Converting oxazoles into imidazoles: New opportunities for diversity-oriented synthesis

Article abstract of DOI:10.1039/c3cc48467j

We report the optimization of a neglected reaction for the rapid and direct conversion of oxazoles into N-substituted imidazoles. The utility of this microwave-promoted reaction for diversity-oriented synthesis is demonstrated in the preparation of >40 N-substituted imidazoles, including α-imidazolyl esters.

Full text of DOI:10.1039/c3cc48467j

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Converting oxazoles into imidazoles: new
opportunities for diversity-oriented synthesis†
Cite this: Chem. Commun., 2014,
50, 1867
Thibaut Alzieu,a Johannes Lehmann,a Ajay B. Naidu,b Rainer E. Martin*a and
Robert Britton*b
Received 5th November 2013,
Accepted 20th December 2013
DOI: 10.1039/c3cc48467j
www.rsc.org/chemcomm
We report the optimization of a neglected reaction for the rapid and
direct conversion of oxazoles into N-substituted imidazoles. The
utility of this microwave-promoted reaction for diversity-oriented
synthesis is demonstrated in the preparation of >40 N-substituted
imidazoles, including a-imidazolyl esters.
Imidazoles are essential heterocycles in modern medicinal
chemistry1 and common in pharmaceuticals2 and natural
products.3 As such, tremendous effort has been devoted to both
imidazole synthesis4 and functionalization,5 from which many robust
processes have emerged.6 For example, the van Leusen imidazole
synthesis7 or multi-component reactions of 1,2-diketones with alde-
hydes and amines8 are standard protocols. Despite these advances,
several fundamental challenges to imidazole synthesis include the
regioselective N-alkylation of asymmetric imidazoles,9 production of
N-tertiary alkyl imidazoles,10 and the rapid synthesis of diverse
libraries of N-substituted imidazoles.11 Considering the ease with
which oxazoles are prepared12,13 and the well-established processes
for their selective functionalization at C214 or C5,15 they represent
potentially useful synthetic isosters for imidazole. Thus, a direct
conversion of oxazoles into imidazoles would provide unique
opportunities for the regioselective synthesis of N-functionalized
imidazoles, and new avenues for incorporating imidazoles into
diversity-oriented drug discovery programs.
Scheme 1 Cornforth’s imidazole synthesis and related reaction of phenyl
oxazole.
During our recent studies of Kondrat’eva reactions involving would involve attack at C2 of an oxazolium intermediate by the amine
cycloalkenes (e.g., 4 + 5 - 7, Scheme 1),16 we examined the 6, followed by ring opening/closing and extrusion of water. Despite
microwave-assisted reaction of aminocyclopentene 6 with phenyl- obvious potential for exploiting this convenient reaction to overcome
oxazole 4. Surprisingly,16 the exclusive product of this reaction was limitations in imidazole synthesis (see above), since the report of a
not the expected cycloalka[c]pyridine 8, but the structurally iso- single related example (see inset, Scheme 1) by Cornforth in 1947,17,18
meric N-cyclopentenyl imidazole 9. Formation of the imidazole 9 only a very limited number of additional examples have been dis-
closed.19,20 In fact, this potentially important transformation is not
a Medicinal Chemistry, Small Molecule Research, Pharma Research & Early
Development (pRED), F. Hoffmann-La Roche AG, Grenzacherstrasse 124,
4070 Basel, Switzerland. E-mail: rainer_e.martin@roche.com
b Department of Chemistry, Simon Fraser University, Burnaby, British Columbia,
Canada. E-mail: rbritton@sfu.ca
considered among the fundamental reactions of oxazole or imidazole
syntheses. Considering the value of imidazoles to the drug discovery
process and the importance of identifying new methods to access
N-functionalized imidazoles, we have investigated the oxazole to imid-
azole transformation, evaluated the scope of this reaction for the pre-
paration of asymmetric N-alkyl imidazoles, and demonstrated the utility
of this reaction for rapidly generating structurally diverse imidazoles.
† Electronic supplementary information (ESI) available: Full experimental
details, and 1H NMR, 13C NMR, and 19F NMR spectra for all new compounds.
See DOI: 10.1039/c3cc48467j
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Table 1 Microwave-assisted oxazole to imidazole transformation
at temperatures ranging from 200 to 240 1C, 14 was produced in
good yield in only 10 min (entries 3–5). Further heating above 240 1C
led to the production of a number of by-products and a reduced yield
of 14 (entry 6). The use of fewer equivalents of TFA and cyclopentyl-
amine was also examined (entries 7–10) and optimally, 2 equivalents
of both the amine and TFA at 200 1C provided the imidazole 14
in reproducibly good yield (79%). A variety of Lewis acids were
examined for their ability to promote the desired transformation, the
most effective of which are summarized in entries 11–14. Several
other solvents were also examined (e.g., xylene, toluene, a,a,a-
trifluorotoluene, N-methyl-2-pyrrolidine), however, the highest yield
of 14 was obtained in o-dichlorobenzene (o-DCB).
Using the optimized reaction conditions (Table 1, entry 10), a
collection of N-substituted imidazoles 15–52 were readily prepared
from the combination of several commercially available 4- and
5-aryloxazoles and a variety of alkylamines. As depicted in Scheme 2,
this microwave-assisted oxazole to imidazole transformation tolerates
both functional and diversifiable groups (e.g., alkene, F, Cl, Br, CN, CF3)
and provides direct access to N-secondary- and N-tertiary alkyl-
imidazoles that would be difficult to prepare via direct alkylation
Entry Ratio 4 : 13 Additive (equiv.) Temp. (1C) Time (min) 14b (%)
1
2
3
4
5
6
7
8
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 1
1 : 1
1 : 1.5
1 : 2
1 : 1
1 : 1
1 : 1
1 : 1
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (3.0)
TFA (1.0)
TFA (2.0)
TFA (1.5)
TFA (2.0)
In(SO3CF3)3 (1.0) 200
InCl3 (1.0) 200
Mg(SO3CF3)2 (1.0) 200
AgSO3CF3 (1.0) 200
150
180
200
220
240
260
200
200
200
200
90
30
10
10
10
10
30
30
30
30
30
30
30
30
13
33
51
67
71
48
65c
55c
73c
79c
59
7
9
10
11
12
13
14
42
52
a
Reactions carried out in a crimped (Teflon seal) vial and heated in an
initiator+ microwave synthesizer (Biotages) in o-DCB. Percent yield (e.g., 23–38). Additionally, N-neopentyl (e.g., 19), N-homoallyl
based on HPLC-MS analysis with internal standard. Isolated yield.
(e.g., 22), and N-homobenzyl imidazoles (e.g., 39–45) were all pre-
b
c
pared regioselectively using these straightforward reaction condi-
The microwave-assisted reaction of cyclopentylamine (13) with tions. The generally improved yields for the transformation of
5-phenyloxazole (4) is summarized in Table 1. While slow conversion 4-aryloxazoles into imidazoles may be attributed to the intermediacy
to the imidazole 14 was observed at 150 and 180 1C (entries 1 and 2), of an aldehyde as opposed to a ketone (e.g., 11, Scheme 1) in the
Scheme 2 Synthesis of 1-alkyl-4-aryl- and 1-alkyl-5-arylimidazoles from 4- and 5-aryloxazoles.
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Scheme 3 Direct synthesis of a-imidazolyl esters from a-amino esters.
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reactions of 5-aryloxazoles, and decreased steric congestion in the
penultimate intermediate 12.
In order to further demonstrate the utility of this process, we
investigated the direct synthesis of 2-imidazolyl carboxylic esters.
Notably, the imidazolyl ester 58 (Scheme 3) is a key intermediate
in the synthesis of a hepatoselective glucokinase activator at
Pfizer.21,22 Employing our optimized conditions, reaction of valine
ethyl ester (53) with 4-phenyloxazole provided the imidazolyl
carboxylic ester 54 in good yield (49%). As analysis of the
enantiomeric purity of 54 indicated partial racemization had
occurred, the reaction was repeated at lower temperature (e.g.,
120 1C), which limited racemization (63% ee) with little effect on
the yield. Optimally, shortening the reaction time to 0.5 h at
100 1C afforded the imidazolyl ester 54 in acceptable yield and
enantiomeric purity (92% ee). Applying these modified conditions
to the reaction of ethyl esters derived from leucine, iso-leucine,
and alanine afforded the corresponding imidazolyl esters 55–57 in
good yield and enantiomeric purity.
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In summary, we have developed a microwave-assisted conver-
sion of oxazoles into imidazoles and evaluated the scope of this
fundamentally important transformation. Considering the ease
with which oxazoles can be selectively functionalized at C2 or C5,
these results highlight the utility of oxazole as a versatile scaffold
for medicinal chemistry and diversity oriented synthesis. Further-
more, owing to the decreased basicity of oxazole (pKa B 1)
relative to imidazole (pKa B 7) and ease of functionalization,14,15
oxazole should be considered a protected imidazole. This
stratagem should prove useful in synthetic sequences where
interaction between the imidazole and a Brønsted or Lewis acid
reagent would otherwise complicate the process, or where late
stage diversification of imidazole is desirable.
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17 J. W. Cornforth and R. H. Cornforth, J. Chem. Soc., 1947, 96.
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20 Examples in patents: (a) N. Hauel, B. Narr, U. Ries, J. C. A. van Meel, 21 J. R. Dunetz, M. A. Berliner, Y. Xiang, T. L. Houck, F. H. Salingue,
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1870 | Chem. Commun., 2014, 50, 1867--1870
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