Synthesis of 2-aminobenzoxazoles using tetramethyl orthocarbonate or 1,1-dichlorodiphenoxymethane

Article abstract of DOI:10.1021/jo1017052

The synthesis of 2-aminobenzoxazoles can be readily achieved by two versatile, one-pot procedures utilizing commercially available tetramethyl orthocarbonate or 1,1-dichlorodiphenoxymethane, an amine, and an optionally substituted 2-aminophenol. The reactions were conducted under mild conditions and provided 2-aminobenzoxazoles in modest to excellent yields. A variety of amines and substituted 2-aminophenols were used to investigate the scope of the reactions.

Full text of DOI:10.1021/jo1017052

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metabolic disorders,4 irritable bowel syndrome (IBS),5 viral
Synthesis of 2-Aminobenzoxazoles
Using Tetramethyl Orthocarbonate
or 1,1-Dichlorodiphenoxymethane
infection,6 thrombosis,7 and insomnia.8
There are several approaches to this scaffold; however,
each one has specific disadvantages (Scheme 1). The classical
route involves nucleophilic displacement of a 2-substituted
benzoxazole (2-substituents include Cl,9 SH,10 SCH3,11 or
OPh12) with an amine. Drawbacks of these routes include
penultimate intermediates that involve multiple steps to
prepare, utilization of harsh reagents and conditions, or
generation of undesirable byproducts. Cyclodesulfurization of an
intermediary thiourea may involve a toxic heavy-metal oxide,13
potentially explosive oxidant,14 or transition metal15 to facilitate
cyclization. Previously reported methods to generate 2-amino-
benzoxazoles directly from 2-aminophenols may require the
preparation of either a thioisocyanate (5),16 N-cyanodithioimido-
carbonate (6),17 or chloroformadinium salt (7)18 prior to cycliza-
tion. 2-Aminobenzoxazoles have also been prepared directly
from benzoxazoles using chloroamines19 or formamides20 as
amine surrogates. However, these reactions either require
initial chlorination of the amine prior to reaction with the
benzoxazole or in the case of formamides Ag2CO3 and
heating at 130 °C to furnish the desired products.
€
Christopher L. Cioffi,* John J. Lansing, and Hamza Yuksel
Department of Discovery Research and Development,
Chemistry, AMRI, 30 Corporate Circle, Albany, New York
12203-5098, United States
christopher.cioffi@amriglobal.com
Received August 30, 2010
The synthesis of 2-aminobenzoxazoles can be readily
achieved by two versatile, one-pot procedures utilizing
commercially available tetramethyl orthocarbonate or
1,1-dichlorodiphenoxymethane, an amine, and an op-
tionally substituted 2-aminophenol. The reactions were
conducted under mild conditions and provided 2-amino-
benzoxazoles in modest to excellent yields. A variety of
amines and substituted 2-aminophenols were used to
investigate the scope of the reactions.
In our laboratories, we required a facile method for
preparing a proprietary 2-aminobenzoxazole that was safe,
efficient, and amenable to large-scale production. We sought
easily handled, commercially available, and relatively inex-
pensive reagents that could facilitate formation of 2-amino-
benzoxazoles from readily available amines and substituted
2-aminophenols in a single step. Herein we report two novel,
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Reiss, D. R.; Harrell, C. M.; Murphy, K. L.; Garson, S. L.; Doran, S. M.;
Prueksaritanont, T.; Anderson, W. B.; Tang, C.; Roller, S.; Cabalu, T. D.;
Cui, D.; Hartman, G. D.; Young, S. D.; Koblan, K. S.; Winrow, C. J.;
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7942 J. Org. Chem. 2010, 75, 7942–7945
Published on Web 10/25/2010
DOI: 10.1021/jo1017052
r
2010 American Chemical Society

Cioffi et al.
JOCNote
SCHEME 1. Examples of Common Methods for Generating
2-Aminobenzoxazoles
TABLE 1. Reactions with 2-Aminophenol and Various Amines
general, and one-pot processes for preparing this scaffold
class21 using either commercially available tetramethyl
orthocarbonate (9) or 1,1-dichlorodiphenoxymethane (10).
We conducted a reaction using 1 equiv of 2-aminophenol,
2 equiv of N-Boc-piperazine, 2 equiv of tetramethyl ortho-
carbonate, and 4 equiv of HOAc heated at 60 °C in CHCl3.
Gratifyingly, we observed clean formation of the desired
product and a 97% isolated yield after SiO2 gel column
chromatography (Table 1, entry 1, Method A). In addition,
we obtained a similar result when conducting an experiment
using 1 equiv of 2-aminophenol, 2 equiv of N-Boc-piperazine,
1 equiv of 1,1-dichlorodiphenoxymethane, with 2 equiv
of Et3N in toluene at room temperature (Table 1, entry 1,
Method B). Using these sets of conditions, we examined the
versatility of both reactions using 2-aminophenol and var-
ious amines (Table 1).21
The desired product was obtained for primary and sec-
ondary amines with yields ranging from modest to excellent
for both methods. Lower yields were obtained with hindered
(Table 1, entries 7 and 12) and volatile amines (Table 1,
entries 4, 11, 13, 16, and 17) when using Method A. However,
yields for volatile amines were generally improved when the
reaction was carried out in a sealed vessel. Volatile pyrroli-
dine performed well when using Method B (Table 1, entry 4),
presumably due to the fact that the reaction was conducted at
room temperature. Nearly all of the amines reacted smoothly
for both methods with the exception of aniline, which
produced unidentified byproducts in addition to the desired
product as observed by TLC. Nonetheless, compound 20 was
isolated in moderate to good yield for both methods after
SiO2 column chromatography (Table 1, entry 12).
aIsolated yield after SiO2 gel column chromatography. bReaction con-
ducted in a sealed vessel. c0.5 M NH3 solution in 1,4-dioxane used as
solvent.
were independently isolated and characterized by 1H NMR
and MS (Scheme 2). We postulate that formation of 33 could
have occurred by direct condensation of 9with 2-aminophenol,22
which was followed by conversion to 34 via a SNAr-type
displacement by the amine, liberating CH3OH as the sole
byproduct (Scheme 2, eq 1). Our proposed pathway was
further substantiated when we observed clean formation of
11 when reacting 33 directly with N-Boc-piperazine in the
presence of 4 equiv of HOAc heated at 60 °C in CHCl3 (data
not shown). HOAc appears to enhance the reaction by either
facilitating condensation of 2-aminophenol and 9 to 33 possi-
bly by forming a reactive cationic species from 9 and/or
On the basis of LC-MS analysis, both reactions appear to
proceed via initial formation of intermediates 33 or 35, which
(22) Kanthlehner, W.; Maier, T.; Loeffler, W.; Kapassakalidis, J. J.
Liebigs Ann. Chem. 1982, 3, 507.
(21) Cioffi, C. WO201010048514A1, 2010.
J. Org. Chem. Vol. 75, No. 22, 2010 7943

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SCHEME 2. Proposed Reaction Pathways
SCHEME 3. Synthesis of the 5-HT3 Receptor Partial Agonist
RR-210 (43)
TABLE 2. Reactions with Substituted 2-Aminophenols
which was circumvented by using 1,1-dichlorodiphenoxy-
methane at room temperature (Table 2, entry 7).
A screening of common organic solvents (CH3OH, EtOH,
EtOAc, toluene, CH3CN, and THF) for the tetramethyl
orthocarbonate method using the same conditions for entry 1
in Table 1 was conducted and all of them performed as well
as chloroform (data not shown). Heating at or above 60 °C
was required for the reaction to proceed; product formation
was not observed when run at room temperature regardless
of the solvent used. In addition, it was discovered that the less
expensive tetraethyl orthocarbonate could also be used in
place of tetramethyl orthocarbonate without compromising
isolated yields (data not shown).
A similar solvent screen was conducted for the 1,1-dichloro-
diphenoxymethane method (CHCl3, CH3CN, and THF)
using the same conditions for entry 1 in Table 1 and it was
observed that the others were inferior to toluene for clean
product formation by LC-MS. Of the bases surveyed (i-Pr2NEt,
DBU, DABCO, and pyridine), i-Pr2NEt and Et3N led to
smoothest product formation by LC-MS (data not shown).
As a demonstration of utility for the tetramethyl ortho-
carbonate approach, we sought to prepare the 5-HT3 receptor
partial agonist 5-chloro-7-methyl-2-(4-methyl-1,4-diazepan-1-yl)-
benzo[d]oxazole (RR-210, 49),24 which was reported to be in
phase I clinical trials for IBS-d (Scheme 3).25 Previously re-
ported methods to prepare 49 involve nucleophilic displacement
of either the 2-mercaptobenzoxazole intermediate 46 or the
2-chlorobenzoxazole intermediate 47 with N-methylhomo-
piperazine (48).23 Our method allowed for the preparation of 49
aIsolated yield after SiO2 gel column chromatography.
displacement of 33 to 34. The reaction involving components
of entry 1 in Table 1 does proceed in the absence of HOAc,
but at a slower rate (incomplete at 48 h), lower isolated
yield (54%), and with unidentified byproducts. We propose
a similar pathway for the reaction involving 10 in that 2-
phenoxybenzoxazole 35 could be formed by direct conden-
sation of 10 with 2-aminophenol23 followed by displacement
of phenol by the amine (Scheme 2, eq 2). The ability of the
1,1-dichlorodiphenoxymethane facilitated reaction to pro-
ceed at room temperature is presumably due to the higher
reactivity of the reagent and/or intermediate 35. Room
temperature displacement of 2-phenoxybenzoxazles such as
35 by amines has been previously reported.12 We also ob-
served clean formation of 11 when reacting 35 directly with
N-Boc-piperazine in the presence of 2 equiv of Et3N in toluene
at room temperature (data not shown).
We further examined the scope of both reactions using
various substituted 2-aminophenols and N-Boc piperazine
(Table 2). Both electron-donating and -withdrawing groups
were well tolerated, again providing yields ranging from modest
to excellent. Entry 7 in Table 2 demonstrates the complemen-
tarity of both reactions as ester 42 underwent competitive amide
formation using the tetramethyl orthocarbonate method,
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S.; Watanabe, T.; Sato, Y. Bioorg. Med. Chem. 2007, 15, 3515. (c) Gao, M.;
Wang, M.; Hutchins, G.; Zheng, Q. Eur. J. Med. Chem. 2008, 43, 1570.
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Cioffi et al.
JOCNote
directly from 2-aminophenol 45 in good yield. We believe
this method to be superior when considering the large-scale
production of 49 in that it is more efficient (one less step), does
not require using the toxic and harsh reagents CS2 or PCl5, and
produces CH3OH as a byproduct instead of H2S (when 46 is
used in the displacement). It is our expectation that the synthesis
of other therapeutically relevant 2-aminobenzoxazoles can be
improved in a similar way using this method.
In summary, we have discovered two novel, versatile,
and facile methods for synthesizing 2-aminobenzoxazoles
using readily available amines, substituted 2-aminophenols,
and commercially available tetramethyl orthocarbonate or
1,1-dichlorodiphenoxymethane. The advantages to our proce-
dures are as follows: (1) the reactions are single-pot and utilize
accessible, easy-to-handle, and off-the-shelf reagents; (2) the
reaction conditions are mild, thus various functional groups
are well tolerated; (3) the reactions are clean, providing virtually
spot-to-spot conversion from 2-aminophenol to 2-methoxy-
benzoxazole or 2-phenoxybenzoxazole to 2-aminobenzoxazole
in most cases (as observed by TLC and LC-MS analysis);
(4) the reactions are versatile for both amines and substituted
2-aminophenols; and (5) the tetramethyl orthocarbonate reac-
tion performs well in a variety of solvents.
The reactions are also complementary. The 1,1-dichloro-
diphenoxymethane reaction performs well at room tem-
perature and is essentially pH neutral, which is desired in
cases when volatile amines are used and/or acid-sensitive
functionality is present. Although the tetramethyl ortho-
carbonate reaction requires mild heating, the byproduct
generated is CH3OH. Such conditions should be amenable
to large-scale production and could present a “greener” and
more environmentally friendly approach to this important
heterocyclic motif. We are currently attempting to extend
these methodologies to other heterocyclic systems, such as
2-aminobenzimidazoles and benzthiazoles.
piperazine-1-carboxylate (6.8 g, 36.6 mmol), and HOAc
(4.2 mL, 73.3 mmol) in CHCl3 (70 mL) was added tetramethyl
orthocarbonate (4.8 mL, 36.6 mmol) at room temperature. A
suspension typically formed, which became a clear solution once
the mixture was heated at 60 °C. The reaction was heated at
60 °C for 16 h and cooled to room temperature. The mixture was
washed with 1 N NaOH, 1 N HCl, and brine. The organic layer
was dried over anhydrous Na2SO4, filtered, and concentrated
under reduced pressure. The resulting residue was chromato-
graphed over SiO2 gel (10% to 30% EtOAc in hexanes) to give
tert-butyl 4-(benzo[d]oxazol-2-yl)piperazine-1-carboxylate as a
white solid (5.38 g, 97%); Rf 0.4 (hexanes/EtOAc, 3:1);
1
mp 132-134 °C; H NMR (300 MHz, CDCl3) δ 7.37 (d, J =
7.1 Hz, 1H), 7.28 (d, J = 7.1 Hz, 1H), 7.20 (t, J = 6.7 Hz, 1H),
7.06 (t, J = 6.7 Hz, 1H), 3.69 (m, 4H), 3.57 (m, 4H), 1.49 (s, 9H);
ESI MS m/z 304 [C16H21N3O3 þ H]þ.
Representative 1,1-Dichlorodiphenoxymethane Procedure. To
a mixture of 2-aminophenol (1.0 g, 9.1 mmol), morpholine
(0.80 g, 9.1 mmol), and Et3N (2.5 mL, 18.3 mmol) in toluene
(36 mL) was added 1,1-dichlorodiphenoxymethane (2.4 g, 9.1
mmol) at room temperature. The reaction continued to stir at
room temperature for 16 h and was washed with 1 N NaOH, 1 N
HCl, and brine. The organic layer was dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. The
resulting residue was chromatographed over SiO2 gel (10% to
30% EtOAc in hexanes) to give 2-morpholinobenzo[d]oxazole
as a white solid (1.32 g, 71%); Rf 0.3 (hexanes/EtOAc, 3:1);
mp 90-92 °C; 1H NMR (500 MHz, CDCl3) δ 7.38 (d, J = 7.1 Hz,
1H), 7.27 (d, J = 7.1 Hz, 1H), 7.19 (t, J = 6.7 Hz, 1H), 7.05 (t,
J = 6.7 Hz, 1H), 3.82 (m, 4H), 3.69 (m, 4H); ESI MS m/z 205
[C11H12N2O2 þ H]þ.
Acknowledgment. We thank Drs. Mark Wolf, David
Manning, and Peter Guzzo (AMRI, Departments of Medic-
inal Chemistry and Discovery Research and Development)
for helpful discussions.
Supporting Information Available: Experimental proce-
dures and characterization data including 1H NMR spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Experimental Section
Representative Tetramethyl Orthocarbonate Procedure. To
a mixture of 2-aminophenol (2.0 g, 18.3 mmol), tert-butyl
J. Org. Chem. Vol. 75, No. 22, 2010 7945