Novel rearrangement during the reaction of diethylmalonate withα-(5-substituted 2-hydroxyphenyl)-n-phenyl nitrones

Article abstract of DOI:10.1080/00397910902985481

The hydroxyl group at the ortho position of the-aryl ring of-(2-hydroxyaryl)-N-aryl nitrones altered the expected course of the reaction between differently substituted-(2-hydroxyaryl)-N-aryl nitrones and diethylmalonate, leading to the formation of an enamine involving an interesting rearrangement under microwave irradiation. The enamines have been characterized by NMR and X-ray analyses, and a reasonable mechanism has been put forwarded to explain the rearrangement.

Full text of DOI:10.1080/00397910902985481

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Novel Rearrangement During the
Reaction of Diethylmalonate with α-(5-
Substituted 2-hydroxyphenyl)-N-phenyl
Nitrones
S. R. Jayapradha a & S. Muthusubramanian a
a Department of Organic Chemistry, School of Chemistry, Madurai
Kamaraj University, Madurai, India
Published online: 06 Jan 2010.
To cite this article: S. R. Jayapradha & S. Muthusubramanian (2010) Novel Rearrangement During the
Reaction of Diethylmalonate with α-(5-Substituted 2-hydroxyphenyl)-N-phenyl Nitrones, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
40:3, 434-441, DOI: 10.1080/00397910902985481
To link to this article: http://dx.doi.org/10.1080/00397910902985481
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Synthetic Communications1, 40: 434–441, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902985481
NOVEL REARRANGEMENT DURING THE REACTION
OF DIETHYLMALONATE WITH a-(5-SUBSTITUTED
2-HYDROXYPHENYL)-N-PHENYL NITRONES
S. R. Jayapradha and S. Muthusubramanian
Department of Organic Chemistry, School of Chemistry, Madurai Kamaraj
University, Madurai, India
The hydroxyl group at the ortho position of the a-aryl ring of a-(2-hydroxyaryl)-N-aryl
nitrones altered the expected course of the reaction between differently substituted a-
(2-hydroxyaryl)-N-aryl nitrones and diethylmalonate, leading to the formation of an
enamine involving an interesting rearrangement under microwave irradiation. The enamines
have been characterized by NMR and x-ray analyses, and a reasonable mechanism has been
put forwarded to explain the rearrangement.
Keywords: Diethyl malonate; enamine formation; microwave-assisted organic reactions; nitrone addition
The enamine group is one of the most versatile functionalities available to the
synthetic chemist for different transformations.[1] Enamines have been intensively
studied in organic synthesis after Stork’s report on the application of enamines
for the alkylation and acylation of carbonyl compounds.[2] Enamines have been
used in natural product synthesis, including total synthesis of fabianine[3] and
quaipyridines.[4]
b-Functionalized enamines or the corresponding imine tautomers derived from
phosphonium salts,[5] phosphine oxides,[6] phosphonates,[7–10] and phosphazenes[11]
have been employed in the synthesis of phosphorus substituted three-, five-, and
six-membered nitrogen heterocycles. The heterocyclic frameworks available through
this methodology include azirines, pyrazoles, pyrroles, 2-pyrrolones, dihydro- and
tetrahydropyridines, 2-pyridones, quinolines, and pyrimidin-2,4-diones. Phosphorus
functionalized enamines are also useful starting materials for the preparation of
phosphorus-containing heterocycles.[12] Very recently, the unexpected diversity of
reaction patterns of heterocyclic enamines when treated with aliphatic and aromatic
dicarboxylic acid chlorides has been discovered.[13] We report the unexpected forma-
tion of ethyl 3-(2-hydroxy-5-substituted phenylanilino)-2-propenoate, a set of novel
enamines obtained by the addition of diethylmalonate to C-aryl-N-aryl nitrones.
Received January 19, 2009.
Address correspondence to S. Muthusubramanian, Department of Organic Chemistry, Madurai
Kamaraj University, Madurai 625 021, India. E-mail: muthumanian2001@yahoo.com
434

NOVEL REARRANGEMENT REACTION
435
Our experience with 2-hydroxy=methoxy-substituted C-aryl-N-aryl nitrones
giving unexpected products during attempted 1,3-dipolar addition reactions[14]
prompted us to investigate the behavior of these nitrones toward other reactions
involving nucleophiles. Addition of carbon nucleophiles to electron-deficient olefins
is a classical and fundamental carbon–carbon bond-forming reaction.[15] This
reaction and its close variants have been extensively used in organic synthesis.[16]
Diethyl malonate adducts have been used in the preparation of a very wide variety
of heterocycles. Addition of diethylmalonate with simple nitrones, leading to the
formation of isoxazolidinone, has already been described.[17,18] In the present work,
when a-(2-hydroxyaryl)-N-aryl nitrones 1 were allowed to react with diethyl
malonate in the hope of preparing 5-oxo-2-phenyltetrahydro-4-isoxazole 2 with
2-hydroxyaryl group at the 3-position and carbethoxy group at the 2-position, the
expected cyclic compound was not obtained, but a novel rearrangement occurred
that led to a new set of enamines.
A mixture of a-(2-hydroxyaryl)-N-aryl nitrone 1, diethyl malonate, and solid
sodium ethoxide was irradiated with microwaves for 2 mins. Only one product, 3,
was obtained in very good yield as evidenced by thin-layer chromatography
1
(TLC) (Scheme 1 and Table 1). The H and 13C NMR spectra along with the
two-dimensional NMR spectra were helpful in assigning the structure of the
product, and the product obtained was enamine 3 and not the expected 2. In
the 1H NMR spectrum of 3a, there is a triplet at 1.17 ppm (J ¼ 7.2 Hz) and a quartet
at 3.98 ppm (J ¼ 7.2 Hz), indicating the presence of a carbethoxy group in the
molecule. There is a singlet appearing at 1.26 ppm for nine hydrogens, due to the t-
butyl group. Two doublets at 4.74 ppm (J ¼ 13.2 Hz) and 8.22 ppm (J ¼ 13.2 Hz)
show the presence of trans coupled olefinic hydrogens. Obviously one is under the
influence of an electron-releasing group and the other is under the influence of an
electron-withdrawing group, shown by the large difference in their chemical shift.
All the aromatic hydrogens appear between 6.97 and 7.35 ppm.
Scheme 1. Reaction of C-(2-hydroxyaryl)-N-aryl nitrone with diethyl malonate.

436
S. R. JAYAPRADHA AND S. MUTHUSUBRAMANIAN
Table 1. Reaction time and yield of enamine formation
Compound
R1
Reaction time (min)
Yield (%)
3a
3b
3c
3d
3e
C(CH3)3
CH2CH3
2
2
2
2
2
85
83
78
80
85
OCH3
C(CH3)2CH2CH3
CH(CH3)2
In the 13C NMR spectrum of 3a, signals appear at 14.3, 31.4, 34.2, 59.6, 94.6,
117.1, 118.5, 123.8, 125.5, 126.5, 127.0, 129.5, 144.5, 145.3, 146.0, 149.3, and
169.0 ppm. The presence of 2-hydroxy-5-substituted aryl group and the phenyl group
are evident from the spectral pattern. The deshielded doublet at 8.22 ppm has hetero-
nuclear multiple bond correlation (HMBC) contours with the carbons at 169.0,
145.3, and 126.5 ppm. All the three carbons are quaternary carbons and have three
bond connectivities with this hydrogen. This clearly suggests that the two aryl groups
and the vinyl group are attached through a common atom. The elemental analysis
performed on the compound indicates the presence of nitrogen, and hence this
common atom could be nitrogen. There are two other quaternary carbons at
144.5 and 149.3 ppm. The former carbon is ipso to the t-butyl group, as it gives a
HMBC contour with the methyl hydrogens of the t-butyl group, and the latter
carbon must be ipso to hydroxyl group. Thus, we conclude the structure of 3 to
be that shown in Fig. 1.
To confirm the assigned structure, single-crystal x-ray analysis was carried out
for compound 3a, ethyl 3-[5-tert-butyl)-2-hydroxyphenylanilino]-2-propenoate.
(Crystallographic data for the structure 3a have been deposited with the Cambridge
Crystallographic Centre as supplementary publication number CCDC 710035.)
The x-ray structure is shown in Fig. 2, confirming the assignments. The crystal data
are provided in Table 2.
Thus, though we aimed at a cyclic compound from the open-chain precursor
nitrone, a different compound was obtained involving an interesting rearrangement.
Obviously, the hydroxyl group present at the ortho position has some role to
play directing the course of the reaction along a different path. Such an influence
by the ortho methoxy group during the 1,3-dipolar cycloaddition reaction of
Figure 1. Selected HMBC correlations of 3a.

NOVEL REARRANGEMENT REACTION
437
Figure 2. ORTEP diagram of ethyl (E)-3-[5-tert-butyl)-2-hydroxyphenylanilino]-2-propenoate 3a.
Table 2. Summary of crystal data, data collection, and structure refinement for 3a
Crystal data
Compound 3a
Empirical formula
Formula weight
C21H25NO3
339.42
Temperature
Wavelength
298(2) K
0.71073 A
Triclinic, P-1
a ¼ 8.4086(4) A, alpha ¼ 73.921(2)ꢀ
b ¼ 11.0725(5) A, beta ¼ 84.265(2)ꢀ
c ¼ 11.3982(5) A, gamma ¼ 70.723(2)ꢀ
962.49(8) A3
2, 1.171 mg=m3
0.078 mmꢁ1
364
Crystal system, space group
Unit cell dimensions
Volume
Z, Calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
0.28 ꢂ 0.25 ꢂ 0.22 mm
Reflections collected (unique)
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
Data=restraints=parameters
Goodness of fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
2.36 to 27.23ꢀ
ꢁ10 ꢃ h ꢃ 10, ꢁ14 ꢃ k ꢃ 14, ꢁ14 ꢃ l ꢃ 14
12273=4208 [R(int) ¼ 0.0259]
25.00 and 99.0%
None
0.9831 and 0.9786
Full-matrix least-squares on F2
4208=1=260
0.783
R1 ¼ 0.0522, wR2 ¼ 0.1359
R1 ¼ 0.0937, wR2 ¼ 0.1740
0.292 and ꢁ0.201 eAꢁ3

438
S. R. JAYAPRADHA AND S. MUTHUSUBRAMANIAN
Scheme 2. Mechanism for the formation of product 3.
a-(2-methoxyphenyl)-N-aryl nitrone with maleic anhydride has already been
noticed.[14] A reasonable mechanism can be provided for the conversion (Scheme 2).
The hydroxyl group present at the 4-position of the nitrone was also expected
to influence the course of the reaction in a similar fashion, and hence the reaction
was carried out with the nitrone 4 generated from 4-hydroxybenzaldehyde. In this
case also, the expected cycloaddition did not take place; the molecule underwent a
rearrangement to give the enamine 5 (Scheme 3). The structure of 5 was confirmed
1
by H and 13C NMR spectral analysis.
It must be mentioned that the reaction of nitrone generated from 2=4-
methoxy benzaldehyde with diethyl malonate did not proceed to give either a
cyclized product or an enamine. The additions of different nucleophiles with one
carbethoxy group and another electron-withdrawing group [viz., ethylchloro
acetate and ethylcyano acetate on a-(2-hydroxyaryl)-N-aryl nitrone] have not
yielded any recognizable products.
Scheme 3. Reaction of C-(4-hydroxyphenyl)-N-aryl nitrone with diethyl malonate.

NOVEL REARRANGEMENT REACTION
439
EXPERIMENTAL
All chemicals were of reagent-grade quality and used with out further purifi-
cation. All melting points were recorded in open capillaries and are uncorrected.
1
The H and 13C NMR spectra were recorded on a Bruker 300-MHz spectrometer
at 300 MHz and 75 MHz respectively in CDCl3 using tetramethylsilane (TMS) as
internal standard. The related two-dimensional (2D) NMR spectra were also
recorded on the same instrument. Chemical shifts are given in parts per million (d
scale), and coupling constants are given in hertz. Microanalyses were carried out
on a Perkin-Elmer instrument. The single-crystal x-ray data set was collected on a
Bruker AXS diffractometer with radiation of k ¼ 0.71073 Aꢀ. All chromatographic
separations were performed on 60- to 120-mesh silica gel using petroleum ether=ethyl
acetate as eluent.
General Procedure for the Preparation of Ethyl 3-[5-Substituted)-
2-hydroxy phenylanilino]-2-propenoate (3)
Diethylmalonate (0.32 g, 0.002 mol) and a catalytic amount of sodium ethoxide
were subjected to microwave irradiation for 1 min, and the resultant mass was taken
out. An equimolar amount of a-(5-substituted-2-hydroxyphenyl)-N-phenyl nitrone 1
was then added and mixed well. The reaction mixture was again irradiated for 1 min,
and the mixture was worked out using petroleum ether to yield the white crystalline
mass of the product ethyl 3-[5-substituted)-2 hydroxy phenylanilino]-2-propenoate 3.
Ethyl 3-[5-tert-Butyl)-2-hydroxyphenylanilino]-2-propenoate (3a)
White crystals (85%), mp 137ꢀC. Anal. calcd. for C21H25NO3: C, 74.31; H,
1
7.42; N, 4.13. Found: C, 74.40; H, 7.45; N, 4.23. H NMR (300 MHz, CDCl3): d
1.17 (t, J ¼ 7.2 Hz, 3H); 1.26 (s, 9H), 3.98 (q, J ¼ 7.2 Hz, 2H); 4.74 (d, J ¼ 13.2 Hz,
1H); 6.97–7.35 (m, 8H), 8.22 (d, J ¼ 13.2 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3):
d 14.3, 31.4, 34.2, 59.6, 94.6, 117.1, 118.5, 123.8, 125.5, 126.5, 127.0, 129.5, 144.5,
145.3, 146.0, 149.3, 169.0 ppm.
Ethyl 3-[5-Ethyl)-2-hydroxyphenylanilino]-2-propenoate (3b)
White crystals (83%), mp 125ꢀC. Anal. calcd. for C19H21NO3: C, 73.29; H,
1
6.80; N, 4.50. Found: C, 73.35; H, 6.80; N, 4.55. H NMR (300 MHz, CDCl3): d
1.15 (t, J ¼ 7.5 Hz, 3H); 1.21 (t, J ¼ 7.5 Hz, 3H); 2.56 (q, J ¼ 7.5 Hz, 2H); 4.00
(q, J ¼ 7.2 Hz, 2H); 4.76 (d, J ¼ 13.8 Hz, 1H); 6.90–7.34 (m, 8H); 8.19 (d, J ¼ 13.8 Hz,
Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): d 14.0, 14.3, 27.7, 59.6, 94.5, 117.4,
118.8, 123.8, 126.9, 127.8, 129.4, 129.5, 137.9, 144.5, 146.1, 149.8, 169.1 ppm.
Ethyl 3-(2-Hydroxy-5-methoxyphenylanilino)-2-propenoate (3c)
White crystals (78%), mp 143ꢀC. Anal. calcd. for C18H19NO4: C, 68.99; H,
1
6.11; N, 4.47. Found: C, 69.05; H, 6.20; N, 4.40. H NMR (300 MHz, CDCl3): d
1.25 (t, J ¼ 7.2 Hz, 3H); 4.05 (q, J ¼ 7.2 Hz, 2H); 3.80 (s, 3H); 4.83 (d, J ¼ 13.2 Hz,

440
S. R. JAYAPRADHA AND S. MUTHUSUBRAMANIAN
1H); 7.00–7.60 (m, 8H); 8.20 (d, J ¼ 13.2 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3):
d 13.9, 41.5, 55.6, 61.3, 94.5, 113.2, 115.9, 117.7, 118.3, 118.6, 119.4, 119.5, 122.5,
123.7, 125.4, 128.7, 129.3, 131.4, 145.9, 154.1, 169.0 ppm.
Ethyl 3-[2-Hydroxy-5-(tert-pentyl)phenylanilino]-2-propenoate (3d)
White crystals (80%), mp 139ꢀC. Anal. calcd. for C22H27NO3: C, 74.76; H,
1
7.70; N, 3.96. Found: C, 74.80; H, 7.65; N, 4.05. H NMR (300 MHz, CDCl3): d
1.20 (t, J ¼ 7.2 Hz, 3H); 1.30 (s, 6H); 1.40 (t, J ¼ 6.9 Hz, 3H); 4.01 (q, J ¼ 7.2 Hz,
2H); 4.40 (q, J ¼ 6.9 Hz, 2H); 4.75 (d, J ¼ 13.2 Hz, 1H); 7.00–7.60 (m, 8H); 8.25 (d,
J ¼ 13.2 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): d ¼ 8.9, 13.8, 14.3, 28.3, 36.8,
59.5, 94.2, 117.2, 123.5, 125.4, 126.4, 127.5, 129.3, 132.6, 146.2, 149.2, 157.1,
169.2 ppm.
Ethyl 3-(2-Hydroxy-5-isopropylphenylanilino)-2-propenoate (3e)
White crystals (85%), mp 139ꢀC. Anal. calcd. for C20H23NO3: C, 73.82; H,
1
7.12; N, 4.30. Found: C, 73.90; H, 7.20; N, 4.40. H NMR (300 MHz, CDCl3): d
1.20 (t, J ¼ 7.2 Hz, 3H); 1.25 (d, J ¼ 7.2 Hz, 6H); 2.70 (sep, J ¼ 7.2 Hz, 1H); 3.95
(q, J ¼ 7.2 Hz, 2H); 4.77 (d, J ¼ 13.2 Hz, 1H); 6.85–7.58 (m, 8H); 8.40 (d, J ¼ 13.2 Hz,
Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): d 14.2, 23.8, 33.0, 59.5, 94.0, 117.4,
118.4, 123.5, 126.4, 127.8, 129.2, 142.4, 144.5, 146.3, 150.0, 169.3 ppm.
Ethyl 3-[4-Hydroxy(phenyl)anilino]-2-propenoate (5)
White crystals (80%), mp 135ꢀC. Anal. calcd. for C17H17NO3: C, 72.07; H,
1
6.05; N, 4.94. Found: C, 71.99; H, 6.95; N, 4.83. H NMR (300 MHz, CDCl3):
1.28 (t, J ¼ 7.2 Hz, 3H); 4.20 (q, J ¼ 7.2 Hz, 2H); 4.83 (d, J ¼ 13.2 Hz, 1H);
6.90–7.38 (m, 9H); 8.15 (d, J ¼ 13.2 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3):
14.4, 59.6, 93.1, 116.5, 122.2, 125.4, 127.3, 128.6, 128.7, 129.3, 147.9, 155.0,
169.8 ppm.
ACKNOWLEDGMENTS
The authors thank the Department of Science and Technology, New Delhi,
India, for the NMR spectrometer facility. Financial support from Madurai Kamaraj
University for one of the authors (S. R. J.) is gratefully acknowledged.
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