Org. Synth. 2002, 78, 113
DOI: 10.15227/orgsyn.078.0113
4-HYDROXY[1-13C]BENZOIC ACID
[
Benzoic-1-13C acid, 4-hydroxy-
]
Submitted by Martin Lang, Susanne Lang-Fugmann, and Wolfgang Steglich
1
.
Checked by Tina M. Marks, Nathan X. Yu, and Edward J. J. Grabowski.
1. Procedure
A.
Diethyl
[2-13C]malonate
(Note 1). A flame-dried, 100-mL, round-bottomed Schlenk
flask equipped with a rubber septum and a magnetic
stirring bar is purged with argon. The flask is charged
with
15 mL of anhydrous tetrahydrofuran
(THF)
(Note 2) and
5.8
mL (28 mmol, 2.5 equiv) of hexamethyldisilazane
(Note 3). After the solution is cooled to 0°C in an ice-water
bath,
9.4 mL (23.5 mmol,
2.1 equiv) of a solution of butyllithium
(2.5 M in
hexanes) (Note 4) is added slowly via a syringe to the stirred
solution. The ice bath is removed and the mixture is allowed
to warm to room temperature. After the solution is stirred for 30 min, it is cooled
to −78°C using an acetone-dry ice bath and equilibrated
for 5 min at the same temperature. Then
1.00 g
(11.2 mmol, 1 equiv) of ethyl [2-13C]acetate
(Note 5) is added within 5 min via a syringe, and the acetate-containing
flask is rinsed with
0.5 mL of anhydrous
THF. Stirring is continued at −78°C for 20 min, and
1.07
mL (11.2 mmol, 1 equiv) of ethyl chloroformate
(Note 6) is added within 5 min via a syringe. The mixture is
stirred for 1 hr at −78°C, and
5 mL
of 6 M hydrochloric acid
(HCl) is added in one portion
(Note 7). The mixture is allowed to warm to room temperature,
and after the addition of 20 mL of water, the pH of the solution is adjusted to 1-2
with 2 M HCl. The mixture is extracted with
diethyl
ether (3 × 50 mL), and the combined organic
phases are washed successively with 2 M HCl, water, and brine
(30 mL each). The HCl and water phases are combined and
reextracted with ether (50 mL). The organic layer is washed with brine
(20 mL) and added to the combined organic phases. The
combined extracts are dried over anhydrous sodium
sulfate
(Na2SO4), filtered and concentrated
under reduced pressure with a rotary evaporator (200 mbar/35°C,
150 mm/35°C). The crude product is distilled in a microdistillation apparatus at 90
mbar (67.5 mm) to give 1.66 g(10.3 mmol, 92%)
of
diethyl [2-13C]malonate
as a colorless liquid (Notes 8 and 9).
B.
Ethyl
4-hydroxy[1-13C]benzoate
.
A 100-mL, single-necked, round-bottomed flask equipped with
a magnetic stirring bar, pressure-equalizing addition
funnel and a reflux condenser beyond the dropping
funnel
(Note 10) is charged with
20
mL of tert-butyl alcohol
(t-BuOH) (Note 11),
1.01 g (10.5
mmol, 1.05 equiv) of 4H-pyran-4-one
(Note 12), and
1.61 g (10.0
mmol, 1 equiv) of diethyl [2-13C]malonate
.
The condenser is sealed with a silica gel drying tube, and
the stirred solution is put in an oil bath at 105°C. A solution
of
0.22 g (2.0 mmol, 0.2 equiv)
of potassium tert-butoxide
(Note 13)
in
20 mL of tert-butyl alcohol
(t-BuOH) is added dropwise via the funnel during 20 min; the mixture turns red and
turbid. After the mixture is heated under reflux for 15 hr, it is allowed to cool
to room temperature. Water (30 mL) is added, followed by
5
mL of 2 M HCl. After removal of most of the solvent with a rotary
evaporator, the aqueous mixture is extracted with
diethyl
ether (3 × 50 mL). The combined organic phases
are washed with water and brine (each 30 mL).
Drying over Na2SO4, filtration, and removal of the solvent under
reduced pressure with a rotary evaporator affords the crude
product. Flash chromatography on silica gel (Note 14) with
ethyl acetate/petroleum ether (4:1)
as eluent affords 1.34 g (8.01 mmol, 80%)
of
ethyl 4-hydroxy[1-13C]benzoate
,
mp 112-113°C
(Note 15).
C.
4-Hydroxy[1-13C]benzoic
acid
. A 25-mL,
one-necked, round-bottomed flask equipped with a magnetic stirring
bar is charged with
1.25 g (7.50
mmol) of ethyl 4-hydroxy[1-13C]benzoate
and
11.3 mL (22.5 mmol, 3
equiv) of 2 M sodium hydroxide
(NaOH). After the solution
is stirred for 24 hr at room temperature,
17 mL
of 2 M HCl is added slowly, whereby the product precipitates. Water is
added (10 mL), and the mixture is extracted with
diethyl
ether (3 × 50 mL). The combined organic phases
are washed with 1 M HCl (2 × 30 mL).
Removal of the solvent under reduced pressure on a rotary evaporator and drying under
reduced pressure affords 1.02 g
(7.34 mmol, 98%) of
4-hydroxy[1-13C]benzoic
acid
(mp 212-213°C)
(Note 16), which can be used for feeding experiments without
further purification.
2. Notes
1.
The procedure follows closely that of Mueller and Leete
2 with slight improvements by the submitters.
2.
Tetrahydrofuran
was distilled from potassium and benzophenone
under an argon atmosphere immediately before use. The checkers
used anhydrous THF purchased from Aldrich Chemical Company, Inc.
3.
Hexamethyldisilazane
(98%), purchased from Lancaster Synthesis Inc.
or Aldrich Chemical Company, Inc.
, was used as received.
4.
Butyllithium (2.5
M in hexanes) was purchased from Aldrich Chemical Company, Inc.
The actual concentration was determined by titration with
diphenylacetic
acid or
4-biphenylmethanol.
3
5.
Ethyl [2-13C]acetate
is commercially available (Aldrich Chemical Company, Inc.),
but expensive. The compound can be prepared by O-ethylation
4,5,6 of the cheaper
sodium [2-13C]acetate (Aldrich
Chemical Company, Inc.) or via
[2-13C]acetyl
chloride.
7
6.
Ethyl chloroformate
(97%), purchased from Aldrich Chemical Company, Inc.
,
was used as received.
7.
The solution should be quenched prior to warming to room temperature.
Solutions of lithiated
ethyl acetate decompose rapidly at 0°C.
8
8.
The receiver is cooled to −10°C. Cooling to −78°C
is not advisable because obstruction may occur. The product (
bp 123-125°C/90 mbar, 67.5 mm) can be separated from the hydrolysis
products
trimethylsilanol and traces of
hexamethyldisiloxane.
9 At the end, the apparatus is rinsed with
diethyl ether
to obtain all the product.
9.
The spectral data are as follows:
1H NMR (300 MHz, CDCl
3) δ:
1.27 (t, 6 H, J = 7.2), 3.34 (d, 2 H, J = 132), 4.19
(q, 4 H, J = 7.2)
;
13C
NMR (75 MHz, CDCl
3) δ: 14.1, 41.7 (
13C),
61.5, 166.7 (d, J = 59)
; MS (EI): 161 [M+] (3), 134 (40), 116 (100),
106 (7), 89 (52), 61 (31)
; IR (KBr) cm
−1: 3465 (br),
2986, 2942, 1754, 1733, 1467,
1448, 1410, 1369, 1319, 1267,
1189, 1151, 1097, 1035, 949,
866, 844, 787, 666, 603
.
10.
Because of the relatively high melting point of the solvent (23-26°C),
it is not advisable to put the condenser directly on the flask. With the addition
funnel between the flask and the condenser, most of the
tert-butyl alcohol
is condensed as a liquid and does not collect as a solid on the cold condenser.
11.
tert-Butyl alcohol
(=99.7%), purchased from Fluka Chemical Corp.
or Fisher Scientific
, was used as received.
12.
4H-Pyran-4-one
is commercially available (98+%, Aldrich Chemical Company, Inc.),
but the substance is expensive. It can be synthesized by decarboxylation of
chelidonic acid monohydrate (Lancaster Synthesis Inc.)
following the procedure of De Souza and co-workers.
10
13.
Potassium tert-butoxide (99%),
purchased from Fluka Chemical Corp. or Aldrich Chemical
Company, Inc.
, was used as received.
14.
Flash chromatography was performed on
E.
Merck silica gel 230-400 mesh: 150 g of silica gel was loaded
on a 7- × 2-in size column using a minimum amount of
ethyl acetate
as loading solvent. The checkers used a 90-g silica column purchased from Biotage.
15.
The spectral data are as follows:
1H NMR (300 MHz, CDCl
3) δ:
1.39 (t, 3 H, J = 7.2), 4.36 (q, 2 H, J = 7.2), 6.50
(s, 1 H), 6.85-6.93 (m, 2 H), 7.93-7.99 (m, 2 H)
;
13C NMR (75 MHz,
CDCl
3) δ: 14.4, 61.1, 115.3 (d, J
= 1.5), 122.7 (
13C), 132.0 (d, J = 60),
160.4 (d, J = 9.1), 167.2 (d, J = 77)
; MS (EI): 167 [M+] (30), 139 (23),
122 (100), 94 (11), 83 (10)
; IR (KBr) cm
−1: 3218 (br),
1672, 1602, 1583, 1441, 1370,
1306, 1286, 1239, 1169, 1104,
1018, 847, 768, 723, 697,
618
.
16.
The spectral data are as follows:
1H NMR (300 MHz, DMSO-d
6) δ:
6.77-6.85 (m, 2 H), 7.75-7.81 (m, 2 H), 10.2 (s, br,
≈0.8 H), 12.4 (s, br, ≈0.8 H)
; (The coupling
pattern of the aromatic protons is even at 600 MHz not clearly resolved.)
13C NMR (75 MHz, DMSO-d
6)
δ: 115.3, 121.6 (
13C), 131.7 (d,
J = 59), 161.8 (d, J = 8.5), 167.3 (d, J = 74)
;
MS (EI) 139 [M+] (100), 122
(94), 94 (20)
; IR
(KBr) cm
−1: 3394 (br), 2966, 2831,
2660, 2562, 1677, 1602, 1588,
1504, 1440, 1421, 1309, 1282,
1243, 1169, 1100, 933, 852,
766, 617, 548
.
Handling and Disposal of Hazardous Chemicals
The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.
These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.
3. Discussion
4-Hydroxybenzoic acid
acts as biosynthetic precursor for several secondary metabolites.
11 Since
in many cases decarboxylation takes place on route to the metabolites,
12 ring-
13C-labeled 4-hydroxybenzoic acids
are in demand for biosynthetic studies.
Previous syntheses of ring-labeled
4-hydroxybenzoic acid use
many steps and show low overall yields.
4-Hydroxy[3-13C]benzoic
acid
was synthesized in six steps from
ethyl [1-13C]acetate
with
2.8% overall yield.
13
4-Hydroxy[3,5-13C2]benzoic acid
was prepared in five steps from
[1,3-13C2]acetone
with an overall yield of less than
4.5%.
12b
4-Hydroxy[2,6-13C2]benzoic
acid
was generated microbiologically from
[1-13C]glucose
by using a mutant strain of
Klebsiella pneumoniae.
14 For 120 mg of product, 18 g of labeled
glucose was necessary.
Methyl
4-methoxy[3,5-13C2]benzoate
was obtained
from
[1,3-13C2]acetone
in five steps with 32% overall
yield.
15 Baldwin
and co-workers
16 synthesized
methyl 4-methoxy[3,4,5-13C3]benzoate
in four steps from
[1,2,3-13C3]acetone
without indicating the overall yield.
The present procedure affords
4-hydroxy[1-13C]benzoic
acid
from
ethyl [2-13C]acetate
in three steps with
72% overall
yield. It is based on an observation of Woodward
17 that
ethyl
4-hydroxybenzoate
is formed by base-catalyzed condensation of
4H-pyran-4-one
with
diethyl malonate
. The submitters studied
several solvent-base combinations for this reaction and found that
tert-butyl
alcohol/
potassium tert-butoxide gave the highest yields.
When a stoichiometric amount of the base is used, an excess of
4H-pyran-4-one
has to be used.
4b This can be avoided by
the use of substoichiometric amounts as given in the procedure.
The use of
ethyl [2-13C]acetoacetate
instead of
diethyl [2-13C]malonate
in the condensation reaction with
4H-pyran-4-one
afforded
ethyl 4-hydroxy[1-13C]benzoate
in 87% yield. In
this case,
1.1 equiv of 4H-pyran-4-one
and
1.1 equiv of potassium tert-butoxide
were optimal. The addition of catalytic amounts of the base was not satisfactory.
Ethyl [2-13C]acetoacetate
was prepared from
ethyl [2-13C]acetate
as described for
diethyl [2-13C]malonate.
18 The
maximum yield for this reaction on a 10-mmol scale was only
70% after distillation.
4H-Pyran-4-one
reacted with
nitromethane and
potassium tert-butoxide
(each 1.1 equiv) to afford
4-nitrophenol
in 75% yield after
purification by flash chromatography. This gives easy access to
4-nitro[4-13C]phenol.
With
2,4-pentanedione
, the
condensation with
4H-pyran-4-one
under the same reaction conditions gave
4-hydroxyacetophenone
in
45-50% yield after purification.
Ethyl 4-hydroxy[1-13C]benzoate
can be converted into other ring-
13C-labeled compounds like
3,4-dihydroxy[1-13C]benzoic
acid,
1,3,4-trihydroxy[1-13C]benzene, and
D,L-[1'-13C]tyrosine.
4b
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
4-Hydroxybenzoic-1-13C acid:
Benzoic-1-13C
acid, 4-hydroxy- (14); (211519-30-1)
Diethyl malonate-2-13C:
Propanedioic-2-13C
acid, diethyl ester (10); (67035-94-3)
1,1,1,3,3,3-Hexamethyldisilazane:
Disilazane,
1,1,1,3,3,3-hexamethyl- (8);
Silanamine, 1,1,1-trimethyl-N-(trimethylsilyl)-
(9); (999-97-3)
Butyllithium:
Lithium, butyl-
(8,9); (109-72-8)
Ethyl acetate-2-13C:
Acetic-2-13C
acid, ethyl ester (9); (58735-82-3)
Ethyl chloroformate:
Formic acid, chloro-,
ethyl ester (8);
Carbonochloridic acid, ethyl ester
(9); (541-41-3)
Ethyl 4-hydroxybenzoate-1-13C:
Benzoic-1-13C
acid, 4-hydroxy-, ethyl ester (14); (211519-29-8)
tert-Butyl alcohol (8);
2-Propanol, 2-methyl-
(9); (75-65-0)
4H-Pyran-4-one (8,9); (108-97-4)
Potassium tert-butoxide:
tert-Butyl alcohol,
potassium salt (8);
2-Propanol, 2-methyl-, potassium salt
(9); (865-47-4)
Diphenylacetic acid:
Acetic acid, diphenyl-
(8);
Benzeneacetic acid, α-phenyl- (9); (117-34-0)
4-Biphenylmethanol (9); (3597-91-9)
Sodium acetate-2-13C:
Acetic-2-13C
acid, sodium salt (9); (13291-89-9)
Acetyl-2-13C chloride:
Acetyl-2-13C
chloride (8,9); (14770-40-2)
Trimethylsilanol:
Silanol, trimethyl-
(8,9); (1066-40-6)
Hexamethyldisiloxane:
Disiloxane, hexamethyl-
(8,9); (107-46-0)
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