Hantzsch synthesis of 2,6-dimethyl-3,5-dimethoxycarbonyl-4-(o- methoxyphenyl)-1,4-dihydropyridine; a novel cyclisation leading to an unusual formation of 1-amino-2-methoxycarbonyl-3,5-bis(o-methoxyphenyl)-4-oxa- cyclohexan-1-ene

Article abstract of DOI:10.3390/12112546

Hantzsch condensation of two equivalents of methyl-3-aminocrotonate with (m- and p)-methoxybenzaldehyde afforded the expected products 2,6-dimethyl-3,5-dimethoxycarbonyl-4-(m-methoxyphenyl)-1,4-dihydropyridine and 2,6-dimethyl-3,5-dimethoxycarbonyl-4-(p-methoxyphenyl)-1,4-dihydropyridine, whereas o-methoxybenzaldehyde produced mainly 1-amino-2-methoxycarbonyl-3,5- bis(o-methoxy-phenyl)-4-oxa-cyclohexan-1-ene. The structure of the product, not previously reported in the literature, was determined by 1D and 2D NMR spectra and its MS fragmentation. This is the first example of cyclisation leading to a substituted pyran rather than 1,4-DHP under typical Hantzsch reaction conditions. A plausible mechanism for its formation is postulated.

Full text of DOI:10.3390/12112546

Molecules 2007, 12, 2546-2558
molecules
ISSN 1420-3049
© 2007 by MDPI
www.mdpi.org/molecules
Full Paper
Hantzsch Synthesis of 2,6-Dimethyl-3,5-dimethoxycarbonyl-4-
(o-methoxyphenyl)-1,4-dihydropyridine; a Novel Cyclisation
Leading to an Unusual Formation of 1-Amino-2-methoxy-
carbonyl-3,5-bis(o-methoxyphenyl)-4-oxa-cyclohexan-1-ene
Mirela Filipan-Litvić 1, Mladen Litvić 1,*, Ivica Cepanec 1 and Vladimir Vinković 2
1 BELUPO Pharmaceuticals, Inc. R&D, Danica 5, 48000 Koprivnica, Croatia;
E-mail: mirela.filipan-litvic@belupo.hr; E-mail: ivica.cepanec@belupo.hr
2 Institute Ruđer Bošković, Bijenička c. 54, 10002 Zagreb, Croatia; E-mail: vvink@irb.hr
* Author to whom correspondence should be addressed. E-mail: mladen.litvic@belupo.hr; Tel: (+385)
48 652 458; Fax: (+385) 48 652 461.
Received: 9 November 2007; in revised form: 22 November 2007 / Accepted: 23 November 2007 /
Published: 26 November 2007
Abstract: Hantzsch condensation of two equivalents of methyl-3-aminocrotonate with (m-
and p)-methoxybenzaldehyde afforded the expected products 2,6-dimethyl-3,5-
dimethoxycarbonyl-4-(m-methoxyphenyl)-1,4-dihydropyridine and 2,6-dimethyl-3,5-
dimethoxycarbonyl-4-(p-methoxyphenyl)-1,4-dihydropyridine,
whereas
o-methoxy-
benzaldehyde produced mainly 1-amino-2-methoxycarbonyl-3,5-bis(o-methoxy-phenyl)-4-
oxa-cyclohexan-1-ene. The structure of the product, not previously reported in the
literature, was determined by 1D and 2D NMR spectra and its MS fragmentation. This is
the first example of cyclisation leading to a substituted pyran rather than 1,4-DHP under
typical Hantzsch reaction conditions. A plausible mechanism for its formation is
postulated.
Keywords: Hantzsch condensation, intramolecular Michael addition, deprotonation,
substituted pyrane.

Molecules 2007, 12
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Introduction
The first synthesis of a 1,4-dihydropyridines (1,4-DHPs) via a three component cyclocondensation
reaction of acetoacetic ester, aldehyde and ammonia was reported by Arthur Hantzsch in 1882 [1].
Since then a lot of new variants of original method have been developed, allowing synthesis of
different substituted 1,4-DHPs (Scheme 1) [2].
1,4-DHPs have now been recognised as vital drugs in treatment of angina and hypertension. Some
of the representatives (nifedipine [3], felodipine [4], nicardipine [5] and amlodipine [6], Figure 1) have
been commercialised and it has been proven that their pharmaceutical action is related to binding to
voltage dependent L-type of calcium channel and thus decreasing the passage of calcium ions to the
cell. The result is relaxation of smooth muscle cells and lowering the blood pressure. Recently, the
other mechanism on molecular level have been found and is based on increasing NO release from the
intact endothelium [7]. As a result, newly synthesized generations of 1,4-DHPs possess different
pharmacological activities such as: anticancer [8], bronchodilating [9], antidiabetic [10], neurotropic
[11], antianginal [12] and other pharmacological activities [13].
Figure 1.
Cl
NO2
Cl
MeO2C
H3C
CO2Me
MeO2C
H3C
CO2Et
N
H
CH3
N
H
CH3
NIFEDIPINE
FELODIPINE
NO2
Cl
MeO2C
H3C
CO2
MeO2C
H3C
CO2Et
Me
N
O
CH2Ph
N
H
CH3
N
H
NH2
AMLODIPINE
NICARDIPINE
As it can be seen from the Scheme 1, methods A and C are only convenient for the synthesis of
symmetrical, nonchiral 1,4-DHPs (R=R') [14-16], while methods B (two step synthesis via
Knoevenagel intermediate IV) and D (three component cyclocondensation) are usually used for the
synthesis of 1,4-DHPs having different ester moieties (R≠R’) [17-20].

Molecules 2007, 12
Scheme 1. Most common variants of the Hantzsch 1,4-DHP synthesis.
2548
X
CO2R
CH3
R'O2C
H3C
CO2R
Knoevenagel
condensation
X
+
2 equiv.
+
O
NH2
O
CH3
CHO
II
V
III
IV
X
NH3
B
A
RO2C
H3C
CO2R'
CH3
N
D
C
H
I
CO2R
CH3
R'O2C
H3C
R'O2C
X
X
+
+
+
2 equiv.
O
NH2
H3C
NH2
CHO
CHO
III
III
V
II
V
As one might predict, symmetrical 1,4-DHPs are always formed as impurities in synthesis of
nonsymmetrical 1,4-DHPs. However, formation of impurities other than intermediates leading to the
1,4-DHP ring is rarely described in Hantzsch condensations [21]. Thus, Angeles et. al. [22] have
isolated four different compounds 1-4 (Figure 2) from the Hantzsch condensation of 2-nitro-
benzaldehyde, NH4OH and acetoacetic acid ethyl ester. The formation of 1,2-dihydropyridine 2, cyclic
amide 3 and substituted hydroxamic acid 4 is possibly due to steric hindrance as well as the oxidative
ability of the nitro group in the ortho- position. Similar observations have not yet been published for
other substituents (OCH3, Cl, CH3 etc.).
Figure 2.
OH
NO2
NO2
H
NH
N
EtO2C
H3C
CO2Et EtO2C
EtO2C
H3C
EtO2C
H3C
N
O
O
N
H
1
CH3
H3C
CH3
N
CH3
N
CH3
CO2Et
4
3
2
Görlitzer et. al. [23] have studied the dimerisation of o-nitrobenzylideneacetic acid esters and
isolated two diastereomeric substituted hexenes which could also be present as impurities in the
classical Hantzsch condensation.
As part of a continuing effort in our laboratory toward the development of new methods for the
synthesis and aromatization of 1,4-DHPs [24], we required gram scale samples of some substituded

Molecules 2007, 12
2549
1,4-DHPs having a OCH3 group in the ortho-, meta- and para- position on the aromatic ring. Taking
into account a wide variety of methods for 1,4-DHP synthesis, as well as our previous experience in
Hantzsch 1,4-DHP synthesis [25], we decided to use method C (Scheme 1) as most convenient for
synthesis when employing sterically demanded ortho-substituted benzaldehydes as starting material.
Results and Discussion
The Hantzsch condensation of all three (o-, m- and p)-methoxybenzaldehyde isomers 5-7 with two
equivalents of methyl-3-aminocrotonate (8) was carried out at reflux temperature in isopropanol as
solvent (Scheme 2). According to our previous experience with the chemistry of 1,4-DHPs the latter
solvent was proven to be better than other alcohols (such as ethanol, methanol, etc.) due to the easier
crystallisation of the product from the cold reaction mixture. With respect to other methods, method C
(Scheme 1) has been rarely mentioned in the modern literature as the method of choice for synthesis of
simple 1,4-DHPs although it has some advantages over the other methods such as the amount of water
formed in the reaction (one mol compared to three moles in method A), which is a key factor in
Hantzsch 1,4-DHP syntheses employing sterically demanding benzaldehydes [25]. The reactions were
monitored by TLC (9:1 CH2Cl2/AcOEt as eluent) until the disappearance of the starting benzaldehydes
and methyl-3-aminocrotonate (8). In the cases of the (m- and p)-methoxybenzaldehydes 6, 7 the
reactions were completed after 23 and 17 h respectively (Table 1). As expected the reaction with the
sterically demanding o-methoxybenzaldehyde (5) was much slower and was complete after 84 h. To
our surprise, a quite intensive methyl-3-aminocrotonate (8) spot of was still present at the end of
reaction but not traces of 5 were noted.
Scheme 2.
MeO
CO2CH3
O
NH2
CO2Me
OMe
i-PrOH, Δ
R
+
9
H2N
CH3
CHO
R
5-7
8
MeO2C
H3C
CO2Me
CH3
N
H
10, 11
The all reaction mixtures were cooled to room temperature to obtain crystalline products which
were collected by filtration. The products thus obtained were analysed by TLC, IR, MS and NMR (1H
13
and C) spectroscopy to determine their structures. The corresponding m- and p-methoxy derivatives
10 and 11 were prepared in 28.8 % and 15.3 % yields, respectively.

Molecules 2007, 12
Table 1. The results of Hantzsch condensation of (o-, m- and p)-methoxybenzaldehydes
2550
5-7 with methyl-3-aminocrotonate (8).
Reaction
time[h]a
Yields after
Entry
R
Product
mp
Rf (TLC)
crystallisation [%]
1
2
3
o-MeOC6H4
m-MeOC6H4
p-MeOC6H4
9
84
23
17
25.6
28.8
15.3
170.0 - 171.5
168.0 - 170.5
172.5 - 175.0
0.11b
0.39c
0.47c
10
11
a Determined by TLC; b CH2Cl2/MeOH, 9:1 as eluent; c CH2Cl2/AcOEt, 9:1 as eluent.
Although the majority of the products were soluble in the mother liquors, the purities of crystals
were very high (>98 %), which was sufficient for further testing. To our surprise the product 9
obtained with o-methoxybenzaldehyde (5) had much higher polarity (Table 1) than was expected [Rf
(CH2Cl2/MeOH, 9:1) = 0.11], compared to similar 1,4-DHP derivatives [Rf (CH2Cl2/MeOH, 9:1) >
0.90], which suggested formation of a product of different structure than 1,4-DHP. By careful
1
13
1
assignment of the H- and C-NMR spectra the structure 9 was elucidated. The H-NMR spectrum
(aromatic region) showed incorporation of two molecues of o-methoxybenzaldehyde (5) and one
molecule of methyl-3-aminocrotonate (8). An interesting observation was made in the aliphatic region
for CDCl3 and DMSO-d6 solutions. Two diastereotopic protons (at position 5) are clearly seen only in
DMSO-d6 solution, suggesting a rigid structure 9B (Figure 3) with an intramolecular hydrogen bond.
Additional proof are the two different shifts of the NH protons. Contrary to 9B, the flexible structure
9A is present in CDCl3 solution, where the NH2 group gives a broad signal at 2.2 ppm and the
diastereotopic protons appear as one signal at 2.5 ppm. To our surprise, from NOESY spectra it was
clearly proved that the two aromatic rings occupy the syn- position. However, it cannot be claimed that
only the syn-conformer is exclusively formed in the reaction. Rather, it is more likely that the anti-
conformer is more soluble in the reaction mixture in comparison to the syn- one and hence the
crystalline product is exclusively the syn-conformer.
Figure 3.
MeO
H
MeO
H
OMe
O
H
O
H
O
CO2CH3
NH2
H
N
H H
OMe
H
OMe
9B
9A
According to MS spectra the high intensity of the molecular ion (m/z = 369, 79 %) indicates the
stability of structure 9. Plausible fragmentation pathways, which are consistent with the spectral data,
together with the structures which are suggested for fragment ions, are shown in Scheme 3.

Molecules 2007, 12
2551
After formation of the molecular ion, fragment ion 13 (m/z 234, 100 %) is formed by unexpected
elimination of o-methoxybenzaldimine (12). This process starts with intramolecular addition of the
amino group to the more reactive benzylic position in the molecule, followed by elimination of 12.
Further fragmentation involves the elimination of one molecule of methyl-2-butynoate (14) and the
formation of 16, a radical cation of o-methoxybenzaldehyde (16) at m/z 136 (19 %). The second
pathway involves one step elimination of a substituted cyclobutane molecule 15 to form the radical
cation 16.
Scheme 3.
+
OMe NH
H
-
MeO
+
CO2CH3
NH2
OMe OH
CO2Me
O
12
OMe
9 M 369
13 m/z 234
OMe
CO2Me
-
-
Me
CO2Me
15
NH2
14
+
OMe O
H
16 m/z 136
Additional proof for the structure 9 was obtained by performing the reaction with two equivalents
of o-methoxybenzaldehyde (5) and one equivalent of freshly prepared methyl-3-aminocrotonate (8),
(Scheme 4).
Scheme 4.
CO2Me
CH3
CO2Me
CH3
NH4OOCNH2
MeOH, rt
H2N
O
17
8
MeO
CO2Me
CH3
CO2CH3
NH2
i-PrOH, Δ
+
2 equiv.
O
OMe
H2N
CHO
OMe
9
8
5

Molecules 2007, 12
2552
The latter compound was prepared by the method developed in our laboratory, via the reaction of
methyl acetoacetate (17) and ammonium carbamate in methanol at room temperature [26]. The crude 8
of high purity (>99%) was then subjected to reaction with 5 for the same time as original reaction (84
h). After crystallisation from reaction mixture the product 9 was obtained in good yield (51 %). These
findings clearly indicate that two molecules of o-methoxybenzaldehyde (5) were incorporated in the
molecule. Additionally, the purity of methyl-3-aminocrotonate (8) also plays an important role in the
overall yield of the product.
As mentioned above, o-methoxy-substituted-1,4-DHP 18 is proven to be formed in the reaction
according to Scheme 2, but only as a mixture with unreacted methyl-3-aminocrotonate (8) and other
impurities (by TLC analysis). A chromatographic separation, due to the small differences in the Rf
values, was not convenient for its isolation from the mother liquor and therefore we decided to
synthesize it employing different Hantzsch conditions (Scheme 5).
Scheme 5.
OMe
MeO2C
H3C
CO2Me
CO2Me
CH3
NH4OAc
+
OMe
i-PrOH, Δ
N
CH3
O
CHO
H
18
17
5
Thus, two equivalents of methyl acetoacetate (12), ammonium acetate and 2-methoxybenzaldehyde
(5) were subjected to reaction in refluxing isopropanol during 22 h to give the expected 1,4-DHP 18 in
37.8 % yield after crystalization from methanol. It is interested to note that even a trace of impurity 9
was observed in this reaction. It seems that its formation is characteristic only for Hantzsch method C,
Scheme 1. To prove this hypothesis we look at the mechanism of its formation according to reaction
condition in Scheme 2. The plausible mechanism is presented in Scheme 6.
The mechanism involves initial nucleophilic attack on o-methoxybenzaldehyde (5) by methyl-3-
aminocrotonate (8) to form the ionic intermediate 19, which upon proton tranfer gives the
iminoalcohol 20. In the next step dehydration leads to formation of the imino-Knoevenagel
intermediate 21. These types of compounds have been described by Kim et. al. [27] as metastable
intermediates in the aza-Diels-Alder condensation, a key step in the synthesis of the antyhypertensive
drug amlodipine. As one could predict, the next step would be Michael addition of a second molecule
of methyl-3-amino-crotonate (8) to 5, according to the proven and accepted mechanism for the
Hantzsch condensation [28]. However, due to steric hindrance by the influence of methoxy group in
the ortho- position, as well as its electron donating ability, the carbon-carbon double bond is poorly
reactive towards Michael addition. Similarly, anthracene-9-carbaldehyde is easily converted to
Knoevenagel intermediate 25 employing Hantzsch reaction conditions, but at this stage the reaction is
stopped although sufficient amounts of acetoacetate ester and ammonia are present in the reaction
medium [29]. For this reason, 9-anthryl-1,4-DHPs have not been prepared so far in Hantzsch
condensations.

Molecules 2007, 12
2553
Scheme 6.
MeO
MeO
HO
proton
transfer
CO2Me
CH3
MeO
CO2Me
CH3
CO2Me
CH3
-O
H
H
O
H
+
H2N
H2N
HN
5
19
20
8
MeO
MeO
MeO
+
CO2Me
NH
:NH3
- NH4
- H2O
CO2Me
NH
CO2Me
NH
:
:
O
MeO
H2C
H
H2C
O
H
OMe
23
21
5
22
MeO
MeO
H+
CO2Me
NH2
CO2Me
NH
O
O
OMe
OMe
9
24
According to TLC analysis of the reaction mixture the formation of 1,4-dihydropyridine 18 is also
observed, suggesting that intermediate 21 (Figure 4) is not as completely unreactive as the 9-anthryl-
analogue 25. However, the presence of higly basic ammonia in the reaction mixture (refluxing i-PrOH)
causes deprotonation of the methyl group rather than Michael addition of the second molecule of
methyl-3-aminocrotonate (8). Thus formed carbanion 21, in which the negative charge is delocalised
and stabilised by interacting with the double bonds and electronegative nitrogen and oxygen atoms in
the molecule is highly nucleophillic and reacts with a second molecule of o-methoxybenzaldehyde (5)
to yield intermediate 23, which upon fast intramolecular Michael addition and protonation gives the
final product 9.
Figure 4.
MeO
COOMe
COOEt
H3C
NH
H3C
O
21
25

Molecules 2007, 12
2554
In summary, the main reasons that determined formation of 9 rather than the expected 1,4-DHP 18
are: a) the steric hindrance and electron donating ability of the o-methoxy group; b) the presence of
basic ammonia formed in reaction; c) the slow bimolecular Michael addition of methyl-3-amino-
crotonate (8) to the imino-Knoevenagel intermediate 21.
Conclusions
We believe that this type of side product formation is characteristic of Hantzsch condensations
employing two equivalents of alkyl-3-aminocrotonates and substituted benzaldehydes, but the amounts
formed may vary drastically, depending on the nature of the substituents present on the aromatic ring.
The preliminary experiments with benzaldehydes having Cl, CH3, and OEt on the ortho- position have
shown formation of traces of impurities with similar polarities as 9 (TLC), but the amounts were not
sufficient for their isolation by crystallisation. Further attempts to isolate analytical samples for
structure determination are now in progress and the results will be published elsewhere in due course.
Experimental
General
1
IR Spectra were recorded on a Perkin-Elmer 297 instrument. H- and 13C-NMR spectra were
1
recorded using a Bruker 600 spectrometer in the solvents indicated (referenced to the residual H
signals in the deuterated solvents) using TMS as internal standard. Chemical shifts are reported in ppm
(δ scale), coupling constant (J) values are given in Hertz (Hz). Electron impact (EI) mass spectra were
recorded using a LC-MS (2795, Waters Micromass Q-ToF) instrument operated at 3.0 kV ionization
energy. Elemental analyses were done by the Central Analytical Service (CAS) at the Ruđer Bošković
Institute, Zagreb. Melting points were measured on a Büchi Melting Point B 540 apparatus and are
uncorrected. Thin layer chromatography (TLC) analyses were performed on Merck aluminium plates
coated with silica gel (60 F254). Compounds were visualised by ultra-violet irradiation at 254 and 366
nm.
General procedure for the Hantzsch condensation
To a solution of corresponding substituted benzaldehyde 5-7 (0.2 mol) in i-PrOH (50 mL) methyl-
3-aminocrotonate (8, 46.05 g, 0.4 mol) was added at once. The reaction mixture was heated at reflux
temperature for the time indicated in Table 1, and then cooled to rt. The obtained crystals of products
9-11 were filtered, washed with i-PrOH (3 x 20 mL) and dried in vacuum to constant weight.
1-Amino-2-methoxycarbonyl-3,5-bis(o-methoxyphenyl)-4-oxa-cyclohexan-1-ene (9). Pale yellow
crystals; yield: 9.47 g (25.6 %); Rf (CH2Cl2/MeOH, 9:1)=0.11; mp 170.0–171.5 °C; IR (KBr): ν=3530,
3399, 3298, 3214, 3075, 2959, 2944, 1658, 1622, 1602, 1585, 1542, 1488, 1459, 1441, 1417, 1364,
1341, 1310, 1293, 1263, 1242, 1186, 1162, 1124, 1099, 1071, 1053, 1026, 999, 968, 947, 929, 888,
1
862, 844, 812, 795, 783, 757, 641, 596, 572, 528 cm-1; H-NMR (CDCl3): δ=2.20 (br., s, 2H, NH2),

Molecules 2007, 12
2555
2.50 (d, 2H, CH2, J=6.8 Hz), 3.50 (s, 3H, OCH3), 3.59 (s, 3H, OCH3), 3.84 (s, 3H, COOCH3), 4.10 (t,
1H, CH, J=7.5 Hz), 5.50 (s, 1H, CH), 6.76 (d, 1H, arom., J=8.2 Hz), 6.85–6.92 (m, 2H, arom.), 6.96
13
(d, 1H, arom., J=7.4 Hz), 7.16–7.24 (m, 3H, arom.), 7.46 (dd, 1H, arom., J1=7.5, J2=1.5); C-NMR
(CDCl3): δ=36.42, 44.44, 50.53, 50.56, 55.13, 55.31, 91.89, 110.07, 110.24, 110.64, 119.48, 120.88,
125.93, 127.61, 128.01, 128.83, 131.78, 132.57, 156.70, 157.16, 157.64, 169.47; MS (3.0 kV, EI,): m/z
(%) for C21H23NO5: 369 (79), 234 (100), 136 (19); anal. calcd. for C21H23NO5: C 68.28, H 6.28, N
3.79, O 21.66; found: C 68.37, H 6.39, N 3.61.
2,6-Dimethyl-3,5-dimethoxycarbonyl–4-(m-methoxyphenyl)-1,4-dihydropyridine (10). Yellow crystals;
yield: 19.06 g (25.6 %); Rf (CH2Cl2/AcOEt, 9:1)=0.39; mp 168.0–170.5 °C; IR (KBr): ν=3348, 3241,
3091, 3029, 3004, 2953, 2837, 1704, 1651, 1626, 1606, 1580, 1486, 1449, 1434, 1380, 1346, 1317,
1296, 1263, 1219, 1181, 1157, 1134, 1120, 1099, 1047, 1016, 964, 949, 895, 871, 810, 775, 752, 735,
716, 682, 637, 584, 563 cm-1; 1H-NMR (CDCl3): δ=2.28 (s, 6H, CH3), 3.65 (s, 6H, COOCH3), 3.75 (s,
3H, OCH3), 5.00 (s, 1H, CH), 6.23 (s, 1H, NH), 6.68 (dd, 1H, arom., J1 = 8.1 Hz; J2 = 1.9 Hz), 6.82–
6.88 (m, 2H, arom.), 7.13 (t, 1H, arom., J = 7.9 Hz); 13C-NMR (CDCl3): δ=19.12, 38.98, 50.80, 54.84,
103.18, 110.65, 113.72, 119.91, 128.73, 144.54, 148.85, 159.13, 167.98; Anal. calcd. for C18H21NO5:
C 65.24, H 6.39, N 4.23, O 24.14; found: C 65.1, H 6.5, N 4.3.
2,6-Dimethyl-3,5-dimethoxycarbonyl–4-(p-methoxyphenyl)-1,4-dihydropyridine (11). Pale yellow
crystals; yield: 10.10 g (15.3 %); Rf (CH2Cl2/AcOEt, 9:1)=0.47; mp 172.5–175.0 °C; IR (KBr):
ν=3365, 3096, 3001, 2950, 2838, 1680, 1653, 1613, 1512, 1486, 1431, 1377, 1336, 1302, 1256, 1229,
1
1174, 1147, 1128, 1093, 1049, 942, 843, 817, 805, 787, 745, 665, 631, 604, 587, 546, 525 cm-1; H-
NMR (CDCl3): δ=2.33 (s, 6H, CH3), 3.65 (s, 6H, COOCH3), 3.75 (s, 3H, OCH3), 4.94 (s, 1H, CH),
13
5.67 (s, 1H, NH), 6.75 (d, 2H, arom.; J=8.5 Hz), 7.18 (d, 2H, arom., J=8.5 Hz); C-NMR (CDCl3):
δ=19.29, 38.24, 50.80, 54.95, 103.78, 113.21, 128.40, 139.81, 143.97, 157.76, 168.01; Anal. calcd. for
C18H21NO5: C 65.24, H 6.39, N 4.23, O 24.14; found: C 65.3, H 6.2, N 4.2.
Synthesis of 1-amino-2-methoxycarbonyl-3,5-bis(o-methoxyphenyl)-4-oxa-cyclohexan-1-ene (9). To a
solution of methyl acetoacetate (12, 5.40 mL, 5.81 g, 50 mmol) in methanol (50 mL) ammonium
carbamate (NH4OOCNH2, 3.90 g, 50 mmol) was added at once. The resulting suspension was stirred at
room temperature untill all solid material was dissolved (about 15 min). After that the solvent was
evaporated off to yield white crystals of crude 8 (yield quantitative) which was suspended in i-PrOH
(40 mL). To the resultant suspension o-methoxybenzaldehyde (5, 12.08 mL, 13.62 g, 100 mmol) were
added dropwise during 15 min and after that the reaction mixture was heated under reflux temperature
for 84 h with stirring. After cooling to room temperature the suspesion was cooled in refrigerator (-
18°C) for 12 h. The obtained crystals were filtered, washed with cold i-PrOH (2 x 10 mL) and dried in
vacuum to constant weight to give 9.42 g (51.0 %) of 9 as a yellow crystals. The analytical data were
in accordance with the sample obtained according to Scheme 2.
Synthesis of 2,6-dimethyl-3,5-dimethoxycarbonyl–4-(o-methoxyphenyl)-1,4-dihydropyridine (18). To a
suspension of ammonium acetate (0.05 mol, 3.85 g) in i-PrOH (50 mL), methyl acetoacetate (17, 0.1
mol; 11.61 g, 10.75 mL) and o-methoxybenzaldehyde (5, 0.05 mol, 6.81 g, 6.04 mL) were added

Molecules 2007, 12
2556
dropwise during 15 min. The resultant suspension was heated under reflux temperature for 22 h with
stirring. After cooling to room temperature reaction mixture was filtered and evaporated to dryness.
The residue was disolved in methanol (25 mL) and the resultant clear solution was cooled in
refrigerator (-18°C) for 12 h. The obtained crystals were filtered, washed with cold methanol (2 x 5
mL) and dried in vacuum to constant weight to give 6.27 g (37.8 %) of 18 as a yellow crystals, Rf
(CH2Cl2/AcOEt, 9:1)=0.46; mp 129.0–131.5 °C; IR (KBr): ν= 3337, 3068, 2993, 2945, 2837, 1706,
1683, 1651, 1584, 1489, 1464, 1433, 1381, 1353, 1335, 1310, 1288, 1212, 1143, 1128, 1096, 1051,
1028, 942, 857, 837, 823, 799, 786, 760, 687, 640, 591 cm-1; 1H-NMR (CDCl3): δ=2.24 (s, 6H, CH3),
3.61 (s, 6H, COOCH3), 3.78 (s, 3H, OCH3), 5.29 (s, 1H, CH), 6.28 (s, 1H, NH), 6.78–6.90 (m, 2H,
arom.), 7.07–7.16 (m, 2H, arom.); 13C-NMR (CDCl3): δ=18.78, 34.60, 50.63m, 55.29, 102.40, 110.71,
120.05, 127.09, 129.47, 134.98, 144.16, 156.69, 168.47; Anal. calcd. for C18H21NO5: C 65.24, H 6.39,
N 4.23, O 24.14; found: C 65.4, H 6.1, N 4.1.
Acknowledgements
The authors wish to express their gratitude to Belupo Pharmaceuticals Inc. for financial support of
this research.
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Sample Availability: Available from the authors.
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