Synthesis of Hexahydropyrimidines and 1,2,3,4-Tetrahydropyridines by Reaction of Ethyl Benzoylacetate with Formaldehyde and Primary Amines

Article abstract of DOI:10.1134/S1070428020100103

Abstract: The reaction of ethyl benzoylacetate with formaldehyde and primary amines in boiling pyridine or methanol afforded new 1,2,3,4-tetrahydropyridine derivatives and substituted hexahydropyrimidines.

Full text of DOI:10.1134/S1070428020100103

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2020, Vol. 56, No. 10, pp. 1733–1737. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Organicheskoi Khimii, 2020, Vol. 56, No. 10, pp. 1557–1563.
Synthesis of Hexahydropyrimidines
and 1,2,3,4-Tetrahydropyridines by Reaction of Ethyl
Benzoylacetate with Formaldehyde and Primary Amines
D. R. Kireevaa,* and A. I. Kamalovaa
a Ufa Institute of Chemistry, Ufa Federal Research Center, Russian Academy of Sciences, Ufa, 450054 Russia
*e-mail: hetcom@anrb.ru
Received June 15, 2020; revised June 28, 2020; accepted July 4, 2020
Abstract—The reaction of ethyl benzoylacetate with formaldehyde and primary amines in boiling pyridine or
methanol afforded new 1,2,3,4-tetrahydropyridine derivatives and substituted hexahydropyrimidines.
Keywords: ethyl benzoylacetate, hexahydropyrimidine, tetrahydropyridine, diethyl 2,4-dibenzoylpentanedioate,
debenzoylation, one-pot synthesis, multicomponent reactions
DOI: 10.1134/S1070428020100103
Six-membered nitrogen heterocycles of the hexa-
hydropyrimidine and 1,2,3,4-tetrahydropyridine series
exhibit a broad spectrum of biological activity. Hexa-
hydropyrimidines were found to show antitumor [1],
cytotoxic [2–4], antibacterial [5, 6], antiviral [7], and
nootropic activities [8]. Tetrahydropyridine derivatives
possessing antimalarial [9], antibacterial [10], insec-
ticidal [11], and analgesic activities [12] have been
reported. Tetrahydropyridines are also promising as
potential anti-Alzheimer and anti-Parkinson drugs
[13, 14]. In recent time, one-pot multi-component
methods for the synthesis of 1,2,3,4-tetrahydropyridine
[15, 16] and hexahydropyrimidine derivatives [17–20]
have attracted much interest.
that hexahydropyrimidine derivatives containing
an amino acid fragment exhibit pronounced cytotoxic-
ity [3]. With the goal of extending the range of biologi-
cally active compounds we examined the reaction of
ester 1 with formaldehyde and primary amines.
The amine components were methylamine (2a),
propylamine (2b), butylamine (2c), and benzylamine
(2d). The reactions were carried out by heating the
reactants at a 1–formaldehyde–amine ratio of 1:15:2 in
boiling methanol for 5 h (Scheme 1). Ethyl benzoyl-
acetate (1) reacted with 33% aqueous formaldehyde
and 25.2% aqueous methylamine in boiling methanol
gave 30% of ethyl 1,3-dimethylhexahydropyrimidine-
5-carboxylate (3a). Apart from compound 3a, diethyl
3-benzoyl-1-methyl-6-phenyl-1,2,3,4-tetrahydropyri-
dine-3,5-dicarboxylate (4a) was formed as minor prod-
uct (16%). It should be noted that products like 4 were
not detected previously in analogous reactions [22].
The present work was aimed at synthesizing a series
of substituted 1,2,3,4-tetrahydropyridines and some
hexahydropyrimidine derivatives using ethyl benzoyl-
acetate (1) as starting material. We previously proposed
a procedure for the synthesis of analogous compounds
on the basis of ethyl acetoacetate [21, 22] and showed
Keto ester 1 reacted with formaldehyde and propyl-
amine (2b) or butylamine (2c) in a similar way. In the
Scheme 1.
O
N
OEt
O
O
Ph
OEt
O
O
MeOH, Δ, 5 h
EtO
+
CH2O
+
RNH2
+
Ph
OEt
N
O
R
R
Ph
N
R
1
2a–2c
3a–3c, 30–54%
4a–4c, 11–16%
R = Me (a), Pr (b), Bu (c).
1733

KIREEVA, KAMALOVA
1734
Scheme 2.
O
O
O
OEt
EtO
Ph
MeOH, Δ, 5 h
1
+
CH2O
+
PhCH2NH2
+
Ph
N
N
Ph
Ph
N
N
Ph
2d
3d, 57%
5d, 12%
Scheme 3.
O
O
O
O
EtO
Ph
OEt
Ph
Ph
OEt
OEt
Pyridine, Δ, 7 h
1
+
CH2O
+
RNH2
+
O
Ph
N
R
O
O
2a–2d
4a–4d, 58–83%
6, 2–13%
R = Me (a), Pr (b), Bu (c), PhCH2 (d).
reaction with propylamine (2b), heterocyclic com-
pounds 3b and 4b were formed in 36 and 5% yields,
respectively, and 54% of 1,3-dibutylhexahydropyrimi-
dine 3c and 11% of 1,2,3,4-tetrahydropyridine 4c were
obtained from butylamine (2c). An interesting result
was obtained in the reaction of 1 with 33% aqueous
formaldehyde and benzylamine (2d) (Scheme 2). In
this case, we isolated 57% of hexahydropyrimidine 3d
and 12% of 5-benzoyl derivative 5d, whereas no
1,2,3,4-tetrahydropyridine derivative was formed.
[22]. Saleh et al. [23] reported an example of deben-
zoylation of 1,3-dicarbonyl compounds in the reactions
with aqueous formaldehyde and aromatic amines in the
presence of a catalytic amount of FeCl3 in methylene
chloride. Elimination of the benzoyl group was ratio-
nalized by FeCl3-catalyzed nucleophilic addition
of water (from formaldehyde solution) to the carbonyl
group [23].
The condensation of ethyl benzoylacetate (1) with
aqueous formaldehyde and amines 2a–2d at a ratio of
2:2:1 in boiling pyridine for 7 h afforded 1,2,3,4-tetra-
hydropyridines 4a–4d as the major products (61, 77,
58, and 83%, respectively). Apart from compounds 4a–
4d, diethyl 2,4-dibenzoylpentanedioate (6) was formed
in 10, 2, 13, and 2% yield, respectively (Scheme 3).
Raising the amine proportion to a 1–CH2O–2b ratio
of 1:15:4 increased the yield of hexahydropyrimidine
3b to 50%, while tetrahydropyridine 4b was formed
only in trace amount. On the other hand, sharp decrease
of the formaldehyde fraction and hence decrease of the
amount water in the system (1–CH2O–2b ratio 2:2:1)
favored formation of tetrahydropyridine 4b (18%),
whereas the yield of 3b was as low as 5%. Furthermore,
in the latter case, the overall yield significantly
decreased. The reaction in DMFA [21] at a 1–CH2O–
2b ratio of 2:2:1 was accompanied by considerable
tar formation, compound 4b was isolated in 10% yield,
and no corresponding hexahydropyrimidine derivative
was detected.
Our results led us to presume that aprotic solvents
such as pyridine and dimethylformamide favor initial
formation of diethyl 2,4-dibenzoylpentanedioate (6) by
aldol condensation of two molecules of 1 with form-
aldehyde. The subsequent reaction of 6 with the corre-
sponding iminium cation yields 1,2,3,4-tetrahydropyri-
dine [21]. When the reaction is carried out in methanol,
the initial formation of iminium cation is followed by
Mannich type aminomethylation of ketone 1 to produce
hexahydropyrimidines 3 and 5. Increased concentration
of formaldehyde in the reaction mixture makes the
Mannich reaction preferred.
The observed debenzoylation (Schemes 1, 2) with
the formation of hexahydropyrimidines 3 is likely to
occur [22] according to the retro-aldol mechanism, and
the benzoyl group under the given conditions appears
to be a better leaving group than the acetyl group in
ethyl acetoacetate; the reactions with the latter gave
mixtures of 3 and 5-acetylhexahydropyrimidines 5
Thus, we have proposed a one-pot synthesis of new
1,2,3,4-tetrahydropyridine derivatives in 40–83% yield
by condensation of ethyl benzoylacetate with formalde-
hyde and primary amines in boiling pyridine. When the
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SYNTHESIS OF HEXAHYDROPYRIMIDINES AND 1,2,3,4-TETRAHYDROPYRIDINES
1735
reaction is carried out in boiling methanol, the corre-
sponding hexahydropyrimidines are formed as the
major products (yield 30–57%).
7.1 Hz), 2.59 m (2H, 4-Hax, 6-Hax), 3.06 m (1H, 5-H),
3.32 d (1H, 2-Hax, J = 8.9 Hz), 3.62 s (4H, CH2Ph),
3.64–3.80 m (3H, 2-Heq, 4-Heq, 6-Heq), 4.19 q (2H,
13
OCH2, J = 7.1 Hz), 7.25–7.40 m (10H, Ph). C NMR
spectrum, δC, ppm: 13.81 (CH3), 37.71 (C5), 56.67
(CH2N), 58.61 (CH2Ph), 61.66 (OCH2), 74.87
(NCH2N), 128.19, 128.30, 128.49, 128.58, 128.79
(CHarom), 137.55(Ci), 170.67 (C=O). Mass spectrum:
m/z 339 [M + H]+. Found, %: C 74.31; H 7.65;
N 8.33. C21H26N2O2. Calculated, %: C 74.52; H 7.74;
N 8.28.
EXPERIMENTAL
The H, 13C, and 15N NMR spectra were recorded
1
on a Bruker Avance III spectrometer (USA) at 500,
125, and 50 MHz, respectively, using CDCl3 as solvent
and tetramethylsilane as internal standard (1H, 13C).
The mass spectra (electrospray ionization) were ob-
tained with a Shimadzu LC-MS-2010EV instrument
(Japan). Elemental analysis was performed with a Euro
EA-3000 CHNS analyzer (HEKAtech, Germany).
Analytical TLC was carried out using Sorbfil PTSKh-
AF-A plates manufactured by Imid Ltd. (Russia);
eluent hexane–ethyl acetate (7:3). Macherey–Nagel
Kieselgel 60 (70–230 mesh) was used for column chro-
matography. Commercially available amines (Acros
Organics) and ethyl benzoyl acetate (Aldrich) were
used without further purification.
Ethyl 5-benzoyl-1,3-dibenzylhexahydropyrimi-
dine-5-carboxylate (5d). Yield 0.14 g (12%), yellow
1
oil. H NMR spectrum, δ, ppm: 1.13 t (3H, CH3, J =
7.1 Hz), 3.15 d (2H, 4-Hax, 6-Hax, J = 12.2 Hz), 3.26 d
(1H, 2-Hax, J = 9.0 Hz), 3.58–3.82 m (3H, 2-Heq, 4-Heq,
6-Heq), 3.61 s (2H, CH2Ph), 3.62 s (2H, CH2Ph),
4.16 q (2H, OCH2, J = 7.1 Hz), 7.22–7.41 m (13H, Ph),
7.80 br.s (1H, o-H), 7.82 br.s (1H, o-H). 13C NMR
spectrum, δC, ppm: 14.18 (CH3), 56.94, 58.16 (CH2Ph),
59.49 (CH2N), 60.54 (OCH2), 74.87 (NCH2N), 126.98,
127.08, 127.18, 128.01, 128.12, 128.26, 128.42,
128.75, 128.95, 132.24 (CHarom), 136.71, 137.55,
138.24 (Ci), 170.67 (CO2), 196.31 (C=O). Mass
spectrum: m/z 443 [M + H]+. Found, %: C 76.01;
H 6.79; N 6.33. C28H30N2O3. Calculated, %: C 75.99;
H 6.83; N 6.33.
Reaction of ethyl benzoylacetate (1) with formal-
dehyde and primary amines 2a–2d (general proce-
dure). Ethyl benzoylacetate (1), 1 mmol, was dissolved
in 20 mL of methanol, 2 mmol of primary amine 2a–2d
and 15 mmol of 33% aqueous formaldehyde were
added with stirring, and the mixture was refluxed for
5 h. The solvent was distilled off under reduced
pressure, and the residue was extracted with 20 mL of
methylene chloride. The extract was washed with water
(3×10 mL), dried over anhydrous sodium sulfate, and
evaporated under reduced pressure, and the residue
was subjected to silica gel column chromatography
using hexane–ethyl acetate (0 to 30% of the latter) as
eluent.
Compounds 4a–4d (general procedure). Ethyl ben-
zoylacetate (1, 2 mmol) and 33% aqueous formalde-
hyde (2 mmol) were added in succession with stirring
to a solution of primary amine 2a–2d (1 mmol) in
6 mL of pyridine, and the mixture was refluxed for 7 h
(TLC). The solvent was distilled off under reduced
pressure, the residue was dissolved in 20 mL of
methylene chloride, and the solution was washed with
water (3×10 mL), dried over anhydrous sodium sulfate,
and evaporated under reduced pressure. The residue
was subjected to column chromatography using
hexane–ethyl acetate (0 to 30% of the latter) as eluent.
Ethyl 1,3-dimethylhexahydropyrimidine-5-car-
boxylate (3a). Yield 0.14 g (30%), light yellow oily
material. Its physicochemical properties and spectral
characteristics coincided with those reported in [22].
Ethyl 1,3-dipropylhexahydropyrimidine-5-car-
boxylate (3b). Yield 0.23 g (36%), light yellow oily
material. Its physicochemical properties and spectral
characteristics coincided with those reported in [22].
Diethyl 3-benzoyl-1-methyl-6-phenyl-1,2,3,4-
tetrahydropyridine-3,5-dicarboxylate (4a). Yield
0.33 g (61%), light yellow crystals. The physicochem-
ical properties and spectral characteristics of the prod-
uct coincided with those reported in [24]. 13C NMR
spectrum, δC, ppm: 13.81 and 13.95 (CH2CH3), 31.01
(C4), 40.40 (NCH3), 55.19 (C2), 55.96 (C3), 58.69,
61.84 (OCH2), 94.53 (C5), 127.94, 128.05, 128.24,
128.48, 128.53 (CHarom), 132.90 (Cp), 135.67, 137.57
(Ci), 156.40 (C6), 167.81 (3-C=O), 171.18 (5-C=O),
195.52 (PhC=O).
Ethyl 1,3-dibutylhexahydropyrimidine-5-car-
boxylate (3c). Yield 0.38 g (54%), light yellow oily
material. Its physicochemical properties and spectral
characteristics coincided with those reported in [22].
Ethyl 1,3-dibenzylhexahydropyrimidine-5-car-
boxylate (3d). Yield 0.50 g (57%), yellow oil.
1H NMR spectrum, δ, ppm: 1.27 t (3H, CH3, J =
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 10 2020

KIREEVA, KAMALOVA
1736
ppm: 13.79, 13.85 (CH2CH3), 31.32 (C4), 52.33 (C2),
55.29 (CH2Ph), 55.56 (C3), 58.81, 61.78 (OCH2),
95.47 (C5), 127.26, 127.37, 128.12, 128.46, 128.56,
128.79, 128.83 (CHarom), 132.97 (Cp), 135.59, 137.41,
137.58 (Ci), 155.83 (C6), 167.78, 171.20 (C=O),
195.29 (PhC=O). Mass spectrum: m/z 498 [M + H]+.
Found, %: C 75.05; H 6.25; N 2.82. C31H31NO5.
Calculated, %: C 74.83; H 6.28; N 2.81.
Diethyl 3-benzoyl-6-phenyl-1-propyl-1,2,3,4-
tetrahydropyridine-3,5-dicarboxylate (4b). Yield
1
0.45 g (77%), yellow oil. H NMR spectrum, δ, ppm:
0.60 t (3H, CH2CH2CH3, J = 7.3 Hz), 0.74 t (3H,
OCH2CH3, J = 7.0 Hz), 1.13 t (3H, OCH2CH3, J =
7.0 Hz), 1.32–1.45 m (2H, NCH2CH2), 2.74 t (2H,
NCH2, J = 7.4 Hz), 3.04 d and 3.33 d (1H each, 4-H,
J = 16.4 Hz), 3.65–3.74 m (2H, OCH2), 3.78 d and
3.83 d (1H each, 2-H, J = 12.7 Hz), 4.12–4.27 m (2H,
5-CO2CH2), 7.06–7.18 m (2H, o-H), 7.28–7.44 m (5H,
Harom), 7.48–7.53 m (1H, p-H), 7.89–7.95 m (2H, o-H).
13C NMR spectrum, δC, ppm: 10.94 (CH2CH2CH3),
13.83, 13.88 (CH2CH3), 22.07 (NCH2CH2), 31.24
(C4), 52.51 (C2), 53.37 (NCH2), 55.73 (C3), 58.50,
61.72 (OCH2), 94.02 (C5), 127.81 (Cp), 128.20, 128.43,
128.51 (CHarom), 133.75 (Cp), 135.76, 137.57 (Ci),
156.04 (C6), 167.67 (3-C=O), 171.12 (5-C=O), 195.42
(PhC=O). 15N NMR spectrum: δN 89.78 ppm. Mass
spectrum: m/z 450.3 [M + H]+. Found, %: C 73.03;
H 6.98; N 3.16. C27H31NO5. Calculated, %: C 72.14;
H 6.95; N 3.12.
Diethyl 2,4-dibenzoylpentanedioate (6) was
isolated as a minor product in the reactions of 1 with
formaldehyde and amines 2a–2d in pyridine. Yield
0.05 g (10%), 0.05 g (2%), 0.06 g (13%), and 0.05 g
(2%), respectively; white amorphous crystals. Its
physicochemical and spectral characteristics coincided
with those reported in [25].
ACKNOWLEDGMENTS
The spectral studies were carried out using the facilities
of the Chemistry joint center (Ufa Institute of Chemistry,
Ufa Federal Research Center, Russian Academy of
Sciences).
Diethyl 3-benzoyl-1-butyl-6-phenyl-1,2,3,4-
tetrahydropyridine-3,5-dicarboxylate (4c). Yield
0.39 g (65%), yellow–orange oil. 1H NMR spectrum, δ,
ppm: 0.67 t (3H, CH2CH2CH3, J = 7.3 Hz), 0.73 t (3H,
CH2CH3, J = 7.1 Hz), 0.93–1.10 m (2H, CH2CH2CH3),
1.13 t (3H, CH2CH3, J = 7.1 Hz), 1.28–1.36 m (2H,
NCH2CH2), 2.73 t (2H, NCH2, J = 7.8 Hz), 3.00 d and
3.27 d (1H each, 4-H, J = 16.4 Hz), 3.64–3.71 m (2H,
OCH2), 3.74 d and 3.79 d (1H each, 2-H, J = 12.7 Hz),
4.12–4.24 m (2H, 5-CO2CH2), 7.06–7.14 m (2H, o-H),
7.28–7.60 m (6H, Harom), 7.86–7.91 m (2H, o-H).
13C NMR spectrum, δC, ppm: 13.58, 13.83, 13.90
(CH3), 19.66 (CH2CH2CH3), 30.98 (CH2CH2CH3),
31.22 (C4), 51.54 (C2), 52.50 (NCH2), 55.78 (C3),
58.64, 61.79 (OCH2), 93.98 (C5), 127.83, 128.17,
128.41, 128.49, 128.52 (CHarom), 132.87 (Cp), 135.91,
137.54 (Ci), 156.13 (C6), 167.86 (3-C=O), 171.25
(5-C=O), 195.65 (PhC=O). Mass spectrum: m/z 464
[M + H]+. Found, %: C 73.01; H 7.17; N 3.04.
C28H33NO5. Calculated, %: C 72.55; H 7.18; N 3.02.
FUNDING
This study was performed in the framework of state
assignment for Ufa Institute of Chemistry, Ufa Federal
Research Center, Russian Academy of Sciences (project
no. AAAA-A20-120012090031-3).
CONFLICT OF INTEREST
The authors declare the absence of conflict of interest.
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