Conversion of oximes to carbonyl compounds by triscetylpyridinium tetrakis(oxodiperoxotungsto) phosphate (PCWP)-mediated oxidation with hydrogen peroxide

Article abstract of DOI:10.3390/molecules13061230

Aromatic and aliphatic oximes have been deoximated in chloroform-water to the corresponding aldehydes with dilute hydrogen peroxide and triscetylpyridinium tetrakis (oxodiperoxotungsto) phosphate as catalyst. The presence of dipolarophiles in the reaction mixtures allows a competitive reaction that converts oximes into isoxazole and isoxazoline derivatives via the intermediate formation of nitrile oxide species.

Full text of DOI:10.3390/molecules13061230

Molecules 2008, 13, 1230-1237; DOI: 10.3390/molecules13061230
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molecules
ISSN 1420-3049
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Article
Conversion of Oximes to Carbonyl Compounds by
Triscetylpyridinium Tetrakis(oxodiperoxotungsto) Phosphate
(PCWP)-mediated Oxidation with Hydrogen Peroxide
Francesco P. Ballistreri *, Ugo Chiacchio, Antonio Rescifina*, Gaetano Tomaselli and Rosa M.
Toscano
Dipartimento di Scienze Chimiche, Università di Catania, Viale Andrea Doria 6, Catania 95125, Italy;
E-mails: uchiacchio@unict.it; gtomaselli@unict.it; rmtoscano@unict.it
* Authors to whom correspondence should be addressed; E-mails: fballistreri@unict.it (Francesco
Ballistreri) and arescifina@unict.it (Antonio Rescifina)
Received: 29 April 2008; in revised form: 12 May 2008 / Accepted: 15 May 2008 / Published: 1 June
2008
Abstract: Aromatic and aliphatic oximes have been deoximated in chloroform-water to the
corresponding aldehydes with dilute hydrogen peroxide and triscetylpyridinium tetrakis
(oxodiperoxotungsto) phosphate as catalyst. The presence of dipolarophiles in the reaction
mixtures allows a competitive reaction that converts oximes into isoxazole and isoxazoline
derivatives via the intermediate formation of nitrile oxide species.
Keywords: Oxidation of oximes; oxodiperoxotungsto complex; 1,3-dipolar cycloaddition;
nitrile oxides; aldehydes.
Introduction
Oximes are frequently used as carbonyl protector groups [1] from which the parent carbonyl
compounds must be regenerated. Regeneration of the carbonyl compound requires the use of soft
reagents that will cleave the oxime bond without modification under mild reaction conditions.

Molecules 2008, 13
1231
Furthermore, since oximes can also be prepared from non-carbonyl compounds, the generation of
carbonyl compounds from them provides an alternative method for the preparation of aldehydes and
ketones [2–5]. The traditional hydrolytic method for deprotection of oximes requires the use of strong
acids and often results in low yields due to the formation of polymeric by-products, so a number of
alternative methods have been developed. Some previously reported carbonyl compound deoximation
methods involve oxidative or reductive protocols using, for example, pyridinium dichromate, t-butyl-
hydroperoxide, and so on. [6,7]. Some of these reactions have different disadvantages such as long
reaction times, difficulties in isolation of products and the possibility of explosions due to the presence
of unstable compounds produced by strong oxidative reagents. Many oxidative deoximation methods of
aldoximes cited in the literature give low yields of aldehydes due to their over-oxidation to acids.
+
Mo(VI) and W(VI) peroxopolyoxo complexes, whose general formula is Q3 {PO4[MO(O2)2]4}3–,
are one of the most promising group of catalysts for the selective transfer oxygen to organic substrates
[8–9]. They can be stoichiometrically used as oxidant agents or as catalysts in the oxidation processes
employing dilute hydrogen peroxide. Lacunary polyoxotungstates have been also recently been
screened as catalysts for H2O2 oxidations under microwave irradiation [10]. Hydrogen peroxide as
oxidant has the great advantage to generate only water as by-product. It has a high content of active
oxygen and it is less expensive than organic peroxides and peracids. Another advantage of using these
salts as oxidants comes from the possibility that the counteraction Q+ itself acts as a phase transfer
agent when Q+ represents a suitable ammonium salt. In this paper we wish to report the oxidative
deprotection reaction of oximes with hydrogen peroxide mediated by triscetylpyridinium
tetrakis(oxodiperoxotungsto) phosphate as catalyst to yield the corresponding carbonyl compounds
under mild conditions and high yields.
Results and Discussion
Aromatic and aliphatic oximes 1 treated in water-chloroform at 30 °C with dilute hydrogen peroxide
(35%, v/v) and 1 mol% of [C5H5N+(CH2)14CH3]3{PO4[WO(O2)2]4}3– (PCWP), used as catalyst, have
been transformed to carbonyl compounds 2 (Table 1).
Table 1. Deoximation of aldoximes 1 by oxidation with diluted hydrogen peroxide and PCWP a
R1
R2
OH
N
O
H2O2, PCWP
H2O/CHCl3, 30 °C
R1
R2
1
2
Oxime
1a
R1
C6H5
R2
H
H
Time (min.)
Conversion (%)
Yield (%)b,c
80
70
90
60
60
90
90
95
80
95
95
85
100
100
95
1b
p-Cl-C6H4
C6H5
1c
CH3
H
1d
CH(Me)(Et)
n-C7H15
70
1e
H
70
1f
–(C5H10)–
93
a All the reactions have been performed in CHCl3/H2O at 30 °C employing [Oximes] = 2.5 mmol; [H2O2] = 20
b
c
mmol and [PCWP] = 0.025 mmol. Isolated yields. Identities of compounds have been confirmed by
comparison of their MS and 1H-NMR spectra with those of authentic samples.

Molecules 2008, 13
1232
The results listed in Table 1 indicate that the reaction is successful for a variety of aliphatic and
aromatic oximes. Moreover, the obtained results suggest that aldoximes are deprotected relatively
faster than ketoximes.
We also explored the possibility of generating nitrile oxides intermediates for the preparation of
isoxazole and isoxazoline derivatives via 1,3-dipolar cycloaddition [11,12], to further expand the
synthetic utility of the PCWP oxidation of aldoximes. N,O-Heterocycles are considered privileged
structures in medicinal chemistry, as they show a wide spectrum of biological activities and have been
used as antimitotic agents, antiviral compounds, antimicotics and so on [13–14]. Moreover, these
compounds have several synthetically useful functionalities, masked in the rings. These functionalities
can be released through ring cleavage giving easy access to a variety of open chain derivatives which
are differently functionalized [15]. Thus, the reaction in chloroform-water of aldoximes 1a,d, used as
model compounds, and treated with dilute hydrogen peroxide (35%, v/v) and 1 mol% of
[C5H5N+(CH2)14CH3]3{PO4[WO(O2)2]4}3– (PCWP), used as catalyst, at 40 °C in the presence of
alkenes 3 (1 equiv.) or alkynes 4 (2.5 equiv.), produced isoxazolines 5 or isoxazoles 6 and 7,
respectively, along with variable amounts of aldehydes 2a,d (Scheme 1, Table 2).
Scheme 1.
R1
N
R2
R3
O
R2
R3
+
+
R1
H
3
4
O
O
H
OH
N
H2O2, PCWP
2a d
5
,
H2O/CHCl3, 40 °C
R1
R1
N
R1
N
R4
1a,d
O
R4
H
+
R1
H
R4
O
2a d
6
7
,
Table 2. Preparation of isoxazole and isoxazoline derivatives by aldoximes oxidation in
the presence of alkenes or alkynes a.
Yields, (%)b,c
Time
Conv.
Entry R1
R2
R3
R4
(h)
9
(%)
84
2
5
6
7
1
2
C6H5
H
CH3(CH2)5
CH3(CH2)5
CO2Me
62 17
56
CH(Me)(Et)
H
5
89
6
d
3
C6H5
CO2Me
CO2Me
9
81
43 46
30 30
67
4
CH(Me)(Et)e
C6H5
CO2Me
6
100
71
5
C6H5
16
6.5
6.5
3
28
9
6
CH(Me)(Et)
C6H5
C6H5
90
49
7
CH3(CH2)5
CH3(CH2)5
CO2Me
CO2Me
83
49
32
5
8
CH(Me)(Et)
C6H5
88
33
9
8
86
30
42 13
25
10
CH(Me)(Et)
3
100
43
8
aAll the reactions have been performed in CHCl3/H2O at 40 °C employing [Oximes] = 2.5 mmol; [H2O2] = 20
mmol and [PCWP] = 0.025 mmol. b Isolated yields. c Identities of the compounds have been obtained comparing
their MS and 1H-NMR spectra with those of authentic samples. dThe reaction is stereospecific; fumarate gave E-
adduct whereas maleate gave Z-adduct. eThe reaction has been performed with fumarate.

Molecules 2008, 13
1233
The results indicate that the aldehydes are the main products of the reaction, except for entries 3 and
4 (cycloadduct-aldehyde ratio 1:1) and entry 9 (cycloadduct-aldehyde ratio 1.8:1). The stereochemistry
of the cycloadducts depends on the stereochemistry of the dipolarophiles (entry 3) – the E-adduct was
obtained using fumarate and the Z-ones has been obtained with maleate. Moreover, all the experiments
show that aromatic oximes lead to a higher yield of cycloadduct than aliphatic ones, and the presence
of electron-withdrawing groups on the dipolarophile moiety increases the yield of cycloadduct (entries
3, 4, 9 and 10). The formation of isoxazole derivatives 5–7 supports the intermediate formation of the
corresponding nitrile oxide [16]. In fact, the reaction of C6H5CH=NOH (2.5 mmol) with H2O2 (20
mmol) and PCWP (0.025 mmol) in CHCl3 at 40 °C, in the absence of dipolarophiles gives as main
product benzaldehyde, along with a small amount of diphenylfuroxan. The same reaction followed by
IR shows two significative bands: at 2250 cm–1 corresponding to benzonitrile oxide [17], and at 1700
cm-1 associated to benzaldehyde. Furthermore, the data point out that the regioselectivity of the
cycloaddition process is in accordance with both steric and frontier molecular orbital interactions of the
reagents [16].
At this stage it is hard to suggest a mechanistic pathway for this reaction. It is known that
peroxopolyoxocomplexes such as PCWP behave as electrophilic oxidants [18,19] and therefore the
oxidation reaction might be triggered by a nucleophilic [18, 20] attack of the oxime nitrogen-atom to
the peroxide oxygen followed by catalytic hydrogen peroxide regeneration of the oxidant and by
subsequent steps for the products formation (Scheme 2). However, it is also known that PCWP is also a
good electron acceptor [21,22] and therefore the involvement of electron transfer events cannot be
excluded a priori.
Scheme 2.
O
+
+
+
+
1)
Nuc
Nuc-O
Ln W
H2O2
Ln W O
H2O
O
O
2)
3)
Ln W O
Nuc-O
Ln W
O
Products
Conclusions
In summary, aromatic and aliphatic oximes 1 can be easily deoximated in water-chloroform to the
corresponding aldehydes 2 with dilute hydrogen peroxide (35%, v/v) mediated by 1 mol% of
[C5H5N+(CH2)14CH3]3{PO4[WO(O2)2]4}3– (PCWP). The presence in the reaction mixtures of alkenes
and alkynes as dipolarophiles allows a competitive reaction path which converts the oximes into
isoxazole and isoxazoline derivatives through the intermediate formation of nitrile oxide species.

Molecules 2008, 13
Experimental
General
1234
Aldoximes, alkynes and alkenes (Aldrich) were distilled or crystallized before use. Chloroform
(Carlo Erba, RPE) was distilled over P4O10, whereas H2O2 (35%, v/v) (Carlo Erba, RPE) was used
1
without further purification. H- and 13C-NMR were obtained on a Varian Unity Inova 200 MHz
spectrometer operating at 200 and 50 MHz, respectively, using CDCl3 as solvent and TMS as internal
standard. GLC analyses were carried out on a programmable Perkin-Elmer 8420 gas chromatograph
equipped with a flame ionization detector and a 25 m DB-1 capillary column. GC/MS analyses were
performed on a Hewlett-Packard model 5890 gas chromatograph, using an HP-1 dimethylpolysiloxane
25 m capillary column, equipped with a Hewlett-Packard MS computerized system Model 5971A,
ionization voltage 70 eV, electron multiplier 1700 V, ion source temperature 280 °C. IR spectra were
recorded on a Perkin-Elmer Paragon 500 FT-IR Spectrometer using potassium bromide discs.
Preparation of triscetylpyridinium tetrakis(diperoxotungsto)phosphate (PCWP)
To a solution of cetylpyridinium chloride (3.1 mmol) in 35% H2O2 (40 mL) has been added
H3PW12O40·nH2O (3 g) in 35% H2O2 (10 mL), and the mixture has been stirred at 40 °C for 4–5 h. The
white precipitate, after filtration, has been washed with water until all the H2O2 was removed and then
dried in vacuo over P4O10. The IR spectrum (KBr) corresponded to the one reported in the literature
[23].
Oxidation and cycloaddition reactions: general procedure for oxidation
To a warm (30 °C) solution containing PCWP (0.025 mmol) in chloroform (5 mL) has been added a
solution of aldoxime (2.5 mmol) and H2O2 (35%, 1.8 mL, 20 mmol). After the addition has been
completed, the mixture has been stirred at 30 °C for an appropriate time, then the organic layer has
been separated, washed with a 10% of aqueous solution of sodium bisulphite and dried over anhydrous
sodium sulphate. The solvent has been removed under reduced pressure to leave a thick oil, which has
been subjected to silica gel chromatography using a 30% ethyl acetate/cyclohexane mixture as eluent.
The 1H-NMR spectra of aldehydes 2a–f corresponded to the ones reported in the literature.
General procedure for oxidation-cycloaddition
A solution of aldoxime (2.5 mmol), alkene (2.5 mmol) or alkyne (7.5 mmol) in chloroform (5 mL)
and H2O2 (35%, 1.8 mL, 20 mmol) were added to a warm (40 °C) solution of PCWP (0.025 mmol) in
chloroform (5 mL). After the addition was completd, the mixture was stirred at 40 °C for an appropriate
time, after which the organic layer was separated, washed with a 10% aqueous solution of sodium
bisulphite and finally dried over anhydrous sodium sulphate. The solvent has been removed under
reduced pressure to leave a thick oil, which was subjected to silica gel chromatography using a 30%
ethyl acetate/cyclohexane mixture as eluent. The 1H-NMR and MS spectra, for the compounds obtained

Molecules 2008, 13
1235
in entries 1 [24], 3 [25, 26], 5 [27], 7 [28], and 9 [29, 30], corresponded to the ones reported in the
literature.
Data for new compounds in Table 2:
1
(5RS)-3-sec-Butyl-5-hexyl-4,5-dihydroisoxazole (entry 2): Light yellow oil; H-NMR: 0.84–0.89 (m,
6H), 1.23 (d, 3H, J = 6.1 Hz), 1.24–1.43 (m, 12H), 2.28 (m, 1H), 2.63 (dd, 1H, J = 8.4 and 16.5 Hz),
13
3.01 (dd, 1H, J = 10.2 and 16.5 Hz), 4.49 (m, 1H); C-NMR: 11.8, 14.1, 18.6, 22.7, 26.0, 26.6, 29.0,
31.7, 35.4, 35.6, 38.4, 80.6, 165.8; Anal. calcd. for C12H25NO: C, 73.88; H, 11.92; N, 6.63%. Found: C,
73.67; H, 11.94; N, 6.62%.
Dimethyl (4RS,5RS)-3-sec-butyl-4,5-dihydroisoxazole-4,5-dicarboxylate (entry 4): Light yellow oil;
1H- NMR: 0.87 (t, 3H, J = 7.1 Hz), 1.15 (d, 3H, J = 6.1 Hz), 1.18–1.40 (m, 2H), 2.58 (m, 1H), 3.72 (s,
3H), 3.81 (s, 3H), 4.63 (d, 1H, J = 5.0 Hz), 5.40 (d, 1H, J = 5.0 Hz); 13C-NMR: 11.8, 18.2, 25.9, 34.8,
51.6, 52.5, 60.1, 80.8, 165.9, 167.6, 168.2; Anal. calcd. for C11H17NO5: C, 54.31; H, 7.04; N, 5.76%.
Found: C, 54.48; H, 7.05; N, 5.75%.
3-sec-Butyl-5-phenylisoxazole (entry 6): Light yellow foam; 1H-NMR: 0.81 (t, 3H, J = 7.1 Hz), 1.28 (d,
13
3H, J = 6.8 Hz), 1.38–1.69 (m, 2H), 2.87 (m, 1H), 6.31 (s, 1H), 7.30–7.78 (m, 5H); C-NMR: 12.4,
20.1, 29.3, 35.6, 102.7, 125.3, 127.8, 131.9, 132.2, 165.4, 167.3; Anal. calcd. for C13H15NO: C, 77.58;
H, 7.51; N, 6.96%. Found: C, 77.35; H, 7.53; N, 6.97%.
1
3-sec-Butyl-5-hexylisoxazole (entry 8): Light yellow oil; H-NMR: 0.88 (t, 3H, J = 7.1 Hz), 1.23 (d,
3H, J = 6.1 Hz), 1.24–1.43 (m, 10H), 1.62 (m, 2H), 2.69 (t, 2H, J = 6.6 Hz), 2.81 (m, 1H), 5.80 (s, 1H);
13C-NMR: 11.6, 14.0, 19.5, 22.4, 26.7, 27.4, 28.7, 29.2, 31.4, 33.1, 98.4, 168.3, 173.2; Anal. calcd. for
C13H23NO: C, 74.59; H, 11.07; N, 6.69%. Found: C, 75.76; H, 11.04; N, 6.68%.
1
Methyl 3-sec-butylisoxazole-5-carboxylate (entry 10): Light yellow oil; H-NMR: 0.91 (t, 3H, J = 7.3
13
Hz), 1.31 (d, 3H, J = 7.1 Hz), 1.44–1.78 (m, 2H), 2.95 (m, 1H), 3.96 (s, 3H), 7.03 (s, 1H); C-NMR:
12.3, 20.4, 30.1, 31.2, 53.1, 112.8, 157.6, 159.4, 171.7; Anal. calcd. for C9H13NO3: C, 59.00; H, 7.15;
N, 7.65%. Found: C, 58.84; H, 7.18; N, 7.63%.
1
Methyl 3-sec-butylisoxazole-4-carboxylate. Light yellow oil; H-NMR: 0.93 (t, 3H, J = 7.3 Hz), 1.34
(d, 3H, J = 7.1 Hz), 1.47–1.81 (m, 2H), 2.97 (m, 1H), 3.86 (s, 3H), 9.02 (s, 1H); 13C-NMR: 12.8, 20.6,
29.4, 30.6, 51.1, 112.5, 154.2, 167.5, 172.1; Anal. Calcd. for C9H13NO3: C, 59.00; H, 7.15; N, 7.65%.
Found: C, 59.11; H, 7.13; N, 7.66%.
Acknowledgements
We thank MIUR and the University of Catania for financial support.

Molecules 2008, 13
1236
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