Vicarious nucleophilic substitution: A dramatically shortened synthesis of 2-amino-6-nitrobenzoic acid labelled with carbon-14

Article abstract of DOI:10.1002/jlcr.1873

Literature preparations of 2-amino-6-nitrobenzoic acid are usually based on phthalic anhydride. In order to make [benzene-¹⁴C(U)]-2-amino-6- nitrobenzoic acid, [benzene-¹⁴C(U)]-phthalic anhydride has to be prepared in multiple steps from [¹⁴C(U)]-benzene, resulting in an unacceptably lengthy 14-step synthesis. We have been able to develop a completely different method of synthesis, producing [benzene- ¹⁴C(U)]-2-amino-6-nitrobenzoic acid from [¹⁴C(U)]-benzene in just four steps with an overall radiochemical yield of 32%.

Full text of DOI:10.1002/jlcr.1873

Research Article
Received 10 October 2010,
Revised 4 December 2010,
Accepted 11 December 2010
Published online 17 February 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1873
Vicarious nucleophilic substitution: a
dramatically shortened synthesis of 2-amino-
6-nitrobenzoic acid labelled with carbon-14
Ã
Terence P. Kelly, Crist N. Filer, and Christopher Wright
Literature preparations of 2-amino-6-nitrobenzoic acid are usually based on phthalic anhydride. In order to make
[benzene-14C(U)]-2-amino-6-nitrobenzoic acid, [benzene-14C(U)]-phthalic anhydride has to be prepared in multiple steps
from [14C(U)]-benzene, resulting in an unacceptably lengthy 14-step synthesis. We have been able to develop a completely
different method of synthesis, producing [benzene-14C(U)]-2-amino-6-nitrobenzoic acid from [14C(U)]-benzene in just four
steps with an overall radiochemical yield of 32%.
Keywords: 14C; vicarious nucleophilic substitution; Umpolung; Fremy’s salt; Zinin reduction
which was from by DeCon Labs. TLC analyses were performed
on Analtech Silica GF and GHLF plates. Solvent was removed
Introduction
2-Amino-6-nitrobenzoic acid (1) has been known for over 100
years,1 but it has been little studied or utilized.2 Interest has
grown considerably in the past few years, however, since this
compound has now been recognized as a valuable tool in
the synthesis of a variety of commercial substances.3 Large
quantities of 2-amino-6-nitrobenzoic acid are therefore needed
today, but it appears that this material is still prepared from
phthalic anhydride via the multi-step route shown in Scheme 14
or slight variations thereof.
We were requested by a customer to prepare a significant
quantity of [benzene-14C(U)]-2-amino-6-nitrobenzoic acid for
commercial development studies. Previously, we had prepared
this same substance on a smaller scale via [benzene-14C(U)]-
phthalic anhydride in a 10.5% radiochemical yield from [14C(U)]-
benzene. This was a viable method for making several hundred
millicuries of product (Scheme 2), but unacceptable for
preparing larger amounts because of 14C precursor and radio-
active waste costs. Our goal was to increase the overall
radiochemical yield of [benzene-14C(U)]-2-amino-6-nitrobenzoic
acid to at least 30%.
during work-up by rotary evaporation. Proton NMR spectra
(300 MHz) were obtained on a Bruker Icon NMR Spectrometer.
Scintillation counting was performed on a PerkinElmer Tri-Carb
3100TR Liquid Scintillation Analyzer.
[Benzene-14C(U)]-2-(2,6-dinitrophenyl)acetonitrile
Solid copper(I) chloride (2970 mg, 30 mmol) was placed in a
goose neck addition funnel and attached to a 500 mL four-neck
round bottom flask with magnetic stir bar, low-temperature
thermometer and rubber septum side arm. The apparatus was
secured to a vacuum line and flame dried under vacuum. The
reaction vessel was cooled to room temperature and placed
under an argon atmosphere. Potassium t-butoxide (42 mmol, 1M
in THF) was introduced via syringe and the solvent removed
under vacuum to give a white solid. The system was again
placed under an argon atmosphere and anhydrous DME
(180 mL) was added via syringe. Copper(I) chloride was then
introduced from the solid additional funnel in one portion,
giving
a dark mixture. Stirring was continued at room
temperature for 45 min. To this mixture was added anhydrous
pyridine (12 mL) followed by [benzene-14C(U)]-1,3-dinitroben-
zene5 (12 mmol, 720 mCi) in 30 mL of anhydrous DME. The
reaction was cooled to an internal temperature of À201C and
bromoacetonitrile (12 mmol, 0.835 mL) in 30 mL of anhydrous
DME was then added via cannula over 15 min, maintaining the
internal temperature at À201C. Stirring was continued at À201C
for 2 h. The reaction was allowed to warm to room temperature,
This paper describes a novel four-step high-yield synthesis
of [benzene-14C(U)]-2-amino-6-nitrobenzoic acid from [14C(U)]-
benzene. Additionally, with regard to the synthesis of unlabelled
2-amino-6-nitrobenzoic acid itself, the last step of our route
(reduction of 2,6-dinitrobenzoic acid to 2-amino-6-nitrobenzoic
acid ) also provides an alternative two-step, large-scale, high-
yield synthesis of 1 from commercially available and inexpensive
2, 6-dinitrotoluene.
Experimental
PerkinElmer Health Sciences, Inc., 940 Winter St., Waltham, MA 02451, USA
General
*Correspondence to: Christopher Wright, PerkinElmer Health Sciences, Inc., 549
Albany St, Boston, MA 02118, USA.
E-mail: crist.filer@perkinelmer.com
Reagents were purchased from Sigma-Aldrich unless otherwise
noted. Solvents were supplied by J. T. Baker, except for ethanol
J. Label Compd. Radiopharm 2011, 54 345–351
Copyright r 2011 John Wiley & Sons, Ltd.

T. P. Kelly et al.
NH2
CO2H
NO2
1
O
O
O
O
CO2H
CO2H
Ac2O
HNO3
O
H2SO4
O
NO2
NO2
2.
1. NH3
O
CONH2
NH2
NaOBr
NH
O
CO2H
CO2H
NO2
NO2
NO2
Scheme 1. Synthesis of 2-amino-6-nitrobenzoic acid from phthalic anhydride (the standard route since 1901).
OH
H
O
O
N
O
N
N
COCl
*
O
CO2H
CO2H
CO2H
O
O
O
*
*
*
*
*
*
6693 mCi
O
CO2CH3
CO2CH3
CO2H
CO2H
CO2H
CO2H
Separate
isomers
O
*
O
O
*
*
*
*
O
NO2
NO2
NO2
NO2
NO2
+ 4-nitro isomer
+ 4-nitro isomer
+ 4-nitro isomer
4 steps
NH2
*
CO2H
NO2
705 mCi
Overall yield 10.5%
Scheme 2. Synthesis of [benzene-14C(U)]-2-amino-6-nitrobenzoic acid via [benzene-14C(U)]-phthalic anhydride.
[Benzene-14C(U)]-2,6-dinitrobenzoic acid
stirred for 0.5 h and then quenched by the addition of 60 mL of
3 N HCl. The mixture was poured into 600 mL water and
extracted with 8 Â 50 mL portions of ether. The extracts were In a 1 L Erlenmeyer flask with a large magnetic stir bar was
dried over anhydrous sodium sulfate, filtered and evaporated to placed Fremy’s Salt (potassium nitrosodisulfonate, 75 mmol,
dryness, affording a dark brown oil. This same reaction was 20.1 g) in 750 mL of 4% aqueous sodium carbonate. Separately,
again repeated on an identical scale. After work-up, the brown [benzene-14C(U)]-2-(2,6-dinitrophenyl)acetonitrile (15 mmol, 900 mCi)
oils from both reactions were combined and purified by silica was dissolved in 75 mL of acetonitrile and added to the Fremy’s
gel flash chromatography using a mobile phase of hexane:ethyl Salt solution over 25 min. The reaction was stirred at room
acetate (3:1) to afford 2.6 g (12.5 mmol, 749 mCi, 52% radio- temperature and the reaction progress was monitored by TLC
chemical yield) of [benzene-14C(U)]-2-(2,6-dinitrophenyl)acetoni- on silica gel GF (chloroform:methanol:ammonium hydroxide
trile as a yellow solid. TLC analysis (silica gel GF, hexane:ethyl (80:20:4)). TLC analysis indicated that the reaction was complete
acetate (3:1)) showed the product to be 498% radiochemically after 3 h, with the product co-eluting at Rf = 0.3 beside
pure. It also co-eluted with previously prepared unlabelled unlabelled standard (ABCR GmbH & Co. KG). The reaction
standard6 (Rf = 0.5). Approximately 30% of the starting material mixture was extracted with ether (1 Â 100 mL, 2 Â 50 mL) and
was recovered from the column purification and recycled.
the organic extracts were discarded. The aqueous layer was then
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Copyright r 2011 John Wiley & Sons, Ltd.
J. Label Compd. Radiopharm 2011, 54 345–351

T. P. Kelly et al.
carefully acidified with concentrated HCl (70 mL). The precipi- bath set at 85–901C and left to reflux for 0.5 h. Methanol and
tated product was extracted with ether (4 Â 100 mL). The water were removed by rotary evaporation. The resulting dark
combined extracts were dried over magnesium sulfate and the oil was dissolved in water (75 mL) with an equal volume of ether
drying agent was removed by filtration, yielding 785 mCi (87% and the mixture was rapidly stirred for 1 h. The layers were
radiochemical yield) of radiochemically pure product. From separated, discarding the organic extracts. The aqueous layer
several combined reactions starting with 41.2 mmol of [benze- was carefully acidified with 6N HCl to a pH of 1.5 and extracted
ne-14C(U)]-2-(2,6-dinitrophenyl)acetonitrile, a total of 35.2 mmol with 4 Â 50 mL portions of ethyl acetate. The combined organic
(2112 mCi, 85% radiochemical yield) of light brown [benzene- extracts were dried over sodium sulfate, filtered and evaprated
14C(U)]-2,6-dinitrobenzoic acid was obtained. TLC analysis (silica to dryness. TLC analysis (silica GF, chloroform:methanol:ammo-
gel GF, chloroform:methanol:ammonium hydroxide (80:20:4)): nium hydroxide (80:20:4), co-spotted with unlabelled standard)
98% radiochemical purity.
showed almost all desired product. Purification was accom-
plished by silica gel flash chromatography using a mobile phase
of hexane:ether:acetic acid (50:50:1). A total of 1118 mCi
(18.6 mmol, 82% radiochemical yield) was obtained and
evaporation afforded the desired product as a deep yellow
solid with a gravimetrically measured specific activity of 60 mCi/
mmol. TLC analysis (silica gel GF, chloroform:methanol:ammo-
nium hydroxide (80:20:4), Rf = 0.2):495% radiochemical purity.
HPLC analysis (reverse phase gradient elution of 0.1% aqueous
trifluoroacetic acid to acetonitrile 0–100% over 60 min with a
retention time of 20 min): 498% radiochemical purity. In both
chromatographic systems the 14C product co-eluted with
authentic standard. 1H NMR (CDCl3)d6.55 (1H, d), 6.61 (1H, d),
6.93 ppm (1H, t).
[Benzene-14C(U)]-2-amino-6-nitrobenzoic acid
In a 125 mL Erlenmeyer flask with magnetic stirring bar was
placed solid sodium sulfide (Alfa Aesar, 5.3 g, 68.1 mmol). The
solid was dissolved in 34 mL water, giving a clear, colorless
solution. The flask was cooled in an ice bath and solid sodium
bicarbonate (5.7 g, 68.1 mmol) was added portionwise over
5 min. When the addition was complete, the resulting solution
was left to stir for 25 min at room temperature. Methanol
(34 mL) was added, resulting in a white precipitate and stirring
of the mixture was continued for an additional 1.5 h. The
reaction was then filtered, giving a clear, colorless solution.
Separately, [benzene-14C(U)]-2,6-dinitrobenzoic acid (22.7 mmol,
1362 mCi) was dissolved in a mixture of methanol (45.4 mL) and
water (9.1 mL). The clear red-orange solution was stirred at room
temperature, and solid sodium bicarbonate (1.9 g, 22.7 mmol)
Results and discussion
was then added slowly over several minutes. When the Retrosynthetic analysis (Scheme 3) suggested four possible
evolution of gas ceased, the reaction was cooled in an ice bath syntheses of [benzene-14C(U)]-2-amino-6-nitrobenzoic acid in
and the sodium hydrogen sulfide solution was rapidly added addition to the route from [benzene-14C(U)]-phthalic anhydride
dropwise over 10 min. Finally, the reaction was placed in an oil (route 1 in Scheme 3). The potential success of each approach
O
O
NH2
*
O
*
*
(1)
O
*
CO2H
NO2
O
NO2
O
NO2
NO2
CH3
*
*
*
*
(2)
CO2H
CH3
NHtBOC
NO2
NO2
*
NO2
NO2
Br
*
NO2
NO2
NO2
*
*
*
NO2
Br
NO2
*
(3)
*
*
(5)
(4)
Scheme 3. Synthetic approaches to [benzene-14C(U)]-2-amino-6-nitrobenzoic acid.
J. Label Compd. Radiopharm 2011, 54 345–351
Copyright r 2011 John Wiley & Sons, Ltd.
www.jlcr.org

T. P. Kelly et al.
was reviewed in light of the requirement for an overall yield of 2. The presence of considerable unreacted starting material
at least 30% from [14C(U)]-benzene.
Route 2 was not promising. The yield for just one step of the
synthesis (conversion of [benzene-14C(U)]-toluene to [benzene-14
C(U)]-2,6-dinitrotoluene) has in our hands been less than 30%.
demonstrated that phenyllithium was not strong enough to
lithiate the substrate 2-position. However, it did undergo a
specific but undefined side reaction (probably with the nitro
group).
Route 3 was also not inviting. To the best of our knowledge 3. NaHMDS was not strong enough to metalate the substrate
no one has reported the direct nitration of bromobenzene to
2,6-dinitrobromobenzene, but we anticipated that it would be
very low.
Routes 4 and 5, based on [14C(U)]-1,3-dinitrobenzene, looked
hopeful and were therefore explored.
2-position, but did undergo multiple side reactions (probably
involving the nitro group).
4. Sodium hydride was not strong enough to metalate the
substrate 2-position and did not react with the nitro group of
the substrate at À781C.
We first investigated route 5, focusing on metalation
chemistry. Although metalation of an aromatic ring in the
presence of a nitro group is notoriously difficult and most often
unsuccessful, careful work by Black and co-workers had
demonstrated that strongly electron deficient systems (which
included a nitro group) could be successfully metalated by the
use of hindered alkali metal amides (e.g. sodium hexamethyldi-
silazide (NaHMDS)) in tetrahydrofuran (THF) at À781C.7
Our reasoning was that by employing this method we might
be able to metalate 3-nitro-t-BOC-aniline using NaHMDS or
an alkyllithium reagent and then react the intermediate with
carbon dioxide (Scheme 4). We felt that the nitro group would
render the 2-position hydrogen acidic enough and that with an
alkylithium base, the ortho-directing effect of the BOC-protected
amino group would further facilitate metalation. The observa-
tion by Black that compounds with similarly excellent directing
groups (e.g. 3-(diethylamido)nitrobenzene) were unreactive with
hindered alkali amide bases was surprising, but we were hopeful
that the desired product could be obtained by careful variation
of reaction conditions.8,9 We therefore attempted the metalation
of 3-nitro-t-BOC-aniline with NaHMDS, alkyllithium reagents (t-
butyl- and phenyl-) as well as sodium hydride at various
temperatures, but no reaction was successful. The technical
details are shown in Table 1.
We therefore decided that this approach was not practical
and turned our attention to route 4.
Route 4 required:
1. The conversion of [14C(U)]-1,3-dinitrobenzene (available in
90% yield from [14C(U)]-benzene) to [benzene-14C(U)]-2,6-
dinitrobenzoic acid.
2. The conversion of [benzene-14C(U)]-2,6-dinitrobenzoic acid to
[benzene-14C(U)]-2-amino-6-nitrobenzoic acid.
We examined the second requirement first, expecting that a
suitable method for the mono-reduction of 2,6-dinitrobenzoic
acid to 2-amino-6-nitrobenzoic acid would readily be found.
However, even though both 2,6-dinitrobenzoic acid and 2-amino-
6-nitrobenzoic acid had previously been described, we were
surprised to uncover no literature example of this reaction.10
We next attempted the mono-reduction of 2,6-dinitrobenzoic
acid to 2-amino-6-nitrobenzoic acid by inorganic sulfide or
disulfide (the Zinin reduction11). Reviewing the literature on the
Zinin reduction, we found many examples of mono-reductions
of 1,3-dinitrobenzene derivatives having a hydrogen or an
electron donating group on the carbon between the nitro
groups. However, we discovered only a single report of a mono-
Reviewing the results we concluded that:
reduction of
a 1,3-dinitrobenzene compound having an
electron-withdrawing group at the 2-position.12–14
1. While t-butyllithium is known to lithiate ortho to a t-BOC-
amino group,8 it reacted preferentially with the nitro group of
the substrate even at À1001C.
Our goal was to obtain a reproducible yield of at least 85% for
the mono-reduction of 2,6-dinitrobenzoic acid to 2-amino-6-
nitrobenzoic acid. Using sodium hydrosulfide as the reducing
agent (prepared in situ from sodium sulfide and sodium
hydrogen carbonate), and by adjusting the reagent ratios and
reaction time,15 we obtained a robust yield of 85–95% after
three trial runs with unlabelled substrate. The crude product was
essentially pure by proton NMR and thin layer chromatography
(TLC; Scheme 5). The optimized conditions were as follows:
3 mmol of sodium hydrosulfide per mmol of 2,6-dinitrobenzoic
NHtBOC
NHtBOC
NH2
1. Base
2. CO2
3. H+
CO2H
CO2H
NO2
NO2
NO2
Scheme 4. General strategy for route 5.
Table 1. Attempted metalation/carbonation of 3-nitro-t-BOC-aniline
Base
Solvent
Temperature (1C)
Resulta
NaHMDS (2.2 eq)
NaHMDS (2.2 )
t-BuLi (2.2 )
t-BuLi (2.2 )
PhLi (2.2 )
PhLi (2.2 )
PhLi (2.2 )
NaH (2.2 )
THF
THF
Ether
Ether
THF
THF
THF
THF
À20
À78
À78
À95
À20
À78
À100
À78
Starting material, unidentified products and no desired product
Starting material, unidentified products and no desired product
Multiple unidentified products
Multiple unidentified products
Starting material, a single unidentified product and no desired product
Starting material, a single unidentified product and no desired product
Starting material, a single unidentified product and no desired product
Starting material only
aReaction run on 1 mmol scale for 30 min., then quenched with water at reaction temperature and analyzed by TLC.
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T. P. Kelly et al.
acid; 5 mL of methanol:water (2:1) per mmol of 2,6-dinitroben- (0.4 mmol) scale, we obtained only a trace of desired product
zoic acid; stirring under gentle reflux for 30 min.
by TLC along with many unidentified by-products. We realized
To complete the synthesis of [benzene-14C(U)]-2-amino-6- that even at ambient temperature, the combined action of
nitrobenzoic acid, we were still required to identify a method for naked t-butoxide and molecular oxygen induced nitro group
the conversion of [14C(U)]-1,3-dinitrobenzene to [benzene-14
C(U)]-2,6-dinitrobenzoic acid. A consistent yield of at least 39%
was needed to achieve an overall 30% yield of [benzene-14C(U)]-
2-amino-6-nitrobenzoic acid from [14C(U)]-benzene.
-
+
NO2-
Cu
NO2
CN
H
X
t-BuOCu
-
Our earlier failure with the metalation/carbon dioxide reaction
of 3-nitro-t-BOC-aniline prompted us to abandon an electro-
philic approach and examine other possibilities. We were
intrigued by Nilsson’s 1994 report16 of the conversion of 1,3-
dinitrobenzene to 2-(2,6-dinitrophenyl)-acetonitrile by copper
mediated vicarious nucleophilic substitution (VNS; Scheme 6).
We believed that if this reaction could be performed in uniform
high yield, we would then be able to oxidize the side chain of
the product to obtain 2,6-dinitrobenzoic acid. It is noteworthy
that the electron flow in this proposed reaction mechanism is
toward (Umpolung17) the aromatic ring (Scheme 7).
Trial runs with unlabelled 1,3-dinitrobenzene demonstrated
that the reaction required careful tuning of experimental
conditions. We were able to obtain reproducible purified yields
exceeding 50% from [14C(U)]-1,3-dinitrobenzene (Scheme 8)
with the following controls in place:
+
K+
CN
X
H
NO2
NO2
-CuX
-t-BuOH
-
NO2
-
NO2
K+
K+
H
NO2
CN
X
CN
NO2
ꢀ
ꢀ
All reagents were fresh and anhydrous.
Oxygen and moisture were rigorously excluded with an
argon atmosphere.
H+
ꢀ
A three-fold excess of CuO-t-Bu (formed in situ from CuCl
and KO-t-Bu) was used to prevent equilibration to the
undesired 4-isomer (see Scheme 7).
1,2-Dimethoxyethane (DME) only was used as solvent (THF
was found not to work).
Traces of ethyl acetate in the [14C(U)]-1,3-dinitrobenzene
(from flash chromatographic purification) were found to
interfere and were removed.
NO2
NO2
CN
ꢀ
ꢀ
Scheme 7. Nilsson’s proposed VNS mechanism.
NO2
To complete the synthesis, one remaining transformation still
had to be accomplished; namely, the oxidation of 2-(2,6-
dinitrophenyl)acetonitrile to 2,6-dinitrobenzoic acid (Scheme 9).
We first tried the method of Gokel and co-workers18 who had
reported the oxidation of phenylacetonitrile to benzoic acid in high
(ꢁ90%) yield by bubbling oxygen at room temperature into a
stirred THF solution of the substrate containing 4 eq of potassium
t-butoxide and 5 mol % of 18-crown-6 (Scheme 10). Employing this
method with unlabelled 2-(2,6-dinitrophenyl)acetonitrile on a small
1.0 equiv.
*
NO2
DME
CuCl
Pyr
idine
KOt-Bu
4.0 equiv.
3.0 equiv.
DME
0 o
C
NO2
BrCH2CN
0 o
C
3N HCl
CO2H
O2N
NO2
*
CO2H
CN
DME
-20 oC
H2N
NO2
H2O/MeOH
r.t.
Filter
NO2
Na2S
+ NaHCO3
Reflux
Scheme 8. VNS reaction of [14C(U)]-1,3-dinitrobenzene with with bromoacetoni-
trile: order of reagents.
Scheme 5. Mono-reduction of 2,6-dinitrobenzoic acid.
NO2
NO2
NO2
t-BuOCu/t-BuOK
CN
CN
CN
CO2H
DME, pyridine
I
+
[O]
O2N
NO2
O2N
NO2
-20 0C
NO2
82%
Scheme 6. 2-Position functionalization of 1,3-dinitrobenzene by VNS (Nilsson16).
Scheme 9. Oxidation of 2-(2,6-dinitrophenyl)acetonitrile to 2,6-dinitrobenzoic acid.
J. Label Compd. Radiopharm 2011, 54 345–351
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T. P. Kelly et al.
displacement and/or decarboxylation and/or ring oxidation. and a reaction time of 3 h. Applying these conditions to the
Clearly, a milder oxidizing agent operating under less basic oxidation of [benzene-14C(U)]-2-(2,6-dinitrophenyl)acetonitrile, we
conditions would be needed.
A
1984 report by Saa obtained an 87% radiochemical yield of [benzene-14C(U)]-2,6-
et al.19 drew our attention. Potassium nitrosodisulfonate (‘Fremy’s dinitrobenzoic acid with a radiochemical purity of approximately
Salt’, 2), a mild free radical oxidant commonly used for conversion 98% by TLC. No purification was necessary. With the final
of aromatic amines and phenols to quinones, had been used to conversion to [benzene-14C(U)]-2-amino-6-nitrobenzoic acid already
oxidize 4-nitrophenylacetonitrile to 4-nitrobenzoic acid in 95% yield established, we ran the optimized reduction, achieving an 82%
(Scheme 11). Although the exact ratios of reagents and solvents radiochemical yield after purification by flash chromatography
were not provided, the solvent mixture of pyridine and 4% (Scheme 14).
aqueous sodium carbonate was only mildly basic and the reaction
time was brief. The single electron transfer mechanism proposed Reaction summary
by the authors is shown in Scheme 12. We ran several reactions on
[Benzene-14C(U)]-2-Amino-6-nitrobenzoic acid, previously pre-
unlabelled 2-(2,6-dinitrophenyl)acetonitrile, adjusting variables
(number of equivalents of Fremy’s Salt, co-solvent, ratio of co-
pared from [14C(U)]-benzene in fourteen steps and 10.5% overall
yield, was successfully prepared in four steps and 32% overall
solvent to 4% aqueous sodium carbonate, reaction time) until
consistently high yields of the desired product were obtained
yield (Scheme 15). The required amount of [benzene-14C(U)]-2-
amino-6-nitrobenzoic acid was delivered to the customer on
(Scheme 13). Optimal conditions were as follows: 5–10 eq of
time and within the quoted specifications.
Fremy’s Salt in acetonitrile : 4% aqueous sodium carbonate (1:10)
CN
CN
CO2H
CO2H
-O3S
-O3S
O2N
NO2
O2N
NO2
HCl
Co-solvent ?
KOt-Bu, O2
N
O
2K+
+
4% aq. Na2CO3
? equiv.
r.t., time ?
18-Crown-6, THF, r.t.
Scheme 10. Oxidation of phenylacetonitrile (Gokel18).
Scheme 13. Optimizing the Fremy’s salt oxidation conditions.
NH2
NO2
-O3S
CO2H
CO2H
N
O
2K+
NaSH
*
-O3S
*
NO2
NaHCO3
NO2
MeOH/H2O
2
82%
Scheme 14. Final reduction step.
CN
CO2H
NO2
NO2
NO2
BrCH2CN
CuCl
-O3S
-O3S
CN
pyridine
N
O
2K+
+
*
*
*
4% aq. Na2CO3
r.t., 15 min.
NO2
52%
NO2
KOtBu
NO2
DME
Pyr
"excess"
90%
95% yield
Fremy's salt
Scheme 11. Fremy’s salt oxidation of a substituted phenylacetonitrile (Saa, 198419).
4% Na2CO3, CH3CN
.
CN
CN
CN
_
NO2
*
NH2
*
CO2H
85%
NO2
CO2H
:B
F. S.
CN
F. S.
NaSH
NaHCO3
NO2
NO2
NO2
NO2
MeOH/H2O
82%
-O3S
O
CN
O
Scheme 15. Improved synthesis summary.
N
CO2H
NO2
SO3-
H2O
CO2H
CH3
CO2H
NaSH (3 equiv.)
MeOH/H2O
H2N
NO2
O2N
NO2
O2N
NO2
Na2Cr2O7
H2SO4
NO2
NO2
Scheme 12. Proposed mechanism for the oxidation of a benzylic methylene by
Fremy’s salt (Saa, 198419).
Scheme 16. Two-step synthesis of 2-amino-6-nitrobenzoic acid, possibly suitable
for a large scale.
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J. Label Compd. Radiopharm 2011, 54 345–351

T. P. Kelly et al.
Finally, with regard to the synthesis of unlabelled 2-amino-6-
nitrobenzoic acid, the final step of our synthesis (reduction of
2,6-dinitrobenzoic acid to 2-amino-6-nitrobenzoic acid by
sodium hydrosulfide15) provides a two-step, high-yield synthesis
of 1 from inexpensive 2, 6-dinitrotoluene. This method would
appear to be suitable for large-scale preparations (Scheme 16).20
converted to carboxylic acids by subsequent introduction of
carbon dioxide.9
[9] G. Koebrich, P. Buck, Angew. Chem. Int. Ed. Engl. 1966, 5, 1044.
[10] A 1984 paper reported that 2-amino-6-nitrobenzoic acid had
been prepared from 2,6-dinitrobenzoic acid by ‘catalytic hydro-
gen transfer from cyclohexene,’ but no synthetic or analytical
information was provided (G. J. Thomas, J. Agric. Food Chem.
1984, 32, 747–749).
[11] H. K. Porter, Org. Reactions 1973, 20, 455–481.
[12] T. C. Schmidt, K. Steinbach, U. Buetehorn, K. Heck, U. Volkwein,
G. Stork, Chemosphere 1999, 38, 3119–3130, wherein 2,4-dinitro-
toluene-3-sulfonic acid was reduced to 4-amino-2-nitrotoluene-3-
sulfonic acid by H2S/NH4OH in low yield (28%).
[13] We are unable to explain this finding, but would note that
studies14 have shown that electron-withdrawing substituents
speed up the rate of the Zinin reaction considerably. Since the
reaction involves several intermediates and has a recognized
propensity for producing varying amounts of side products
(e.g. azo and azoxy compounds), it may be that it has been
difficult to ‘tune’ the conditions to produce satisfactory yields of
desired product when there is an electron-withdrawing moiety at
the 2-position. Steric bulk could also be a factor.
[14] M. Hojo, Y. Takagi, Y. Ogata, J. Am. Chem. Soc. 1960, 82,
2459–2462.
[15] US and PCT patent applications have been filed (‘Methods for
Producing Aminonitrobenzoic acids,’ P2000-701010/WO, October
14, 2009).
[16] O. Haglund, M. Nilsson, Synthesis 1994, 242–244.
[17] D. Seebach, Angew. Chemie Int. Ed. 1979, 18, 239–258.
[18] S. A. DiBiase, R. P. Wolak, Jr, D. M. Dishong, G. W. Gokel, J. Org.
Chem. 1980, 45, 3630–3634.
[19] L. Castedo, A. Peralta, R. Suau, J. M. Saa, Anal. Quim. 1984, 80,
148–150.
References
[1] H. Seidel, Chem. Ber. 1901, 34, 4351–4353.
[2] A recent SciFinder search revealed a total of only 62 references to
this compound.
[3] Between 2006 and 2010, 10 use patents have cited 2-amino-6-
nitrobenzoic acid as a chief building block material in the
preparation of pharmaceuticals, agricultural compounds and
imaging agents.
[4] See, for example, G. W. Muller, H.-W. Man, WO 2008/039489 A2
(2008) and M. Muthuppalniappan, A. Thomas, K. Sukeerthi,
S. Margal, N. Khairatkar-Joshi, I. Mukhopadhyay, S. Gullapalli, WO
2010/004390 A1. This has been the standard method of synthesis
since 1901 (see reference 1 above).
[5] Prepared from [14C(U)]-benzene in 90% yield (technical details are
proprietary).
[6] The 1H NMR spectrum (300 MHz) of the unlabelled standard
in CDCl3 was identical with the spectrum obtained by Nilsson
(see reference 16).
[7] W. C. Black, B. Guay, F. Scheuermeyer, J. Org. Chem. 1997, 62, 758–760.
[8] Black and co-workers trapped the metalated substrates, several
of which were ortho-nitroaryl-metalated, by in situ trimethylsilyla-
tion. We were not convinced, however, that immediate trapping
of the metalated substrate by an in situ electrophile was
necessary, since earlier workers had found that ortho-nitroaryl-
lithiated substrates were stable enough at low temperature to be
[20] The first step has been accomplished in 58% yield: R. N. Warrener,
R. A. Russell, S. M. Marcuccio, Aust. J. Chem. 1980, 33,
2777–2779.
J. Label Compd. Radiopharm 2011, 54 345–351
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