Nitrone cycloaddition to quinones: A novel strategy for the synthesis of benzisoxazolidenes

Article abstract of DOI:10.1002/jhet.255

(Chemical Equation Presented) 1,3-Dipolar cycloaddition reaction involving nitrones and benzoquinones resulting in the formation of benzisoxazolidene is described. As the nitrone is selectively added to carbon-carbon double bond of the benzoquinone, the quinone-nitrone reaction is considered as a special case among quinone-1,3-dipole cycloaddition reactions. Molecular orbital calculation was performed to examine the electronic effects involved in the regioselectivity of the reaction.

Full text of DOI:10.1002/jhet.255

396
Nitrone Cycloaddition to Quinones: A Novel Strategy for the
Synthesis of Benzisoxazolidenes
Vol 47
Rony Rajan Paul,a Vimal Varghese,a P. B. Beneesh,a C. R. Sinu,a E. Suresh,b
and E. R. Anabhaa*
aOrganic Chemistry section, National Institute for Interdisciplinary Science and Technology
(formerly Regional Research Laboratory), Thiruvananthapuram 695019, India
bAnalytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar
364002, India
*E-mail: anabhaer@rediffmail.com
Received January 12, 2009
DOI 10.1002/jhet.255
Published online 4 March 2010 in Wiley InterScience (www.interscience.wiley.com).
1,3-Dipolar cycloaddition reaction involving nitrones and benzoquinones resulting in the formation of
benzisoxazolidene is described. As the nitrone is selectively added to carbon–carbon double bond of the
benzoquinone, the quinone-nitrone reaction is considered as a special case among quinone-1,3-dipole
cycloaddition reactions. Molecular orbital calculation was performed to examine the electronic effects
involved in the regioselectivity of the reaction.
J. Heterocyclic Chem., 47, 396 (2010).
INTRODUCTION
The quinones and their derivatives have attracted the
continuous attention in view of their antitumor activi-
tites [5]. The biological processes involved with the
antitumor activities of quinones are DNA intercalation,
bioreductive alkylation of biomolecules, and generation
of oxyradicals through redox cycling. The chemistry of
o-quinones has invoked considerable interest, and the
cycloaddition reactions of these versatile compounds
have been the subject of a number of investigations
[1,6]. A wide variety of dipolar species, including diazo-
methane [3], nitrile oxides [7–10], and mesoionic com-
pounds [11–15], have been used in these reactions. Most
of the dipolar cycloadditions to quinones, however,
involved addition across carbon–oxygen double bonds.
In contrast, nitrone cycloaddition occur across carbon–
carbon double bond of the o-quinones to afford substi-
tuted benzisoxazolidenes, and it is the subject matter of
present investigations.
The 1,3-dipolar cycloaddition reactions of nitrones with
alkenes leading to the formation of isoxazolidenes is a
fundamental reaction in organic chemistry [1]. In 1982
Deshong et al. reported the first 1,3-dipolar cycloaddition
reaction of electron rich alkenes, such as vinyl acetate and
vinyl ethers, with nitrones [2]. This work was extended to
other functionalized alkenes synthesizing a variety of iso-
xazolidenes that served as key synthetic intermediates in
the synthesis of c-amino acids, b-lactams, amino sugars,
and alkaloids [3]. A variety of isoxazolidines have been
prepared using 1,3-dipolar nitrone cycloaddition to func-
tionalized alkenes. In most cases, the nitrone cycloaddi-
tions to alkenes proceeded with high regioselectivity to
yield isoxazolidenes with three new contiguous stereo-
genic centers. The stereoselectivity seems to be influenced
by both electronic and steric factors. Recently, several
asymmetric syntheses of isoxazolidines giving special em-
phasis on their effective use as chiral auxilaries in the syn-
thesis of biologically active molecules have been reported
[3e,4]. Shortly, decades ago itself nitrone cycloaddition
chemistry was enriched with versatility of substrates, cata-
lysts, and solvents used in the reaction. Still, organic
chemistry demands for new reactions for the synthesis of
appropriately substituted isoxazolidenes.
RESULTS AND DISCUSSION
Present studies were initiated by the reaction of 3,5-
di-tert-butyl-1,2-benzoquinone with 1,2-diphenylnitrone.
Preliminary investigations showed that the nitrone cyclo-
addition to 1,2-benzoquinone occured at carbon–carbon
C
V
2010 HeteroCorporation

March 2010 Nitrone Cycloaddition to Quinones: A Novel Strategy for the Synthesis of Benzisoxazolidenes
397
Scheme 1
Table 1
The reaction of 3,5-di-tert-butyl-1,2-benzoquinone 2 with
1,2-diphenylnitrone 1.
Yield (%)
Entry
Reaction conditions
3a
4a
Figure 1. Energy level diagram for HOMOnitrone-LUMOquinone
interaction. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
1
2
3
4
DCM, rt, 48 h
0
0
0
0
0
12
48
7
DCM, BF3.Et2O, rt, 1 h
DCM, Net3, rt, 1 h
Acetonitrile, 70 ꢀC,
18 h
Benzene, 70 ꢀC, 18 h
Toluene, 110 ꢀC,
18 h, Ar
nitrone in toluene was refluxed at 110ꢀC for 18 h. The
yield was further optimized by screening the number of
equivalents of the substrates. Thus, when 3,5-di-tert-
butyl-1,2-benzoquinone (2 mmol) was treated with 1,2-
diphenyl nitrone (1 mmol) in toluene (20 mL) for 18 h
at 110ꢀC, bezisoxazolidene 3a was obtained in 57%
yield. The product 3a was characterized on the basis of
common spectroscopic analysis and ultimately by single
crystal X-ray analysis (Fig. 1).
5
6
0
15
0
61
7
Toluene (excess),
110 ꢀC, 18 h
32
57
19
23
a8
Toluene (excess),
110 ꢀC, 18 h
Toluene, 110 ꢀC, 48 h
Xylene, 140 ꢀC, 2 h
9
10
Complex reaction mixture
Complex reaction mixture
a Quinone nitrone ¼ 2:1.
double bond to afford benisoxazolidene 3a, which is a
rare case among the 1,3-dipole-quinone cycloaddition
reactions. Then, suitable reaction condition for the pro-
posed cycloaddition was identified by treating 3,5-di-
tert-butyl-1,2-benzoquinone with 1,2-diphenylnitrone
under different conditions (Scheme 1, Table 1). Under
most of the reaction conditions, the quinone was easily
reduced to corresponding catechol. To our surprise, even
under perfectly dry conditions using aprotic solvents
like benzene and toluene, the catechol formation domi-
nated over the product formation. Close investigations
on the reaction made us to conclude that any proton
source including the cycloaddition product facilitate the
catechol formation, and so, the yield of the reaction is
decreased. At higher temperature, quinone underwent
cycloaddition with another molecule of quinone, and the
nitrones were fragmented to corresponding amines and
aldehydes. However, the expected cycloaddition product
was obtained in moderate yield, when a dilute solution
of 3,5-di-tert-butyl-1,2-benzoquinone and 1,2-diphenyl
Figure 2. ORTEP diagram (40% probability factor for the thermal
ellipsoids) of compound 6,7a-di(tert-butyl)-2,3-diphenyl-2,3,3a,7a-tetra-
hydro-1,2-benzisoxazole-4,5-dione 3a. [Color figure can be viewed in
the online issue, which is available at www.interscience.wiley.com.]
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

398
R. R. Paul, V. Varghese, P. B. Beneesh, C. R. Sinu, E. Suresh, and E. R. Anabha
Vol 47
Scheme 2
ered as a special case among quinone-1,3-dipole cyclo-
addition reactions.
EXPERIMENTAL
General remarks. Melting points were recorded on a Bu¨chi
melting point apparatus and are uncorrected. NMR spectra
were recorded at 300 MHz (1H) and 75 MHz (13C), respec-
tively on a Bru¨ker Avance DPX-300 MHz NMR spectrometer.
Chemical shifts are reported (d) relative to TMS (1H) and
CDCl3 (13C) as the internal standards. Coupling constants (J)
are reported in Hertz (Hz). High-resolution mass spectra were
recorded under EI/HRMS (at 5000 resolution) using JEOL
JMS 600H mass spectrometer. IR spectra were recorded on
Nicolet Impact 400D FTIR spectrophotometer. Commercial
grade solvents were distilled before use.
General procedure for the synthesis of 6,7a-di(tert-butyl)-
2,3-diaryl-2,3,3a,7a-tetrahydro-1,2-benzisoxazole-4,5-diones
(3a-e) and 5,7a-Di(tert-butyl)-2,3-diphenyl-2,3,3a,7a-tetra-
hydro-1,2-benzisoxazole-4,7-dione (5a-b). A solution of 3,5-
di-tert-butyl-1,2-benzoquinone (220 mg, 1 mmol) and 1,2-dia-
ryl nitrone (0.5 mmol) in toluene (10 mL) was refluxed at
110ꢀC for 18 h. The solvent was removed under vacuum, and
the crude reaction mixture was purified by silica gel (100-200
mesh) column chromatography using hexane-ethyl acetate
(98:2) as the eluent to get the title compounds in good to mod-
erate yields.
Table 2
Synthesis of tetrahydrobenzisoxazolediones 3 and 5.
Entry
3, 5
R1
R2
Yield (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
C6H5
4-CH3C6H5
4-ClC6H5
4-CH3C6H5
2-Naphthyl
9-Anthracenyl
C6H5
C6H5
C6H5
C6H5
4-CH3C6H5
C6H5
C6H5
57
72
98
33
45
0
5a
5b
C6H5
C6H5
22
9
4-CH3OC6H5
To explain the mechanism of cycloaddition of nitrone
to quinone, we have carried out some MNDO and AM1
calculations on 3,5-di-tert-butyl-1,2-benzoquinone and
1,2-diphenyl nitrone using MOPAC (version 5.01) pro-
gram. The HOMO and LUMO energies were derived
from this program. Most of the 1,3-dipolar cycloaddition
reactions of o-quinones undergo inverse electron
demand Diels-Alder reaction (Type II mechanism) [4–
13] at carbon–oxygen double bond. The molecular or-
bital coefficients calculated from the eigen vectors for
the orbital interactions of 3,5-di-tert-butyl-1,2-benzoqui-
none and 1,2-diphenyl nitrone (Fig. 2) favoured the
addition of nitrone oxygen to a highly substituted
carbon atom to form 6,7a-di(tert-butyl)-2,3-diphenyl-
2,3,3a,7a-tetrahydro-1,2-benzisoxazole-4,5-dione 3a,
which follows a normal diels-Alder mode of cycliza-
tion involving HOMOnitrone-LUMOquinone interaction
(Type I mechanism).
6,7a-Di(tert-butyl)-2,3-diphenyl-2,3,3a,7a-tetrahydro-1,2-
benzisoxazole-4,5-dione (3a). This compound was obtained
as yellow crystalline solid; mp: 144–146ꢀC; yield: 119 mg
(57%); 1H NMR: d ¼ 0.82 (s, 9H, CH3), 1.05 (s, 9H, CH3),
3.88 (d, 1H, J ¼ 9 Hz, CH), 4.77 (d, 1H, J ¼ 9 Hz, CH),
6.78–6.85 (m, 4H, ArH), 7.12–7.19 (m, 3H, ArH), 7.23–7.31
(m, 4H, ArH) ppm; 13C NMR: d ¼ 25.9, 28.1, 35.0, 37.3,
66.5, 71.6, 93.7, 112.6, 121.2, 126.1, 128.3, 128.9, 129.2,
140.0, 147.5, 150.2, 152.3, 181.5, 191.0 ppm; hrms (EI): m/z
calcd for C27H31NO3: 417.2304; found: 417.1793; ir (KBr):
3030, 2914, 1663, 1643, 1566 cmꢁ1
.
6,7a-Di(tert-butyl)-3-(4-methylphenyl)-2-phenyl-2,3,3a,7a-
tetrahydro-1,2-benzisoxazole-4,5-dione (3b). This compound
was obtained as yellow crystalline solid, mp: 132–134ꢀC;
1
yield: 155 mg (72%); H NMR: d ¼ 0.89 (s, 9H, CH3), 1.11
(s, 9H, CH3), 2.34 (s, 3H, CH3), 3.93 (d, 1H, J ¼ 7.8 Hz,
CH), 4.80 (d, 1H, J ¼ 7.8 Hz, CH), 6.85 (d, 2H, J ¼ 8 Hz,
ArH), 6.89 (s, 1H, vinylic), 7.15-7.33 (m, 7H, ArH) ppm; 13C
NMR: d ¼ 21.2, 25.8, 28.2, 35.0, 37.3, 67.1, 93.7, 112.8,
125.8, 126.1, 128.1, 129.6, 129.8, 130.8, 136.9, 138.0, 150.1,
152.4, 181.5, 190.9 ppm; hrms (EI): m/z calcd for C28H33NO3:
431.2460; found: 431.2700; ir (KBr): 3029, 2916, 1659, 1645,
On extending the strategy to other benzoquinones and
diaryl nitrones, tetrahydrobenzisoxazolediones 3a-f and
5a-b were synthesized (Scheme 2, Table 2). However,
the presence of bulky substituents like anthraceneyl
groups on nitrone and diphenylmethane group on qui-
none adversely affected the product formation.
1563 cmꢁ1
.
3-(4-Chlorophenyl)-6,7a-di(tert-butyl)-2-phenyl-2,3,3a,7a-
tetrahydro-1,2-benzisoxazole-4,5-dione (3c). This compound
was obtained as yellow crystalline solid, mp: 126–128ꢀC;
1
CONCLUSION
yield: 221 mg (98%); H NMR: d ¼ 0.88 (s, 9H, CH3), 1.11
(s, 9H, CH3), 3.90 (d, 1H, J ¼ 9 Hz, CH), 4.82 (d, 1H, J ¼ 9
Hz, CH), 6.81-6.93 (m, 4H, ArH), 7.19–7.26 (m, 2H, ArH),
7.30–7.36 (m, 4H, ArH) ppm; 13C NMR: d ¼ 25.9, 28.2, 35.1,
37.3, 66.2, 93.7, 112.6, 121.4, 127.4, 128.3, 129.0, 129.4,
129.8, 134.3, 138.5, 147.3, 150.3, 152.0, 181.2, 190.7 ppm;
The reaction of nitrones with benzoquinones resulted
in the formation of benzisoxazolidenes. As the nitrone is
selectively added to carbon–carbon double bond of the
benzoquinone, the quinone–nitrone reaction is consid-
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DOI 10.1002/jhet

March 2010 Nitrone Cycloaddition to Quinones: A Novel Strategy for the Synthesis of Benzisoxazolidenes
399
hrms (EI): m/z calcd for C27H30ClNO3: 451.1914; found:
451.0599; ir (KBr): 3026, 2915, 1659, 1643, 1560 cmꢁ1
matrix least squares on F2; goodness of fit on F2: 1.053; final R
.
indices [I > 2r (I)] R1 ¼ 0.0623, wR2 ¼ 0.1329; largest differ-
ence peak and hole o.422 and ꢁ0.224 eAꢀꢁ3. Selected bond
lengths (Aꢀ) and angles (ꢀ): O(1)-C(3): 1.454(2), C(1)-C(2):
1.557(2), O(1)-N(1): 1.4296(18), N(1)-C(1): 1.478(2), C(3)-
C(7): 1.506(2), C(3)-C(20): 1.5496(2), C(5)-C(24): 1.532(2),
C(1)-C(14): 1.514(2), N(1)-C(8): 1.417(2), O(1)-C(3)-C(4):
106.51(13), O(1)-C(3)-C(2): 102.19(13), C(2)-C(7)-C(6):
116.44(14), C(1)-C(2)-C(7): 111.06(14), N(1)-C(1)-C(2):
104.95(13), O(1)-N(1)-C(1): 107.33(12), C(4)-C(5)-C(24):
123.40(16), C(4)-C(3)-C(20): 113.92(14), O(1)-C(3)-C(20):
106.48(13).
6,7a-Di(tert-butyl)-2,3-di(4-methylphenyl)-2,3,3a,7a-tetra-
hydro-1,2-benzisoxazole-4,5-dione (3d). This compound was
obtained as yellow crystalline solid, mp: 102–104ꢀC; yield: 73
mg (33%); 1H NMR: d ¼ 0.91 (s, 9H, CH3), 0.96 (s, 9H,
CH3), 2.26 (s, 3H, CH3), 2.34 (s, 3H, CH3), 3.92 (d, 1H, J ¼
9 Hz, CH), 4.77 (d, 1H, J ¼ 9 Hz, CH), 6.75 (d, 2H, J ¼ 9Hz,
ArH), 6.86 (s, 1H, vinylic), 7.00 (d, 2H, J ¼ 9Hz, ArH), 7.15
(d, 2H, J ¼ 9Hz, ArH), 7.24 (d, 2H, J ¼ 9Hz, ArH) ppm; 13C
NMR: d ¼ 21.2, 25.9, 28.2, 30.1, 35.0, 37.4, 71.6, 93.3, 113.0,
119.1, 122.3, 126.1, 126.6, 129.3, 129.9, 130.6, 137.0, 137.9,
147.8, 149.8, 181.5, 191.0 ppm; hrms (EI): m/z calcd for
C29H35NO3: 445.5931; found: 445.5021; IR (KBr): 3029,
Acknowledgment. The authors thank the Council of Scientific
and Industrial Research, New Delhi, for research fellowship,
Mrs. Saumini Mathew for NMR spectra and Mrs. S. Viji for high
resolution mass spectral analyses.
2914, 1659, 1642, 1561 cmꢁ1
.
6,7a-Di(tert-butyl)-3-(2-naphthyl)-2-phenyl-2,3,3a,7a-tetra-
hydro-1,2-benzisoxazole-4,5-dione (3e). This compound was
obtained as yellow crystalline solid, mp: 114–116ꢀC; yield:
1
105 mg (45%); H NMR: d ¼ 0.91 (s, 9H, CH3), 1.14 (s, 9H,
CH3), 4.03 (d, 1H, J ¼ 9 Hz, CH), 5.01 (d, 1H, J ¼ 9 Hz,
CH), 6.87 (s, 1H, vinylic), 6.90 (d, 2H, J ¼ 9Hz, ArH), 7.21
(t, 3H, J ¼ 9Hz, ArH), 7.46–7.52 (m, 3H, ArH), 7.79–7.90 (m,
4H, ArH) ppm; 13C NMR: d ¼ 25.4, 28.3, 35.1, 37.4, 67.0,
93.8, 112.4, 123.1, 124.3, 125.1, 125.9, 127.0, 127.2, 127.8,
128.1, 129.2, 133.4, 137.2, 150.3, 152.3, 181.5, 190.9 ppm;
hrms (EI): m/z calcd for C31H33NO3: 467.5987; found:
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(22%); 1H NMR: d ¼ 1.09 (s, 9H, CH3), 1.27 (s, 9H, CH3),
3.89 (d, 1H, J ¼ 9 Hz, CH), 4.52 (d, 1H, J ¼ 9 Hz, CH), 6.60
(s, 1H, vinylic), 6.88 (d, 2H, J ¼ 9Hz, ArH), 7.12 (t, 3H, J ¼
9Hz, ArH), 7.25–7.38 (m, 5H, ArH) ppm; 13C NMR: d ¼
26.0, 30.0, 35.5, 36.5, 68.4, 73.8, 92.5, 115.3, 122.0, 127.0,
128.5, 128.5, 129.1, 136.1, 138.9, 149.9, 158.8, 194.7, 199.0
ppm; hrms (FAB): m/z calcd. for C27H31NO3: 417.2304;
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yield: 20 mg (9%); H NMR: d ¼ 1.09 (s, 9H, CH3), 1.26 (s,
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(m, 3H, ArH), 6.99 (d, 2H, J ¼ 9Hz, ArH), 7.09–7.20 (m, 5H,
ArH) ppm; 13C NMR: d ¼ 26.3, 33.0, 35.8, 37.5, 55.7, 68.2,
73.6, 92.7, 115.8, 121.0, 127.0, 128.5, 128.5, 129.4, 133.1,
133.9, 149.9, 158.8, 159.3, 194.7, 199.0 ppm; hrms (FAB): m/
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3029, 1844, 1444, 687 cmꢁ1
.
3a: X-ray crystallographic data. Single crystals were
grown from CDCl3. Crystal system: triclinic; space group: P-1;
T ¼ 100 (2) K; a ¼ 9.8727 (8) Aꢀ, b ¼ 10.6520 (8) Aꢀ, c ¼
22.0592 (17) Aꢀ, a ¼ 94.8130 (10)ꢀ, b ¼ 92.3670 (10)ꢀ, c ¼
90.1150 (10)ꢀ, z ¼ 4, Dcalcd ¼ 1.201 mg/m3; crystal size 0.66 ꢂ
0.45 ꢂ 0.12 mm; y range for data collection 1.85ꢀ to 28.27ꢀ.
Limiting indices ꢁ13h12, ꢁ14k13, ꢁ29l29; reflections collected
20146, independent reflections: 10519. Refinement method: full-
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet