A Straightforward Access to [14C]- and [13C]-Labeled 2-Aminobenzoxazoles and Benzothiazoles via a KCN Polarity Inversion Strategy

Article abstract of DOI:10.1055/s-0035-1562116

A convenient one-pot synthetic access to 2-aminobenzoxazoles and 2-aminobenzothiazoles has been developed. The protocol uses KCN as starting material and proceeds through an in situ polarity inversion step. This approach provides a new valuable and straightforward entry to carbon-14-radiolabeled pharmaceutically relevant heterocycles and substantially reduces as well the amount of radioactive wastes generated.

Full text of DOI:10.1055/s-0035-1562116

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
O. Loreau et al.
Letter
Synlett
A Straightforward Access to [14C]- and [13C]-Labeled 2-Aminobenz-
oxazoles and Benzothiazoles via a KCN Polarity Inversion Strategy
Olivier Loreau
reducing radioactive waste
Nathalie Camus
O
Frédéric Taran
N
Cl
N
one pot
O
R
Davide Audisio*
NH2
C
K*CN
*
X
N
H
C
*
O
OH
CEA Saclay, iBiTec-S, Service de Chimie Bioorganique
et de Marquage, Bât. 547, PC # 108, Gif-sur-Yvette,
91191, France
13C/14
C
X = O, S
*
45% RCY
from [14C]-K*CN
10 examples
up to 62% yield
davide.audisio@cea.fr
Received: 18.12.2015
O
O
Accepted after revision: 04.04.2016
Published online: 10.05.2016
DOI: 10.1055/s-0035-1562116; Art ID: st-2015-b0983-l
S
NH
N
O
NH
HO
O
lipoxygenase inhibitor
Cl
Abstract A convenient one-pot synthetic access to 2-aminobenzoxaz-
oles and 2-aminobenzothiazoles has been developed. The protocol uses
KCN as starting material and proceeds through an in situ polarity inver-
sion step. This approach provides a new valuable and straightforward
entry to carbon-14-radiolabeled pharmaceutically relevant heterocy-
cles and substantially reduces as well the amount of radioactive wastes
generated.
O
N
O
F
F
N
H
O
O
O
O
Cl
herbicide
N
O
Cl
S
N
H
Key words 2-aminobenzoxazoles, 2-aminobenzothiazoles, electro-
philic cyanating agent, isotopic labeling, carbon, cyanide
Cl
fructose-1,6-bisphosphatase inhibitor
HO
O
NH
N
N
S
2-Aminobenzoxazoles and 2-aminobenzothiazoles are
key heterocyclic scaffolds broadly found in biological chem-
istry, medicinal, and agrochemical areas.1 These moieties
are privileged pharmacophores as well as valuable reactive
intermediates, and a plethora of biologically active deriva-
tives have shown remarkable potential as antibacterial
agents,2 anticancer agents,3 and antihistamine drugs (Fig-
ure 1).4 Not surprisingly, to date a large number of synthetic
routes are available to access those heterocycles. The most
common report the use of toxic cyanogen bromide and
di(imidazole-1-yl)methanimine in the presence of bisnuc-
leophiles,5 the coupling with carbon disulfide under metal
or metal-free conditions,6 the use of isothiocyanates7 and
isocyanides.8 Though these protocols have proved high effi-
ciency for the synthesis of the unlabeled scaffolds, unfortu-
nately most of them are not optimal for carbon-14 labeling.
Carbon-14 [14C] is the long-lived radioactive isotope of
carbon. It is historically known for the key role played in the
determination of how plants utilize carbon dioxide in the
process of photosynthesis, by Nobel laureate Melvin Cal-
vin.9 Nowadays [14C] is the radioelement of choice for in
NH
F
Cl
N
antiplatelet agent
Cl
Figure 1 Pharmaceutically relevant compounds containing 2-amino-
benzoxazole and benzothiazole scaffolds
vivo metabolism and pharmacokinetic studies in drug de-
velopment.10 Due to its long half-life decay (t1/2 = 5730
years), the ideal [14C]-radiolabeling strategy should produce
a very limited amount of radioactive chemical waste, use
the simplest (and cheapest) labeled starting material avail-
able (i.e., BaCO3 and KCN), and be performed at a late stage
of the synthesis.11
In order to meet this synthetic challenge, we decided to
develop an alternative synthetic access to 2-aminobenzox-
azoles and benzothiazoles starting from 2-aminophenol or
2-aminothiophenol and KCN, one of the fundamental
sources of [14C]. In the event, KCN should be converted into
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
O. Loreau et al.
Letter
Synlett
Table 1 Optimization of the Reaction Conditions for the One-Pot Syn-
thesis of 2-Aminobenzoxazoles 1a
in situ polarity
inversion
[
14C]-KCN
CN+
N
N
N
Advantages:
NH2
OH
NH2
XH
N
- use of simple and cheap building block
- one-pot procedure
- reduced generation of radioactive wastes
Cl
BtCl
N
O
N
K*CN
*C NH2
N
MeCN
step 1
*CN
step 2
BtCN
1a
H
N
N
NH2
CN
CN
Entry
Step 1
Step 2
Yield (%)a,b
NH2
X
or
XH
X
1
2
3
4
5
0 °C 5 min, r.t. 2 h
0 °C 3 h, r.t. 20 h
0 °C 3 h, r.t. 20 h
0 °C 3 h, r.t. 20 h
0 °C 3 h, r.t. 20 h
reflux, 1 h
reflux, 1 h
reflux, 20 h
25 °C, 3 h
reflux, 3 h
53
59
55
49
62
X = O, S
Scheme 1 One-pot syntheses of 2-aminobenzoxazoles and 2-amino-
benzothiazoles through a polarity inversion strategy; X = S, O
a All reactions performed using [12C]-KCN.
b Isolated yield.
an electrophilic cyanating agent, undergoing in situ polari-
ty inversion. Subsequent cyanation of a bisnucleophile such
as 2-aminophenol or thiophenol and further intramolecu-
lar cyclization should deliver the desired 2-aminobenzoxaz-
oles or benzothiazoles (Scheme 1).12
ates well electron-rich substituents on the aromatic ring,
such as OMe and Me, delivering 1b and 1d in 53% and 56%
yield, respectively (Table 2, entries 2 and 4).
Inspired by the seminal work of Katritzky,13 we decided
to investigate the use of N-cyanobenzotriazole (BtCN) as an
electrophilic cyanating reagent. BtCN is a useful reagent
with ample reactivity and easy preparation.14 In 1998, the
group of Cava reported a convenient access to BtCN, in one
simple step, from potassium cyanide and chlorobenzotri-
azole.15 The authors reported that, after sublimation, BtCN
could be isolated in 70% yield. In order to meet the radio-
synthetic requirements and reduce the generation of radio-
active wastes we adapted the procedure in order to avoid all
unnecessary purification steps by using the in situ prepared
electrophilic cyanating agent directly in the subsequent
step. To our delight, preliminary experiments met with en-
couraging results. The addition of KCN to a homogeneous
solution of N-chlorobenzotriazole (BtCl) in acetonitrile at
low temperature followed by the addition of 2-aminophe-
nol smoothly delivered the desired 2-aminobenzoxazoles in
53% yield in one step from potassium cyanide.
With this preliminary result, we further screened a se-
ries of conditions in order to optimize the yield. After some
experimentation, it was found that the optimal protocol re-
quired 20 hours for the formation of BtCN and three hours
at reflux for the subsequent step. Under this protocol we
could isolate 1a in 62% yield (Table 1, entry 5). The experi-
ment with [13C]-KCN yielded the labeled [13C]-2-aminoben-
zoxazoles with almost identical 61% yield (Table 2, entry 1).
Spurred by this preliminary success, we proceeded to
examine the scope of the transformation. As shown in Table
2, the one-pot preparation of BtCN appears to be a very
valuable tool to achieve the straightforward preparation of
such fundamental heterocyclic scaffold. The reaction toler-
Table 2 Substrate Scope of the Transformation
N
NH2
N
N
OH
N
N
N
R
Cl
13 C NH2
O
N
K
13CN
reflux, 3 h
MeCN
R
0 °C , 3 h
then r.t., 20 h
13CN
1
BtCN
Entry
1
Substrate
Product
Yield (%)a
61
N
NH2
13C NH2
O
OH
1a
MeO
N
13
MeO
NH2
OH
C
NH2
2
3
53
62
O
1b
Br
N
Br
NH2
13 C NH2
O
OH
1c
N
NH2
OH
13
C
NH2
4
5
56
47
O
1d
N
13
NH2
C
NH2
O
OH
1e
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
O. Loreau et al.
Letter
Synlett
Table 2 (continued)
Concerning the reaction mechanism, two possible path-
ways could be envisioned (Scheme 2). The amine of the
bidentate nucleophile could react with BtCN delivering the
transient guanidine 2; subsequent cyclization would gener-
ate the 2-aminobenzoxazole 1 (Scheme 2, pathway A). Re-
ports by the group of Katritzky suggest that intermediate 2,
in equilibrium with its tautomeric form 3, is highly stable
and reluctant to react with primary and secondary
amines.18 According to this data, pathway B (Scheme 2)
seems to be more likely. In the event, the attack of the phe-
nol or thiophenol to the electrophilic BtCN would generate
the reaction intermediate 4, which further evolves to the
phenyl cyanate 5 and finally cyclizes to give 1.
Entry
Substrate
Product
Yield (%)a
49
Cl
N
13 C NH2
O
Cl
NH2
OH
6
1f
Cl
N
Cl
NH2
OH
13
C
C
NH2
7
8
traces
19
O
1g
NO2
NO2
O2N
N
13
O2N
NH2
NH2
O
OH
1h
H2N
Bt
HN
Bt
N
13
N
NH2
NH2
NH
XH
C
NH2
ref. 18
pathway A
BtCN
9
10
11
traces
41
N
H
XH
1i
1j
2
3
N
13
NH2
SH
C
NH2
N
X
NH2
XH
S
NH2
1
Cl
N
X = O, S
Cl
NH2
SH
13 C NH2
S
10
NH2
pathway B
BtCN
NH2
X
1k
N
a Isolated yield.
X
– BtH
HN
Bt
4
5
The tricyclic aminobenzoxazole 1e could be isolated in a
good 47% overall yield (Table 2, entry 5). The presence of
halogens proved suitable for this reaction, the desired prod-
ucts 1c and 1f were obtained in a good 62% and 49% yield
from [13C]-KCN (Table 2, entries 3 and 6). Notably, halogens
proved to be valuable appendages for further metal-cata-
lyzed manipulation of this scaffold.16 Not surprisingly, the
presence of electron-withdrawing substituents was delete-
rious for this transformation, and 5-nitro aminobenzoxaz-
ole 1h could be isolated in a modest 19% yield while only
traces amounts of 1g were observed (Table 2, entry 7 and
8).17 Noteworthy, the protocol was successfully applied to
the synthesis of 2-aminobenzothiazoles, though in a less ef-
ficient manner with moderate isolated yields (1j, 1k, Table
2, entries 10 and 11). In agreement with previous observa-
tions in the aminobenzoxazole series (see 1a and 1f, Table
2, entries 1 and 6), the presence of inductive electron-with-
drawing chlorine substituent is not favorable for the reac-
tion; the 5-chloro substituted 2-aminobenzothiazole 1k
was isolated in a meager 10% yield (Table 2, entry 11). Inter-
estingly, the utilization of o-phenylenediamine did not al-
low the formation of the corresponding 2-aminobenzimid-
azole 1i even in presence of base or prolonged reaction time
(Table 2, entry 9).
Scheme 2 Proposed plausible reaction pathway
The potential of the current methodology was high-
lighted by the straightforward radiosynthesis of optically
active amide 6. Compound 6 is an advanced intermediate
common to a vast family of potent herbicides used for the
protection of rice, wheat, and barley crops that have shown
high selectivity against weeds.19 Following our optimized
procedure, [14C]-1f was prepared in one operationally sim-
ple step starting from 296.6 MBq of [14C]-radiolabeled KCN.
After a simple basic workup [14C]-1f was directly used in
the subsequent peptide-bond coupling reaction, in the
presence of N,N′-dicyclohexylcarbodiimide (DCC) and hy-
droxybenzotriazole (HOBt) in EtOAc–DMF. After column
purification, the [14C]-labeled intermediate 6 was isolated
in 98% radiochemical purity and 45% radiochemical yield
from K14CN (134.5 MBq, Scheme 3). The measured specific
activity was determined by ESI-MS to be 286.3 MBq mmol–1.
It is worth mentioning that the one-pot protocol enables
the reduction of radioactive waste generation, particularly
polluting and challenging to treat.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
O. Loreau et al.
Letter
Synlett
O
O
HO
O
Cl
N
1. Bt-Cl
MeCN
O
OH
Cl
N
O
C
N
DCC, HOBt
EtOAc–DMF
*
O
[
14C]-K*CN
H
*C NH2
2.
Cl
OH
14C]-6
296.6 MBq
NH2
OH
[
45% RCY
134.5 MBq
[
14C]-1f
O
Cl
N
O
O
F
ref. 19
F
N
H
O
selective herbicide agent
Scheme 3 One-pot synthesis of [14C]-radiolabeled herbicide intermediate 6
In summary, we have described a valuable one-pot syn-
thetic access to 2-aminobenzoxazoles and 2-aminobenzo-
thiazoles.20 This procedure uses KCN as starting material
and proceeds through an in situ polarity inversion step. This
mild and facile protocol has demonstrated to be wide in
scope. Notably, this methodology has proven to be very
valuable for carbon-14-radiolabeling, as shown by the se-
quential synthesis of an advanced herbicide intermediate in
45% radiochemical yield starting from KCN. In addition, the
one-pot procedure substantially reduces the amount of ra-
dioactive waste generated. We believe the novelty of the
current methodology will provide a suitable alternative to
previous synthetic protocols and will have broad applica-
bility in the field of radiolabeling.
(d) Hohlfeld, K.; Wegner, J. K.; Kesteleyn, B.; Linclau, B.; Unge, J.
J. Med. Chem. 2015, 58, 4029. (e) Singh, M.; Singh, S. K. Antican-
cer Agents Med. Chem. 2014, 14, 127.
(2) Yang, S.-K.; Kang, J. S.; Oelschlaeger, P.; Yang, K.-W. ACS Med.
Chem. Lett. 2015, 6, 455.
(3) (a) Chanda, K.; Maiti, B.; Yellol, G. S.; Chien, M.-H.; Kuo, M.-L.;
Sun, C. M. Org. Biomol. Chem. 2011, 9, 1917. (b) Tao, Z.-F.;
Hasvold, L.; Wang, L.; Wang, Z.; Petros, A. M.; Park, C. H.;
Boghaert, E. R.; Catron, N. D.; Chen, J.; Colman, P. M.; Czabotar,
P. E.; Deshayes, K.; Fairbrother, W. J.; Flygare, J. A.; Hymowitz, S.
G.; Jin, S.; Judge, R. A.; Koehler, M. F. T.; Kovar, P. J.; Lessene, G.;
Mitten, M. J.; Ndubaku, C. O.; Nimmer, P.; Purkey, H. E.;
Oleksijew, A.; Phillips, D. C.; Sleebs, B. E.; Smith, B. J.; Smith, M.
L.; Tahir, S. K.; Watson, K. G.; Xiao, Y.; Xue, J.; Zhang, H.; Zobel,
K.; Rosenberg, S. H.; Tse, C.; Leverson, J. D.; Elmore, S. W.;
Souers, A. J. ACS Med. Chem. Lett. 2014, 5, 1088.
(4) Liu, K. G.; Lo, J. R.; Comery, T. A.; Zhang, G. M.; Zhang, J. Y.;
Kowal, D. M.; Smith, D. L.; Di, L.; Kerns, E. H.; Schechter, L. E.;
Robichaud, A. J. Bioorg. Med. Chem. Lett. 2009, 19, 1115.
(5) (a) Kaupp, G.; Schmeyers, J.; Boy, J. Chem. Eur. J. 1998, 4, 2467.
(b) Wu, Y. Q.; Limburg, D. C.; Wilkinson, D. E.; Hamilton, G. S.
J. Heterocycl. Chem. 2003, 40, 191. (c) Rynearson, K. D.;
Charrette, B.; Gabriel, C.; Moreno, J.; Boerneke, M. A.; Dibrov, S.
M.; Hermann, T. Bioorg. Med. Chem. Lett. 2014, 24, 3521.
(6) (a) Zhao, H.; He, W.; Yao, R.; Cai, M. Adv. Synth. Catal. 2014,
3092. (b) Guntreddi, T.; Allam, B. K.; Singh, K. N. RSC Adv. 2013,
3, 9875.
(7) (a) Ding, Q.; He, X.; Wu, J. J. Comb. Chem. 2009, 11, 587.
(b) Evindar, G.; Batey, R. A. J. Org. Chem. 2006, 71, 1802.
(8) Vlaar, T.; Cioc, R. C.; Mampuys, P.; Maes, B. U. W.; Orru, R. V. A.;
Ruijter, E. Angew. Chem. Int. Ed. 2012, 51, 13058.
(9) (a) Calvin, M. Bull. Soc. Chim. Biol. 1956, 38, 1233. (b) Wilson, T.;
Calvin, M. J. Am. Chem. Soc. 1955, 77, 5948.
(10) (a) Penner, N.; Xu, L.; Prakash, C. Chem. Res. Toxicol. 2012, 25,
513. (b) Isin, E. M.; Elmore, C. S.; Nilsson, G. N.; Thompson, R. A.
Weidolf. L. Chem. Res. Toxicol. 2012, 25, 532.
(11) Voges, R.; Heys, J. R.; Moenius, T. In Preparation of Compounds
Labeled with Tritium and Carbon-14; John Wiley and Sons:
Chichester, 2009.
Acknowledgment
The authors thank Céline Puente, Elodie Marcon and David-Alexandre
Buisson (CEA, iBiTecS, Service de Chimie Bioorganique et de Mar-
quage) for liquid scintillation measurements.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1562116.
S
u
p
p
ortioInfgrmoaitn
S
u
p
p
ortiInfogrmoaitn
References and Notes
(1) (a) Luci, D. K.; Jameson, J. B.; Yasgar, A.; Diaz, G.; Joshi, N.; Kantz,
A.; Markham, K.; Perry, S.; Kuhn, N.; Yeung, J.; Kerns, E. H.;
Schultz, L.; Holinstat, M.; Nadler, J. L.; Taylor-Fishwick, D. A.;
Jadhav, A.; Simeonov, A.; Holman, T. R.; Maloney, D. J. J. Med.
Chem. 2014, 57, 495. (b) Yang, W.; Wang, Y.; Lai, A.; Qiao, J. X.;
Wang, T. C.; Hua, J.; Price, L. A.; Shen, H.; Chen, X.; Wong, P.;
Crain, E.; Watson, C.; Huang, C. S.; Seiffert, D. A.; Rehfuss, R.;
Wexler, R. R.; Lam, P. Y. S. J. Med. Chem. 2014, 57, 6150. (c) Liu,
K.; Robichaud, A. J.; Elokdah, H. M. US 0281945 Al, 2007.
(12) During the preparation of this manuscript, the group of Kast-
huri reported a synthesis 2-aminobenzoxazoles using N-cyano-
N-phenyl-p-toluenesulfonamide (NCTS) as electrophilic cyanat-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

E
O. Loreau et al.
Letter
Synlett
ing agent. Despite the high efficiency of their protocol, NCTS is
prepared from N-phenyl urea and, for this reason, is not suitable
for carbon-14 radiolabeling, see: Kasthuri, M.; Babu, H. S.;
Kumar, K. S.; Sudhakar, C.; Nagendra Kumar, P. V. Synlett 2015,
26, 897.
(20) General Procedure: The One-Pot Synthesis of 2-Aminoben-
zoxazoles
To a stirred suspension of finely ground [12C]-KCN or [13C]-KCN
(1 mmol, 1 equiv) in anhydrous MeCN (5 mL) was added 1-
chloro-1H-benzo[d][1,2,3]triazole (1 mmol, 1 equiv) at 0 °C.
After 3 h, the mixture was allowed to warm to room tempera-
ture and stirred overnight (20 h). The 2-aminophenol or 2-ami-
nothiazole derivative (1.2 mmol, 1.2 equiv) was added to the
suspension, and the resulting mixture was refluxed for 3 h.
After dilution with EtOAc (50 mL), the organic phase was
washed three times with 1 N NaOH aq solution (25 mL), dried
over MgSO4, filtered, and concentrated under reduced pressure.
Purification of the crude material by column chromatography
(13) Katritzky, A. R.; Brzezinski, J. Z.; Lam, J. N. Rev. Roum. Chim.
1991, 36, 573.
(14) (a) Dehmel, F.; Abarbri, M.; Knochel, P. Synlett 2000, 345.
(b) Cushman, M.; Mohanakrishnan, A. K.; Hollingshead, M.;
Hamel, E. J. Med. Chem. 2002, 45, 4748. (c) Pearson, A. J.; Kim, J.
B. Tetrahedron Lett. 2003, 44, 8525. (d) Ren, S.; Mcnamara, P.;
Koharski, D. J. Labelled Compd. Radiopharm. 2010, 53, 247.
(e) Suzuki, Y.; Shimawaki, M.; Miyazaki, E.; Osaka, I.; Takimiya,
K. Chem. Mater. 2011, 23, 795. (f) Adams, G. L.; Carroll, P. J.;
Smith, A. B. III. J. Am. Chem. Soc. 2012, 134, 4037.
(15) (a) Hughes, T. V.; Hammond, S. D.; Cava, M. P. J. Org. Chem. 1998,
63, 401. (b) Hughes, T. V.; Cava, M. P. J. Org. Chem. 1999, 64, 313.
(16) Martin, M.; Worrall, C. P.; Gancedo, S. D. R.; Ren, P. WO
2013071272, 2013.
(17) It cannot be excluded that intramolecular hydrogen bonding
may play an additional role in diminishing the efficiency of the
transformation in the case of product 1g (Table 2).
(18) Katritzky, A. R.; Rogovoy, B. V.; Chassaing, C.; Vvedensky, V.
J. Org. Chem. 2000, 65, 8080.
on silica gel (CH2Cl2–MeOH
= 20:1) gave the expected
benzo[d]oxazol-2-amine or benzo[d]thiazol-2-amine deriva-
tive.
[2-13C]-Benzo[d]oxazol-2-amine (1a)
White solid (82 mg, 61% yield); mp 130–131 °C. Isotopic enrich-
ment: 99% (based on MS-ESI); TLC (silica gel Merck 60F254,
CH2Cl2–MeOH = 20:1); Rf = 0.27. 1H NMR (400 MHz, CDCl3): δ =
7.35 (m, 1 H), 7.28 (m, 1 H), 7.19 (dt, J = 7.7, 1.2 Hz, 1 H), 7.07
(dt, J = 7.7, 1.2 Hz, 1 H), 5.35 (br s, 2 H). 13C NMR (100 MHz,
CDCl3): δ = 162.18 (13C enriched), 148.45 and 148.40 (d, J = 4.6 Hz),
142.49, 123.92, 121.14, 116.24 and116.15 (d, J = 9.2 Hz), 108.93
and 108.88 (d, J = 4.6 Hz). IR: ν = 3319, 3161, 1679, 1640, 1547,
(19) Kim, J.; Kim, D.; Kim, H.; Kim, K.; Yoon, C.; Hwang, I. WO
2009005297, 2009.
1463, 1379, 1358, 1255, 1110, 959, 897, 740, 669, 536 cm–1
.
ESI-MS: m/z 136 [M + H]+. MS (EI): m/z = 135 (100) [M]+.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E