Dakin-west trick in the design of novel 2-alkyl(aralkyl) derivatives of oxazolo[3,2-a]pyridines

Article abstract of DOI:10.3390/10091109

A series of previously unavailable derivatives of 2-alkyl- and 2-benzylderivatives of oxazolo[3,2-a]pyridines III were obtained via tandem ring opening and ring closure from stable mesoionic 3-acyloxazolo[3,2-a]pyridinium- 2-olates I. The key intermediate

Full text of DOI:10.3390/10091109

Molecules 2005, 10, 1109-1118
molecules
ISSN 1420-3049
http://www.mdpi.org
Dakin-West Trick in the Design of Novel 2-Alkyl(aralkyl)
Derivatives of Oxazolo[3,2-a]pyridines
Zeinul-Gabiden M. Kazhkenov, Alexander A. Bush, Eugene V. Babaev*
Chemistry Department, Moscow State University, Moscow 119992, Russia
* Author to whom correspondence should be addressed. E-mail: babaev@org.chem.msu.su
Received: 23 April 2005 / Published: 30 September 2005
Abstract: A series of previously unavailable derivatives of 2-alkyl- and 2-benzylderivatives
of oxazolo[3,2-a]pyridines III were obtained via tandem ring opening and ring closure from
stable mesoionic 3-acyloxazolo[3,2-a]pyridinium-2-olates I. The key intermediates of this
tandem transformation are N-(b-oxoethyl)pyridones-2 II obtained by Dakin-West acylation
of (pyridone-2-yl-1)acetic acid. An example of further utilization of this strategy is
illustrated by preparation of unknown 2-benzylimidazo[1,2-a]pyridine from the salt I and
ammonia.
Keywords: Oxazolo[3,2-a]pyridinium salts, mesoionic oxazolo[3,2-a]pyridinium-2-olates,
munchnones, recyclization, hydrolytical cleavage of oxazolone, Dakin-West reaction
Introduction
The oxazolo[3,2-a]pyridinium system has attracted our attention due to its ambident behavior
toward nucleophiles and the ability to undergo opening of either the pyridine or oxazole ring forming
various azolopyridines or hetaryldienes (Scheme 1, see [1-4] and review [5]):

Molecules 2005, 10
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Scheme 1.
O
HNR2
Z = O
z
R
NR2
N+
R
N
HNR2
XH-
X
Z=S, NR
Y
O
R
- H2O
N
N
R
Y=S+, NR+, N, CR
The most efficient route to the oxazolo[3,2-a]pyridinium ring system is the method of Bradsher [6]
involving the cyclocondensation of 2-halopyridinium salts (A) or pyridones-2 (B) bearing a phenacyl-
group at the nitrogen atom:
Scheme 2.
Cl
O
N
Cl
O
Ar
N+
N+
Ar
A
ArCOCH2Br
OMe
O
O
O
N
OMe
N+
N
Ar
Ar
B
The intermediates A and B in turn could be easily prepared by alkylation of 2-substituted pyridines
with α-bromoketones. This strategy gives the best results if aromatic bromoketones (phenacyl
bromides) are used, leading to 2-aryl derivatives of the oxazolopyridinium salts.
2-Alkyl(aralkyl) derivatives of oxazolopyridines C would be interesting target compounds capable
to further transformations. However, it is evident that the Bradsher method (e.g. cyclization of
oxoalkylpyridones D) is hardly to apply for preparation of this class. Indeed, the precursors of the
pyridones D would be the unsymmetrical bromoketones E, that are difficult (or even impossible) to
prepare by selective bromination of methylalkylketones F.

Molecules 2005, 10
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Scheme 3.
O
O
Br
O
COOH
N
O
N
X
R
O
D
G
E
R
R
H2O
- CO2
- H2O
O
O
CH3
O
N+
CH2
O
F
CH2R
N+
R
H
O
C
R
Key intermediates D can be considered as a specific type of α-aminoketones (fragment shown by
red on Scheme 3). Therefore, an alternative strategy to compounds D would be Dakin-West reaction
[7] i.e. acylation of a corresponding aminoacid, in our case - pyridone-2-acetic acid G. Meanwhile, the
reaction of the acid G with acyl chlorides and anhydrides leads to stable mesoionic bicycles (fused
munchnones) H [8-10]. The possibility of conversion of bicyclic munchnones H to the pyridones D
has received little attention. Thus, Boyd and Wright reported that munchnones H may undergo
hydrolytical cleavage under the action of hot water [9]. In our early work [11-13] we have observed
that some bicyclic munchones can undergo cleavage of the oxazolone fragment in the acidic media.
In the present communication we report our success in applying the sequence G-H-D-C as a new
strategy to prepare previously unknown class of 2-benzyl and 2-alkyl derivatives of oxazolo[3,2-a]-
pyridines.
Results and Discussion
Acylation of oxazolo[3,2-a]pyridinium-2-olate.
At the initial phase we have synthesized broad series of munchnones of the type H according to
Scheme 4. According to literature data the munchnones I were prepared by acylation of unstable key
intermediate Ih, which in turn was generated at low temperature (0°C) [9] (Scheme 4). The known
instability of the unsubstitued munchone Ih is connected with its ability to undergo self-acylation
leading to undesired dimer Ii. The presence of dimer Ii (poorly soluble in most of solvents) makes
difficulties in purification of mesoionic compounds Ia-g. Nevertheless, we found, that considerable
decrease of the temperature (up to -45oC) depressed the dimerization process, and the desired pure
munchnones I can be easily obtained by this modified protocol.
1H-NMR spectra of fused munchnones are shown in Table 1. Specific feature for the entire series
of mesoionic structures I is the strong downfield shift of the signal of proton H-5 (observed at 9.8-9.9
ppm) due to the presence of magnetically anisotropic acyl group at peri-position 3.

Molecules 2005, 10
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Scheme 4.
O
O
Et3N
CH2Cl2,t=-45 oC
Ac2O
O
N+
O
N+
HClO4
N
O
-
ClO4
Ih
RCOCl
CH2Cl2, Et3N, t=-45 oC
COOH
dimerization
O
O+
O
N+
O
N+
O
R
N
O
O
Ii
Ia-g
Table 1. Characteristics of mesoionic compounds I
Chemical shift, p.p.m.; J, Hz
№
R
Yield, %
Mp, °С
Н5
Н6
Н7
Н8
RСО
m; 7.24 (2H);
m; 6.77 (2H);
s; 3.99 (2H);
s; 3.74 (3H)
m; 7.34 (2H);
m; 7.24 (2H);
s; 4.06 (2H)
m; 8.12 (2H);
m; 7.59 (2H);
s; 4.24 (2H)
d; 9.86;
J56=6.4
d; 7.61;
J78=8.6
Iа
4-МеОPhCH2
64
60
171-173
m; 7.51
m; 7.96
d; 9.83;
J56=6.3
d; 7.64;
J78=8.4
Ib
4-ClPhCH2
194-196
m; 7.52
m; 7.99
Ic
4-NO2PhCH2
Me
82
78
225-227
m; 9.81
m; 7.55
m; 7.55
m; 8.01
m; 8.01
m; 7.65
d; 9.78;
J56=6.5
d; 7.68;
J78=8.5
Id
170-172*
s; 2.36 (2H)
q; 2.75 (2H);
J=8.0;
t; 1.12 (3H);
J=7.0
t; 2.74 (2H);
J=7.7;
m; 1.64 (2H);
t; 0.94 (3H);
J=7.1
153-
155**
d; 9.90
J56=6.4
d; 7.60;
J78=8.7
Ie
If
Et
Pr
48
62
m; 7.52
m; 7.55
m; 7.96
m; 8.00
d; 9.83;
J56=6.2
d; 7.68;
J78=9.0
136-137
140-142
t; 2.74 (2H);
J=7.3;
m; 1.59 (2H);
m; 1.35 (2H);
t; 0.91; (2H);
J=7.1
d; 9.84;
J56=6.2
d; 7.67;
J78=8.0
Ig
Bu
86
m; 7.55
m; 8.00
*Lit. Mp 170-171 [8]; **lit. Mp 146-147.5 [8].
Hydrolytic cleavage of oxazolones I.
Our attempt to perform the hydrolytical cleavage of mesoionic oxazolones either in pure water or
in strong sulfuric acid were unsuccessful. In the first case no indication of the reaction was observed,
and in the second case undesired side reactions occurred (in particular, during the hydrolysis of

Molecules 2005, 10
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munchnone Ib the strong smell of phenylacetic acid indicated that the concurrent cleavage of the acyl
group might take place). After several attempts we have found that the optimal condition of hydrolysis
of munchones I is the use of diluted (0.06 M) aqueous solution of hydrochloric acid leading to desired
products II in good yields.
Scheme 5.
O
O
O
HCl
COOH
R
O
N+
0.06 M
N
N
- CO2
R
R
O
O
O
II
Table 2. Characteristics of pyridones II
Chemical shift, p.p.m.; J, Hz
№
R
Yield, %
Mp, °С
Н3
Н4
Н5
Н6
RСОCH2
m; 7.14 (2H);
m; 6.83 (2H);
s; 4.76 (2H);
s; 3.76 (5H)
m; 7.30 (2H);
m; 7.22 (2H);
s; 4.81 (2H);
s; 3.86 (2H)
m; 8.16 (2H);
m; 7.50 (2H);
s; 4.87 (2H);
s; 4.06 (2H)
s; 4.72 (2H);
s; 2.18 (3H)
s; 4.71 (2H);
q; 2.52 (2H);
J=7.1;
t; 1.05 (3H);
J=7.1
s; 4.70 (2H);
t; 2.45 (2H);
J=7.9;
m; 1.61 (2H);
t; 0.95 (3H);
J=7.1
d; 7.42;
J34=7.0
d; 6.35;
J56=8.6
IIa
4-МеОPhCH2
86
84-85
m; 7.35
m; 6.13
d; 7.45;
J34=7.2
d; 6.36;
J56=9.5
IIb
4-ClPhCH2
90
133-135
m; 7.37
m; 6.15
d; 6.36;
J56=9.0
IIc
IId
4-NO2PhCH2
Me
91
67
170-172
-
m; 7.50
m; 7.37
m; 7.36
m; 6.16
m; 6.14
d; 7.44;
J34=6.9
d; 6.36;
J56=9.2
d; 7.46;
J34=7.0
d; 6.35;
J56=8.7
IIe
IIf
Et
Pr
52
59
-
-
m; 7.36
m; 7.36
m; 6.15
m; 6.14
d; 7.45;
J34=7.1
d; 6.34;
J56=9.2
s; 4.70 (2H);
t; 2.49 (2H);
J=7.8;
m; 1.56 (2H);
m; 1.35 (2H);
t; 0.93 (3H);
J=7.1
d; 7.44;
J34=6.6
d; 6.35;
J56=8.6
IIg
Bu
76
-
m; 7.36
m; 6.14
It should be mentioned, that the formed pyridones II are more water soluble than the parent
munchones, therefore in the course of reaction the initial suspension was usually dissolved (the
exception was only the case of 4-nitrobenzyl derivatives Ic, IIc, both poorly soluble in water). In the

Molecules 2005, 10
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1H-NMR spectra of pyridones II the signals of protons of N-CH2-CO group (4.7-4.8 ppm) are usually
clear distinguishable from the signals of protons from another methylene group RCH2CO belonging to
aliphatic or aromatic fragment (Table 2).
Synthesis of 2-substitued oxazolo[3,2-a]pyridinium perchlorates.
According to Bradsher [6] the cyclodehydratation of N-phenacylpyridones B to 2-aryloxazolo-
pyridinium salts can be achieved by the action of concentrated sulfuric acid. However, all our attempts
to perform conversion of N-(b-oxoethyl)pyridones-2 II to the desired 2-alkyloxazolopyridinium salts
III failed. In all cases the degree of conversion has never exceeded 10-15%. Probably in these cases
the completeness of the reaction was connected with the reversibility of the ring closure, and complete
removal of water would shift the equilibrium. Indeed, the use of 8% oleum (completely capturing all
the water) led to desired products III in good yields.
Scheme 6.
H+
O
OH
R
O
O
R
N+
N+
III
N
- H2O
H
H
R
O
8% of SO3
in H2SO4
The only exception was the case of pyridone IIa (with an anisyl group) that in reaction with oleum
or concentrated sulfuric acid gave a red oil, probably due to sulfonation of the benzene ring [6]. In the
1H-NMR spectra of the aromatic bicycles III novel aromatic singlet of the oxazole ring was observed
at 8.5-8.7 ppm, and the most downfield doublet (at 9.2 ppm) was the signal of a-pyridinium proton H-
5.
Reaction of 2-(4’-chlorobenzyl)oxazolo[3,2-a]pyridinium perchlorate (IIIb) with ammonia.
In order to investigate the reactivity of the salts III we performed simple test experiment with
ammonia. Reaction of salt IIIb with solution of anhydrous ammonia in DMF resulted in formation of
corresponding imidazo[1,2-a]pyridine IV (Scheme 7, Table 3), the first representative of a previously
unknown class of 2-benzylimidazopyridines. Clearly, this structural type would be difficult to obtain
by standard Chichibabin reaction from the practically unavailable bromoketone E (R = p-ClPh). Our
attempts to perform this reaction in ethanolic or aqueous ammonia resulted in a number of byproducts.

Molecules 2005, 10
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Scheme 7.
O
?
Br
N+
NH2
N
NH3/DMF
40%
N
N
O
Cl
Cl
Cl
IV
Table 3. Characteristics of the salts III and imidazopyridine IV
Chemical shift, p.p.m.; J, Hz
№
R
Yield, %
Mp, °С
Н3
Н5
Н6
Н7
Н8
R
m; 7.44 (2H);
m; 7.35 (2H);
s; 4.40 (2H)
m; 8.21 (2H);
m; 7.72 (2H);
s; 4.60 (2H)
d; 9.15;
J56=6.1
m;
8.45
d; 8.27;
J78=9.0
IIIb 4-ClPhCH2
IIIc 4-NO2PhCH2
87
162-164 s; 8.64
132-134 s; 8.71
m; 7.90
d; 9.18;
J56=6.7
m;
8.46
d; 8.30;
J78=9.0
84
m; 7.91
d; 9.13;
J56=6.3
d; 9.13;
J56=6.1
m;
8.44
m;
d; 8.29;
J78=8.9
d; 8.29; q; 3.03 (2H); J=7.8;
IIId
IIIe
Me
Et
42
67
127-128* s; 8.56
118-120 s; 8.61
m; 7.90
m; 7.91
s; 2.68 (3H)
8.45
J78=8.9
t; 1.40 (3H); J=7.7
t; 2.98 (2H); J=7.1;
m; 1.85; (2H);
t; 1.09; (3H); J=8.1
t; 2.99 (2H);
d; 9.16;
J56=5.9
m;
8.46
d; 8.30;
J78=8.9
IIIf
IIIg
IV
Pr
Bu
74
48
40
98-100 s; 8.65
m; 7.91
m; 7.92
J=7.3;
d; 9.19;
J56=6.3
m;
8.46
d; 8.28;
J78=8.9
-
s; 8.68
m; 1.78 (2H);
m; 1.48 (2H);
t; 0.98 (3H); J=7.3
m, 7.30 (2H);
m, 7.24 (2H);
s, 4.01 (2H)
d, 8.32,
J56=7.1
d, 7.38,
J78=9.3
4-ClPhCH2
162-164 s; 7.49
m, 7.11 m, 6.73
*Lit. Mp 128 [6]
Conclusions
We have found new simple route to 2-substituted oxazolo[3,2-a]pyridinium salts which, in turn
could be precursors of previously unavailable structures, e.g. 2-benzylimidazo[1,2-a]pyridines. The
group CH2R at the imidazopyridine and oxazolopyridines originates from the residue of an organic
acid.
O
O
O
CH2R
O
N+
N+
N
O
O
R
R

Molecules 2005, 10
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Experimental
General
1H- and 13C-NMR spectra were obtained using a Bruker AC 400 NMR spectrometer and were
recorded at 360 MHz. All reagents and chemicals were obtained from Acros or Merck and were used
as received unless otherwise noted. Ether was purified by refluxing over sodium with benzophenone
for 2-2.5 hours followed by distillation. Triethylamine was distilled over sodium hydroxide, sodium
and finally phthalic anhydride under an argon atmosphere. Acetic anhydride was purified by
distillation over sodium acetate. DMF was stirred with calcium hydride powder for 2-3 hours at 70-
80ºC, decanted from the precipitate and distilled under reduced pressure. Acyl chlorides were prepared
according to a standard procedure [14].
Preparation of 2,3-dihydro-2-oxooxazolo[3,2-a]pyridinium perchlorate.
N-(Pyridone-2-yl-1)acetic acid (6 g, 39 mmol) was suspended in acetic anhydride (40 mL, freshly
distilled over sodium acetate) and 70% HClO4 (6 mL, 59 mmol) was carefully added dropwise at 30-
40°C. After addition of all the HClO4 the reaction was stirred for 30 min and diluted with dry Et2O (50
mL). Product was filtered off, washed with dry Et2O (12×15 mL) and dried in vacuo. Yield 8.95 g
(97%). m.p. 180-182 °C (Lit. [9] m.p. 178-180 °C).
Typical procedure for the preparation of mesoionic 3-acyloxazolo[3,2-a]pyridinium-2-olates (I).
A suspension of 2,3-dihydro-2-oxooxazolo[3,2-a]pyridinium perchlorate (7.0 g, 30 mmol) in dry
CH2Cl2 (100 mL) was cooled to –60 °C and freshly distilled Et3N (14.1 mL, 101 mmol) was added
dropwise at –58 to -52 °C. The thus obtained slightly yellow solution was treated with a solution of the
corresponding acyl chloride (30 mmol) in dry CH2Cl2 (40 mL) at –58- to -52 °C. The suspension or
solution obtained was allowed to warm to RT. The solvent was evaporated to dryness, and the residue
was washed with water (2×25 mL), recrystallized from iso-propanol and dried on air. In the case of
synthesis of Ic, after warming the suspension the precipitate was just filtered off and washed with iso-
propanol (2×25 mL). Yields and NMR data for compounds I are given in Table 1.
Procedures for the preparation of N-alkylated pyridones (II)
N-(3-(4’-chlorophenyl)-β-oxopropyl)-2-pyridone (IIa)
Mesoionic compound Ia (8.7 g, 30 mmol) was suspended in a 0.06 M aqueous solution of HCl
(500 mL) and refluxed for 8 h (TLC control). The hot aqueous phase was decanted from the brown
oily mass and cooled to RT. The white needle-like crystals obtained were filtered off and washed with
water (10 mL). The mother liquors were added to the oily mass and refluxed for 2 h until all the oil
was dissolved and TLC shown full conversion of the starting material. The solution was cooled to RT,
and the gray needles thus obtained were filtered off and recrystallized from iso-propanol.

Molecules 2005, 10
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N-(3-(4’-nitrophenyl)-β-oxopropyl)-2-pyridone (IIb)
Mesoionic compound Ib (6.1 g, 20 mmol) was suspended in 600 mL 0.06 M aqueous solution of
HCl and refluxed for 8 h (TLC control). Hot solution was decanted from solid and cooled to RT.
Obtained white needle crystals were filtered off and washed with water (10 mL). Mother liquor was
added to residuary starting substance and refluxing was continued for 8 h. Procedure was repeated
until TLC shown full conversion of starting product. Obtained precipitate was recrystallized from iso-
propanol.
N-(3-(4’-methoxyphenyl)-β-oxopropyl)-2-pyridone (IIc)
Mesoionic compound Ic (7.0 g, 25 mmol) was suspended in 0.06 M aqueous solution of HCl (500
mL) and refluxed for 4 h. After all the starting substance was dissolved the solution was refluxed for 1
h (TLC control) and then cooled to RT. The obtained mixture was extracted with CHCl3 (3×100 mL),
dried over Na2SO4. The extract was evaporated obtaining yellow oil. The residue was recrystallized
from iso-propanol giving white needles.
Synthesis of N-β-oxoalkyl-2-pyridones IId-g
Mesoionic compound Id-g (25 mmol) was suspended in 0.06 M aqueous solution of HCl (250 mL)
and refluxed for 2 h. After all the starting substance was dissolved and solution was refluxed for 1 h
(TLC control) and then cooled to RT. Obtained mixture was extracted with CHCl3 (3×75 mL), dried
over Na2SO4. The extract was evaporated to give a yellow oil. Yields and NMR data for compounds II
are given in Table 2.
Typical procedure for the preparation of 2-substituted oxazolo[3,2-a]pyridinium perchlorates (III):
The N-β-oxoalkyl-pyridone-2 (II, 0.74 mmol) was dissolved in 8% oleum (2 mL) and kept for 20-
25 hours. Then 70% HClO4 (0.15 mL, 1.5 mmol) was carefully added to the mixture and after 20 hours
the solution was poured into vigorously stirred diethyl ether (200 mL). After decanting the oily residue
was mixed again with a fresh portion of ether (200 mL); this procedure should be repeated until a pure
white powder formed. The precipitate was filtered, washed with ether and dried in vacuum over P2O5.
Yields and NMR data for compounds III are given in Table 3.
Reaction of 2-(4’-chlorobenzyl)oxazolo[3,2-a]pyridinium perchlorate with ammonia.
2-(4’-Chlorobenzyl)oxazolo[3,2-a]pyridinium perchlorate (2.4 mmol) was dissolved in anhydrous
distilled DMF (3 mL) and mixed at -25°C with a solution (4 mL) prepared by saturation of DMF with
gaseous ammonia (prepared at -60°C). The reaction mixture immediately turned to deep red color.
This mixture was kept for 10 hours and then poured into cold water. The product precipitated as a
yellow powder, which was separated, washed with water and recrystallized from petroleum ether.
Yields and NMR are given in Table 3.

Molecules 2005, 10
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