Practical Synthesis of 1,8-Naphthyridine-2,7-dialdehydes Syntone

Article abstract of DOI:10.14233/ajchem.2019.22229

Several synthetic approaches for synthesizing 1,8-naphthyridines were tested. Reliable protocols for synthesis of some new 1,8-naphthyridine-2,7-dialdehydes were developed starting from 2-amino-6-methylpyridine and acetoacetate. The dialdehyde were found

Full text of DOI:10.14233/ajchem.2019.22229

Asian Journal of Chemistry; Vol. 31, No. 11 (2019), 2596-2600
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.22229
Practical Synthesis of 1,8-Naphthyridine-2,7-dialdehydes Syntone
2,*
I.I. LEVINA1,* and A.A. GRIDNEV
1Institute of Biochemical Physics of Russian Academy of Sciences, Moscow, Russia
2Federal Research Center of Chemical Physics of Russian Academy of Sciences, Moscow, Russia
*Corresponding author: Tel:+7 495 9397592; E-mail: 99gridnev@gmail.com
Received: 13 May 2019;
Accepted: 8 July 2019;
Published online: 28 September 2019;
AJC-19592
Several synthetic approaches for synthesizing 1,8-naphthyridines were tested. Reliable protocols for synthesis of some new 1,8-
naphthyridine-2,7-dialdehydes were developed starting from 2-amino-6-methylpyridine and acetoacetate. The dialdehyde were found to
react with pyrrole to form either a polymer or a macrocycle depending on conditions.
Keywords: 1,9-Naphthyridine, 2-Amino-6-methylpyridine, Dialdehyde.
aldehydes to obtain one of such multi-aromatic chelate (II).
Chelate II was planned to synthesize by reaction of 1,8-naphth-
yridine dialdehyde (III) with pyrrole.
Several authors obtained dialdehyde III by oxidation of
corresponding dimethyl precursors IV with SeO2 (eqn. 1) [3-9].
INTRODUCTION
Synthesis of conductive and semi-conductive polymers
requires molecules with extended system of conjugation of
π-electrons [1]. Their synthesis can be easily arranged by a con-
densation reaction of dialdehydes of different heterocycles with
nucleophiles like pyrroles or thiophenes. Besides synthesis of
conductive materials, 1,8-naphthyridine 2,7-dialdehydes, poten-
tially, can be used for synthesis of multi-aromatic polynuclear
chelates of general formula I [2]. In particular, we are interested
in simple yet effective synthesis of 1,8-naphthyridine-2,7-di-
SeO2
O
O
(1)
N
N
N
N
IV
III
N
H
N
N
N
M
M
HN
N
NH
N
N
N
H
N
N
N
N
O
O
M = Transition metal
I
II
III
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Vol. 31, No. 11 (2019)
Practical Synthesis of 1,8-Naphthyridine-2,7-dialdehydes Syntone 2597
O
Hence, compound IV could be the key intermediate in
O
H3PO4
90oC
synthesis of dialdehyde III. From literature, several reaction
pathways were tested in order to synthesize naphthyridine IV.
However, a Fridlander synthesis reported for the synthesis of
aminonicotinaldehyde (eqn. 2) looked the most attractive
[10,11].
+
+
(7)
N
H2N
NH2
N
N
H2N
O
The only fessible route that gave appropriate results in
present experiments was the one based on condensation of
2-aminopicoline and acetoacetic aster [5,7]. However, pursuing
this route, several difficulties was occured, so the optimization
of literature synthetic procedures is obviously required.
EWG
EWG
O
+
(2)
N
N
NH2
N
O
EXPERIMENTAL
All solvents were distilled and stored in air-tight bottles.
Solid (Acros) reagents were used as received. NMR spectra
were recorded in CDCl3 using Bruker Avance spectrometer
with 500 MHz working frequency. Molecular weights were
determined by HPLC chromtograph "Waters" in THF solu-
tions at 35 ºC with UV-detector and styrogel 103 and 105 Å
columns calibrated with polystyrene standards. High resolution
mass spectra (HRMS) were registered on a Bruker Daltonics
micrOTOF-Q II instrument using electrospray ionization (ESI).
The measurements were done in positive ion mode. Interface
capillary voltage: 4500 V; mass range from m/z 50 to 3000;
external calibration (electrospray calibrant solution, Fluka);
nebulizer pressure: 0.4 bar; flow rate: 3 µL/min; dry gas: nitrogen
(6 L/min); interface temperature: 180 ºC. Samples were injected
into the mass spectrometer chamber from the Agilent 1260
HPLC system equipped with Agilent Poroshell 120 EC-
C18 column (3.0 × 50 mm; 2.7 µm) and an identically packed
security guard using an autosampler. The samples were injec-
ted from 50 % acetonitrile (LC-MS grade) in water (MilliQ
ultrapure water, Merck Millipore KGaA, Germany) solution.
The column temperature was 30 ºC and 5 µL of sample solution
was injected. The column was eluted in a gradient of concen-
trations of A (acetonitrile) in B (water) with the flow rate of
400 µL/min in the following gradient parameters: 0-15 % A
for 6.0 min, 15 %-85 % A for 1.5 min, 85 %-0 % A for 0.1
min, 0 % A for 2.4 min.
2,6-dimethyl-4H-pyrido[1,2-a]pyrimidin-4-one (V): In
1 L flask, 190 g of 115 % polyphosphoric acid is placed and
the flask is kept at 60 ºC for 20 min. A solution of 32.4 g (0.3
mol) of 2-aminopicoline in 41 g (0.031 mol) of ethyl acetoacetate
is added by small portions with intensive mixing. Formed homo-
geneous mixture with some foam is left for 0.5 h. Mixing is
stopped and temperature raised to 100 ºC and the flask is
opened to air letting vapours of ethanol out. After 1 h vine
mixture is chilled and compound V is isolated by either of the
two following methods.
Method A: The reaction mixture is neutralized with 5 M
NaOH to pH 4-5 upon chilling with water bath. Colour of the
solution is changed from vine to yellow and some oil is separated.
The overnight crystallization at room temperature gave yellow
crystals. The crystals were filtered under pressure and dried
in air. Reasonably dry crystals were dissolved in chloroform
or methylene chloride, filtered and the solution was evaporated.
Method B: The reaction mixture was neutralized with 5 M
solution of KOH till pH 6-7 extracted with methylene chloride
and the solution is evaporated under vacuum at 90 ºC. Brown
viscous liquid was allowed to stand overnight at room temper-
However, corresponding aminoaldehyde (methylamino-
nicotene aldehyde) turned out to be a problem. Synthesis of
this compound using lithiation (eqn 3) was not successful as
only the starting aminomethylpyridine was recovered with
traces of aldehyde derivative [12].
1. 2BuLi
O
O
2. DMF
3. H+
(3)
NH2
N
N
N
H
Synthesis of 1,8-naphthyridine [13] is carried out by using
the following reaction (eqn 4) which is followed by replacing
of chlorine atom with methyl group (eqn 5), however these
reactions did not work either. Synthesis of naphthyridine using
eqn 4 is failed in our lab. Several products were detected in
the reaction mixture but none of them consist of aldehyde as
confirmed by NMR analysis.
CHO
POCl3 + DMF
(4)
N
N
Cl
N
H
N
O
CHO
Cl
CHOH
CH3MgCl
(5)
N
N
N
N
However, an attempt is made for synthesis of compound
IV (Skroup synthesis) according to Fox et al. [6] using pyridine
and crotonaldehyde gave substantial amount of black sticky
gum which was very difficult to separate from the desired
naphthyridine (eqn 6).
O
+
NH2
(6)
N
N
O
N
Brown [14] successfully applied dimethylal of crotonal-
dehyde to synthesize 2-amino-7-methyl-1,8-naphthyridine
(eqn 7). Our attempt to apply this approach to synthesize 2,7-
dimethyl-1,8-naphthyridine resulted in recovering of starting
pyridine.Above reaction (eqn 7) was also attempted at different
temperatures (90, 150 and 250 ºC). Presumably, cyclization
failed because of less electron density on aminomethylpyridine
as compared with diaminopyridine.

2598 Levina et al.
Asian J. Chem.
ature to form a mixture of crystals and brown oil. The mixture
is placed onto sufficient amount of filter paper and kept until
all oil was absorbed by filter paper.
CH,Ar, J = 8.3 Hz). 13C NMR (100 MHz, CDCl3) δ ppm: 25.00
(CH3), 25.47 (CH3), 31.02 (CH3, t-Bu), 34.04 (Cq, t-Bu),
104.07(CH, Ar ), 111.69 (Cq, Ar ), 119.83 (2CH, Ph), 120.77
(CH, Ph), 126.60 (2CH, Ph), 130.73 (CH, Ar), 148.24 (Cq, Ar),
151.26 (Cq, Ar), 156.10 (Cq, Ar), 161.83 (Cq, Ar), 162.54 (Cq,
Ar), 162.83 (Cq, Ar). HRMS (ESI-TOF) m/z: [M + H]+ calcd
for C20H23N2O+ 307.1810, found 307.1821.
Yield: about 13 g (25 %) of dark yellow crystals of 2,6-
dimethyl-4H-pyrido[1,2-a]pyrimidin-4-one pure enough for
the next step.Analytical pure compound V can be obtained by
1
crystallization from hot heptane as transparent needles. H
NMR (500 MHz, CHCl3) δ ppm: 2.14 (s, 3H, CH3), 2.82 (s,
3H, CH3), 5.91 (s, 1H, CH, Ar), 6.41 (d, 1H, CH, Ar, J = 8.3 Hz),
7.09 (d, 1H, CH,Ar, J= 8,3Hz), 7.20 (t, 1H, CH,Ar, J = 8.3 Hz).
2,7-Dimethyl-1,8-naphthyridin-4-ol (VI):About 25 mL
of diphenyl ether and 26 g of 2,6-dimethyl-4H-pyrido[1,2-a]-
pyrimidin-4-one in 100 mL flask with short Vigreux column
without a stopper (to let ethanol vapour out) and without a stirrer
are placed into preheated to about 360 ºC at aluminum block.
After 4 h of refluxing, the fask content was chilled down and
diluted with 30 mL of methylene chloride. The solid product
was filtered off and dried in air. Yield: 10.3 g (40 %), poorly
soluble in acetone, but soluble in DMSO and DMF.
4-Chloro-2,7-dimethyl-1,8-naphthyridine (VII): To a
100 mL flask containing 21 g (0.11 mol) of dry compound VI,
about 65-70 mL of POCl3 was added and a shortVigreux column
with a stopper is put into the flask and warmed the reaction
contents upto ~70 ºC. Then the flask was kept for 1 h at 90 ºC,
cooled to room temperature and then dark reaction solution
was poured by portions into ~100 g of sodium carbonate in
500 mL of ice-water. Water bath can be used instead of ice to
maintain temperature below ~45 ºC. The solution is adjusted
to neutral with sodium carbonate and extracted three times
with methylene chloride or chloroform. Combined organic
solution was dried with sodium sulfate over night, evaporated
to dryness and the product was extracted twice with hot heptane.
Crystallization of 4-chloro-2,7-dimethyl-1,8-naphthyridine is
slow and can take 12 h and even more hours. Precipitated white-
orange or white with black coating needles can be recrystal-
lized from hot heptane to yield colourless crystals.Yield: 15.2
g (72 %). 1H NMR (500 MHz, CDCl3) δ ppm: 2.76 (s, 3H, CH3),
2.80 ( s, 3H, CH3), 7.40 (s, 1H, CH, Ar), 7.39 (d, 1H, CH, Ar,
J = 8.3 Hz), 8.39 (d, 1H, CH, Ar, J = 8.3 Hz).
4-(4-(tert-Butyl)phenoxy)-2,7-dimethyl-1,8-naphth-
yridine (VIIIa): About 4 g of powdered KOH was placed into
a 250 mL flask and 22 g of 4-(tert-butyl)phenol was put with
it. The mixture was kept at 160 ºC for 2 h periodically shaking.
To this hot solution, 7.8 g (0.04 mol ) of compound VII was
added. After 2 h, the mixture was cooled down forming a glass
at room temperature. Magnetic stirrer is placed onto the glassy
mass, then 40 mL of chloroform and 30 mL of water were added.
Stirring this mixture results the dissolution of glassy mass.
Then water layer was separated together with white precipi-
tation of poorly soluble in water of sodium tert-butylphenolate.
The washing repeated several times with new portions of 5 M
NaOH solution until no white precipitation formed. Chloroform
layer finally evaporated to dryness and pure compound VIIIa
was obtained by crystallization from hot heptane as beige wax.
Yield: 10.6 g (84 %). 1H NMR (500 MHz, CDCl3) δ ppm: 1.39
(s, 9H, t-Bu), 2.63 (s, 3H, CH3), 2.80 (s, 3H, CH3), 6.47 (s, 1H,
CH, Ar), 7.11 (d, 2H, CH, Ar, J = 8.3 Hz), 7.33 (d, 1H, CH,
Ar, J = 8.3 Hz), 7.49 (d, 2H, CH, Ar, J = 8.3 Hz), 8.53 (d, 1H,
4-Butoxy-2,7-dimethyl-1,8-naphthyridine (VIIIb): To
a 5.5 g (0.0285mol) of compound VII, 70 mL of 0.7 M NaOBu
in butanol was added and the solution was kept over night at
room temperature. Most of the butanole was removed under
vacuum, 5 mL of water was added and all volatiles were removed
under high vacuum at 70-80 ºC. The product VIIIb was extracted
three times with hot heptane.After left for overnight, light brown
crystals were collected and dried in air.Yield: 4.8 g (73 %). 1H
NMR (500 MHz, CDCl3) δ ppm: 1.03 (t, 3H, CH3, J = 7.4 Hz),
1.58 (m, 2H, CH2), 1.91 (m, 2H, CH2), 2.71 (s, 3H, CH3), 2.74
(s, 3H, CH3), 4.17 (t, 2H, CH2, J = 6.4 Hz), 6.62 (s, 1H, CH,Ar),
7.23 (d, 1H, CH,Ar, J= 8.3 Hz), 8.36 (d, 1H, CH,Ar, J= 8.3 Hz).
13C NMR (100 MHz, CDCl3) δ ppm: 13.27 (CH3, Bu), 18.75
(CH2, Bu), 24.90 (CH3,Ar), 25.65 (CH3,Ar), 30.37 (CH2, Bu),
67.78 (OCH2, Bu), 110.71 (CH, Ar ), 111.55 (Cq, Ar), 120.22
(CH, Ar), 130.79 (CH, Ar), 156.18 (Cq, Ar ), 161.57 (Cq, Ar),
161.94 (Cq, Ar), 162.89 (Cq, Ar). HRMS (ESI-TOF) m/z:
[M + H]+ calcd for C20H19N2O+ 335.1395, found 335.1409.
4-(4-(tert-Butyl)phenoxy)-1,8-naphthyridine-2,7-dicarb-
aldehyde (IXa): In 100 mL flask, 5 g (0.016 mol) of compound
VIIIa, 5 g (0.05 mol) of SeO2 and 60 mL of dioxane treated
with solid NaOH were added. The mixture was refluxed for
90 min. Dark brown suspension was centrifuged and clear
solution was evaporated to dryness. The solid mass refluxed
with 15 mL of chloroform for 20 min, then 100 mL of hot
heptane was added and after 15 min, refluxed hot mixture was
decanted. The extraction was repeated 2 more times. Combined
heptane solution stayed over night and precipitated solid recry-
stallized from hot heptane. Overage yield: 3 g (64 %) of light
brown 4-(4-(tert-butyl)phenoxy)-1,8-naphthyridine-2,7-
dicarbaldehyde. 1H NMR (500 MHz, CDCl3) δ ppm: 1.42 (s,
9H, t-Bu), 7.15 (d, 2H, CH, phenyl, J = 8.7 Hz), 7.35 (s, 1H,
CH, Ar), 7.55 (d, 2H, CH, phenyl, J = 8.7 Hz ), 8.28 (d, 1H,
CH,Ar, J = 8.5 Hz), 9.03 (d, 1H, CH, Ar, J = 8.5 Hz), 10.24 (s,
1H, CHO), 10.38 (s, 1H, CHO). 13C NMR (100 MHz, CDCl3)
δ ppm: 30.89 (3CH3, t-Bu), 34.18 (Cq, t-Bu), 101.13 (CH, Ar),
111.69 (Cq, Ar ), 119.74 (2CH, phenyl), 120.77 (CH, Ar),
127.20 (2CH, phenyl), 130.73 (CH,Ar), 149.50 (Cq,Ar), 150.21
(Cq, Ar), 155.54 (Cq, Ar), 156.20 (Cq, Ar), 156.46 (Cq, Ar),
163.63 (Cq, Ar), 192.56 (CHO), 192.58 (CHO). HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C20H19N2O3+ 335.1396, found
335.1412.
4-Butoxy-1,8-naphthyridine-2,7-dicarbaldehyde (IXb):
The procedure is essentially the same as for compound IXa.
1H NMR (500 MHz, CHCl3) δ ppm: 1.06 (t, 3H, CH3, J = 7.4 Hz),
1.63 (m, 2H, CH2), 1.98 (m, 2H, CH2), 4.37 (t, 2H, CH2, J =
6.4 Hz), 7.53 (s, 1H, CH, Ar), 8.19 (d, 1H, CH, Ar, J = 8.3
Hz), 8.83 (d, 1H, CH, Ar, J = 8.3 Hz), 10.27 (s, 1H, CHO), 10.34
(q, 1H, CHO). 13C NMR (100 MHz, CDCl3) δ ppm: 13.21
(CH3, Bu), 18.768 (CH2, Bu), 30.14 (CH2, Bu), 69.38 (OCH2,
Bu), 98.30 (CH,Ar), 118.33, (Cq,Ar), 120.25 (CH,Ar), 133.56

Vol. 31, No. 11 (2019)
Practical Synthesis of 1,8-Naphthyridine-2,7-dialdehydes Syntone 2599
(CH, Ar), 155.17 (Cq, Ar), 155.95 (Cq, Ar), 156.57 (Cq, Ar),
163.20 (Cq, Ar), 192.70 (CHO), 192.91 (CHO). HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C14H15N2O3+ 259.1083, found
259.1106.
Polymer composition made from compound IX: To
solution of 0.258 g (1 mmol) of compound IXb in 2 mL of
chloroform 0.06 g (1 mmol) of pyrrole in 1 mL of chloroform
was added. The solution was kept three days at room tempe-
rature. Evaporated solution was found to be a polymer with
pick molecular weight of 1340 Daltons.
Finally, corresponding dialdehydes IX were obtained by
oxidation with selenium dioxide.All new naphthyridines, VIII
and IX, showed excellent solubility in solvents of intermediate
polarity, like acetone or methylene chloride at room temperature
and in non-polar solvent (heptane) at ~100 ºC.
Reproduction of literature reactions (eqns 8-11) gave less
yields in several cases. Also, pursuing this route, several diffi-
culties were faced so optimization of literature synthetic proce-
dures was obviously required.
For better results of reaction (eqn 8), 115 % polyphosphoric
acid should be used. Less concentrated, e.g. 105 % polyphos-
phoric acid provided poor yields. Temperature of mixing of
all reagents is important. Best results were obtained at 60 ºC
vs. 90 ºC. Based on this observation, temperature less than 60
ºC might be even better but crystallinity and/or high viscosity
of polyphosphoric acid at such temperatures prevent mixing
to reach complete homogeneity which is important for good
results.During the reaction substantial foaming may occur, so
the volume of reaction mixture may increase 3-5 times. To prevent
foaming and runaway of the reaction solution, mixing is recom-
mended to be stopped at 100 ºC. Major byproduct in eqn 2, is
a non-crystallize liquid that does not seem to effect next reaction
so that high purity compound V is not necessary to synthesize.
For the reaction (eqn 9), thermal isomerization of compound
Vas recommended in literature 350 ºC requires very high dilution
(6 % solution) of compound V in paraffin oil. An increase of
compound V concentration in paraffin results in fast sublimation
of compound V. We found that isomerization can be conducted
at 300 ºC but sublimation is still substantial at this temperature.
Reaction (eqn 3) can be conducted in refluxing diphenyl ether
at 275 (beginning of the process)-265 ºC (end of the process).
These temperatures require more time for completion but prevent
Macrocyclic compound Xb: A 0.002 M solutions of
pyrrole and dialdehyde IXb are prepared separately in 20 mL
of methylene chloride, then 1 mL of each solution were added
into nitrogen-flashed flask. After 5 such additions amount of
aliquots was increased to 2 mL. When about 1/2 of the solutions
have been mixed, addition continued the same way with 4 mL
of aliquots. The final mixture allowed to stay 4 more days and
evaporated in high vacuum at room temperature to form trans-
parent colourless waxy solid. The solid consist mostly of comp-
ound Xb with traces of higher oligomers.Yield: 72 % (HPLC).
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C36H39N6O6+ 651.2931,
found 651.2902.
RESULTS AND DISCUSSION
According to the literature reaction schemes [5,7], pyrido-
[1,2-a]pyrimidine (V) was obtained by the following reaction
(eqn 8) followed by isomerization of compound Vinto compound
VI at 350 ºC (eqn 9).
N
O Et
O
O
+
N
N
NH2
(8)
sublimation completely. Also, byproducts of reaction (eqn 3)
are not soluble in hexane so that the isolation of compound
VI by dilution of thermal isomerization reaction mixture with
methylene chloride is recommended.
O
V
OH
N
For reaction (eqn 10), the replacement of hydroxyl in
compound VI with chlorine undergoes easier than in many
other hydroxypyridines, so that reaction temperature at 90 ºC
provides less byproducts as compared with high temperature
(100-110 ºC). In the reaction (eqn. 11) of compound VII with
4-tert-butylphenol is complicated by the formation of strong
complex VII with excess of phenol. The only method to remove
phenol is to do the repeated washing using chloroform solution
of reaction products with aqueous KOH or NaOH. Potassium
and especially, sodium phenolates are very poorly soluble in
water and forms white powder on boundary between chloroform
and aqueous phase. Removal of tert-butylphenol by sublima-
tion did not provide pure compound VIII. In this connection,
aliphatic alcohols seem to be more convenient.
Oxidation reaction of methyl groups of compound VIII
depends on quality of SeO2 (eqn 1). Different samples of SeO2
may provide different results. Because of that we prepared
selenium dioxide ourselves. Too much of SeO2 results in form-
ation of carboxylic acid, while insufficient amount of selenium
dioxide causes formation of hydroxymethyl groups. The latter
is more difficult to separate from dialdehyde IX than from acid
impurity. The problem of separation of colloidal selenium from
∆
N
(9)
N
N
O
VI
Poorly soluble compound VI, then was converted into corr-
esponding ethers, first, by converting into 4-chloro derivative
VII (eqn 10), followed by the reaction of compound VII with
alcoholates to produce compound VIII (eqn 11).
Cl
OH
N
POCl3
(10)
(11)
N
N
VII
N
Cl
N
OR
-
RO
N
N
N
VIIIa
- R=4-(tert-butyl)phenyl
VIIIb - R=butyl

2600 Levina et al.
Asian J. Chem.
dialdehydes IX was achieved by extraction of dialdehydes IX
with hot heptane.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests
regarding the publication of this article.
Naphthyridines IX reacted with pyrrole without any acidic
catalysts at room temperature to form polyaddition compounds
(eqn 12). Cyclic derivative of this reaction (eqn 6) was obtained
applying low concentrations of both reagents, however, the
reaction was not optimized. Cycles X are formed as two regio
isomers because of non-symmetrical substitution in naphth-
yridine IX. Besides, presence of oligomers and four chiral
centers in X (naphthyridine-C*H(OH)-pyrrole) make NMR
spectra very broad.All together, it complicates not only isolation
of X but also their next derivatives. However, there are no
doubts that cyclic compound X was synthesized under condi-
tions indicated in the experimental sections as HRMS spectra
indicate.
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IXa - R=4-(t-butyl)phenyl
IXb
OR
Xa - R=4-(t-butyl)phenyl
Xb
- R=butyl
- R=butyl
Conclusion
Synthetic procedures for making 1,8-naphthyridine-2,7-
diladehyles were optimized. Several new naphthyridines were
synthesized with high solubility in non-polar solvents. It was
showed that 1,8-naphthyridine-2,7-diladehyles reacts readily
with pyrrole with formation of polymer or macrocyclic product
under mild conditions.