Unexpected pyrylium to pyrylium domino transformation. Synthesis of pyrano[3,4-c]pyran-7-ium cation and its recyclization to 2,7-naphthyridine derivative

Article abstract of DOI:10.1007/s10593-017-2033-9

[Figure not available: see fulltext.]4-Ethyl-3-formyl-2,6-diphenylpyrylium perchlorate was obtained from 2,6-diphenylpyrylium perchlorate in three steps. Its reaction with triethyl orthoformate was accompanied by rearrangement of the initial pyrylium ring and led to pyrano[3,4-c]pyran-7-ium perchlorate system through a domino process involving ethanol addition-elimination. A plausible mechanism of the reaction is suggested on the basis of DFT quantum-chemical calculations. Reaction of the obtained pyrano[3,4-c]pyran-7-ium perchlorate with ammonium acetate led to a 2,7-naphthyridine derivative without a skeletal rearrangement.

Full text of DOI:10.1007/s10593-017-2033-9

DOI 10.1007/s10593-017-2033-9
Chemistry of Heterocyclic Compounds 2017, 53(2), 156–160
Unexpected pyrylium to pyrylium domino transformation.
Synthesis of pyrano[3,4-c]pyran-7-ium cation
and its recyclization to 2,7-naphthyridine derivative
Konstantin F. Suzdalev1, Alina V. Krachkovskaya1*, Mikhail E. Kletskii1,
Oleg N. Burov1, Artem V. Tatarov,1 Sergey V. Kurbatov1
1
Department of Chemistry, Southern Federal University,
7 Zorge St., Rostov-on-Don 344090, Russia
е-mail: lienekekrachkovskysfedu@gmail.com
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2017, 53(2), 156–160
Submitted October 9, 2016
Accepted after revision November 17, 2016
4-Ethyl-3-formyl-2,6-diphenylpyrylium perchlorate was obtained from 2,6-diphenylpyrylium perchlorate in three steps. Its reaction with
triethyl orthoformate was accompanied by rearrangement of the initial pyrylium ring and led to pyrano[3,4-c]pyran-7-ium perchlorate
system through a domino process involving ethanol addition-elimination. A plausible mechanism of the reaction is suggested on the basis
of DFT quantum-chemical calculations. Reaction of the obtained pyrano[3,4-c]pyran-7-ium perchlorate with ammonium acetate led to a
2,7-naphthyridine derivative without a skeletal rearrangement.
Keywords: naphthyridine, pyrylium salts, triethyl orthoformate, domino process, recyclization.
Pyrylium salts have been investigated quite extensively
by many research groups. Being aromatic, they are easily
produced from simple starting materials.1 The reactivity of
pyrylium salts toward nucleophiles makes them useful for
obtaining other compounds with strong aromatic character.
These cations react with nucleophiles at positions 2, 4, and
6, inducing ring-opening2 and recyclization reactions
affording pyridines, phosphinines,3 new pyridinium4 and
thiopyrylium salts.5 Reduction of pyrylium salts leads to
hydrocarbons, hydrogenated pyridines, tetrahydropyrans,
and thiopyrans.6 There are many papers devoted to the
conversion of pyrylium salts to furan derivatives.7
our study of 4-ethyl-3-formyl-2,6-diphenylpyrylium per-
chlorate reaction with triethyl orthoformate.
The initial purpose of our study was to obtain a
pyrylium salt containing aldehyde and alkyl groups in
vicinal positions and to investigate its reaction with triethyl
orthoformate. We synthesized such a salt in three steps
from 2,6-diphenylpyrylium perchlorate (1) according to the
method developed in our earlier work.13 First we obtained
4H-pyran 2 by reaction of perchlorate 1 with ethyl-
magnesium bromide (Scheme 1).
Scheme1
Pyrylium derivatives have been used as fluorescent
biomarkers8 and nonlinear optical chromophores.9 Recently,
pyrylium salts have been employed as ionic liquids10 and in
supramolecular chemistry studies, acting as guests in
cucurbituril hosts.11 Pyrylium salts bound with macrocyclic
crown compounds have been used as reagents for the
recognition of transition metal cations.12 Despite the large
number of publications on recyclization reactions of
pyrylium cations, no information is yet available about
transformations involving the reorganization of carbon
skeleton and conversion of one pyrylium cation to another.
Such a recyclization was surprisingly discovered during
The second step was a Vilsmeier formylation of pyran 2.
However, this reaction resulted in the formation of two
products: the desired 3-formylpyran 4 and aldehyde 7,
which were separated by alumina column chromatography
(Scheme 2). Obviously, two parallel processes occur:
electrophilic attack at position 3 of pyran 2 to form a
Vilsmeier intermediate 3 and an oxidation of pyran system
0009-3122/17/53(02)-0156©2017 Springer Science+Business Media New York
156

Chemistry of Heterocyclic Compounds 2017, 53(2), 156–160
Scheme 2
to produce salt 5. The ethyl-substituted salt 5 reacted with
DMF to give the intermediate condensation product 6. The
formation of such products from alkyl-substituted pyrylium
salts has been investigated earlier.14 Treatment of the
reaction mixture with alkali resulted in the formation of
aldehydes 4 and 7.
Figure 1. Molecular structure of product 19.
addition of triethyl orthoformate to the double bond, gave
the intermediate 10. After ethanol elimination and cycli-
zation through intermediates 11 and 12, product 13 was
expected to form. However, X-ray structural analysis showed
that structure 13 was incorrect. Figure 1 demonstrates that the
phenyl groups are located on different pyran rings. It means
that structure 19 was actually formed. We have proposed a
plausible reaction mechanism shown in Scheme 3 and have
performed DFT calculations of the important stationary
points (Table 1) on the potential energy surface (PES) of
the reacting system (Scheme 3). Presumably, the initial salt
8 was deprotonated to ethylidenepyran 9; the eliminated
proton reacted with triethyl orthoformate to give a diethoxy
carbenium cation, which then added to the double bond of
3-Formylpyran 4 was successfully oxidized to the
desired 3-formylpyrylium perchlorate 8 by the action of
trityl perchlorate (Scheme 3).
The reaction of pyrylium salt 8 with triethyl orthofor-
mate gave a compound that was characterized by 1H NMR
spectroscopy, with a methyl group singlet at 2.68 ppm and
ethyl group signals at 1.24 and 3.89–4.54 ppm. Initially,
according to the data of our earlier work,15 the reaction pro-
duct was incorrectly assigned the structure 13 (Scheme 3).
It was assumed that 4-ethylidene-4H-pyran 9 formed after
the deprotonation, followed by a conventional acid-induced
Scheme 3
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Chemistry of Heterocyclic Compounds 2017, 53(2), 156–160
Table 1. Total (E) and relative (Erel) energies of the stationary
fact, the bifurcation point of the minimum energy pathways
leading to products 13 and 19 (Supplementary materials,
Table S2). According to our calculations, the formation of
product 13 is unrealistic, since intermediate 11, lying on
the endothermic path to the expected product 13, was
substantially (22.6 kcal/mol) less stable. At the same time,
the formation of the experimentally detected product 19
was predicted to be exothermic (Table 1).
points on PES according to Scheme 3. B3LYP calculations
by 6-31G** basis set in 1,2-dichloroethane
System
CH(OEt)3
8
E, arb unit
−502.06080
−922.66729
−155.05017
−761.30830
−760.8755
Erel*, kcal/mol
0.0
EtOH
–
−
HClO4
In the reaction of pyrano[3,4-c]pyran-7-ium salt 19 with
an excess of ammonium acetate, both rings underwent
recyclization. Here it is appropriate to note that only a recyc-
lization the pyrylium moiety occurred in pyrano[2,3,4-de]-
chromen-4-ium system containing condensed pyran and
pyrylium rings.17 There was a methyl group singlet
observed at 2.68 ppm and three aromatic CH singlets at
–
ClO4
−
−1269.67139
−1114.59176
−1114.62488
−1424.27841
−1114.63030
4.1
22.6
1.8
−10.6
−1.6
10
11
13
14
19
1
8.29, 8.63, 9.39 ppm in H NMR spectrum of the product.
These chemical shifts were in good agreement with the
literature data for 2,7-naphthyridines.18 However, the
recyclization may produce predominantly one of the
isomeric systems 20 or 21, because both six-membered
rings may be cleaved and recyclized (Scheme 4).
* The total energy of isolated system comprised of compound 8
and triethyl orthoformate was set to zero. The calculations of Erel
took into account the reaction stoichiometry.
compound 9 to form pyrylium salt 10 and ethanol. The
ethanol molecule liberated in this step reacted as a
nucleophile and started the subsequent domino process. Salt
10 gave the addition product – 6H-pyran 14, because
according to our DFT quantum-chemical calculations with
B3LYP functional and 6-31G** basis set, while assuming
1,2-dichloroethane as solvent, the largest positive charge
(0.346 e–) was located at position 6 of the pyrylium ring.
Ring opening, elimination of ethanol, and protonation
gave cation 15, which was stabilized by three vinyl groups
(Scheme 3). Rotation around the single bond in cation 15
led to structure 16, which cyclized to pyrylium perchlorate
17. Perchlorate 17 rearranged to the observed product 19
through hemiacetal 18.
Scheme 4
In order to establish the exact naphthyridine structure
resulting from this reaction, we performed X-ray structural
analysis of the product, with the solved structure presented
in Figure 2. It turned out that structure 21 represented the
actual product obtained, proving the preferred formation of
structures 19 and 21 with phenyl substituents in different
rings.
In a recent DFT study of the complexes of some
−
heterocyclic cations with Cl−, NO3 , and BF4− anions it has
been shown that the stabilization energies of the ionic pairs
varied in the range from 1.2 to 2.0 kcal/mol, depending on
the position of anions relative to the phenyl-substituted
pyrylium cation. These values are certainly much smaller
than the expected activation energies of the pyrylium
system 8 toward the nucleophilic rearrangements under
2,7-Naphthyridines are a relatively unexplored class of
N-heterocycles, and methods for their synthesis are insuf-
ficiently developed.19 The most common route of synthesis
involves a reaction of ammonia with o-ethynylpyridine-
carbaldehydes.20 However, the preparation of these starting
materials requires a multistep functionalization of pyridine
derivatives in the presence of palladium complexes. In our
reaction sequence, an aldehyde group necessary for
annulation of the second ring was introduced into the pyran
−
consideration.16 Obviously, the same applies to the ClO4
anion, which is located not less than 3.0–3.5 Å from the
plane of the heterocyclic cation, according to our X-ray
structural analysis data. Moreover, the selected basis set
provided accurate geometrical characteristics of these
pyrylium salts, in particular cation 19 (Supplementary
materials, Fig. S13). On one hand, it confirmed the suitabi-
lity of the chosen computational method and on the other
this indicated the practical independence of the cation structure
rearrangements from the presence of counterion. Therefore,
the sum of total energy values for cation 8 and triethyl
orthoformate was adopted as the starting point for relative
energy calculations.
Obviously, at the initial stage of the reaction, inter-
mediate 10 was formed, which was less stable by 4.1 kcal/mol,
compared to the system of reagents (Table 1). A stationary
point of the PES corresponding to intermediate 10 was, in
Figure 2. Molecular structure of 2,7-naphthyridine 21.
158

Chemistry of Heterocyclic Compounds 2017, 53(2), 156–160
ring by using the classic Vilsmeier reaction and did not
alumina column chromatography (eluent CHCl3). Product 4
was eluted as a part of the red fraction with Rf 0.9 and
purified by crystallization from 2-propanol. Product 7 was
eluted as a part of the yellow fraction with Rf 0.7 and
purified by crystallization from 2-propanol.
require transition metal catalysis.
In summary, we have synthesized 4-ethyl-3-formyl-
2,6-diphenylpyrylium perchlorate, a new representative of
pyrylium salts bearing alkyl and formyl functional groups,
which can be further modified. An unexpected reaction of
this perchlorate, including a rearrangement of the initial
pyrylium ring leading to 1-ethoxy-5-methyl-3,8-diphenyl-
1H-pyrano[3,4-c]pyran-7-ium perchlorate, has been disco-
vered. Both rings of the bicyclic pyrano[3,4-c]pyran-7-ium
perchlorate underwent recyclization to form 4-methyl-
1,6-diphenyl-2,7-naphthyridine without a skeletal rearran-
gement. This reaction can be used for further developing
the synthesis of 2,7-naphthyridines.
4-Ethyl-2,6-diphenyl-4H-pyran-3-carbaldehyde (4).
Yield 2.90 g (37%), colorless crystals, mp 155–156°C.
1H NMR spectrum (CDCl3), δ, ppm (J, Hz): 0.95 (3H, t,
J = 7.5, 4-CH2CH3); 1.46–1.88 (2H, m, 4-CH2CH3); 3.52
(1H, td, J = 5.9, J = 4.5, H-4); 5.73 (1H, d, J = 5.5, H-5);
7.12–7.95 (10H, m, H Ph); 9.63 (1H, s, 3-CHO). 13С NMR
spectrum (CDCl3), δ, ppm: 9.5; 29.0; 30.5; 103.8; 116.3;
124.4 (2C); 128.5 (4C); 128.7; 130.2 (2C); 131.0; 131.8;
133.0; 148.8; 168.4; 191.6. Found, %: С 82.60; H 6.14.
C20H18O2. Calculated, %: C 82.73; H 6.25.
Experimental
2-(2,6-Diphenyl-4H-pyran-4-ylidene)propanal (7). Yield
1
1H and 13C NMR spectra were acquired on a Bruker
DPX-250 spectrometer (250 and 63 MHz, respectively).
The internal standard was TMS. Elemental analysis was
performed on an Elementar varioMICROcube CHNS-
analyzer. The content of halogens was determined
separately by the Schöniger method. Melting points were
determined in open capillaries on a Khimlaborpribor PTP
apparatus.
3.03 g (39%), yellow crystals, mp 188–189°C. H NMR
spectrum (CDCl3), δ, ppm (J, Hz): 1.89 (3H, s,
4-C(CH3)CHO); 6.98 (1H, d, J = 1.9, H-5); 7.35–7.68 (6H,
m, H Ph); 7.91 (1H, d, J = 1.8, H-3); 7.93–8.11 (4H, m,
H Ph); 10.29 (1H, s, 4-C(CH3)CHO). 13С NMR spectrum
(CDCl3), δ, ppm: 9.4; 101.1; 104.0; 117.6; 125.3 (2C);
125.5 (2C); 128.9 (4C); 130.1; 130.5; 132.3; 132.5; 142.2;
153.4; 155.6; 187.2. Found, %: С 83.18; H 5.73. C20H16O2.
Calculated, %: C 83.31; H 5.59.
All commercially available compounds were used
without further purification. The starting 2,6-diphenyl-
pyrylium perchlorate (1) was obtained by a previously
published procedure.21
4-Ethyl-3-formyl-2,6-diphenylpyrylium perchlorate (8).
Trityl perchlorate (3.42 g, 0.01 mol) was added to a solution
of 3-formylpyran 4 (2.88 g, 0.01 mol) in anhydrous
1,2-dichloroethane (4 ml). The obtained yellow crystalline
precipitate was filtered off, washed with diethyl ether, and
4-Ethyl-2,6-diphenyl-4H-pyran (2). Ethyl bromide
(14.17 g, 9.4 ml, 0.13 mol) was added to a suspension of
magnesium (3.0 g, 0.13 mol) with few iodine crystals in
anhydrous diethyl ether (40 ml). After the interaction of the
reagents, pyrylium salt 1 (16.62 g, 0.050 mol) was added to
the reaction mixture, while cooling with ice water. Then it
was treated with 20% aqueous solution of ammonium
chloride. The organic phase was dried over anhydrous
calcium chloride, and the solvent was removed by
distillation using a water bath, providing a yellow solid.
The residue was recrystallized from EtOH. Yield 11.2 g
1
air-dried. Yield 2.70 g (70%), mp 187–189°С. H NMR
spectrum (CF3COOD), δ, ppm (J, Hz): 1.81 (3H, t,
4-CH2CH3); 3.64–3.89 (2H, m, 4-CH2CH3); 7.81–8.36 (8H,
m, H Ph); 8.62 (2H, d, J = 7.8, H Ph); 8.79 (1H, s, H-5);
10.33 (1H, s, 3-CHO). 13С NMR spectrum (CF3COOD),
δ, ppm: 11.7; 29.1; 119.1; 125.7; 126.8; 126.9; 129.2;
129.9; 130.3; 131.4; 135.7; 137.7; 174.1; 177.2; 179.9;
190.2. Found, %: С 61.61; H 4.25; N 8.91. C20H17ClO6.
Calculated, %: C 61.78; H 4.41; N 9.12.
1
(85%), colorless crystals, mp 57–59°C. H NMR spectrum
1-Ethoxy-5-methyl-3,8-diphenyl-1H-pyrano[3,4-c]pyran-
7-ium perchlorate (19). An excess of triethyl orthoformate
(4 ml, 20 mol) was added to a boiling solution of
3-formylpyrylium salt 8 (2.40 g, 7 mmol) in anhydrous
dichloromethane (4 ml). After cooling, the formed orange
crystalline precipitate was filtered off, washed with diethyl
ether, and air-dried. Yield 1.65 g (58%), mp 233–234°C.
1H NMR spectrum (acetone-d6), δ, ppm (J, Hz): 1.24 (3H,
t, J = 7.1, OCH2CH3); 2.68 (3H, s, CH3); 3.89–4.54 (2H, m,
OCH2CH3); 6.80 (1H, s, H Ar); 7.57 (1H, s, H Ar); 7.60–
8.41 (10H, m, H Ar); 9.20 (1H, s, H Ar). 13С NMR
spectrum (CD3CN), δ, ppm: 13.8; 15.6; 66.4; 97.0; 98.7;
117.7; 126.8; 129.8 (2С); 129.9; 130.9 (4С); 131.0 (2С);
132.3; 134.9; 136.1; 155.4; 159.7; 169.0; 171.1. Found, %:
С 61.91; H 4.59; Cl 7.79. C23H21ClO7. Calculated, %:
C 62.10; H 4.76; Cl 7.97.
(CDCl3), δ, ppm (J, Hz): 1.00 (3H, t, J = 7.4, 4-CH2CH3);
1.46–1.68 (2H, m, 4-CH2CH3); 2.97–3.24 (1H, m, H-4);
5.38 (2H, d, J = 3.8, H-3,5); 7.26–7.47 (6H, m, H Ph); 7.70
(4H, d, J = 6.9, H Ph). 13С NMR spectrum (CDCl3),
δ, ppm: 10.2; 31.1; 33.1; 100.8 (2C); 124.5 (4C); 128.2
(2C); 128.3 (4C); 134.8 (2C); 148.9 (2C). Found, %:
С 86.80; H 6.75. C19H18O. Calculated, %: C 86.99; H 6.92.
Synthesis of 4-ethyl-2,6-diphenyl-4H-pyran-3-carb-
aldehyde (4) and 2-(2,6-diphenyl-4H-pyran-4-ylidene)-
propanal (7). POCl3 (8.28 g, 5.0 ml, 54 mmol) was added
to a solution of DMF (3.95 g, 3.8 ml, 54 mmol) in
anhydrous 1,2-dichloroethane (25 ml). The reaction mixture
was kept at room temperature for 1 h and then 4H-pyran 2
(7.07 g, 27 mmol) was added. The reaction mixture was
refluxed for 2 h, cooled, kept at room temperature for 24 h,
and then treated with a solution of NaOH (2.16 g, 54 mmol)
in water (10 ml). The organic phase was dried over anhydrous
calcium chloride, and the solvent was removed by distilla-
tion using a water bath. The crude residue was purified by
4-Methyl-1,6-diphenyl-2,7-naphthyridine (21). Per-
chlorate 19 (0.30 g, 0.67 mmol) was added to a mixture of
glacial acetic acid (5.25 g, 5 ml, 87 mmol) with an excess
of ammonium acetate (0.30 g, 3.89 mmol). The reaction
159

Chemistry of Heterocyclic Compounds 2017, 53(2), 156–160
Science of Synthesis; Houben-Weyl Methods of Molecular
mixture was refluxed for 1 h, cooled, treated with solid
Na2CO3 (4.63 g, 44 mmol) until alkaline reaction, and
extracted with chloroform (2×20 ml). The organic fraction
was dried over anhydrous calcium chloride, and the solvent
was removed by distillation using a water bath. The residue
was recrystallized from EtOH. Yield 0.165 g (83%), light-
green crystals, mp 164–165°C. 1H NMR spectrum (acetone-d6),
δ, ppm: 2.68 (3H, s, CH3); 7.25–7.59 (6H, m, H Ar); 7.63–
7.85 (2H, m, H Ar); 8.16–8.30 (2H, m, H Ar); 8.35 (1H, s,
H Ar); 8.56 (1H, s, H Ar); 9.41 (1H, s, H Ar). 13С NMR
spectrum (CDCl3), δ, ppm: 15.5; 111.8; 120.2; 125.6; 127.4
(2C); 128.6 (2C); 129.0 (2C); 129.2; 129.4; 130.2 (2C);
138.1; 138.9; 141.0; 145.9; 153.2; 154.4; 160.1. Found, %:
С 84.92; H 5.29; N 9.29. C21H16N2. Calculated, %:
C 85.11; H 5.44; N 9.45.
X-ray structural study of compounds 19 and 21 was
performed on a Bruker SMART APEX II CCD diffracto-
meter (λ(MoKα) 0.71072 Å, ω-scans, 2θ<56°). For com-
pound 19, the intensities of 31372 reflections were
recorded and 10656 independent reflections (Rint 0.0435)
were used in further refinement. For compound 21, the
intensities of 15515 reflections were recorded and 3986
independent reflections (Rint 0.0351) were used in further
refinement. The structures were solved by direct method
and refined by the full-matrix least-squares technique
against F2 in isotropic-anisotropic approximation. The
positions of hydrogen atoms were calculated geometrically
and refined with the riding model. All calculations were
performed using the SHELXTL PLUS 5.0 software suite.22
The X-ray structural analysis datasets for compounds 19
and 21 were deposited at the Cambridge Crystallographic
Data Center (deposits CCDC 1494063 and CCDC
1494064, respectively).
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