Synthesis of benzoxazole-[phenyl-13C6] by directed ortho-metalation chemistry

Article abstract of DOI:10.1002/jlcr.792

A new method for the preparation of benzoxazole-[phenyl-¹³C ₆] (1) starting from aniline-[¹³C₆] (4) has been developed involving directed ortho-metalation (DoM) chemistry. The synthesis comprises four steps and

Full text of DOI:10.1002/jlcr.792

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2004; 47: 19–24.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.792
Research Article
Synthesis of benzoxazole-[phenyl-13C6] by directed
ortho-metalation chemistry
Volker Derdau*
Aventis Pharma Deutschland GmbH, Chemical Development, Frankfurt am
Main, Germany
Summary
A new method for the preparation of benzoxazole-[phenyl-13C6] (1) starting from
aniline-[13C6] (4) has been developed involving directed ortho-metalation (DoM)
chemistry. The synthesis comprises four steps and an overall yield of 39% was
obtained. Copyright # 2003 John Wiley & Sons, Ltd.
Key Words: benzoxazole; ortho-metalation; 13C-labelling
Introduction
Benzoxazole (1) has been used as a building block in various pharmacolo-
gically relevant molecules as demonstrated by several examples in medicinal
chemistry journals and the patent literature.1 For the development of these
potential drugs, pharmacokinetic studies with isotopically labelled material are
necessary. The synthesis of specifically 13C-labelled benzoxazole (1), as part of
these potential drugs, becomes of interest as a result of the use of these labelled
drug molecules as internal standard in LC/MS assays (Figure 1).
The retrosynthetic analysis of benzoxazole (1) suggests aminophenol (2 and
derivatives) as possible precursor. There are examples described in the
literature wherein the ring-closure of aminophenol (2) was carried out using
triazine or ortho-ester in moderate to good yields.2 Aminophenol (2) could be
prepared by two possible reaction pathways, through a nitration/reduction
sequence starting from phenol (3) or by metalation/oxidation reaction starting
from aniline (4). The nitration of phenols is frequently described in the
literature, but in most cases a mixture of regioisomers was obtained.3 The
subsequent reduction of 2-nitrophenol was successfully performed under
several reaction conditions with high yields.4 Due to the low costs of the
*Correspondence to: V. Derdau, Aventis Pharma Deutschland GmbH, Chemical Development, Industriepark
.
Hochst G 876, 65926 Frankfurt, Germany. E-mail: volker.derdau@aventis.com
Copyright # 2003 John Wiley & Sons, Ltd.
Received 21 August 2003
Revised 29 September 2003
Accepted 5 October 2003

20
V. DERDAU
OH
nitration/
reduction
3
4
OH
O
N
NH2
2
1
metalation/
oxidation
NH2
Figure 1. Retrosynthesis of benzoxazole
starting materials, most described syntheses are performed on multigram scale
with distillations as the purification method which can be unfavourable on a
small scale.
There are some examples of the directed ortho-lithiation of Boc-protected
anilines,5 but to our knowledge this method has not been used for the synthesis
of aminophenol (2) yet. Therefore we developed a short and convenient
lab procedure starting from aniline (4) using metalation chemistry for
the introduction of substituents and chromatography as the purification
method.
Results and discussion
Aniline-[13C6] (4) was Boc-protected by reaction with Boc-anhydride to give
N-Boc-aniline-[phenyl-13C6] (5) in 91% yield (Figure 2). N-Boc-aniline-
[phenyl-13C6] (5) was ortho-metalated5 using t-BuLi (reactions with n-BuLi
or sec-BuLi gave only minor metalation products as reported by6) and
quenched with boronic ester to give, after aqueous workup, the boronic acid
derivative as a crude product, which was directly converted into the phenol (6)
by reaction with hydrogen peroxide. Over several attempts, we were able to
increase the initially low yield of the reactions by variation of solvents and
electrophiles (Table 1). A direct oxidation of the benzene–lithium species (8)
with gaseous oxygen gave only inferior results (entries 6,7).7 We finally
performed the synthesis of 13C-labelled N-Boc-aminophenol (6) as described in
entry 2 (for details see experimental part) and isolated the desired product
after purification by chromatography in 52% yield, while 31% of N-Boc-
aniline-[phenyl-13C6] (5) was also recovered.
Best results for the deprotection of the amino function were achieved with
TFA in dichloromethane (94% yield). Without further purification of
aminophenol (2) the reaction with triazine (7) yielded benzoxazole-
[phenyl-13C6] (1) as crude product which was purified by chromatography
using pentane/diethylether as eluent to give (1) in 78% yield (purity >98%).
Copyright # 2003 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 19–24

SYNTHESIS OF BENZOXAZOLE
21
1. t-BuLi
2. B(OMe)3
H
H
NH2
N
Boc
NBoc
Boc2O
91 %
13C
13C
13C
H O , H O
3.
2
2
2
OH
4
5
52 %
6
N
N
O
7
N
NH2
OH
N
TFA, CH2Cl2
94 %
13C
13C
toluene, 110˚C
78 %
2
1
Figure 2. Metalation chemistry route for the synthesis of 13C-benzoxazole
Table 1. Optimization of the metalation/oxidation reaction of N-Boc-aniline (5)
H
H
NBoc
Boc
N
1. E
H2O2
BuLi
13C
6
5
13C
2. H2O
E
Li
9
8
Entry
Reagent
Equiv
E
Additive
Solvent
5 [%]
6 [%]
1
2
3
4
n-BuLi
t-BuLi
t-BuLi
t-BuLi
2.2
3.3
3.3
3.3
B(OMe)3
}
}
}
}
THF
THF
Et2O
Et2O
100
31
14
23
}
52
47
35
B(OMe)3
B(OMe)3
B(OiPr)3
OH
5
t-BuLi
3.3
B(OiPr)3
Et2O
43
32
OH
Ti(OiPr)4
6
7
t-BuLi
t-BuLi
3.3
3.3
O2
O2
}
Et2O
Et2O
74
65
8
11
Experimental
General
1
The H-NMR and 13C-NMR spectra were recorded on Bruker 500 and 600
1
nuclear magnetic resonance spectrometers. 13C-decoupled H-NMR experi-
ments were accomplished and compared with literature data or an authentic
sample if possible. The purification of the compounds was done on open
columns with silica gel 60 (Merck) and the eluents described. The purity
analyses were performed on a Gilson HPLC system using a Luna C18 RP
column with an acetonitrile/water gradient as mobile phase. t-BuLi was
purchased from Aldrich and used directly.
Copyright # 2003 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 19–24

22
V. DERDAU
N-Boc-aniline-[phenyl-13C6] (5). 26.5 g (0.12 mol) Boc-anhydride was added
into a 250 ml round bottom flask flushed with argon. The solid was dissolved
in 10 ml chlorobenzene and 10.0 g (0.10 mol) aniline-[13C6] (4) (CDN isotopes,
99% isotopically pure) was added in portions at room temperature. The
reaction mixture was heated to 508C for 1 h and to 808C for a further 36 h.
Then methylcyclohexane (10 ml) was added and the solution allowed to cool to
room temperature. The reaction mixture was centrifuged, the colourless solid
was washed twice with 5 ml methylcyclohexane and dried in vacuo to give
18.4 g (92.4 mmol, 91%) N-Boc-aniline-[phenyl-13C6] (5) as colourless needles
(>97% purity). Identity was checked by comparison with an authentic sample
1
purchased from Aldrich using a H-NMR (13C-decoupled) spectrum.8
2-N-Boc-aminophenol-[phenyl-13C6] (6). 4.54 g (22.8 mmol) N-Boc-aniline-
[phenyl-13C6] (5) was introduced into a 250 ml round bottom flask and the
flask flushed with argon for 10 min. After addition of 60 ml dry THF the solid
was dissolved and the solution cooled to –788C. Then, 46.9 ml (70.0 mmol) t-
BuLi were added dropwise via syringe (15 min) and stirred for 15 min. The
solution was allowed to come up to –208C and stirred for a further 2 h. After
addition of 7.04 ml (62 mmol) trimethylborate the solution was stirred for 8 h
at room temperature. Then, 8 ml water was added, the phases separated and
the aqueous phase extracted three times with 20 ml dichloromethane. The
combined organic layers were dried over anhydrous Na2SO4, filtered and the
solvent removed in vacuo. The oily residue was dissolved in 7.9 ml ethanol/
water (1:1) and cooled to 08C. After dropwise addition of 7.9 ml hydrogen
peroxide (30% in water) the solution was stirred at room temperature for two
hours. The aqueous phase was extracted 5 times with 20 ml dichloromethane,
the combined organic phases dried over anhydrous Na2SO4 and the solvent
removed in vacuo. The crude product was purified by silica gel chromato-
graphy (heptane/ethyl acetate 4:1) to give 2.57 g (12.3 mmol, 52%) of (6)
(purity 94%) and 1.37 g (7.09 mmol, 31%) of recovered starting material (5).
13C-decoupled 1H-NMR spectra of (6) are in accordance with literature data.9
2-Aminophenol-½13C6ꢀ (2). 3.35 g (15.6 mmol) 2-N-Boc-aminophenol-[phe-
nyl-13C6] (6) were dissolved in 14 ml dichloromethane/TFA (1:1) and stirred
at room temperature for 1 h (TLC-control, silica, dichloromethane/MeOH
2:1). After addition of 1 ml TFA the solution was stirred for one additional
hour. Subsequently 2 N NaOH solution was added at 08C to give a solution of
pH 8. The aqueous phase was extracted 2 times with 30 ml dichloromethane
and 2 times with 20 ml ethyl acetate. The combined organic phases were dried
over anhydrous Na2SO4 and the solvent removed in vacuo to give 1.69 g
(14.7 mmol, 94%) of (2) as a brown solid (>93% purity by HPLC, contained
Copyright # 2003 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 19–24

SYNTHESIS OF BENZOXAZOLE
23
4% TFA as determined by 19F-NMR). 13C-decoupled 1H-NMR spectra of (2)
are in accordance with literature data.4b,c
Benzoxazole-[phenyl-13C6] (1). 1.74 g (15.1 mmol) Aminophenol-[13C6] (2)
were suspended in 9 ml dry toluene and 253 mg (3.12 mmol) 1,3,5-triazine
(Merck) (7) and 2.2 ml triethylamine added. The reaction mixture was heated
to reflux for 15 h and a further 84 mg (1.00 mmol) 1,3,5-triazine and 1.0 ml
triethylamine added. After 3 h reflux the reaction mixture was cooled to room
temperature and 3.5 ml water added. The phases were separated, the organic
layer dried over anhydrous Na2SO4 and directly purified by open column silica
gel chromatography using pentane/ether 5:1 as mobile phase. The relevant
fractions were combined and the solvent removed by short path distillation
(bath temperature should not exceed 558C). Traces of residual solvent were
evaporated in vacuo (30–45 s) to give 1.47 g (11.8 mmol, 78%) of (1) as a
1
colourless oil which crystallizes after a few hours (99% purity). H-NMR
(600 MHz, MeOD) d=8.49 (dd, J=4.8 Hz, J=4.8 Hz, 1 H), 7.92–7.82 (m,
1 H), 7.64–7.55 (m, 2 H), 7.35–7.28 (m, 1 H); 1H-NMR (600 MHz, MeOD, 13
C
decoupled) d=8.47 (s, 1 H), 7.77 (d, J=7.8 Hz, 1 H), 7.69 (d, J=7.8 Hz, 1 H),
7.48–7.41 (m, 2 H); in accordance with literature data2d; 13C-NMR (150 MHz,
CDCl3) d=154.1 (ratio 1:100, N=CH–O), 150.5–149.4 (m, 1Cq), 140.6–
139.5 (m, 1Cq), 126.1–124.0 (m, 2CH), 121.1–120.1 (m, 1CH), 111.4–110.4
(m, 1CH) ppm.
Conclusion
We have developed a short synthesis for benzoxazole-[phenyl-13C6] (1) based
on a common 13C-starting material. The synthesis of aminophenol-13C6 (2) by
metalation chemistry has been optimized to obtain 52% yield as the best
result.
Acknowledgements
I would like to thank Silvia Weber and Gerald Scholz for valuable
experimental support and Dr. Michael Kossenjans for helpful discussions.
References
1. (a) Hirst GC, Rafferty P, Ritter K, Claderwood D, Wishart N, Arnold LD,
Friedman MM. US Pat Appl Publ 2002, US 2002156081; (b) Barta T, Becker DP,
Bedell LJ, Boehm TL, Fobian YM, Freskos JN, Hockerman SL, Kassab DJ,
Kolodziej SA, McDonald JJ, Norton MB, Rico JG, Talley JJ, Villamil CI, Wang
TJ. PCT Int Appl 2002, WO 0292588 A2 20021121; (c) Neya M, Sawada A, Ohne
K, Abe Y, Mizutani T, Ishibashi N, Inoue M. PCT Int Appl 2003, WO 0322842, A1
20030320; (d) Phoon CW, Ng PY, Ting AE, Yeo SL, Sim MM. Bioorg Med Chem
Lett 2001; 11: 1647; (e) Akahoshi F, Ashimori A, Sakashita H, Yoshimura T,
Copyright # 2003 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 19–24

24
V. DERDAU
Imada T, Nakajima M, Mitsutomi N, Kuwahara S, Ohtsuka T, Fukaya C,
Miyazaki M, Nakamura N. J Med Chem 2001; 44: 1286.
2. (a) Ried W, Storbeck W, Schmidt E. Arch Pharm 1962; 295: 143; (b) Ito Y, Ito I,
Hirao T, Saegusa T. Syn Commun 1974; 4: 97; (c) Grundmann C, Kreutzberger A.
J Am Chem Soc 1955; 77 : 6559; (d) El-Sheikh MI, Marks A, Biehl ER. J Org Chem
1981; 46: 3256.
3. (a) Pervez H, Onyiriuka SO, Rees L, Rooney JR, Suckling CJ. Tetrahedron 1988;
44: 4555; (b) Poirier JM, Vottero C. Tetrahedron 1989; 45: 1415; (c) Joshi AV,
Baidoosi M, Mukhopadhyay S, Sasson Y. Org Proc Res Dev 2003; 7: 95; (d)
Shackelford SA, Anderson MB, Christie LC, Goetzen T, Guzman MC, Hananel
MA, Kornreich WD, Li H, Pathak VP, Rabinovich AK, Rajapakse RJ, Truesdale
LK, Tsank SM, Vazir HN. J Org Chem 2003; 68: 267 and references cited therein.
4. (a) George J, Chandrasekaran S. Syn Commun 1983; 13: 495; (b) Ren P-D, Pan S-
F, Dong T-W, Wu S-H. Syn Commun 1995; 25: 3799; (c) Yu Z, Liao S, Xu Y, Yang
B, Yu D. J Mol Cat A 1997; 120: 247; (d) Nagaraja D, Pasha MA. Tetrahedron Lett
1999; 40: 7855 and references cited therein.
5. (a) Eskildsen J, Ostergaard N, Vedso P, Begtrup M. Tetrahedron 2002; 58: 7635; (b)
Sato N. Tetrahedron Lett 2002; 43: 6403 (c) Peterson MA, Nilsson BL. Syn
Commun 1999; 29: 3821.
6. (a) Lane C, Snieckus V. Syn Lett 2000; 9: 1294; (b) Muchowski JM, Venuti MC. J
Org Chem 1980; 45: 4798.
.
7. Moller M, Husemann M, Boche G. J Organomet Chem 2001; 624: 47.
8. Moraczewski AL, Banaszynski LA, From AM, White CE, Smith BD. J Org Chem
1998; 63: 7258.
9. Buon C, Chacun-Lefevre L, Rabot R, Bouyssou P, Coudert G. Tetrahedron 2000;
56: 605.
Copyright # 2003 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2004; 47: 19–24