intramolecular nitrile
oxide or silyl nitronate cycloadditions.13–19
t-Bu
Et
4g
t-Bu
neopentyl
4h
a Isolated yield.
To achieve suitable conditions for the above transforma-
tion, in our initial study, the reaction of cyclohexyl isocy-
anide, dimethyl acetylenedicarboxylate (DMAD), and 3-
phenylisoxazol-4-(5H)-one was used as a model to opti-
mize the reaction conditions. We examined the reaction in
various solvents at different temperatures to increase the
product yields and to reduce the reaction time (Table 2).
We subsequently used various dialkyl acetylenedicarbox-
ylates and isocyanides with 3-phenylisoxazol-4-(5H)-one
SYNLETT 2010, No. 10, pp 1477–1480
x
x
.x
x
.2
0
1
0
Advanced online publication: 26.05.2010
DOI: 10.1055/s-0029-1220072; Art ID: D07610ST
© Georg Thieme Verlag Stuttgart · New York

1478
A. A. Esmaeili et al.
LETTER
in MeCN under reflux conditions, leading to the forma-
tion of the corresponding 4H-pyrano[3,2-d]isoxazole de-
rivatives 4a–h, in good yields (Table 1). All reactions
proceed to completion within three hours. The structures
of the products were deduced from their IR, 1H NMR, and
13C NMR spectra. The mass spectra of these compounds
displayed molecular ion peaks at the appropriate m/z val-
ues. For example, the 1H NMR spectrum of 4a contained
multiplet signals for the cyclohexyl ring (d = 1.33–2.02
ppm), two singlets for two methoxy groups (d = 3.42 and
3.71 ppm), multiplets for NCH of the cyclohexyl ring
(d = 3.84 ppm), the allylic methine (d = 4.79 ppm), and
five aryl protons (d = 7.47, 7.78 ppm). An exchangeable
doublet for the NH group (d = 8.94 ppm) was also ob-
served. The chemical shift of NH group indicates that it
participates in an intramolecular hydrogen bond with the
vicinal carbonyl group. The 13C NMR spectrum of 4a
showed 20 distinct resonances in agreement with the pro-
posed structure, partial assignment of these resonances is
given in the experiment section. Characteristic 13C NMR
signals were due to two ester carbonyls at d = 169.45 and
172.59 ppm. The IR spectrum of 4a showed strong ab-
sorptions at 1750 and 1740 cm–1 owing to the carbonyls
and the amino group at 3250 cm–1 as a weak broad band.
The 1H decoupled 13C NMR spectra of 4b–h are similar to
those of 4a except for the R1 or R2 groups, which exhibit
characteristic signals with appropriate chemical shifts.
CO2R2
R1
N
3
R1
N
C
CO2R2
CO2R2
+
CO2R2
2
1
5
Ph
CO2R2
H
CO2R2
CO2R2
CO2R2
Ph
N
C
O
N
C
N
O
R1
N
O
O
6
7
8
R1
CO2R2
Ph
CO2R2
N
O
O
NH
R1
4
Scheme 1 Proposed mechanism
In summary, we have developed a new efficient one-pot
synthesis of 4H-pyrano[3,2-d]isoxazoles 4 from 3-phe-
nylisoxazol-4-(5H)-one (3), isocyanides 1, and dialkyl
acetylenedicarboxylates 2, via a three-component, one-
pot reaction without any specific modifications. Good
yields of the products, the readily availability of the start-
ing materials, and experimental simplicity are the main
advantages of this method. To the best of our knowledge,
this method represents the first example of an efficient,
three-component method for the synthesis of 4H-pyra-
no[3,2-d]isoxazole derivatives 4a–h. We believe this ap-
proach will be of value to those requiring access to novel
synthetic of pyrano[3,2-d]isoxazoles.
Table 2 Optimization Conditions for the Synthesis of 4a
Entry
Solvent
MeCN
nitrobenzene
benzene
DMF
Temp (°C)
Yield (%)
1
2
82
80
80
80
66
80
79
110
56
40
82
71
63
20
31
12
40
45
55
50
Dimethyl 6-(Cyclohexylamino)-3-phenyl-4H-pyrano[3,2-
d]isoxazole-4,5-dicarboxylate (4a); Typical Procedure
3
4
To a magnetically stirred solution of 3-phenylisoxazol-4-(5H)-one
(0.16 g, 1 mmol) and dimethyl acetylenedicarboxylate (0.14 g, 1
mmol) in MeCN (5 mL) was added dropwise a solution of cyclo-
hexyl isocyanide (0.11 g, 1 mmol) in MeCN (1 mL) at r.t. over 10
min. The reaction was then refluxed for 3 h. The solvent was re-
moved under reduced pressure, and the residue was purified by sil-
ica gel (Merck 230–240 mesh) column chromatography using a
short column eluting with n-hexane–EtOAc–Et3N (75:5:1) to yield
5
THF
6
DMSO
EtOH
7
8
toluene
acetone
CH2Cl2
4a as a white solid (0.34 g, 82%); mp 122–124 °C. IR (KBr): nmax
=
9
3250, 1750, 1740 cm–1. MS: m/z (%) = 412 (5) [M+], 353 (98), 271
(60), 239 (100), 105 (60), 77 (30), 55(45). 1H NMR (300.13 MHz,
CDCl3): d = 1.25–2.02 (10 H, m, 5 CH2 of cyclohexyl), 3.42 (3 H,
s, OCH3), 3.71 (3 H, s, OCH3), 3.84 (1 H, m, CHNH), 4.79 (1 H, s,
CHCO2CH3), 7.47 (3 H, m, arom.), 7.78 (2 H, m, arom.), 8.94 (1 H,
d, 3JHH = 7.8 Hz, CHNH) ppm. 13C NMR (75.47 MHz, CDCl3): d =
24.3, 24.34, 25.28, 33.42, 33.69 (5 CH2 of cyclohexyl), 37.4
(CHCO2CH3), 50.82, 51.19 (2 OCH3), 52.08 (CHNH), 71.69,
87.32, 127.72, 128.21, 128.50, 130.19, 159.03, 161.29, 164.64 (C
alkene, C Ar), 169.45, 172.59 (2 C=O) ppm. Anal. Calcd (%) for
C22H24N2O6: C, 64.07; H, 5.87; N, 6.79. Found: C, 63.67; H, 5.91;
N, 6.75.
10
Although the mechanism of this reaction has not been es-
tablished experimentally, a possible pathway is proposed
in Scheme 1. Mechanistically, it is conceivable that the re-
action involves the initial formation of a 1:1 zwitterionic
intermediate 5 between the isocyanide and DAAD proto-
nation of 5 by 3 and subsequent attack of the resulting nu-
cleophile generated 6, to the positively charged ion 7
afforded ketenimine 8 (Scheme 1). Such an addition prod-
uct may tautomerize and cyclize, under the reaction con-
ditions employed, to produce 4.
Diisopropyl-6-(cyclohexylamino)-3-phenyl-4H-pyrano[3,2-
d]isoxazole-4,5-dicarboxylate (4c)
White powder (0.36 g, 77%), mp 102–104 °C. IR (KBr): nmax
=
3250, 1750, 1740 cm–1. MS: m/z (%) = 468 (5) [M+], 381 (100), 339
Synlett 2010, No. 10, 1477–1480 © Thieme Stuttgart · New York

LETTER
Highly Functionalized 4H-pyrano[3,2-d]isoxazoles
1479
(28), 257 (40), 239 (50), 105 (40). 1H NMR (300.13 MHz, CDCl3):
d = 0.58, 1.03. 1.19, 1.28 [12 H, 4 d, 3JHH = 6.2 Hz OCH(CH3)2,],
1.29–2.02 (10 H, m, 5 CH2 of cyclohexyl), 3.78 (1 H, br s, CHNH),
MHz, CDCl3): d = 0.66. 0.95 [18 H, 2s, 2 CH2C(CH3)3], 1.48 [9 H,
s, NC(CH3)3], 3.61 [2 H, AB system, CHAHBC(CH3)3,2JHH = 10.5
Hz], 3.80 [2 H, AB system, CHAHBC(CH3)3,2JHH = 10.5 Hz], 4.85
[1 H, s, CHCO2CH2C(CH3)3], 7.45–7.48 (3 H, m, arom.), 7.85–7.89
(2 H, m, arom.), 9.12 (1 H, s, CHNH) ppm. 13C NMR (75.47 MHz,
CDCl3): d = 26.04, 26.54 [2 CH2C(CH3)3], 30.44 [NC(CH3)3],
31.04, 31.42 [2 CH2C(CH3)3], 37.74 [CHCO2CH2C(CH3)3], 73.58,
74.59 [2 OCH2C(CH3)3], 73.41, 87.71, 127.62, 128.54, 128.88,
130.36, 160.7, 161.02, 164.79 (C alkene, C Ar), 169.25, 171.79 (2
C=O) ppm. Anal. Calcd (%) for C28H38N2O6: C, 67.45; H, 7.68; N,
5.62. Found: C, 66.79; H, 7.95; N, 5.47.
3
4.70 [1 H, sept, JHH = 6.2 Hz OCH(CH3)2], 4.72 [1 H, s, CHC
O2CH(CH3)2], 5.05 [sept, 3JHH = 6.2 Hz, OCH(CH3)2], 7.47 (3 H, m,
arom.), 7.78 (2 H, m, arom.), 8.96 (1 H, d, 3JHH = 7.8 Hz, CHNH)
ppm. 13C NMR (75.47 MHz, CDCl3): d = 20.58, 21.55, 21.90, 22.24
(4 CH3), 24.43, 25.37, 33.53, 33.81, 37.67 (5 CH2 of cyclohexyl),
37.7 [CHCO2CH(CH3)2], 50.22 (CHNH), 67.17, 68.56 [2
OCH(CH3)2], 72.17, 87.16, 128.16, 128.49, 128.67, 130.14, 158.97,
161.39, 164.79 (C alkene, C Ar), 168.8, 171.9 (2 C=O) ppm. Anal.
Calcd (%) for C26H32N2O6: C, 66.65; H, 6.88; N, 5.98. Found: C,
66.56; H, 6.93; N, 5.96.
Acknowledgment
Di-tert-butyl-6-(cyclohexylamino)-3-phenyl-4H-pyrano[3,2-
d]isoxazole-4,5-dicarboxylate (4e)
We gratefully acknowledge financial support from the Research
Council of the University of Birjand.
White powder (0.28 g, 57%), mp 134–135 °C. IR (KBr): nmax
=
3250, 1750, 1740 cm–1. MS: m/z (%) = 496 (5) [M+], 409 (100), 257
1
(24), 105 (25), 77 (5) cm–1. H NMR (300.13 MHz, CDCl3): d =
References
1.10, 1.50 [18 H, 2 s, 2 OC(CH3)3], 1.26–2.00 (10 H, m, 5 CH2 of
cyclohexyl), 3.76 (1 H, br s, CHNH), 4.63 [1 H, s, CHCO2C(CH3)3],
7.48 (3 H, t, m, arom.), 7.81 (2 H, m, arom.), 8.87 (1 H, d, CHNH,
3JHH = 7.8 Hz) ppm. 13C NMR (75.47 MHz, CDCl3): d = 24.65,
25.42 (2 CH2 of cyclohexyl), 27.48, 28.51 [2 OC(CH3)3], 33.73,
33.97, 38.74 (3 CH2 of cyclohexyl), 38.79 [CHCO2C(CH3)3], 50.33
(CHNH), 73.39 (C alkene), 80.08, 81.03 [2 OC(CH3)3], 87.58,
128.01, 128.63, 128.96, 130.09, 158.85, 161.28, 164.99 (C alkene,
C Ar), 168.92, 171.89 (2 C=O) ppm. Anal. Calcd (%) for
C28H36N2O6: C, 67.72; H, 7.31; N, 5.64. Found: C, 67.35; H, 7.37;
N, 5.59.
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azole-4,5-dicarboxylate (4f)
White powder (0.31 g, 81%), mp 124–126 °C. IR (KBr): nmax
=
3250, 1750, 1740 cm–1. MS: m/z (%) = 386 (5) [M+], 328 (20), 327
(72), 271 (83), 239 (100), 105 (25), 77 (18). 1H NMR (300.13 MHz,
CDCl3): d = 1.48 [9 H, s, C(CH3)3], 3.42, 3.70 (6 H, 2 s, 2 OCH3)
4.79 (1 H, s, CHCO2CH3), 7.46 (3 H, m, arom.), 7.77 (2 H, m,
arom.), 9.09 (1 H, s, CHNH) ppm. 13C NMR (75.47 MHz, CDCl3):
d = 30.44 [C(CH3)3], 37.44 (CHCO2CH3), 51.38 [C(CH3)3], 52.15,
53.57 (2 OCH3), 72.56, 87.34, 127.78, 128.29, 128.58, 130.26,
160.39, 161.38, 164.41 (C alkene, C Ar), 169.61, 172.58 (2 C=O).
Anal. Calcd (%) for C20H22N2O6: C, 62.17; H, 5.74; N, 7.25. Found:
C, 61.66; H, 5.75; N, 7.19.
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Diethyl-6-(tert-butylamino)-3-phenyl-4H-pyrano[3,2-d]isox-
azole-4,5-dicarboxylate (4g)
White powder (0.33 g, 80%), mp 109–111 °C. IR (KBr): nmax
=
3250, 1750, 1740 cm–1. MS: m/z (%) = 414 (5) [M+], 342 (28), 341
(100), 285 (92), 239 (73), 105 (30), 77 (15). 1H NMR (300.13 MHz,
3
CDCl3): d = 0.86 (3 H, t, JHH = 7.1 Hz, OCH2CH3), 1.24 (3 H, t,
3JHH = 7.1 Hz, OCH2CH3), 1.48 (9 H, s, NC(CH3)3], 3.85 (m, 2 H,
OCH2CH3), 4.15 (m,
2 H, OCH2CH3), 4.76 (1 H, s,
CHCO2CH2CH3), 7.47 (3 H, m, arom.), 7.77 (2 H, m, arom.), 9.10
(1 H, s, CHNH) ppm. 13C NMR (75.47 MHz, CDCl3): d = 13.56,
14.39 (2 OCH2CH3), 30.45 [C(CH3)3], 37.54 [CHCO2C(CH3)3],
53.45 [C(CH3)3], 59.95, 60.99 (2 CO2CH2CH3), 72.71, 87.22,
127.98, 128.47, 128.53, 130.19, 160.27, 161.47, 164.41 (C alkene,
C Ar), 169.26, 172.42 (2 C=O) ppm. Anal. Calcd (%) for
C22H26N2O6: C, 63.76; H, 6.32; N, 6.76. Found: C, 63.24; H, 6.41;
N, 7.23.
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Dineopentyl-6-(tert-butylamino)-3-phenyl-4H-pyrano[3,2-
d]isoxazole-4,5-dicarboxylate (4h)
White powder (0.31 g, 62%), mp 138–139 °C. IR (KBr): nmax
=
3250, 1750, 1740 cm–1. MS: m/z (%) = 498 (5) [M+], 382 (30), 383
1
(100), 327 (48), 257 (47), 239 (34), 105 (28). H NMR (300.13
Synlett 2010, No. 10, 1477–1480 © Thieme Stuttgart · New York

1480
A. A. Esmaeili et al.
LETTER
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