Rearrangement of oxazolo[3,2-a]pyridines as an approach of synthesizing aza[3.3.2]cyclazines

Article abstract of DOI:10.1007/s10593-015-1685-6

[MediaObject not available: see fulltext.]5-Methyloxazolo[3,2-a]pyridinium salts were shown to react with (methylamino)acetaldehyde dimethyl acetal leading to the formation of functionalized 5-aminoindolizines, which in turn are capable of closing the pyr

Full text of DOI:10.1007/s10593-015-1685-6

DOI 10.1007/s10593-015-1685-6
Chemistry of Heterocyclic Compounds 2015, 51(3), 269–274
Rearrangement of oxazolo[3,2-a]pyridines
as an approach of synthesizing aza[3.3.2]cyclazines
Eugene V. Babaev1*, Aleksandra A. Nevskaya1, Ilya V. Dlynnikh1
1 Lomonosov Moscow State University
1, Build. 3 Leninskie Gory, Moscow 119991, Russia;
e-mail: babaev@org.chem.msu.ru
Translated from Khimiya Geterotsiklicheskikh Soedinenii,
2015, 51(3), 269–274
Submitted January 9, 2015
Accepted February 13, 2015
OMe
OMe
, 20 min
Me
Ar
H
N
Ar
Ar
N+
N
N
Me
TFA
OMe
OMe
O
N
N+
Me
Me
ClO4–
CF3CO2
–
5-Methyloxazolo[3,2-a]pyridinium salts were shown to react with (methylamino)acetaldehyde dimethyl acetal leading to the formation
of functionalized 5-aminoindolizines, which in turn are capable of closing the pyrimidine ring in acidic media forming aza[3.3.2]
cyclazines.
Keywords: 5-aminoindolizines, oxazolo[3,2-a]pyridines, pyrimido[6,1,2-cd]indolizines, cyclization, NMR spectroscopy, rearrangement.
Evolution of the theory of aromaticity has raised interest
in the synthesis of pericondensed indolizines such as
cyclazines 1 and their analogs.1 Yet, chemistry of hetero
counterparts of such structures, including antiaromatic
ones, are still poorly understood. Figure 1 shows the
currently known representatives of [3.3.2]cyclazine series
1a-i.2
cylization schemes shown in Figure 2.
Theoretically, indolizines 2 containing a heteroatom at
position 5 could serve as precursors of [3.3.2]cyclazines,
but these compounds are not readily available. Our
research group has developed a new strategy for the
synthesis of 5-substituted indolizines by recyclization of
oxazolopyridinium salts 3 by the action of nucleophiles
(amines and alkoxides).3 The use of bifunctional reagents
(Nu – a nucleophilic heteroatom, E – an electrophilic
carbon atom) in such a reaction could lead to indolizines 2,
potentially capable of cyclization into cyclazine 1 analogs
via attack by an electrophilic atom on the π-excessive
pyrrole moiety (Scheme 1).
Amine derivatives having a carbonyl group at the chain
end could serve as suitable reagents. Of course, such agents
must be stable under recyclization conditions (high
temperature and concentration of the reagents). Of
secondary amines, N-methylaminoacetaldehyde (acetal
The synthesis of such cyclazines includes addition of a
five- and/or six-membered ring,2 in accordance with
1
3
1
2
N+
a
2
c
N
d
5
3
5
4
1a
X
Z
Y
1
N
Y
N
N
Y
X
X
O
1b –X–Y– = –CH=CH–
c –X–Y– = –S–CH2–
d –X–Y– = –O–CH2–
e –X–Y– = –CH2–O–
1f X = exo-C=O
g X = S
1h Y = S
i Y = NMe
Figure 1. [3.3.2]cyclazines described in literature.
Figure 2. Principles of construction of [3.3.2]cyclazine system.
0009-3122/15/5103-0269©2015 Springer Science+Business Media New York
269

Chemistry of Heterocyclic Compounds 2015, 51(3), 269–274
Scheme 2
Scheme 1
form) and N-methylglycine (sarcosine) esters, for instance,
satisfy our requirements. This strategy, however, has not
yet been employed in the synthesis of cyclazine hetero
analogs. The purpose of this study was to investigate this
approach.
As demonstrated in our laboratory, 5-substituted
indolizines are unstable compounds which are readily
oxidized in air. One example of a stable system is a
5-substituted indolizine possessing a p-nitrophenyl group at
position 2.3 Therefore, we chose this model compound, the
synthesis of which is described in the literature, for further
studies.
the reaction mixture decomposed upon evaporation.
An identical reaction of indolizine 2a with CF3COOH
was carried out in CDCl3 solution at 20°C in an NMR
ampoule. Initially, the disappearance of the signal of the
proton H-3 and the emergence of a new signal of a CH2
group at 5.63 ppm was observed, which corresponds to the
protonated form of indolizine.4 After one day, however, the
color of the solution changed to bright-red, and signals of
1
the starting indolizine were no longer observed in the H
Oxazolopyridinium perchlorate 3a was reacted with
N-(methylamino)acetaldehyde dimethyl acetal. The
reaction proceeded in the presence of the amine by heating
under reflux for 20 min, whereupon the color changes first
to a bright-cherry color before turning brown. Formation of
new compound was registered by TLC which gave blue
color by Ehrlich's reagent (qualitative reaction to a free
position 3 and/or 1 of the indolizine system). Oily product
2a was isolated by chromatography, but the yield was low
NMR spectrum. Proton signals of the dimethyl acetal group
at 3.74 ppm had disappeared; the characteristic signals of
protons of cleaved methanol appeared instead. In the
spectrum of the resulting compound, signals of a new CH2
group at 5.10 ppm and of a new CH group at 6.27 ppm
were conspicuously evident. One of the proton signals of
indolizine cycle had disappeared while others had shifted
upfield that appears to indicate the formation of a
benzylidene structure. On the basis of these data, we
assumed that the structure of the resulting compound
corresponds to the structure 1j. Full interpretation of
spectra is difficult because of the presence of degradation
products of the starting compound 2a in the mixture.
It is known that 5-amino-substituted indolizines are unstable
compounds, easily oxidized in air.5 The most stable
compounds of the series contain electron-withdrawing groups
in the pyridine ring, therefore, it is necessary to use the
appropriate pyridine derivatives for their synthesis.6,7 In the
current work, we utilized the easily accessible homolog
3-cyanopyridin-2-one, which we succeeded in turning into an
oxazolopyridinium salt 3b by phenacylation and subsequent
1
(30%) (Scheme 2). In H NMR spectrum of the obtained
compound, the signals of the aromatic protons of indolizine
are observed at 6.19-8.06 ppm, the doublet of the CH2
group protons is at 3.25 ppm, while the triplet of the acetal
CH proton is at 4.69 ppm. Acetal methoxy protons appear
as a singlet at 3.42 ppm (Table 1).
Compound 2a was quite unstable and easily oxidized in
air which complicated further work. Upon an attempt to
cyclize the obtained indolizine 2a by heating with
hydrochloric acid (for 20 min), a mixture of difficult to
identify products formed. Likewise, processing of
compound 2a with trifluoroacetic acid under argon led to
the formation of a new compound as shown by TLC, but
Table 1. 1H NMR spectra of (СDCl3) indolizines 2а–с
Chemical shifts, δ, ppm (J, Hz)
Com-
pound
H-1
(1H, s)
Ar
(2H, m)
H-3
(1H, s)
H-8
(1H)
NCH3
CH2
(2H)
CH
(1Н)
7-CH3
(3Н, s)
ArCH3
(3H, s)
OAlk
(3H, s)
2a*
2b
6.79
6.66
6.69
7.83–7.85;
8.25–8.27
8.06
7.72
7.83
7.19
(d, J = 8.8)
2.97
3.19
3.21
3.25
4.69
3.42
(6H, s, 2CH3)
–
–
(d, J = 4.8) (t, J = 4.8)
7.15–7.17;
7.54–7.56
7.03 (s)
7.01 (s)
3.44 4.63
3.31
(6H, s, 2CH3)
2.41
2.41
2.42
2.43
(d, J = 5.3) (t, J = 5.3)
4.05 (s)
7.22–7.24;
7.57–7.59
–
1.51
(9H, s, C(CH3)3)
2c
* Other signals: 6.19 (1H, d, J = 8.8, H-6); 6.74–6.76 (1Н, m, H-7).
270

Chemistry of Heterocyclic Compounds 2015, 51(3), 269–274
cyclocondensation by a method developed by us previously.7
obtained in this case was quite stable, it was formed in low
yield (20%) and required chromatographic purification. In
the H NMR spectrum of compound 2c, indolizine proton
signals are visible (Table 1), including protons H-1, H-8,
and H-3 at 6.69, 7.01, and 7.83 ppm, respectively. Also,
signals of sarcosine tert-butyl ester fragment are observed:
CH2 group proton singlets at 4.05 ppm, N-methyl group at
3.21 ppm, and those of tert-butyl group at 1.51 ppm.
Notably, sarcosine tert-butyl ester was obtained by the
reaction of tert-butyl chloroacetate with aqueous
methylamine in the presence of KI in only 7% yield.
Therefore, in order to reduce the required amounts of
sarcosine ester and increase recyclization product 2c yield,
we attempted to optimize the recyclization conditions.
Heating compound 3b in acetonitrile or DMF with 3 equiv
of a base (4-dimethylaminopyridine, triethylamine) and
1 equiv of sarcosine tert-butyl ester did not lead to the
expected derivative 2c. In all cases, the dimerization
product was the only product isolated from the reaction
mixture. Thus, although indolizine 2c possessing an amino
acid residue has been synthesized (and its structure
unequivocally proved), the low yield and time-consuming
purification did not allow us to synthesize the compound in
amounts necessary for further transformations.
Using the sodium salt of the precursor cyanopyridone
(promotes alkylation at the nitrogen atom), and carrying
out the reaction in an ion-solvating solvent (e.g., DMF)
was expected to increase the yield of N-alkylated isomer
4b. Despite the low yield of compound 4b (and the
formation of the O-alkylation by-product 4a) in the first
step, we found this method convenient since it does not
require the use of chromatography and can be done in a
larger scale. N-Phenacylpyridone 4b underwent cyclization
to oxazolopyridinium salt 3b by the action of an acid
(Scheme 3). The hydrolysis of the cyano group is possible
in this case; however, the cyclization proceeded to
completion by the action of the acid for 30 min and no
hydrolysis was observed (as in the case of a p-bromophenyl
derivative).7
For recyclization of salt 3b, N-(methylamino)acet-
aldehyde dimethyl acetal and an ester of sarcosine were
selected as the secondary amines. Recyclization was
conducted by heating under reflux with an excess of the
amine (Scheme 3). Structures of the obtained compounds
2b,c were confirmed by IR, NMR spectroscopy and
elemental analysis.
1
Reaction of perchlorate 3b with N-(methylamino)
acetaldehyde dimethyl acetal by heating in acetonitrile led
to only trace amounts of the recyclization product 2b.
However, by heating under reflux without solvent in an
excess of the amine, target indolizine 2b was obtained in
To conclude, indolizines substituted at position 5 with
sarcosine
tert-butyl
ester
and
N-methyl-2,2-di-
methoxyethanamine moieties have been obtained for the
first time. Derivatives of the previously unknown
2H-pyrimido[6,1,2-cd]indolizinium heterocyclic system have
been synthesized and studied by NMR spectroscopy.
1
25% yield (Scheme 3). H NMR spectrum of dimethyl
acetal derivative 2b is in good agreement with the spectrum
of the previously obtained compound 2a (Table 1).
Experimental
When indolizine 2b was treated with CF3COOH, the
reaction solution turned bright-red after a day of the
experiment, and the starting material had completely
converted to pyrimidoindolizinium trifluoroacetate 1k
(Scheme 3). The formation of indolizine 2b degradation
products was not observed, allowing acquisition and
interpretation of 1H and 13C NMR spectra of the cyclization
product 1k (Fig. 3). An attempt to convert the salt 1k to the
base form by the action of a base (alkali or triethylamine)
led to its decomposition. Probably, the instability of the
bases corresponding to cations 1j,k stems from anti-
aromaticity of these systems.
IR spectra were registered on a UR-20 spectrometer in
petroleum jelly. 1H and 13C NMR spectra were acquired on
a Bruker AM 400 (400 and 100 МHz, respectively) in
DMSO-d6 (1H NMR spectra of salts 3b, 4a,b) and in
CDCl3 (remaining NMR spectra) with TMS as internal
standard. Elemental analysis was performed on a Vario
multi cube analyzer. Melting points were determined on an
Electrothermal IA910 apparatus. Refractive indices of
obtained liquid products were measured on a IRF-22
apparatus. Silufol plates were used for thin-layer
chromatography (visualization with UV light); Acros (0.04
–0.06 mm) silica gel was used for column chromatography.
All used solvents were purified by distillation.
Sarcosine was used in the form of its tert-butyl ester in
order to prevent self-condensation. While compound 2c
Me
Me
Scheme 3
CF3CO2H
Ar
Ar
N
N
NC
NC
OMe
OMe
Me
NC
Me
O
N
N+
Me
–
Me
CF3CO2
N
1k
2b
O
+
1) EtONa, EtOH
rt, 30 min
MeNHCH2CH(OMe)2
, 30 min
Ar
4a
Me
NC
Me
NH
1) H2SO4
rt, 30 min
2) ArCOCH2Br
DMF, 80°C, 2 h
Me
NC
Me
Me
NC
Me
Me
NC
MeNHCH2CO2Bu-t
, 30 min
O
Ar
N+
N
O
N
2) HClO4
H2O
–
O
ClO4
Ar
O
O
N
Me
Ar
Ar = 4-MeC6H4
4b
3b
OBu-t
2c
271

Chemistry of Heterocyclic Compounds 2015, 51(3), 269–274
Figure 3. Spectroscopic study of the reaction of compound 2b with СF3СООН in an
NMR ampoule: а) 1H NMR spectrum of precursor 2b; b) 1H NMR spectrum of
compound 1k; c) 13C NMR spectrum of compound 1k.
5-Methyl-2-(4-nitrophenyl)oxazolo[3,2-a]pyridinium
Indolizine 2a cyclization reaction by the action of
СF3COOH (in NMR ampoule). 1H NMR spectrum of
indolizine 2a (20 mg) solution in СDCl3 was acquired.
Subsequently, 1 drop of CF3COOH was added, the mixture
stirred, and 1H NMR spectrum acquired repeatedly after 1 h
and 1 day. Spectra can be found in the Supporting
information file.
Phenacylation of 4,6-dimethyl-2-oxo-1,2-dihydro-
pyridine-3-carbonitrile and separation of the isomer
mixture (according to a literature method)7. 4,6-Dimethyl-
2-oxo-1,2-dihydropyridine-3-carbonitrile (20.0 g, 0.135 mol)
was added to a solution of EtONa prepared from Na (3.1 g,
0.135 mol) and EtOH (50 ml); the mixture was stirred for
30 min, and the solvent evaporated to dryness. 4-Methyl-
phenacyl bromide (28.8 g, 0.135 mol) and dry DMF (100 ml)
was added to the resulting salt. The reaction mixture was
heated on water bath at 80°C for 2 h, cooled to room
temperature, poured into ice water, and the formed
precipitate was filtered off. The mixture of N- and
О-alkylated isomers obtained in this way could be
perchlorate (3а)3 (yield 69%), 3-cyano-4,6-dimethyl-
pyridin-2(1H)-one8 (yield 98%), and tert-butyl chloro-
acetate9 (yield 53%) were prepared using known methods.
Oxazolopyridinium salt 3a recyclization. N-(Methyl-
amino)acetaldehyde dimethyl acetal (6 ml) was added to
oxazolopyridinium perchlorate 3a (0.4 g, 1.13 mmol) and the
reaction mixture heated under reflux for 20 min. The starting
perchlorate quickly dissolves, the color of the solution changes
from yellow to cherry-red that stays for 10 min; then the
solution turns brown. The reaction mixture was cooled to
room temperature and poured into Н2О (10 ml). The formed
emulsion was extracted with PhH (2×10 ml), and the organic
phase concentrated. The oily residue was purified by
column chromatography (eluent CHCl3, Rf 0.6) to give
N-(2,2-dimethoxyethyl)-N-methyl-2-(4-nitrophenyl)indo-
lizin-5-amine (2a). Yield 0.12 g (30%), yellow oil that
decomposes in air. 13C NMR spectrum, δ, ppm: 146.0; 144.7;
142.7; 139.3; 135.7; 126.1; 124.2; 119.0; 114.4; 108.3; 102.5;
99.7; 97.6; 55.0; 53.9; 40.4.
272

Chemistry of Heterocyclic Compounds 2015, 51(3), 269–274
separated by two methods. Method I is based on difference
amino})acetate (2c) was recrystallized from EtOH. Yield
20 mg (20%), yellow crystals, mp 132–133°С. IR
spectrum, ν, cm–1: 1625, 1715 (С=О), 2225 (C≡N).
13C NMR spectrum, δ, ppm: 168.8; 148.7; 136.9; 134.5;
131.5; 129.6; 127.1; 126.2; 124.6; 117.4; 114.1; 107.9;
98.7; 82.2; 76.7; 55.4; 39.7; 28.1; 21.2; 20.0. Found, %:
С 74.02; H 7.21; N 10.39. C24H27N3O2. Calculated, %:
С 74.01; H 6.99; N 10.79.
in solubility of these substances (isomer 4b is less soluble
in CHCl3 and EtOAc than isomer 4а). To separate the
isomers, the obtained precipitate was placed on a Schott
filter and washed with CHCl3–EtOAc, 1:1, several times.
The substance remaining on the filter was recrystallized
from EtOH. Method II involves the use of column
chromatography (gradient eluting through SiO2, eluent
CHCl3 followed by EtOAc).
Recyclization of oxazolopyridinium perchlorate 3b
by the action of N-(methylamino)acetaldehyde dimethyl
acetal. N-(Methylamino)acetaldehyde dimethyl acetal
(2 ml) was added to oxazolopyridinium perchlorate 3b (0.2 g,
0.55 mmol), and the mixture heated under reflux for
30 min. The color of the mixture turns from yellow to
cherry-red, then turns brown. The mixture was cooled,
poured into Н2О, and the formed precipitate filtered off.
The product was separated by column chromatography
(eluent CHCl3, Rf 0.7). The obtained 5-[(2,2-dimethoxy-
ethyl)(methyl)amino]-7-methyl-2-(4-methylphenyl)indo-
lizine-6-carbonitrile (2b) was recrystallized from EtOH.
Yield 50 mg (25%), colorless crystals, mp 107–108°С. IR
spectrum, ν, cm–1: 1625, 2225 (С≡N). 13C NMR spectrum,
δ, ppm: 147.1; 137.0; 133.4; 131.4; 129.6; 127.0; 126.2;
124.5; 117.4; 114.1; 107.5; 102.7; 98.7; 91.2; 54.5; 54.0;
40.4; 21.2; 20.0. Found, %: С 72.79; H 7.07; N 11.35.
С22H25N3O2. Calculated, %: С 72.70; H 6.93; N 11.56.
8-Cyano-1,7-dimethyl-4-(4-methylphenyl)-2H-pyri-
mido[6,1,2-cd]indolizin-1-ium trifluoroacetate (1k). A
drop of CF3COOH was added to a solution of compound
2b in CDCl3, agitated, and kept in dark. 1H NMR spectrum
was acquired right after adding the acid, as well as after 1 h
and 1 day. The last spectrum corresponds to complete
transformation of starting compound 2b into a mixture of
pyrimidoindolizinium salt 1k and methyl trifluoroacetate.
1H NMR spectrum, δ, ppm (J, Hz): 7.34-7.36 (4H, m,
H Ar); 7.35 (4H, dd, J = 4.0, J = 8.3, Н Ar); 6.93 (1H, s,
H 5); 6.78 (1H, s, H-6); 6.61 (1H, t, J = 4.0, 3-СH); 5.07
(2H, d, J = 4.0, 2-СH2); 4.00 (6H, s, CF3CO2CH3); 3.55
(3H, s, 1-CH3); 2.62 (3H, s, CH3); 2.43 (3H, s, CH3).
4,6-Dimethyl-2-[2-(4-methylphenyl)-2-oxoethoxy]nico-
tinonitrile (4а). Yield 12.0 g (31%, method II), colorless
crystals, mp 133–135°С, Rf 0.5 (CHCl3). 1H NMR
spectrum, δ, ppm (J, Hz): 7.87 (2H, d, J = 7.9, H Ar); 7.32
(2H, d, J = 7.9, H Ar); 6.86 (1H, s, 5-CH); 5.73 (2H, s,
CH2); 2.48 (3H, s, CH3); 2.44 (3H, s, CH3); 2.31 (3H, s,
CH3).
4,6-Dimethyl-1-[2-(4-methylphenyl)-2-oxoethyl]-2-oxo-
1,2-dihydropyridine-3-carbonitrile (4b). Yield 9.3 g
(25%, method I), colorless crystals, mp 182–183°С, Rf 0.2
1
(CHCl3). H NMR spectrum, δ, ppm (J, Hz): 7.98 (2H, d,
J = 7.9, Н Ar); 7.37 (2H, d, J = 7.9, H Ar); 6.28 (1H, s,
5-СH); 5.59 (2H, s, CH2); 2.46 (3H, s, СН3); 2.40 (3H, s,
СН3); 2.30 (3H, s, СН3). 13C NMR spectrum, δ, ppm:
191.1; 160.7; 158.7; 150.9; 145.4; 132.0; 129.6; 128.2;
115.2; 109.3; 101.3; 50.3; 21.7; 20.9 (2С). Found, %:
С 72.64; H 5.93; N 10.13. С17H16N2O2. Calculated, %:
С 72.84; H 5.75; N 9.99.
8-Cyano-5,7-dimethyl-2-(4-methylphenyl)oxazolo[3,2-а]-
pyridin-4-ium perchlorate (3b). Conc. H2SO4 (5 ml) was
added to N-phenacylpyridone 4b (0.6 g, 2 mmol); the
mixture was stirred at room temperature for 30 min, poured
into Н2О (50 ml), and 70% HClO4 added (2 ml). The
formed precipitate was filtered off, washed with PhH, and
dried in air. Yield 0.7 g (90%), colorless crystals, mp 249–
1
250°С. H NMR spectrum, δ, ppm (J, Hz): 9.49 (1H, s,
H-3); 7.96 (2H, d, J = 7.9, H Ar); 7.47 (2H, d, J = 7.9,
H Ar); 2.95 (3H, s, CH3); 2.87 (3H, s, CH3); 2.49 (3H, s,
CH3). Found, %: С 56.10; H 4.11; N 7.80. С17H15N2O5Cl.
Calculated, %: С 56.29; H 4.17; N 7.72.
Preparation of sarcosine tert-butyl ester. A mixture of
tert-butyl chloroacetate (30 ml, 0.21 mol), KI (34.9 g,
0.21 mol), and 40% aqueous MeNH2 (1 l) was stirred at
room temperature for 1 day. The reaction mixture was then
extracted with CH2Cl2 (3×100 ml), the extract washed with
Н2О, dried over Na2SO4, filtered, and concentrated on a
rotary evaporator. The residue was distilled in vacuo. Yield
2.3 g (7%), colorless liquid, bp 74–75°С (26 mmHg),
nD19 1.4164 (bp 76.578°С (40 mmHg), nD23 1.4148).10
Recyclization of oxazolopyridinium perchlorate 3b by
the action of sarcosine tert-butyl ester. Sarcosine tert-butyl
ester (0.7 ml, 4.50 mmol) was added to perchlorate 3b
(0.1 g, 0.27 mmol), and the mixture heated under reflux for
30 min. The color of the mixture turns from yellow to cherry-
red, then turns brown. The mixture was cooled, poured into
Н2О (20 ml), and the formed emulsion extracted with
EtOAc (2×10 ml). The organic extract was evaporated, the
residue extracted several times with petroleum ether, and
the extract evaporated. The obtained tert-butyl (2-{(methyl)-
[6-cyano-7-methyl-2-(4-methylphenyl)indolizin-5-yl]-
Supporting information file with data of the NMR study
of the cyclization reaction of indolizine 2a by the action of
СF3COOH is available for authorized users.
This work was supported financially by the Russian
Foundation for Basic Research (grant 12-03-00644-а)
and by the Megagrant program (grant 2012-220-03-360).
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