Preparation of mono-labelled aliphatic polyacids via isoxazole derivatives

Article abstract of DOI:10.1016/j.tetlet.2010.09.113

A method allowing the selective oxygen labelling of aliphatic polyacids, for example, succinic and glutaric acids, is reported. This process is based on the hydrolysis of a 3-alkyl-4-nitroisoxazol-5-yl group by Na¹⁸OH and affords the products i

Full text of DOI:10.1016/j.tetlet.2010.09.113

Tetrahedron Letters 51 (2010) 6310–6312
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Tetrahedron Letters
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Preparation of mono-labelled aliphatic polyacids via isoxazole derivatives
Mauro F. A. Adamo a, , Piero Sarti-Fantoni b, Stefano Chimichi b, Alessandro Sandrelli b
⇑
a Centre for Synthesis and Chemical Biology (CSCB), Department of Pharmaceutical and Medicinal Chemistry, The Royal College of Surgeons in Ireland,
123 St. Stephen’s Green, Dublin 2, Dublin, Ireland
b Dipartimento di Chimica Organica ‘U. Schiff’, Università di Firenze, Via della Lastruccia, Sesto Fiorentino 50019, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A method allowing the selective oxygen labelling of aliphatic polyacids, for example, succinic and glutaric
acids, is reported. This process is based on the hydrolysis of a 3-alkyl-4-nitroisoxazol-5-yl group by
Na18OH and affords the products in high yields. The same procedure was applied for the preparation
of labelled aliphatic carboxylic acids.
Received 29 May 2010
Revised 24 August 2010
Accepted 24 September 2010
Available online 18 October 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Polyfunctional scaffolds
4-Nitroisoxazole
18O-bilabelled carboxylates
3-Methyl-4-nitro-5-styrylisoxazoles 1 (Fig. 1) are valuable syn-
thons that can be prepared on multigram scale from commercially
available isoxazole 2 and an aromatic aldehyde. For example, we
have shown that 1 could be used for the preparation of spiroisox-
azolines,1 heteroarylpropionic acids,2–4 3-indolepropionic acids5
and 3-arylglutaric acids.6 In these syntheses the 4-nitroisoxazolyl
core is used as an activator of the exocyclic alkene and is converted
into a carboxylate by reaction with excess sodium hydroxide.7
During previous studies we showed that alkaline hydrolysis of
3-methyl-4-nitro-5-styrylisoxazole 1 gave the corresponding cin-
namic acid 6 (Scheme 1).8 In particular, when Na18OH was used,
the 18O-bilabelled carboxylic group in the above compound was
obtained.9 This was demonstrated by the presence of a signal at
m/z 152 in the mass spectrum which was in agreement with the
proposed bilabelled cinnamic acid.9 Similarly, 3-methyl-4-nitro-
5-styrylisoxazoles of type 1 were successfully employed to prepare
3-[2H5]-phenyl-2-propenoic-[18O]-bilabelled acids.9 Bromination
of compound 1 gave the corresponding dibromo derivative 4 which
in turn was converted into arylpropionic acid. Hydrolysis of 4 using
Na18OH gave the corresponding 18O-bilabelled acids (Scheme 1).
Methods affording labelled carboxylates are of significant inter-
est for investigating the metabolism of fatty acids and for proteo-
mics studies.10 Current literature describes:11,12 (i) generation of
an alkyl-metal reagent and its reaction with C18O2, (ii) hydrolysis
of an acid derivative, for example, an ester or an acyl chloride
and (iii) exchange procedures using H218O run on a preformed
carboxylate.
NO2
NO2
O
N
N
Ar
O
O
Ar
1
2
3
Figure 1. The polyfunctional 5-styryl-4-nitroisoxazole scaffold 1.
These methods worked well for the preparation of simple la-
belled carboxylates, however, they could not be applied to the
selective labelling of molecules possessing two or more carboxyl-
ates. Conversely, our strategy, in which a carboxylate is introduced
as a 4-nitroisoxazole, could be employed for the preparation of
such compounds. In order to demonstrate this point, we applied
this procedure to the preparation of mono-labelled succinic acids
9a–e and glutaric acids 12a–e (Scheme 2, Table 1). These syntheses
were based on previous studies demonstrating the reactivity of
isoxazoles 1a–e as Michael acceptors toward classic donors, such
as nitromethane3 and diethyl malonate.4 Hence, compounds 1a–e
were reacted with nitromethane to afford adducts 7a–e, which in
turn were converted into acids 8a–e using the Victor Meyer proce-
dure. Compounds 8a–e were subsequently reacted with 5 equiv of
Na18OH, readily generated by the addition of Na0 to commercially
available enriched 18OH2. Delightfully, this reaction furnished
2-arylsuccinic acids 9a–e in high isolated yields (Table 1). Impor-
tantly the mass spectra of compounds 9a–e and 12a–e displayed
peaks demonstrating the double addition of 18O to C-5 of the
isoxazole. For example, the mass spectrum of compound 9a
showed a peak at m/z = 198 only, clearly indicating the presence
of two atoms of 18O in the mass ion M+. The fragmentation showed
two main peaks at m/z = 150 and m/z = 154 which were attributed
⇑
Corresponding author. Tel.: +353 1 4022208; fax: +353 1 4022168.
E-mail address: madamo@rcsi.ie (M.F.A. Adamo).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.113

M. F. A. Adamo et al. / Tetrahedron Letters 51 (2010) 6310–6312
6311
NO2
NO2
H18O18OC
N
Br
Ph
N
C18O18OH
O
Ph
O
Ph
Br
Ph
6
1
4
5
Scheme 1. Preparation of [18O]-bilabelled cinnamic 6 and phenylpropionic acid 5.
NO2
NO2
NO2
H18O18OC
N
N
N
O
Na18OH
O
O
CH3NO2
Piperidine
conc HCl
Ar
COOH
120 °C, 3 h
120 °C, 36 h
Ar
Ar
COOH
Ar
1a-e
7a-e
9a-e
8a-e
NO2
NO2
NO2
CH2(CO2Et)2
Piperidine
H18O18OC
Na18OH
N
N
conc HCl
O
O
COOH
120 °C, 16 h
EtO2C
120 °C, 3 h
Ar
Ar
CO2Et
Ar
COOH
10a-e
11a-e
12a-e
Scheme 2. Preparation of [18O]-mono-labelled phenylsuccinic acids 9a–e and phenylglutaric acids 12a–e.
Table 1
Yields and significant m/z data of phenylsuccinic acids 9a–e and phenylglutaric acids 12a–e
Entry
Ar
Reactant
Product
Yielda (%)
m/z
[M+]
[M+ÀC16O2]
[M+ÀC18O2]
1
2
3
4
5
6
7
8
9
C6H5
8a
8b
8c
8d
9a
9b
9c
9d
83
76
69
75
61
79
76
65
77
62
198 (38%)
232 (21%)
228 (21%)
212 (27%)
243 (17%)
212 (15%)
246 (18%)
242 (10%)
226 (23%)
257 (11%)
154 (30%)
188 (25%)
184 (16%)
168 (28%)
199 (12%)
168 (10%)
202 (16%)
198 (8%)
150 (15%)
184 (63%)
180 (47%)
164 (33%)
195 (57%)
164 (24%)
198 (32%)
194 (39%)
182 (25%)
209 (14%)
p-Cl–C6H4
p-CH3O–C6H4
p-CH3–C6H4
p-NO2–C6H4
C6H5
p-Cl–C6H4
p-CH3O–C6H4
p-CH3–C6H4
p-NO2–C6H4
8e
9e
11a
11b
11c
11d
11e
12a
12b
12c
12d
12e
182 (13%)
213 (6%)
10
a
Isolated yield after crystallisation.
to [M+ÀC18O2] and [M+ÀC16O2]. The absence of a signal at m/z =
152 attributed to [M+ÀC18O16O] ruled out the presence of mono-
labelled carboxylic acids which were not formed under the
adopted conditions. Similarly, isoxazoles 1a–e were reacted with
diethyl malonate to afford adducts 10a–e which in turn were
hydrolysed to give compounds 11a–e. Reaction of compounds
11a–e with 5 equiv of Na18OH afforded the expected glutaric acids
12a–e which showed mass spectra consistent with double addition
of 18O to C-5 of the isoxazole (Table 1). As a final extension of this
study, we have shown that 4-nitroisoxazoles could be used as
starting materials for the preparation of simple aliphatic
18O-bilabelled acids (Scheme 3). Therefore, 3,5-dimethyl-4-nitrois-
oxazole (2), 3,5-diethyl-4-nitroisoxazole (14) and 3-methyl-5-
butyl-4-nitroisoxazole (15) were prepared according to literature
procedures.13
Alkaline hydrolysis of compounds 2, 14 and 15 with Na18OH led
to labelled acetic, propionic and pentanoic acids, respectively. The
mass spectra of those acids showed peaks at m/z values in agree-
ment with the presence of a C18O18OH group.
In conclusion, we have shown that 4-nitroisoxazoles could be
employed for introduction of labelled carboxylates at specific posi-
tions of carbon frameworks. The desired labelled carboxylates were
obtained in high isolated yields. This study adds to the numerous
applications of 4-nitroisoxazoles in synthesis and furnishes exam-
ples involved in the study of fatty acid metabolism and represents
an efficient means to prepare labelled carboxylic acids.
1. General procedure for the preparation of compounds 9a–e
and 12a–e
4-Nitroisoxazoles 8a–e or 11a–e (0.5 mmol), in
a round-
bottomed flask, were suspended in 1 N Na18OH solution (2.5 mL).
The reaction mixture was refluxed for 3 h, then allowed to cool
to room temperature and the pH was adjusted to 1 by addition
of 6 N HCl. The solvent was evaporated to yield a solid which
was purified by crystallisation.
R
NO2
R1
Na18OH
N
H18O18OC-R1
O
16 R = R1 = Me;
2
R = R1 = Me;
14 R = R1 = Et;
15 R = Me , R1 = n-Bu;
1.1. 2-Phenylsuccinic acid (9a):14
17 R = R1 = Et;
18 R = Me , R1 = n-Bu;
Colourless solid (82 mg, yield 83%) mp 164–166 °C; mmax (KBr)/
cmÀ1: 3600–3200 broad, 1694, 726, 700; dH (200 MHz, CDCl3)
Scheme 3. Preparation of [18O]-labelled aliphatic acids 16–18.

6312
M. F. A. Adamo et al. / Tetrahedron Letters 51 (2010) 6310–6312
11.20 (2H, br s), 7.22–6.99 (5H, m), 3.95–3.78 (1H, dd, J = 9.5,
J = 5.5 Hz), 3.06–2.57 (2H, m); dC (50.3 MHz, CDCl3) 175.2, 173.9,
139.0, 129.1, 128.3, 127.7, 47.7, 38.2; m/z 198 (M+, 38%). Elemental
Anal. Calcd for C10H10O218O2: C, 60.6; H, 5.1. Found: C, 60.2; H, 5.2.
129.1, 128.4, 37.5, 20.3; m/z 246 (M+, 18%). Elemental Anal. Calcd
for C11H11ClO218O2: C, 53.6; H, 4.5. Found: C, 53.3; H, 4.7.
1.6. 3-(4-Methoxyphenyl)pentanedioic acid (12c):6
1.2. 2-(4-Chlorophenyl)succinic acid (9b):15
Colourless solid (78 mg, 65% yield), mp 99–103 °C; mmax (KBr)/
cmÀ1: 3560, 1736, 1289; dH (400 MHz, CDCl3) 12.15 (2H, br s),
7.71 (2H, d, J = 8.0), 7.39 (2H, J = 8.0), 3.81 (3H, s), 3.54–3.47 (1H,
m), 2.66–2.62 (2H, m), 2.58–2.54 (2H, m); dC (100.6 MHz, CDCl3)
174.9, 158.8, 141.6, 128.9, 121.1, 61.5, 35.4, 20.2; m/z 242 (M+,
10%). Elemental Anal. Calcd for C12H14O318O2: C, 59.5; H, 5.8.
Found: C, 59.1; H, 6.0.
Colourless solid (88 mg, yield 76%) mp 201–203 °C; mmax (KBr)/
cmÀ1: 3620–3190 broad, 1708; dH (200 MHz, DMSO-d6) 12.5 (2H,
br s), 7.37–7.21 (4H, m), 3.93–3.71 (1H, m), 2.91–2.42 (2H, m); dC
(50.3 MHz, DMSO-d6) 178.9, 173.1, 133.9, 132.1, 129.6, 128.8,
46.9, 37.9; m/z 232 (M+, 21%). Elemental Anal. Calcd for
C
10H9ClO218O2: C, 51.63; H, 3.90. Found: C, 51.2; H, 3.7.
Acknowledgement
1.3. 2-(4-Methoxyphenyl)succinic acid (9c):16
We are grateful to the PTRLI cycle III for a grant to MFAA.
References and notes
Colourless solid (79 mg, yield 69%) mp 205–206 °C; mmax (KBr)/
cmÀ1: 3620–3190 broad, 1704s; dH (200 MHz, acetone-d6) 11.1
(2H, br s), 7.28–7.26 (2H, m), 6.91–6.87 (2H, m), 3.99 (q,
J = 4.0 Hz, 1H), 3.76 (s, 3H), 3.10 (dd, J = 8.0 Hz, J = 4.0 Hz, 1H),
2.61 (d, J = 12.0 Hz, 1H); dC (50.3 MHz, acetone-d6) 174.6, 172.9,
159.9, 131.5, 129.7, 114.8, 55.5; m/z 228 (M+, 21%). Elemental Anal.
Calcd for C11H12O318O2: C, 57.9; H, 5.3. Found: C, 57.5; H, 5.4.
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1.4. 3-Phenylpentanedioic acid (12a):6
Colourless solid (84 mg, 79% yield), mp 103–107 °C; mmax (KBr)/
cmÀ1: 3665, 1743, 1270; dH (400 MHz, CDCl3) 11.90 (2H, br s),
7.20–7.16 (5H, s), 3.35–3.31 (1H, m), 2.66–2.63 (2H, m), 2.47–
2.45 (2H, m); dC (100.6 MHz, CDCl3) 175.1, 141.6, 135.5, 128.0,
126.6, 36.6, 20.2; m/z 212 (M+, 15%). Elemental Anal. Calcd for
C
11H12O218O2: C, 62.3; H, 5.7. Found: C, 62.5; H, 6.0.
1.5. 3-(4-Chlorophenyl)pentanedioic acid (12b):6
12. Ott, D. G. In Synthesis with Stable Isotopes; J. Wiley & Sons, Wiley-Interscience
Publications: New York, 1981. p. 16–84.
13. Kashima, C. Heterocycles 1979, 12, 1343.
14. Freeman, J. P.; Kassner, J. A.; Grabiak, R. C. J. Org. Chem. 1975, 40, 3402.
15. Kamlet, M. J. J. Am. Chem. Soc. 1955, 77, 4896.
16. Yuan, G.-Q. Electrochim. Acta 2008, 53, 2170.
Colourless solid (94 mg, 76% yield), mp 131–136 °C; mmax (KBr)/
cmÀ1: 3480, 1745, 1286; dH (400 MHz, CDCl3) 12.25 (2H, br s), 7.32
(2H, d, J = 8.0), 7.26 (2H, J = 8.0), 3.41–3.35 (1H, m), 2.66–2.64 (2H,
m), 2.54–2.47 (2H, m); dC (100.6 MHz, CDCl3) 172.3, 142.9, 130.5,