Synthesis, spectroscopic and conformational analysis of 1,4-dihydroisonicotinic acid derivatives

Article abstract of DOI:10.1016/j.molstruc.2014.06.044

Structural and conformational properties of 1,4-dihydroisonicotinic acid derivatives, characterized by ester, ketone or cyano functions at positions 3 and 5 in solid and liquid states have been investigated by X-ray analysis and nuclear magnetic resonance

Full text of DOI:10.1016/j.molstruc.2014.06.044

Journal of Molecular Structure 1074 (2014) 549–558
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic and conformational analysis
of 1,4-dihydroisonicotinic acid derivatives
a
a
a
Inguna Goba a,b, , Baiba Turovska , Sergey Belyakov , Edvards Liepinsh
⇑
a Latvian Institute of Organic Synthesis, 21 Aizkraukles, LV-1006 Riga, Latvia
b Faculty of Chemistry, University of Latvia, 48 Kr. Valdemara, LV-1013 Riga, Latvia
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
The present work offers methods for
the synthesis of novel 1,4-DHINA
derivatives.
Crystals suitable for X-ray were
obtained by slow evaporation from
saturated solvents.
Conformational properties have been
investigated by X-ray, NMR and
theoretical calculations.
R5
1
R
3C
H
R3
C
R4
C(4) dev
N
C
C
N(1) dev
C
C
3C
H
R2
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 May 2014
Received in revised form 12 June 2014
Accepted 12 June 2014
Available online 21 June 2014
Structural and conformational properties of 1,4-dihydroisonicotinic acid derivatives, characterized by
ester, ketone or cyano functions at positions 3 and 5 in solid and liquid states have been investigated
by X-ray analysis and nuclear magnetic resonance and supported by quantum chemical calculations.
The dihydropyridine ring in each of the compounds exists in flattened boat-type conformation. The
observed ring distortions around the C(4) and N(1) atoms are interrelated. The substituent at N(1) has
great influence on nitrogen atom pyramidality. The 1H, 13C and 15N NMR chemical shifts and coupling
constants are discussed in terms of their relationship to structural features such as character and position
of the substituent in heterocycle, N-alkyl substitution and nitrogen lone pair delocalization within the
conjugated system.
Keywords:
1,4-Dihydroisonicotinic acid
NMR
X-ray
Quantum-chemical calculations
Conformation
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
residue – glutapyrone, both possess antimutagenic properties
[1–3]. Besides, glutapyrone shows a wide variety of pharmacolog-
1,4-Dihydroisonicotinic acid derivatives (1,4-DHINA) are
important precursors for the synthesis of biologically active
1,4-dihydropyridines (1,4-DHPs). The parental compound,
2,6-dimethyl-3,5-diethoxycarbonyl-1,4-dihydroisonicotinic acid
sodium salt, as well as 1,4-DHINA coupled with glutamic acid
ical properties, amongst which the neuromodulatory one is the
most pronounced [4,5]. Taurine derivative of 1,4-dihydroisonicoti-
nic acid (tauropyrone) shows anti-platelet properties and is active
as an anti-aggregant at concentrations that are six times lower
than those for taurine [6].
Since the discovery of pharmacological effects of this class of
substances, it has been of interest to know which conformation
produces the optimum effect. The conformation of the dihydropyr-
idine ring, the nature of the substituents and their mutual
⇑
Corresponding author at: Latvian Institute of Organic Synthesis, 21 Aizkraukles,
LV-1006, Riga, Latvia. Tel.: +371 29107428.
E-mail address: inguna@osi.lv (I. Goba).
http://dx.doi.org/10.1016/j.molstruc.2014.06.044
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

550
I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
orientation play a key role for the biological activity of 1,4-DHPs
[7]. A number of studies have been devoted to the conformational
analysis of numerous 4-phenyl and 4-heteroaryl substituted
1,4-DHP derivatives [8–19], however the crystal structures of
1,4-DHINA derivatives are not analysed in literature until now.
In the course of studies on the chemistry of 1,4-DHINA we
focused on conformation and spectroscopic properties of several
symmetrically and unsymmetrically substituted derivatives
(Table 1).
Synthesis
2,6-Dimethyl-1,4-dihydropyridine-3,4,5-tricarboxylic
acid
3,5-dimethyl ester (1a) was synthesized by the procedure given
in literature [22]. Yield 38%, colourless solid, mp 233–234 °C (from
MeOH). 1H NMR d 2.22 (s, 6H, 2,6-CH3), 3.60 (s, 6H, 3,5-COOCH3),
4.58 (s, 1H, 4-H), 8.93 (s, 1H, NH), 11.95 (s, 1H, 4-COOH). 13C
NMR 17.85, 39.16, 50.79, 96.98, 146.17, 167.24, 174.42. Found: C,
53.52; H, 5.54; N, 5.16. Calc. for C12H15NO6: C, 53.53; H, 5.62; N,
5.20%. IR spectrum,
m
max/cmÀ1: 3346 (NH), 1700 (CO), 1662 (CO).
acid
2,6-Dimethyl-1,4-dihydropyridine-3,4,5-tricarboxylic
Experimental section
trimethyl ester (1b) was synthesized by the procedure given in lit-
erature [23]. Yield 93%, colourless solid, mp 158–159 °C (from
MeOH). 1H NMR d 2.23 (s, 6H, 2,6-CH3), 3.50 (s, 3H, 4-COOCH3),
3.62 (s, 6H, 3,5-COOCH3), 4.69 (s, 1H, 4-H), 9.01 (s, 1H, NH). 13C
NMR 17.89, 39.38, 50.89, 51.75, 96.30, 146.83, 166.97, 173.35.
Found: C, 55.17; H, 5.98; N, 4.82. Calc. for C13H17NO6: C, 55.12;
General information
Previously we have reported the synthesis and X-ray analysis of
compounds 1d–1f [20] and 5a, 5b, 5d and 5e [21]. The symmetrical
4-phenyl substituted 1,4-DHP derivatives 1c–3c were synthesized
at Latvian Institute of Organic Synthesis and obtained as stable sol-
ids with good yields.
All chemicals were purchased from commercial sources
(Sigma–Aldrich and Acros) and used without further purification.
Reactions were monitored using analytical TLC plates (Merck, silica
gel 60 F254), and visualized with ultraviolet light (254 nm). Column
chromatography was carried out using Acros silica gel (particle size
0.035–0.070 mm). IR spectra were recorded on a IRPrestige-21
Shimadzu spectrometer in Nujol. Melting points were determined
H, 6.05; N, 4.94%. IR spectrum,
1647 (CO).
m
max/cmÀ1: 3338 (NH), 1709 (CO),
3,5-Diacetyl-2,6-dimethyl-1,4-dihydropyridine-4-carboxylic
acid (2a) was synthesized by the procedure given in literature [22].
Yield 25%, yellow solid, mp 173–174 °C (from MeOH) (lit. [22],
170 °C). 1H NMR d 2.23 (s, 6H, 2,6-CH3), 2.27 (s, 6H, 3,5-COCH3),
4.61 (s, 1H, 4-H), 8.91 (s, 1H, NH), 12.11 (s, 1H, 4-COOH). 13C
NMR 18.75, 29.65, 40.47, 107.67, 144.84, 174.41, 196.42. Found:
C, 60.55; H, 6.36; N, 5.80. Calc. for C12H15NO4: C, 60.75; H, 6.37;
N, 5.90%. IR spectrum, m
max/cmÀ1: 3318 (NH), 1715 (CO), 1601 (CO).
with
a SRS Stanford Research Systems OptiMelt Automated
3,5-Diacetyl-2,6-dimethyl-1,4-dihydropyridine-4-carboxylic
acid methyl ester (2b) was synthesized by the procedure given in
literature [21]. Yield 2.4 g (63%), yellow solid, mp 151–153 °C
(from MeOH). 1H NMR d 2.24 (s, 6H, 2,6-CH3), 2.28 (s, 6H, 3,5-
COCH3), 3.50 (s, 3H, 4-COOCH3), 4.74 (s, 1H, 4-H), 9.03 (s, 1H,
NH). 13C NMR 18.78, 29.74, 40.36, 51.75, 107.18, 145.43, 173.32,
196.06. Found: C, 61.87; H, 6.82; N, 5.52. Calc. for C13H17NO4: C,
Melting Point System instrument. Elemental analysis was carried
out on EA 1106 (Carlo Erba Instruments) automatic analyzer.
Table 1
Chemical structures for 1,4-DHP derivatives.
R5 R4
62.14; H, 6.82; N, 5.57%. IR spectrum,
1728 (CO), 1675 (CO).
m
max/cmÀ1: 3307 (NH),
R3
H3C
R2
3,5-Diacetyl-1,2,6-trimethyl-1,4-dihydropyridine-4-carbox-
ylic acid methyl ester (2d). To a stirred solution of compound 2b
(1.0 g, 0.004 mol) in anhydrous CH3CN (20 mL) at r.t. was added
sodium methoxide (0.3 g, 0.005 mol), as a result of which a vigor-
ous reaction with hydrogen evolution was observed, and strongly
colored anion, which gives characteristic orange fluorescence,
was formed. At the end of hydrogen evolution CH3I (1.2 mL,
0.02 mol) was added. Reaction mixture was stirred for 12 h and
evaporated under reduced pressure. The crude residue was puri-
fied by column chromatography on silica gel (CH2Cl2-petroleum
ether-acetone, 9:7:1) to give compound 2d. Yield 0.6 g (57%), yel-
low solid, mp 106–108 °C (from CH2Cl2). 1H NMR d 2.31 (s, 6H,
2,6-COCH3), 2.36 (s, 6H, 2,6-CH3), 3.18 (s, 3H, NACH3), 3.51 (s,
3H, 4-COOCH3), 4.73 (s, 1H, 4-H). 13C NMR 16.27, 29.94, 33.97,
40.47, 51.89, 109.62, 148.99, 172.86, 196.77. Found: C, 63.42; H,
7.32; N, 5.21. Calc. for C14H19NO4: C, 63.38; H, 7.22; N, 5.28%. IR
N
R
CH3
1
1,4-DHP
R1
R2
R3
R4
R5
1a
1b
1c
1d
1e
1f
H
H
H
CH3
CH3
CH3
H
H
H
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COCH3
COCH3
COCH3
COCH3
CN
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
COCH3
COCH3
COCH3
COCH3
CN
CN
CN
CN
CN
CN
CN
CN
CN
H
H
H
H
COOH
COOCH3
C6H5
COOCH3
COOCH3
COOCH3
COOH
COOCH3
C6H5
COOCH3
COOH
COOCH3
C6H5
COOCH3
COOCH3
COOH
COOCH3
C6H5
COOCH3
COOH
COOCH3
C6H5
COOCH3
COOCH3
COOH
COOCH3
C6H5
CH3
CH(CH3)2
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
2a
2b
2c
2d
3a
3b
3c
3d
3e
4a
4b
4c
4d
5a
5b
5c
5d
5e
6a
6b
6c
6d
CH3
H
H
CN
CN
CN
CN
spectrum, m
max/cmÀ1: 1736 (CO), 1649 (CO).
H
CH3
CH3
H
H
H
CH3
H
H
3,5-Dicyano-2,6-dimethyl-1,4-dihydropyridine-4-carboxylic
acid monohydrate (3a) was synthesized by the procedure given in
literature [22]. Yield 64%, colourless solid, mp 189–190 °C (from
MeOH) (lit. [24], 190 °C). 1H NMR d 2.03 (s, 6H, 2,6-CH3), 4.02 (s,
1H, 4-H), 9.62 (s, 1H, NH), 13.09 (s, 1H, 4-COOH). 13C NMR 17.71,
40.57, 78.00, 119.03, 148.42, 172.01. Found: C, 54.21; H, 4.89; N,
18.86. Calc. for C10H11N3O3: C, 54.30; H, 5.01; N, 18.99%. IR spec-
COOCH3
COOCH3
COOCH3
COOCH3
COCH3
COCH3
COCH3
COCH3
COCH3
COOCH3
COOCH3
COOCH3
COOCH3
CN
CN
CN
CN
H
H
H
CH3
H
H
trum, m
max/cmÀ1: 3245 (NH), 2211 (CN), 1714 (CO).
CH3
CH3
H
H
H
3,5-Dicyano-2,6-dimethyl-1,4-dihydropyridine-4-carboxylic
acid methyl ester monohydrate (3b) was synthesized by the pro-
cedure given in literature [23]. Yield 3.2 g (86%), colourless solid,
mp 176–178 °C (from MeOH) (lit. [24], 184 °C). 1H NMR d 2.04 (s,
6H, 2,6-CH3), 3.70 (s, 3H, 4-COOCH3), 4.24 (s, 1H, 4-H), 9.72 (s,
CN
COCH3
COCH3
COCH3
COCH3
H
H
CH3
COOCH3

I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
551
1H, NH). 13C KMR 17.74, 40.45, 52.58, 77.30, 118.79, 148.95,
170.98. Found: C, 56.01; H, 5.42; N, 17.79. Calc. for C11H13N3O3:
C, 56.16; H, 5.57; N, 17.86%. IR spectrum,
2204 (CN), 1747 (CO).
The solution was extracted with EtOAc (3 Â 200 mL). The com-
bined organic extracts were dried (Na2SO4), and concentrated
under reduced pressure. The residue was washed with H2O, filtered
and dried to give the desired product 4a. Yield 3.4 g (68%), colour-
less solid, mp 202–203 °C (from EtOH). 1H NMR d 2.02 (s, 3H,
6-CH3), 2.23 (s, 3H, 2-CH3), 3.57 (s, 3H, 3-COOCH3), 4.04 (s, 1H,
4-H), 9.24 (s, 1H, NH), 12.50 (s, 1H, 4-COOH). 13C NMR 17.49,
18.17, 41.21, 50.91, 79.31, 96.64, 119.67, 146.05, 148.28, 167.00,
173.47. Found: C, 52.02; H, 5.46; N, 10.84. Calc. for C11H14N2O5:
m
, cmÀ1: 3200 (NH),
3,5-Dicyano-1,2,6-trimethyl-1,4-dihydropyridine-4-carbox-
ylic acid methyl ester (3d). To a stirred solution of compound 3b
(1.0 g, 0.005 mol) in anhydrous CH3CN (20 mL) at r.t. was added
sodium methoxide (0.3 g, 0.005 mol), as a result of which a vigor-
ous reaction with hydrogen evolution was observed, and strongly
colored anion, which gives characteristic green fluorescence, was
formed. At the end of hydrogen evolution CH3I (1.4 mL, 0.02 mol)
was added. Reaction mixture was stirred for 2 h and evaporated
under reduced pressure. The crude residue was treated with
CH2Cl2 (20 mL), filtered and evaporated under reduced pressure.
The residue was recrystallized from MeOH to afford compound
3d. Yield 0.6 g (55%); colourless solid, mp 148–150 °C (from
MeOH). 1H NMR d 2.24 (s, 6H, 2,6-CH3), 3.16 (s, 3H, NACH3), 3.68
(s, 3H, 4-COOCH3), 4.23 (s, 1H, 4-H). 13C NMR 18.44, 34.52, 39.65,
52.58, 79.33, 119.33, 152.89, 170.89. Found: C, 62.23; H, 5.61; N,
18.17. Calc. for C12H13N3O2: C, 62.33; H, 5.67; N, 18.17%. IR
C, 51.97; H, 5.55; N, 11.02%. IR spectrum,
2213 (CN), 1699 (CO).
m
max/cmÀ1: 3419 (NH),
5-Cyano-2,6-dimethyl-1,4-dihydropyridine-3,4-dicarboxylic
acid dimethyl ester (4b) was synthesized by the procedure given
in literature [23]. Yield 1.9 g (92%), colourless solid, mp 141–143 °C
(from CH2Cl2). 1H NMR d 2.02 (s, 3H, 6-CH3), 2.24 (s, 3H, 2-CH3),
3.57 (s, 3H, 3-COOCH3), 3.62 (s, 3H, 4-COOCH3), 4.17 (s, 1H, 4-H),
9.31 (s, 1H, NH). 13C NMR 17.53, 18.14, 41.18, 51.06, 52.11,
78.21, 95.53, 119.04, 146.99, 149.12, 166.58, 172.19. Found: C,
57.66; H, 5.55; N, 11.11. Calc. for C12H14N2O4: C, 57.59; H, 5.64;
N, 11.19%. IR spectrum,
(CO).
m
max/cmÀ1: 3222 (NH), 2211 (CN), 1742
spectrum, m
max/cmÀ1: 2195 (CN), 1762 (CO).
3,5-Dicyano-1,2,4,6-tetramethyl-1,4-dihydropyridine-4-car-
boxylic acid methyl ester (3e). To a stirred solution of compound
3b (1.0 g, 0.005 mol) in anhydrous CH3CN (20 mL) at r.t. was added
sodium methoxide (0.5 g, 0.01 mol), as a result of which a vigorous
reaction with hydrogen evolution was observed, and strongly col-
ored anion, which gives characteristic green fluorescence, was
formed. At the end of hydrogen evolution CH3I (2.3 mL, 0.04 mol)
was added. Reaction mixture was stirred for 12 h and evaporated
under reduced pressure. The crude residue was purified by column
chromatography on silica gel (CH2Cl2-petroleum ether-acetone,
9:7:1) to give compound 3e. Yield 0.3 g (22%); beige solid, mp
103–105 °C (from MeOH). 1H NMR d 1.54 (s, 3H, 4-CH3), 2.25 (s,
6H, 2,6-CH3), 3.17 (s, 3H, NACH3), 3.68 (s, 3H, 4-COOCH3). 13C
NMR 18.81, 25.19, 34.67, 41.98, 52.92, 85.47, 118.46, 151.09,
172.77. Found: C, 63.53; H, 6.07; N, 17.08. Calc. for C13H15N3O2:
5-Cyano-2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3-car-
boxylic acid methyl ester (4c). A mixture of 3-phenyl-2-propio-
nyl-acrylic acid methyl ester (0.8 g, 0.004 mol) and 3-
aminocrotononitrile (0.3 g, 0.004 mol) in EtOH (10 mL) was stirred
at 60 °C for 10 h and evaporated under reduced pressure. The crude
residue was purified by column chromatography on silica gel (CH2-
Cl2-petroleum ether-acetone, 9:7:2) to give compound 4c. Yield:
0.8 g (77%), yellow crystals, mp 165–167 °C (from acetone). 1H
NMR d 2.01 (s, 3H, 6-CH3), 2.27 (s, 3H, 2-CH3), 3.47 (s, 3H, 3-
COOCH3), 4.44 (s, 1H, 4-H), 7.17–7.31 (m, 5H, C6H5), 9.19 (s, 1H,
NH). 13C NMR 17.43, 18.37, 41.61, 50.72, 84.32, 99.52, 119.92,
126.74, 126.96, 128.49, 145.68, 146.24, 146.37, 167.02. Found: C,
71.49; H, 6.04; N, 10.29. Calc. for C16H16N2O2: C, 71.62; H, 6.01;
N, 10.44%. IR spectrum,
(CO).
m
max/cmÀ1: 3301 (NH), 2192 (CN), 1696
C, 63.66; H, 6.16; N, 17.13%. IR spectrum,
1725 (CO).
m
max/cmÀ1: 2198 (CN),
5-Cyano-1,2,6-trimethyl-1,4-dihydropyridine-3,4-dicarbox-
ylic acid dimethyl ester (4d). To a stirred solution of compound 4b
(1.0 g, 0.004 mol) in anhydrous CH3CN (20 mL) at r.t. was added
sodium methoxide (0.3 g, 0.005 mol), as a result of which a vigor-
ous reaction with hydrogen evolution was observed, and strongly
colored anion, which gives characteristic green fluorescence, was
formed. At the end of hydrogen evolution CH3I (1.3 mL, 0.02 mol)
was added. Reaction mixture was stirred for 2 h and evaporated
under reduced pressure. The crude residue was treated with CH2-
Cl2 (20 mL), filtered and evaporated under reduced pressure. The
residue was recrystallized from MeOH to afford compound 4d.
Yield: 0.6 g (57%), colourless crystals, mp 103–105 °C (from
MeOH). 1H NMR d 2.22 (s, 3H, 6-CH3), 2.45 (s, 3H, 2-CH3), 3.17 (s,
3H, N-CH3), 3.61 (s, 6H, 3,4-COOCH3), 4.23 (s, 1H, 4-H). 13C NMR
15.84, 18.30, 34.14, 40.17, 51.41, 52.22, 79.59, 98.11, 119.77,
150.65, 152.78, 166.62, 171.75. Found: C, 58.92; H, 6.09; N,
10.41. Calc. for C13H16N2O4: C, 59.08; H, 6.10; N, 10.60%. IR spec-
5-Cyano-2,6-dimethyl-1,4-dihydropyridine-3,4-dicarboxylic
acid 3-methyl ester monohydrate (4a). A mixture of methyl ace-
toacetate (5.4 mL, 0.05 mol) and glyoxylic acid monohydrate (4.6 g,
0.05 mol) in glacial acetic acid (10 mL) was stirred at 80 °C for 6 h.
The reaction mixture was cooled and 3-aminocrotononitrile (4.1 g,
0.05 mol) was added under stirring. The mixture was stirred over-
night and evaporated under reduced pressure. The remaining ace-
tic acid was not fully removed by lyophilisation though. The crude
residue was dissolved in anhyd CH3CN (45 mL), and dicyclohexyl-
amine (9.9 mL, 0.05 mol) was added. The mixture was stirred at r.t.
for 1 h, filtered and washed with CH3CN (10 mL). The residue was
recrystallized from EtOH to afford 3-cyano-5-methoxycarbonyl-
2,6-dimethyl-1,4-dihydro-pyridine-4-carboxylate dicyclohexy-
lammonium salt. Yield 8.2 g (39%), yellow solid, mp 227–229 °C
(from EtOH). 1H NMR d 1.09–1.24 (m, 10H, CH), 1.57 (m, 2H, CH),
1.88 (m, 4H, CH), 1.69 (m, 4H), 1.97 (s, 3H, 2-CH3), 2.16 (s, 3H, 6-
CH3), 2,78 (m, 2H), 3.53 (s, 3H, 5-COOCH3), 3.74 (s, 1H, 4-H), 8.91
(s, 1H, NH). 13C NMR 17.38, 17.97, 24.30, 25.24, 30.36, 41.99,
50.54, 51.80, 81.61, 98.63, 120.57, 143.73, 146.26, 167.66, 173.81.
Found: C, 66.16; H, 8.45; N, 10.06. Calc. for C23H35N3O4: C, 66.19;
trum,
5-Acetyl-2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3-car-
bonitrile (5c). mixture of 3-benzylidene-2,4-pentanedione
m
max/cmÀ1: 2201 (CN), 1743 (CO).
A
(1.8 g, 0.01 mol) and 3-amino-crotononitrile (0.8 g, 0.01 mol) in
glacial acetic acid (10 mL) was stirred for 12 h and evaporated
under reduced pressure. The crude residue was purified by column
chromatography on silica gel (CH2Cl2-petroleum ether-acetone,
9:7:1) to give compound 5c. Yield: 1.1 g (46%), yellow crystals,
mp 54–56 °C (from diethyl ether). 1H NMR d 1.99 (s, 3H, 2-CH3),
2.00 (s, 3H, 5-COCH3), 2.27 (s, 3H, 6-CH3), 4.58 (s, 1H, 4-H), 7.17–
7.34 (m, 5H, C6H5), 9.13 (s, 1H, NH). 13C NMR 17.37, 19.18, 29.83,
40.74, 85.21, 108.87, 119.93, 126.83, 127,03, 128.67, 144.59,
145.84, 145.93, 196.88. Found: C, 75.92; H, 6.44; N, 10.94. Calc.
H, 8.54; N, 10.23%. IR spectrum,
(CN), 1701 (CO), 1620 (CO).
m
max/cmÀ1: 3185 (NH), 2197
To a stirred solution of compound 3-cyano-5-methoxycar-
bonyl-2,6-dimethyl-1,4-dihydropyridine-4-carboxylate dicyclo-
hexyl-ammonium salt (8.2 g, 0.02 mol) in H2O (200 mL) at 70 °C
was added dropwise 1 N NaOH until the mixture became basic
(pH 8). It was then extracted with Et2O (3 Â 200 mL). The aqueous
layer was separated and acidified with concentrated HCl to pH 3.

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I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
for C16H16N2O: C, 76.16; H, 6.39; N, 11.10%. IR spectrum, mmax
/
2d) and diethyl ether (5c), however suitable crystals for 6d were
not obtained.
cmÀ1: 3286 (NH), 2197 (CN), 1667 (CO).
5-Acetyl-2,6-dimethyl-1,4-dihydropyridine-3,4-dicarboxylic
X-ray crystal structure determination was performed on Bru-
ker-Nonius KappaCCD automated diffractometer using graphite
acid 3-methyl ester (6a). A mixture of methyl acetoacetate
(5.4 mL, 0.05 mol) and glyoxylic acid monohydrate (4.6 g,
0.05 mol) in glacial acetic acid (10 mL) was stirred at 80 °C for
6 h. The reaction mixture was cooled and acetylacetonamine
(7.4 g, 0.07 mol) was added under stirring. The mixture was stirred
overnight and evaporated under reduced pressure. The crude resi-
due was treated with H2O (30 mL), the precipitate was removed
by filtration, washed with H2O, and dried to give the product 6a.
Yield: 6.8 g (53%), yellow solid, mp 189–190 °C (from MeOH). 1H
NMR d 2.19 (s, 3H, 6-CH3), 2.25 (s, 3H, 2-CH3), 2.26 (s, 3H, 5-COCH3),
3.61 (s, 3H, 3-COOCH3), 4.58 (s, 1H, 4-H), 8.91 (s, 1H, NH), 12.06 (s,
1H, 4-COOH). 13C NMR 17.88, 18.71, 29.44, 40.06, 50.88, 97.63,
106.83, 144.74, 146.21, 167.15, 174.38, 196.75. Found: C, 56.88;
H, 5.86; N, 5.49. Calc. for C12H15NO5: C, 56.91; H, 5.97; N, 5.53%.
monochromated Mo K
a radiation (k = 0.71073 Å). The crystal
structures of compounds 1a–6c were solved by the direct method
and refined by full-matrix least squares. The details of X-ray struc-
tures, data collections and refinements are listed in Supplementary
material Figs. 1–6, Table 1 and 2.
NMR spectroscopy
The NMR spectra were measured at 298 K. Sample concentra-
tions were 0.02 M. The 1,4-DHINA derivatives were dissolved in
deuterated dimethyl sulphoxide (DMSO-d6) for NMR experiments.
The 1H chemical shifts were referenced to tetramethylsilane at
0.0 ppm, the 13C chemical shifts to the solvent DMSO-d6 at
39.5 ppm and the 15N chemical shifts to liquid NH3 at 0.0 ppm.
The isotropic 1H, 13C and 15N chemical shifts were determined
with an experimental error 0.01 ppm, 0.1 ppm and 0.2 ppm
accordingly; the experimental error in the determination of the
1J(15NA1H) and 1J(13C4A1H) values was 0.1 Hz.
IR spectrum, m
max/cmÀ1: 3310 (NH), 1734 (CO), 1672 (CO).
5-Acetyl-2,6-dimethyl-1,4-dihydropyridine-3,4-dicarboxylic
acid dimethyl ester (6b) was synthesized by the procedure given
in literature [21]. Yield 4.0 g (87%), yellow solid, mp 158–160 °C
(from MeOH). 1H NMR d 2.21 (s, 3H, 6-CH3), 2.25 (s, 3H, 2-CH3),
2.26 (s, 3H, 5-COCH3), 3.50 (s, 3H, 4-COOCH3), 3.62 (s, 3H, 3-
COOCH3), 4.71 (s, 1H, 4-H), 8.99 (s, 1H, NH). 13C NMR 17.89,
18.79, 29.58, 40.05, 50.95, 51.76, 96.98, 106.42, 145.35, 146.78,
166.95, 173.32, 196.22. Found: C, 58.35; H, 6.36; N, 5.07. Calc. for
The 1H, 15N and 13C NMR experiments were carried out on Var-
ian INOVA 600 MHz instrument operating at 599.9 MHz (1H),
60.8 MHz (15N) and 150.8 MHz (13C) and equipped with cryoprobe.
Accurate one bond 1J(15NA1H) and 1J(13CA1H) coupling constants
were directly extracted from conventional 1D 1H NMR spectra.
The 1HA15N gradient-selected heteronuclear single quantum cor-
relation (gHSQC) experiments were used to measure the d(15N)
chemical shifts. Experiments were adjusted for a direct nJ(15NA1H)
coupling constants of 3.0 Hz and 95.0 Hz. The spectral widths for
1H and 15N were 8000 and 20,000 Hz, respectively. Each 1HA15N
gHSQC spectrum was collected with 2048 points in the direct 1H
dimension and 256 increments in the indirect 15N dimension.
Before Fourier transformation, the dataset was zero-filled to a
2048 Â 2048 matrix and subjected to the application of cosine
window function.
C
13H17NO5: C, 58.42; H, 6.41; N, 5.24%. IR spectrum,
3344 (NH), 1707 (CO), 1676 (CO).
5-Acetyl-2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3-car-
m :
max/cmÀ1
boxylic acid methyl ester (6c). A mixture of 3-benzylidene-2,4-
pentanedione (1.8 g, 0.01 mol) and methyl-3-aminocrotonate
(1.1 g, 0.01 mol) in EtOH (20 mL) was stirred at 60 °C for 6 h and
evaporated under reduced pressure. The residue was recrystallized
from MeOH to afford compound 6c. Yield: 1.3 g (48%), yellow crys-
tals, mp 172–173 °C (from MeOH) (lit. [18], 170–172 °C). 1H NMR d
2.12 (s, 3H, 5-COCH3), 2.23 (s, 3H, 2-CH3), 2.28 (s, 3H, 6-CH3), 3.58
(s, 3H, 3-COOCH3), 4.94 (s, 1H, 4-H), 7.11–7.21 (m, 5H, C6H5), 8.87
(s, 1H, NH). 13C NMR 18.14, 19.08, 30.02, 38.78, 50.68, 102.48,
111.42, 125.98, 127.07, 128.13, 144.89, 145.12, 147.40, 167.39,
196.36. Found: C, 71.41; H, 6.81; N, 4.83. Calc. for C17H19N2O3: C,
Theoretical calculations
71.56; H, 6.71; N, 4.91%. IR spectrum,
1682 (CO), 1655 (CO).
m
max/cmÀ1: 3270 (NH),
For theoretical calculations the geometries of various conforma-
tions of symmetrically and unsymmetrically substituted 1,4-DHIN-
A derivatives have been optimized at density functional theory
(DFT) level using B3LYP with 6-31GÃ basis set method in both
gas and liquid – CHCl3 and DMSO (Poisson Boltzmann Finite ele-
ment method) phases. The geometry optimization of 1,4-DHINA
derivatives has been carried out with different starting points –
several initial structures/conformations for each molecule. The
starting structures of 1,4-DHINA derivatives have been based on
some known typical values of bond lengths and angles. All quan-
tum chemical calculations were performed by using Jaguar 8.0 pro-
gram package [25]. The optimized geometries were used to obtain
the Mulliken charges on nitrogen and carbon atoms in heterocycle.
The 15N and 13C NMR chemical shifts were calculated with gauge-
including atomic orbitals (GIAO) approach by subtracting the shiel-
5-Acetyl-1,2,6-trimethyl-1,4-dihydropyridine-3,4-dicarbox-
ylic acid dimethyl ester (6d). To a stirred solution of compound 6b
(2.0 g, 0.008 mol) in anhydrous CH3CN (40 mL) at r.t. was added
sodium methoxide (0.5 g, 0.009 mol), as a result of which a vigor-
ous reaction with hydrogen evolution was observed, and strongly
colored anion, which gives characteristic orange fluorescence,
was formed. At the end of hydrogen evolution CH3I (1.2 mL,
0.04 mol) was added. Reaction mixture was stirred for 12 h and
evaporated under reduced pressure. The crude residue was puri-
fied by column chromatography on silica gel (CH2Cl2-petroleum
ether-acetone, 9:7:1) to give compound 6d. Yield: 1.2 g (53%), col-
ourless crystals, mp 79–81 °C (from MeOH). 1H NMR d 2.28 (s, 3H,
5-COCH3), 2.33 (s, 3H, 6-CH3), 2.45 (s, 3H, 2-CH3), 3.17 (s, 3H,
N-CH3), 3.50 (s, 3H, 4-COOCH3), 3.65 (s, 3H, 3-COOCH3), 4.77 (s,
1H, 4-H). 13C NMR 15.71, 16.37, 29.77, 33.98, 39.84, 51.26, 51.86,
99.32, 109.27, 148.76, 151.01, 166.85, 172.78, 196.96. Found: C,
59.69; H, 6.84; N, 4.87. Calc. for C14H19NO5: C, 59.78; H, 6.81; N,
dings of the individual atoms from the shielding of the standard d
15
(
N
) = 254.4 ppm, d (13CTMS) = 189.5 ppm calculated using the
NH3
same method in the gas phase.
4.98%. IR spectrum,
m
max/cmÀ1: 1742 (CO), 1692 (CO).
Results and discussion
X-ray crystallography
Synthesis
Single crystals suitable for X-ray analysis were obtained by slow
evaporation from saturated MeOH (1a-3a, 6a, 1b-3b, 6b, 2c, 6c,
3d-4d, 6d, 3e), EtOH (1c, 4a), acetone (3c–4c), CH2Cl2 (4b and
The symmetrical 1,4-DHINA 1a-3a were prepared (25–64%
yield) according to the modified Hantzsch condensation of one

I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
553
Table 2
The X-ray geometric and puckering parameters of the 1,4-DHP ring in compounds 1a–6a, 1b–6b, 1c–6c, 1d–5d, 1e, 3e, 5e and 1f.
1,4-DHP
C(4) deva, Å
N(1) deva, Å
u
1, (°)
u
2, (°)
P, (°)
s, (°)
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
3a
3b
3c
3d
3e
4a
0.233(9)
0.313(4)
0.341(9)
0.520(7)
0.419(4)
0.325(3)
0.341(1)
0.332(3)
0.341(2)
0.553(3)
0.319(1)
0.134(3)
0.107(4)
0.374(3)
0.203(2)
0.255(4)
0.182(4)
0.232(4)
0.214(3)
0.260(3)
0.427(3)
0.012(4)
0.213(3)
0.303(3)
0.009(2)
0.445(3)
0.022(3)
0.347(3)
0.232(8)
0.364(2)
0.102(7)
0.132(3)
0.168(8)
0.283(6)
0.209(3)
0.163(3)
0.152(1)
0.159(2)
0.143(2)
0.262(3)
0.171(1)
0.052(2)
0.045(3)
0.169(3)
0.118(2)
0.100(3)
0.066(4)
0.091(3)
0.116(2)
0.123(3)
0.223(2)
0.023(3)
0.077(3)
0.136(3)
0.015(2)
0.218(3)
0.028(3)
0.153(3)
0.125(6)
0.154(2)
167.5(5)
5.4(6)
175.5(5)
69.3 (7)
91.9(4)
102.0(8)
30.4(3)
122.4(5)
94.6(3)
102.0(1)
99.8(3)
99.7(2)
161.9(2)
95.8(2)
38.5(3)
31.1(3)
109.4(3)
61.6(2)
74.3(3)
52.3(4)
67.4(4)
65.0(3)
77.2(3)
128.0(3)
14.9(3)
61.4(4)
89.8(4)
14.8(2)
131.3(4)
14.7(3)
103.0(3)
70.7(6)
106.7(2)
79.2
82.8
85.8
85.6
88.4
83.2
76.5
82.3
83.8
85.5
90.0
72.3
88.4
75.6
71.9
80.4
80.3
79.9
76.9
76.1
79.0
72.9
74.2
79.6
87.7
86.2
89.5
86.3
84.2
85.0
À171.5(3)
176.6(6)
162.4(6)
À26.9(6)
À28.7(3)
167.7(8)
À14.9(3)
160.8(2)
À159.8(2)
–
–
–
–
–
175.9(3)
2.5(5)
2.7(5)
4.7(4)
2.3(4)
À21.5(3)
À174.5(5)
À172.2(5)
À168.4(5)
À32.4(2)
À33.3(4)
À134.1(4)
0.8(4)
6.0(1)
À15.0(1)
37.6(6)
À41.9(3)
170.0(8)
171.0(2)
À14.9(3)
À15.2(3)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4b mol A
4b mol B
4c mol A
4c mol B
4d
5a
5b mol A
5b mol B
5c
5d
5e
6a
6b
6c
–
169.9(2)
À30.7(7)
À174.8(2)
À3.4(7)
À3.3(3)
a
Deviation from the least-squares plane defined by C(2)AC(3)AC(5)AC(6).
equivalent of a glyoxylic acid with two equivalents of a methyl
b-aminocrotonate, acetylacetone imine or 3-amino-crotononitrile,
as reported in Scheme 1.
structures of the prepared compounds as well as compounds
which were reported previously are summarized in Table 1.
The unsymmetrical 1,4-DHINA 4a and 6a were synthesized by
condensing first glyoxylic acid in a Knoevenagel manner with
methyl b-aminocrotonate. The condensation of Knoevenagel
adduct with 3-aminocrotononitrile or acetylacetone imine gives
1,4-DHINA 4a and 6a in 68% and 53% overall yield (Scheme 2).
The esterification of the carboxylic acids 1a–4a and 6a was real-
ized in a similar manner to the procedures described previously
[21,23] in 63–93% yield.
The N-substituted and 4,4-disubstituted 1,4-DHINA derivatives
2d–4d, 6d (ꢁ55% yield) and 3e (22% yield) were obtained by the
treatment of 1,4-DHINA methyl esters 2b–4b or 6b with sodium
methoxide and methyl iodide.
X-ray crystallography
Many X-ray structural analyses of 1,4-DHPs show that the dihy-
dropyridine ring has a flattened boat conformation [8–19]. The
X-ray structural studies of 4-phenyl substituted 1,4-DHPs indicate
that the degree of puckering of the 1,4-DHP ring is related to phar-
macological activity, the latter increasing with increasing ring pla-
narity [8,26]. The sum of the absolute values of the ring internal
torsion angles (P) was suggested as a quantitative measure for
the evaluation of the flatness of the six-membered ring [9]. These
values range from 14.7(3)° (5e) to 161.9(2)° (2d) in Table 2, indi-
cating a significant amount of flattening from the ideal boat confor-
mation (P = 90°). A mean value of 77(2)° was found previously for
1,4-DHP rings, although the P values generally vary over a wide
range from 4° to 130° [27]. For example, in nifedipine, a calcium
channel blocking drug, the value of P is 72° [8,26].
The unsymmetrical 4-phenyl-1,4-DHP derivatives 4c–6c were
prepared (48–77% yield) by the modified method of the Hantzsch
condensation which involved Michael addition of a Knoevenagel
adduct with an enamine.
The structures of the title compounds were confirmed by IR, 1H,
13C and 15N NMR spectroscopy, X-ray and elemental analysis.
All final products were pure and stable compounds. Chemical
The crystal structures for the compounds studied (1a–6a,
1b–6b, 1c–6c, 1d–5d, 1e, 3e, 5e and 1f) reveal that the 1,4-DHP
ring exists in a boat form flattened at the N(1) and puckered at
Scheme 1. Synthesis of symmetrically substituted 1,4-dihydroisonicotinic acid 1a–3a.

554
I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
Scheme 2. Synthesis of unsymmetrically substituted 1,4-dihydroisonicotinic acid 4a and 6a.
the C(4) atoms as evidenced by the distances of N(1) and C(4) to
the main plane C(2)AC(3)AC(5)AC(6) of the olefinic double bonds
(Table 2). The C(4) puckering, which varies considerably among the
compounds, is linearly related to that at the nitrogen (Fig. 1); the
greater the displacement of C(4) from the ring, the larger is the ring
puckering at N(1).
Comparative analysis of the data reported in Table 2 indicates
that the deviations of the C(4) and N(1) atoms from the planarity
follow the order COCH3 > COOCH3 > CN. So, the substitution with
ester and acetyl groups at C(3) and C(5) positions induces more
deviation on C(4) and N(1) atoms from the plane. A completely pla-
nar 1,4-DHP ring was found in the structures of 5a, 5c and 5e, but
N(1) substitution with methyl group (1d–5d) causes the 1,4-DHP
ring to become more puckered (Table 2).
Comparative analysis with 4-phenyl group substituted 1,4-DHP
derivatives 1c–6c was carried out to elucidate the individual effect
of 4-carboxyl and 4-methoxycarbonyl groups on the 1,4-DHP ring
planarity. No relation between the nature of C(4) substituent and
the puckering of the C(4) and N(1) atoms could be established.
Similarly to 4-phenyl substituted 1,4-DHP derivatives 1c–6c the
4-carboxyl and 4-methoxycarbonyl groups in compounds 1a–6a,
1b–6b, 1d–5d and 3e prefer a pseudo-axial position on C(4). The
The C(4) hydrogen substitution with alkyl group in compounds
1e, 1f, 3e and 5e causes the 1,4-DHP ring to become more flat
(Table 2).
The carbonyl groups at the C(3) and C(5) positions of the 1,4-
DHP ring are of crucial importance to the pharmacological effect,
therefore important aspect is the rotation of the carbonyl group.
It has been shown that unsymmetrical substituents at the C(3)
and C(5) positions alter the activity [28].
Assuming a favoured coplanar arrangement of the ester or
acetyl carbonyl with the double bound of the dihydropyridine ring,
in principle, three different conformations at 3(5)-positions are
possible for the carbonyl groups: s-trans/s-trans, s-cis/s-cis, and
enantiomeric s-cis/s-trans and s-trans/s-cis. X-ray structural investi-
gations, theoretical calculations and in vitro analyses of fused
1,4-DHPs indicate that, probably, at least one of the carbonyl must
be in the s-cis arrangement making hydrogen bond to the receptor
[28].
The orientation of ester and acetyl carbonyls is defined by tor-
sion angles C(2)AC(3)AC(7)AO(8)
(u1) and C(6)AC(5)AC(9)A
O(10) ( 2) in Table 2. An analysis of results of 22 X-ray structures
u
shows a preference for the s-trans/s-cis arrangement in the crystal
of 1,4-DHPs 1a–1f, 2a–2d, 6a–6c, and a preference for the s-cis
arrangement is found for unsymmetrical ones 4a–4d, 5a–5e
(Table 2). Only in two cases of 1a and 2a both carbonyl groups have
s-trans/s-trans arrangement. Examination of u1 and u2 (Table 2)
revealed that ester and acetyl groups remain nearly coplanar with
the plane of the 1,4-DHP ring due to the electron delocalization in
the conjugated double bond system.
dihedral angle (s) between the least-squares plane of the 1,4-
DHP ring and the 4-carboxyl or 4-methoxycarbonyl groups range
from 71.9° (3e) to 90.0° (3a) indicating that both groups are almost
perpendicular to the 1,4-DHP ring. It is worth noting that both 4-
carboxyl and 4-methoxycarbonyl groups can rotate freely about
their adjacent C(4)ACO bond.
However, the X-ray structures of 4,4-disubstituted 1,4-DHP
derivatives 1e, 1f and 5e in comparison with those of 4-monosub-
stituted 1d and 5d revealed drastic changes in conformation
caused by the second 4-substitution. In 4-alkyl-4-methoxycar-
bonyl disubstituted 1,4-DHP (1e, 1f and 5e) 4-methoxycarbonyl
group moved from pseudo-axial into the pseudo-equatorial confor-
mation, as already observed in other 4-phenyl-4-methyl disubsti-
tuted 1,4-DHP structure [15]. It seems that the steric influence of
alkyl group dominates the conformation.
It is worth noting that in the crystal structure of compounds 3a,
3b and 4a a water molecule has been found, which forms an
intermolecular hydrogen bond system with 1,4-DHP carboxylic
acid and NH group. The lengths of these hydrogen bonds are
O(12)AHÁ Á ÁO(1w) 1.73(2) Å; O(12)Á Á ÁO(1w) 2.65(2) Å (3a);
O(11)AHÁ Á ÁO(1w) 1.70(2)Á Á ÁÅ; O(11)Á Á ÁO(1w) 2.58(3) Å (4a) and
N(1)AHÁ Á ÁO(1w) 2.01(1) Å; N(1)Á Á ÁO(1w) 2.82(2) Å (3b). In 4b
and 4c there are two independent molecules with similar confor-
mation in the crystal asymmetric unit.
Quantum chemical conformational analysis
The prepared compounds as well as compounds which were
reported previously were subjected to conformational analysis by
quantum chemical calculations. The high level density functional
theory (DFT) B3LYP method with 6-31GÃ basis set was used to
obtain the optimum structures of the 1,4-DHINA derivatives in
gas and liquid (CHCl3 and DMSO) phases. The calculations in gas
phase produced the same relative conformation patterns with
respect to the liquid phase (CHCl3 and DMSO) calculations (Supple-
mentary material Tables 3–5). The results of calculations in gas
phase are summarized in Table 3, and the optimized structures
for some representative 1,4-DHINA derivatives 1b–6b are demon-
strated in Scheme 3.
The 1,4-DHP ring puckering analysis confirmed the known pref-
erence for a flattened boat conformation for all of the considered
structures 1a–6a, 1b–6b, 1c–6c, 1d–6d, 1e, 3e, 5e and 1f. Stronger
Fig. 1. The correlations of the NH (
idine ring distortion of C(4) with that at N(1).
D) and NCH3 substituted (j) 1,4-dihydropyr-

I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
555
Table 3
The gas phase optimized geometric and puckering parameters for the 1,4-DHP ring in
compounds 1a–6a, 1b–6b, 1c–6c, 1d–6d, 1e, 3e, 5e and 1f using B3LYP/6-31G*
method.
1,4-DHP C(4) deva, Å N(1) deva, Å
u
1, (°)
u
2, (°)
P, (°)
s, (°)
1a
1b
1c
1d
1e
1f
0.389
0.400
0.298
0.469
0.494
0.582
0.429
0.429
0.349
0.492
0.273
0.278
0.202
0.354
0.336
0.262
0.279
0.252
0.382
0.307
0.314
0.269
0.452
0.401
0.389
0.397
0.338
0.470
0.171
0.176
0.128
0.246
0.252
0.285
0.184
0.186
0.145
0.244
0.123
0.119
0.102
0.165
0.160
0.116
0.122
0.120
0.195
0.120
0.120
0.117
0.213
0.182
0.161
0.163
0.144
0.237
2.85
2.45
À1.60
6.02
7.65
7.50
1.27
12.31
114.8 75.6
118.0 75.7
87.1
89.9
140.8 83.1
À27.83 À145.99 143.2 90.0
146.99
À5.54
À5.16
5.99
28.19
À4.78
À3.68
À3.62
166.1 70.1
125.8 76.0
126.1 75.4
101.2 89.9
145.9 88.8
2a
2b
2c
2d
3a
3b
3c
3d
3e
4a
4b
4c
4d
5a
5b
5c
5d
5e
6a
6b
6c
6d
À14.07 À5.76
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
80.1
81.0
60.4
82.6
83.8
88.3
Fig. 2. The correlations of the NH (
D
) and NCH3 substituted (j) 1,4-DHP ring
103.4 85.5
distortion of C(4) with that at N(1) obtained by calculations in gas phase.
–
87.5
76.9
81.9
74.9
83.4
79.3
79.3
86.2
5.18
5.26
À2.85
À8.14
0.38
0.46
4.06
16.33
132.34
À7.25
À6.38
À3.85
À7.71
the N-methyl substituent occupies a pseudo-equatorial position
that agrees with X-ray structural analysis.
113.5 83.1
The sum of the absolute values of the ring internal torsion
angles (P) ranges from 60.4° to 166.1° in Table 3. These findings
are in good agreement with the results of X-ray studies (Table 2).
Both X-ray structural analysis and quantum chemical calcula-
tions show that the change of the planarity of 1,4-DHP ring has
small effect on bond lengths in 1,4-DHP heterocycle (Supplemen-
tary material Tables 2–5).
88.6
90.1
78.6
86.9
87.7
82.7
132.6 86.8
115.0 70.9
113.5 81.3
115.7 81.3
À11.26
À11.53
1.91
À14.94
98.6
86.1
Regarding the orientation of the 4-carboxyl and 4-methoxycar-
bonyl groups with respect to the 1,4-DHP ring, the computational
data confirmed the preference of C(4) substituent for pseudo-axial
position. The pseudo-equatorial conformation of 4-methoxycar-
bonyl group was observed in 4-alkyl-4-methoxycarbonyl substi-
tuted 1,4-DHPs 1e, 1f and 5e what is in agreement with the
X-ray results.
140.0 86.3
a
Deviation from the least-squares plane defined by C(2)AC(3)AC(5)AC(6).
distortion from planarity around the C(4) was highlighted for all
compounds (Table 3). The C(4) puckering is linearly related to that
at the N(1) (Fig. 2).
The dihedral angle (s) between the C(4) substituent and
1,4-DHP ring ranges from 70.1° to 90.0° which indicates that they
are nearly perpendicular to each other (Table 3) as it was found by
the X-ray analysis (Table 2).
The N(1) atom substitution with methyl group (1d–5d) causes
the 1,4-DHP ring become more puckered (Fig. 2, Table 3). The
quantum chemical calculations of these compounds show that
Scheme 3. The DFT optimized structures of 1,4-DHINA derivatives 1b–6b.

556
I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
Unlike calculations reported in literature [29–32], the orienta-
are summarized in Table 4 (fully in Supplementary material Tables
6 and 7).
tion of the carbonyl groups at the C(3) and C(5) positions of the
studied 1,4-DHP derivatives 1a–1f, 2a–2d and 6a–6c, the s-cis/s-
cis arrangement seemed to be the preferred one both in gas and
liquid phases, with the s-cis/s-trans arrangement showing up only
in compounds 1e and 1f (Table 3). Interestingly, the s-trans/s-trans
conformation was obtained for none of the 1,4-DHP derivatives. In
the optimized structures of unsymmetrical 1,4-DHPs 4a–4d and
5a–5d the carbonyl group at C(3) or C(5) position is oriented in
s-cis arrangement, only in 5e the carbonyl group is oriented in
s-trans conformation (Table 3). These small differences relative to
X-ray studies (Table 2) could be explained by crystal packing
effects in solids. Quite recently, it was found by quantum chemical
calculations that C(3) and C(5) carboxyl group rotation around
C(3,5)ACO has energy barrier 8–10 kcal/mol [33].
This conformational flexibility of 1,4-DHINA derivatives could
be of great relevance and could be exploited in the interaction pro-
cess with the biological receptor.
Most conclusions about conformational characteristics of
1,4-DHP derivatives come from X-ray analysis and theoretical cal-
culations, however these data may not reflect the conformation in
solution. In order to improve and to complete the insight into the
conformation of 1,4-DHPs, we present here NMR study of these
compounds.
The electronegativity of the substituents at C(3,5) positions of
heterocycle has a straightforward effect on the N1H and C4A1H
proton chemical shifts (Table 4). The N1H chemical shifts in 3(5)-
cyano-1,4-DHP analogues (3a–3c, 4a–4c and 5a–5c) are shifted
downfield compared with 3(5)-acetyl- or 3(5)-methoxycarbonyl-
ones (1a–1c, 2a–2c and 6a–6c) (Table 4). This indicates that both
double bonds and the p-orbital of the nitrogen are in stronger
conjugation in 3(5)-cyano-1,4-DHP analogues (3a–3c, 4a–4c and
5a–5c). Substituents at C(3,5) positions have a direct influence
on the chemical shift of C4A1H, leading to upfield shifts for
3(5)-cyano-1,4-DHP derivatives (3a–3c, 4a–4c and 5a–5c) and
downfield shifts for 3(5)-acetyl- or 3(5)-methoxy-carbonyl-ones
(1a–1c, 2a–2c and 6a–6c) (Table 4). This effect can be attributed
at least partially to the anisotropic effect of the C(3,5) substituents.
The chemical shift of the C4A1H protons varies with the nature
of the C(4) substituent (Table 4). In the case of the 4-phenyl substi-
tuted 1,4-DHPs 1c–6c the signal corresponding to the C4A1H atom
in the 1,4-DHP ring is shifted downfield by 0.3–0.4 ppm as com-
pared with 4-carboxyl substituted 1,4-DHP analogues (1a–6a),
due to the influence of the anisotropic effect of the phenyl group.
The effect of the substituent at C(4) position of heterocycle on
the N1H chemical shifts is small (Table 4).
The signals of carbon-13 resonance in the 1,4-DHP moiety were
found in the range from 144.4 to 152.9 ppm for the C(2,6) atoms, in
the range 77.3–112.5 ppm for the C(3,5) atoms, and in the range
from 38.4 to 41.9 ppm for the C(4) atom (Table 4). As the
electron-withdrawing characteristics of the C(3,5) substituents
are increased in the series COCH3 < COOCH3 < CN the signals
corresponding to the C(2,6) atoms, as well as the C(4) carbon are
shifted downfield. The upfield shift was observed for the carbons
located at the C(3,5) positions of the 1,4-DHP ring (Table 4). In
the case of N-methyl 1,4-DHP derivatives 1d–6d the signals corre-
sponding to the C(2,6) atoms are shifted downfield, while C(3,5)
signals are shifted upfield, reflecting substantial polarization of
the C(2,6)@C(3,5) double bonds under the influence of N-methyl
group (Table 4). The introduction of a methyl group to the nitrogen
Spectral analysis
Several experimental studies have shown the usefulness of the
1H, 13C and 15N NMR in structural and conformational studies of
1,4-DHP derivatives; in fact, the chemical shifts of these nuclei
are highly influenced by structural changes [33–40]. In particular,
1H and 13C chemical shifts for a series of 4-phenyl substituted
1,4-DHPs have been studied [33–39], however, no systematic
investigation has been made of substituent effects on NMR data
for 1,4-DHINA derivatives.
The representative 1H, 13C, 15N NMR chemical shifts and direct
1J(15NA1H), 1J(13C4A1H) spin–spin coupling constants measured
for 1,4-DHP derivatives 1a–6a, 1b–6b, 1c–6c, 1d–6d, 3e and 5e
Table 4
The 1H, 15N and 13C NMR chemical shifts and coupling constants 1J(15NA1H), 1J(13C4A1H) in DMSO-d6 for compounds 1a–6a, 1b–6b, 1c–6c, 1d–6d, 3e and 5e.
1,4-DHP
d, ppm
1J, Hz
15N
N1H
C4A1H
C(2)
C(6)
C(3)
97.0
96.3
101.5
99.1
107.7
107.2
112.5
109.6
78.0
77.3
82.7
79.3
85.5
96.6
95.5
99.5
98.1
106.3
106.2
108.9
108.3
116.7
97.6
97.0
102.5
99.3
C(5)
97.0
96.3
101.5
99.1
107.7
107.2
112.5
109.6
78.0
77.3
82.7
79.3
85.5
79.3
78.2
84.3
79.6
C(4)
15NA1H
13C4A1H
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
3e
4a
4b
4c
4d
5a
5b
5c
5d
5e
6a
6b
6c
6d
139.7
140.2
138.9
126.5
141.5
141.9
141.2
128.2
132.3
133.0
130.5
123.3
122.7
134.8
134.9
133.7
124.7
136.0
136.3
134.3
124.5
122.0
140.3
140.8
140.0
126.8
8.93
9.01
8.87
–
8.91
9.03
8.88
–
9.62
9.72
9.52
–
4.58
4.69
4.88
4.83
4.61
4.74
5.03
4.73
4.02
4.24
4.39
4.23
1.54
4.04
4.17
4.44
4.23
4.16
4.27
4.58
4.35
1.25
4.58
4.71
4.94
4.77
146.2
146.8
145.7
150.9
144.8
145.4
144.4
149.0
148.4
148.9
146.7
152.9
151.1
146.1
147.0
145.7
150.7
145.0
145.7
144.6
148.2
148.9
146.2
146.8
145.1
151.0
146.2
146.8
145.7
150.9
144.8
145.4
144.4
144.0
148.4
148.9
146.7
152.9
151.1
148.3
149.1
146.4
152.8
148.3
148.8
145.9
152.6
150.6
144.7
145.3
144.9
148.8
39.2
39.4
38.5
38.9
40.5
40.4
38.7
40.5
40.6
40.5
40.9
39.7
42.0
41.2
41.2
40.6
40.2
41.0
41.2
40.7
40.4
45.4
40.1
40.1
38.8
39.8
À94.3
À94.4
À94.3
–
À93.8
À93.8
À93.6
–
À96.0
À96.4
À96.4
–
135.7
136.7
136.2
135.8
131.8
133.1
132.2
131.2
138.8
140.1
138.2
138.7
–
137.1
139.3
136.5
137.4
136.0
137.6
134.9
135.0
–
–
–
9.24
9.31
9.19
–
9.21
9.29
9.13
–
À95.2
À95.7
À95.3
–
À94.9
À95.2
À94.9
–
79.4
78.9
85.2
80.0
–
86.3
–
8.91
8.99
8.87
–
106.8
106.4
111.4
109.3
À94.4
À94.1
À93.8
–
133.8
134.9
134.0
133.5

I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
557
atom significantly changes the depth of the 1,4-DHP boat (X-ray
data in Table 2) that reduces the role of the nitrogen unshared elec-
tron pair in the overall conjugation of the heterocycle. In the case
of 4,4-disubstituted 1,4-DHP 3e and 5e introduction of methyl
group instead of proton at C(4) position causes drastic downfield
shift to the C(3,5) and C(4) resonances (Table 4). This effect could
be probably explained by the shift of pseudo-equatorial into
pseudo-axial conformation of 4-methoxycarbonyl group at C(4)
position in 3e and 5e that could influence the 13C chemical shifts.
The 13C chemical shift values within the phenyl ring can be used
to analyze the effect of 1,4-DHP ring on the phenyl ring itself. In the
case of 4-phenyl substituted 1,4-DHP derivatives studied herein
(compounds 1c–6c) the electron withdrawing characteristics of
the C(3,5) substituent are increased in row COOCH3 < COCH3 < CN,
as the signals corresponding to the meta- and para-carbon atoms in
the phenyl ring are shifted downfield (Supplementary material
Table 7). The meta-carbon resonance is less affected, but the chem-
ical shift of the carbon atom in para position varies significantly
under the influence of the substituent at C(3,5). In the case of the
meta-carbon resonance this effect can be explained in terms of
inductive electron withdraw by the complex 4-(1,4-DHP) substitu-
ent, however the chemical shift of para-carbon atom could be
related to the hyperconjugation and substantial polarization of
Fig. 3. Relationship between experimental (DMSO-d6) and calculated (gas phase)
15N chemical shifts of NH (
D) and NCH3 (j) 1,4-DHPs.
agreement with experimental values (Table 4) and confirm the
shielding effect of methyl group (Fig. 3).
The computed atomic Mulliken charges (Supplementary mate-
rial Table 8) for the B3LYP/6-31GÃ geometry optimized structures
of 1a–6a, 1b–6b, 1c–6c, 1d–6d, 3e and 5e showed correlation with
the experimental 15N chemical shifts (Supplementary material
Fig. 7).
p
-electron system according to usual resonance concepts.
The computed atomic Mulliken charges and 13C chemical shifts
The range of 1J(15NA1H) values corresponds to the nearly planar
arrangement around the nitrogen. It was found that direct
1J(15NA1H) spin–spin coupling constants are larger for 3(5)-
cyano-1,4-DHP derivatives (3a–3c, 4a–4c and 5a–5c) in compari-
son to 3(5)-acetyl- or 3(5)-methoxycarbonyl-ones (1a–1c, 2a–2c
and 6a–6c) (Table 4). Substituents giving the flattened 1,4-DHP
boat and/or pyramid at nitrogen lead to the larger absolute value
of the 1J(15NA1H) coupling constant that is observed for 3(5)-
cyano-1,4-DHP derivatives (3a–3c, 4a–4c and 5a–5c).
(Supplementary material Table 8) for the B3LYP/6-31GÃ geometry
optimized structures of 1a–6a, 1b–6b, 1c–6c, 1d–6d, 3e and 5e do
not show any correlations, probably due to the influence of the
anisotropic effects of the ester, ketone or cyano functional groups
at positions C(3,5).
The direct 1J(13C4A1H) spin–spin coupling constants measured
for the 1,4-DHP derivatives 1a–6a, 1b–6b, 1c–6c and 1d–6d
change considerably as the substituent at C(3,5) position was var-
ied (Table 4). It was found that 1J(13C4A1H) coupling constants are
larger for 3(5)-cyano-1,4-DHP derivatives (3a–3d, 4a–4d and
5a–5d) (Table 4). Supporting X-ray data have shown that 3(5)-
cyano group is a more effective boat flattening substituent than
3(5)-methoxycarbonyl or 3(5)-acetyl groups in 1,4-DHPs. Such
increasing of planarity at C(4) increases 1J(13C4A1H) values
(Table 4).
It should be noted that the sign of 1J(15NA1H) is negative [41];
this leads to the opposite appearance of the influence of the
electronic effects in 1,4-DHP ring to 1J(15NA1H) and d(15N),
1J(15NA1H) and d(N1H) values in correlations (Figs. 4 and 5).
A regular dependence of the 1J(15NA1H) coupling constants on
the d(15N) and d(N1H) chemical shifts is observed (Figs. 4 and 5).
The increase of 1J(15NA1H) coupling constants by the introduction
of cyano group in molecule demonstrates that 3(5)-cyano group is
a more effective boat and/or nitrogen pyramid flattening substitu-
ent than 3(5)-methoxycarbonyl or 3(5)-acetyl groups in 1,4-DHPs
probably due to its smaller volume.
There are two effects possible that could act on d(15N) in 1,4-
DHPs: first, the change of the flattened boat conformation of the
1,4-DHPs upon the electronic effects of substituents at C(3,5) posi-
tions, and second, the change of pseudo-axial/pseudo-equatorial
equilibrium on the nitrogen, depending on both the conformation
of the flattened boat and the electronic effects of substituents at
C(3,5) positions.
In general the extensive NMR studies of 1H, 13C and 15N chem-
ical shifts of 1,4-DHINA derivatives, both concerning 1,4-DHP ring
The 1,4-DHP derivatives bearing methoxycarbonyl and acetyl
groups at C(3) and/or C(5) position of heterocycle (1a–1c, 2a–2c
and 6a–6c) have 15N resonating the farthest downfield relative to
the ones bearing cyano group (3a–3c, 4a–4c and 5a–5c) (Table 4).
N-methyl substitution results in an upfield shift as compared with
NH analogues (Table 4). The changes in geometry and different
ability of the lone pair delocalization in N-methyl derivatives must
be considered as the dominant d(15N) shielding contributor.
The X-ray structures of N-methyl derivatives (1d–6d) show that
the nitrogen atom has larger displacement from the base of the
boat plane as compared with the NH analogues (1b–6b) (Table 2).
The upfield shift of the N-methyl nitrogen may therefore be ratio-
nalized in terms of partial increase in sp3 character of the N-methyl
nitrogen relative to the NH analogues and preferred pseudo-equa-
torial conformation of N-methyl group.
The effect of the substituent at C(4) position of heterocycle on
the d(15N) chemical shifts is very small (Table 4).
Our quantum chemical calculations show that the calculated
d(15N) values (Supplementary material Table 8) are in good
Fig. 4. A plot of the observed 15N chemical shifts of 1,4-DHPs against 1J(15NA1H)
coupling constants in DMSO-d6.

558
I. Goba et al. / Journal of Molecular Structure 1074 (2014) 549–558
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.molstruc.2014.06.044.
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Verlag, Berlin-Heidelberg-New York, 1981.
Supplementary crystallographic data for this paper can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retriev-
ing.html (or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or
E-mail: deposit@ccdc.cam.ac.uk). Supplementary data associated