Photochemistry of 1,4-dihydropyridine derivatives: Diradical formation, delocalization and trapping as a route to novel tricyclic and tetracyclic nitrogen heterocyclic ring systems

Article abstract of DOI:10.3390/molecules21070866

Irradiation of an acetonitrile solution of 4-aryl-3,5-dibenzoyl-1,4-dihydropyridine derivatives 1a-c and maleimides 2a-c using medium pressure Hg-arc lamp (λ > 290) nm afforded three different cycloadducts 4, 5, 6 in addition to the oxidation products 3.

Full text of DOI:10.3390/molecules21070866

molecules
Article
Photochemistry of 1,4-Dihydropyridine Derivatives:
Diradical Formation, Delocalization and Trapping as
a Route to Novel Tricyclic and Tetracyclic Nitrogen
Heterocyclic Ring Systems
Nader A. Al-Jalal *, Yehia A. Ibrahim, Nouria A. Al-Awadi, Maher R. Ibrahim and
Osama M. Sayed
Department of Chemistry, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait;
yehiaibrahim2002@yahoo.com (Y.A.I.); n.alawadi@ku.edu.kw (N.A.A.-A.); maherriyad@hotmail.com (M.R.I.);
dr.osama_qc@yahoo.com (O.M.S.)
* Correspondence: nader.aljalal@ku.edu.kw; Tel.: +965-2498-7079; Fax: +965-2481-6482
Academic Editors: Scott Reed and Marino Resendiz
Received: 2 June 2016; Accepted: 25 June 2016; Published: 30 June 2016
Abstract: Irradiation of an acetonitrile solution of 4-aryl-3,5-dibenzoyl-1,4-dihydropyridine
derivatives 1a
three different cycloadducts
that compounds 1a undergoes intermolecular cycloaddition reaction through three biradical
–
c
and maleimides 2a
–
c
using medium pressure Hg-arc lamp (λ > 290) nm afforded
4,
5,
6
in addition to the oxidation products
3. These results indicate
–
c
intermediates and behave photochemically different than those reported previously for the analogous
3,5-diacetyl and 3,5-dicarboxylic acid derivatives. The present work also offers simple access to
novel tricyclic and tetracyclic nitrogen heterocyclic ring systems of potential biological and synthetic
applications. The structure of the photoproducts was established spectroscopically and by single
crystal X-ray crystallography.
Keywords: photochemistry; 1,4-dihydropyridines; dihydropyrrolo[3’,4’4,5]cyclopenta[1,2-b]-pyridine;
4,11-diazatetracyclo[5,3,1,02,6,08,10]undecane; tetrahydropyrrolo[3’,4’:5,6]pyrano[4,3-b]-pyridine
1. Introduction
1,4-Dihydropyridines have historically played a very important role in the synthesis and
mechanistic organic chemistry. Derivatives of 1,4-dihydropyridine (DHP) are drugs belonging to
a class of pharmaceutical agents known as calcium channel blockers. They are inhibitors of calcium
ion penetration inside cells and weaken the contractility of the cardiac muscle. These compounds have
been shown to be very effective vasodilators and are useful in the treatment of hypertension, ischemic
heart disease, and other cardiovascular disorders [1–6]. Among the most important representative of
this group of drugs are nifedipine (NPDHP) and felodipine (CPDHP).
Most photolytic reactions of 1,4-dihydropyridine (DHP) take place with elimination of
molecular hydrogen followed by dimerization or aromatization products [7–9]. Moreover, Ponticelli
reported that [10 13] photocycloaddition of acrylonitrile to 1-benzyl-1,4-dihydropyridine dicarboxylate
–
derivatives afforded two diastereomeric azabicyclo[4.2.0]octane derivatives. Using Z or E-but-2-enenitrile
in this photocycloaddition reaction gave products retaining the original alkene stereochemistry, thus
proving the concerted nature of this [2 + 2] cycloaddition [13]. Also, 1,4-dihydropyridine dicarboxylate
derivatives have been reported to undergo head-to-tail [2 + 2] photodimerization to give anti- and
syn-dimers, and a cage dimer (formed by further intramolecular [2 + 2] photodimerization of the
syn-dimer) (Scheme 1) [14–17].
Molecules 2016, 21, 866; doi:10.3390/molecules21070866
www.mdpi.com/journal/molecules

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R
R
H
H
H
R = CO2Et
CONHR
+
CN
H
N
N
CN
CH2Ph
CH2Ph
Ar
R
hν
λ = 365 nm
CN
H
N
R
Ar
Ar
R = CO2Et
H
hν
hν
λ = 254 nm
N
H
R
R
R
R
R
R
λ > 280 nm
Ar
N
N
Ar
R
H
R'
R
N
R = Alkyl, Aryl
N
R
R
H
R
R
Ar
R'
Ar
N
Ar
R'
R
N
R
Scheme 1. Reported photochemistry of 1,4-dihydropyridines.
In this work, we describe the photochemical behavior of 4-aryl-3,5-dibenzoyl-1,4-dihydropyridine
derivatives 1a–c and attempts to trap the possible intermediate diradicals with maleimides 2a–c.
2. Results and Discussion
Several photoirradiation experiments were carried out with 1a and 2a in order to optimize the
reaction conditions, involving changes of solvent (CH2Cl2, toluene, CHCl3, and CH3CN), irradiation
time, type of lamp (low or medium pressure Hg-arc lamps), the molar ratio of the two reactants
(see Supplementary Materials). The optimum reaction conditions was achieved by using acetonitrile
as a solvent, 3 h irradiation time, a 400 W medium pressure mercury arc-lamp (λ > 290 nm), a 1:3 molar
ratio of 1,4-dihydropyridine 1a with maleimide 2a under a nitrogen atmosphere. These conditions
were then applied to all photoreactions and the results are summarized in Table 1. Thus, irradiation
of 1,4-dihydropyridines 1a
produced novel fused heterocyclic ring systems 4a
–
c
with maleimides 2a
–
c
–
using 400 W medium pressure mercury arc-lamp
5a and 6a, together with the photooxidation
h,
–e
products, the 3,5-dibenzoyl-4-aryl-pyridine derivatives 3a–c (Scheme 2, Table 1).
Table 1. Photoproducts from irradiation of 1,4-DHPs 1a–c with maleimides 2a–c.
Reactants
1a–c 2a–c
1a 2a
Photoproducts (Yield %)
Entry *
3a–c
4a–h
5a–e
6a
1
2
3
4
5
6
7
8
9
3a (48)
3b (45)
3c (44)
3a (41)
3b (41)
3c (39)
3a (37)
3b (39)
3c (42)
4a (23)
4b (28)
4c (29)
4d (32)
4e (27)
4f (34)
-
5a (15)
5b (12)
6a (13)
1b
1c
1a
1b
1c
1a
1b
1c
2a
2a
2b
2b
2b
2c
2c
2c
-
-
-
-
-
-
-
-
-
-
5c (13)
-
5d (23)
5e (10)
-
4g (29)
4h (27)
* Reaction conditions: a solution of 1a
–
c
(1 mmol) with 2a
–c
(3 mmol) was irradiated at
λ
> 290 nm in acetonitrile
(100 mL) for 3 h under N2.

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Scheme 2. Photoproducts upon irradiation of 1,4-dihydropyridines 1a–c with maleimides 2a–c.
The structure of all products has been established based on the 1H-NMR, 13C-NMR, HRMS and
the X-ray crystal structures of photoproducts 4d 5a
5b and 6a (Figure 1). The 1H-NMR spectra of
compounds 5a showed interesting small coupling constants (J 0-2) for the vicinal ring protons
,
,
–
e
˝
(C-4, C-5 or C-1, C-29 shown in the X-ray of 5a) consistent with a dihedral angle around 80 as found
by the X-ray structure of two of these compounds 5a,b.
(4d)
(5a)
Figure 1. Cont.

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(5b)
(6a)
Figure 1. X-ray crystal structures of (4d), (5a), (5b) and (6a).
The formation of the different photo-adducts 4a
by the formation of the diradical intermediates
excitation of the (n, *), undergoes a hydrogen shift from the
then undergoes cycloaddition with followed by dehydrogenation to give
resonate to diradicals and . Trapping of the diradical with 2a yields 6a, diradical
may also go through loss of H2 to give the pyridine derivatives 3a–c
–
h, 5a
–
e, and 6a can be satisfactorily interpreted
, which is formed from the initial
-carbon to form diradical , which
. Diradical can also
which
. Finally,
A
–
D
. Diradical
A
β
π
B
A
C
2
4
C
,
D
E
C
is in resonance with
trapping of diradical
system 5 (Scheme 3).
D
E
with the appropriate maleimide gives the corresponding interesting tetracyclic
Scheme 3. The diradical photoreaction mechanism.

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Furthermore irradiation of 1a alone in acetonitrile solution leads to the corresponding
oxidized pyridine derivative 3a. Under the same conditions irradiation of diethyl 4-phenyl-1,4
-dihydropyridine-3,5-dicarboxylate (
Figure 2. These facts showed different photochemical behavior for compounds 1a
7
) produced the cage compound
8
as shown in Scheme 4 and
than those
–
c
reported for the other dihydropyridine derivatives. The possibility of the formation of compound 6a
by [4 + 2] thermal cycloaddition reaction has also been excluded, when attempts to react 1a with 2a at
˝
temperature of 180 C for 1 h, gave only the unreacted starting material.
Scheme 4. Formation of cage structure 8.
Figure 2. X-ray crystal structure of cage compound 8.
3. Experimental Section
3.1. General Information
Melting points were recorded on a Gallenkamp apparatus (London, United Kingdom) The UV-Vis
absorption spectra were scanned by using a Cary 5 instrument (Agilent, Santa Clara, CA, USA) with
dry, clean quartz cuvettes of 1.0 cm path length. IR spectra were recorded in KBr disks on JASCO
1
FTIR 6300 spectrophotometer (JASCO, Easton, MD, USA). H-NMR (400 MHz or 600 MHz) and
13C-NMR (100 MHz or 150 MHz) spectra were recorded on a DPX 400 MHz NMR spectrometer
(Bruker, Karlsruhe, Germany). Mass spectra were measured on (GC-MS DFS) (high resolution, high
performance, tri-sector GC/MS/MS (Thermo, Bremen, Germany) and by LC-MS using LC-MS DFS
(Thermo). with an API-ES/APCI ionization mode. X-Ray single crystals data were performed using a
Rapid II (Rigaku, Tokyo, Japan) and X8 Prospector diffractometer (Bruker) An annular reactor model
APQ40 (Applied Photo-Physics Ltd., RG 49PA, England, UK) fitted with a 400 W (λ > 290 nm) medium

Molecules 2016, 21, 866
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pressure mercury arc-lamp was used for the irradiation. The starting 1,4-dihydropyridine derivatives
1a–c were synthesized as described recently [18].
3.2. General procedure for the photoreaction of 1,4-dihydropyridines 1a–c with maleimides 2a–c
A solution of each of 1a
in a Pyrex tube was purged with nitrogen for 20 min, and then irradiated under N2 atmosphere
for 3 h at room temperature using a 400 W ( > 290 nm) medium pressure mercury arc-lamp. The
–c (1 mmol) and the appropriate 2a–c (3 mmol) in acetonitrile (100 mL)
λ
reaction progress was monitored by TLC. The solvent was removed under reduced pressure and
the resulting residue was subject to column chromatography on silica gel (70–230 mesh) using ethyl
˝
acetate/petroleum ether (b.p. 60–80 C) as eluent to give the corresponding photoproducts.
(4-Phenylpyridine-3,5-diyl)bis(phenylmethanone) (3a) [18].
(4-(p-Tolyl)pyridine-3,5-diyl)bis(phenylmethanone) (3b) [18].
(4-(4-Chlorophenyl)pyridine-3,5-diyl)bis(phenylmethanone) (3c) [18].
3-Benzoyl-5-hydroxy-4,5-diphenyl-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]pyridine-6,8(5H,7H)-dione
(
4a): Colorless solid from ethyl acetate; mp. 266–268 ˝C; max (KBr)/cm´1 3258, 1723, 1692, 1583; δH
ν
(600 MHz, CDCl3) 3.6 (s, 1H), 3.9 (d, 1H, J 6), 4.7 (d, 1H, J 6), 6.8–7.5 (m, 15H), 8.1 (s, 1H), 8.8 (s, 1H); δC
(150 MHz, DMSO-d6) 36.4, 47.9, 49.1, 74.2, 110.8, 115.2, 126.5, 127.2, 128.3, 128.5, 128.6, 128.8, 128.9,
129.4, 130.1, 133.6, 140.9, 146.3, 147.8, 152.1, 175.7, 176.1, 190.5; MS m/z 460.3 (M+, 88%), 431.1 (100%),
105.0 (65%), 77.1 (75%); HR-MS (EI) m/z [M+] calcd for C29H20N2O4 460.1423, found 460.1416.
3-Benzoyl-5-hydroxy-7-methyl-4,5-diphenyl-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]pyridine-6,8
ν
(5H,7H)-dione (4b): Colorless solid from ethyl acetate; mp. 260–262 ˝C; max (KBr)/cm´1 3464, 1702,
1667, 1572; δH (600 MHz, CDCl3) 3.1 (s, 3H), 3.7 (s, 1H), 3.9 (d, 1H, J 8.4), 4.7 (d, 1H, J 8.4), 6.8–7.5 (m,
15H), 8.8 (s, 1H); δC (150 MHz, CDCl3) 25.6, 51.5, 56.5, 77.23, 83.4, 123.9, 126.9, 127.4, 127.9, 128.2, 128.3,
129.6, 132.9, 133.5, 136.9, 137.2, 138.3, 146.0, 148.5, 150.6, 157.1, 173.5, 175.4, 195.7; MS m/z 474.1 (M+,
6%), 365.2 (83%), 288.0 (100%); HR-MS (EI) m/z [M+] calcd for C30H22N2O4 474.1579, found 474.1574.
3-Benzoyl-5-hydroxy-4,5,7-triphenyl-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]pyridine-6,8(5H,7H)-dione
˝
(
4c): Colorless solid from ethyl acetate; mp. 168–170 C;
ν
(KBr)/cm´1 3062, 1717, 1669; δH
max
(600 MHz, CDCl3) 3.6 (s, 1H), 4.0 (d, 1H, J 8.4), 4.8 (d, 1H, J 8.4), 6.8–7.5 (m, 20H), 8.8 (s, 1H); δC
(100 MHz, CDCl3) 51.5, 56.7, 83.9, 124.0, 126.5, 127.0, 127.4, 127.9, 128.2, 128.3, 128.9, 129.2, 129.7, 131.2,
131.6, 133.0, 133.5, 136.9, 137.3, 138.4, 145.7, 148.4, 150.8, 157.4, 172.4, 174.3, 195.7; MS m/z 536.1 (M+,
10%); HR-MS (EI) m/z [M+] calcd for C35H24N2O4 536.1736, found 536.1730.
3-Benzoyl-5-hydroxy-5-phenyl-4-(p-tolyl)-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]pyridine-6,8(5H,
7H)dione (4d): Colorless solid from ethyl acetate; mp. 249–251 ˝C; max (KBr)/cm´1 3520, 1716, 1653;
ν
δH (400 MHz, DMSO-d6) 2.0 (s, 3H), 2.1 (s, 1H), 3.7 (d, 1H, J 6.2), 4.6 (d, 1H, J 6.2), 6.9–7.5 (m, 14H), 8.6
(s, 1H), 11.2 (s, 1H); δC (150 MHz, DMSO-d6) 20.4, 52.3, 60.5, 82.1, 93.9 125.8, 126.9, 128.3, 129.4, 130.6,
133.6, 135.1 135.5, 136.2, 138.6, 145.6, 146.5, 148.9, 158.9, 166.6, 174.7, 175.4, 186.6, 195.7; MS m/z 474.2
(M+, 100%), 105.0 (50%); HR-MS (EI) m/z [M+] calcd for C30H22N2O4 474.1579, found 474.1574.
3-Benzoyl-5-hydroxy-7-methyl-5-phenyl-4-(p-tolyl)-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]pyridine-
˝
´1
(KBr)/cm 3504, 1703,
max
6,8(5H,7H)-dione (4e): Colorless solid from ethyl acetate; mp. 236–238 C;
ν
1667; δH (400 MHz, CDCl3) 2.2 (s, 3H), 3.1 (s, 3H), 3.7 (s, 1H), 3.9 (d, 1H, J 8.4), 4.7 (d, 1H, J 8.4), 6.7–7.5
(m, 14H), 8.8 (s, 1H); δC (150 MHz, CDCl3) 21.1, 25.6, 51.4, 56.3, 83.4, 124.0, 127.4, 127.9, 128.2, 128.3,
128.7, 129.7, 130.1, 133.4, 136.9, 137.9, 138.4, 139.1, 146.3, 148.7, 150.6, 157.1, 173.57, 175.3, 195.7; MS

Molecules 2016, 21, 866
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m/z 488.4 (M+, 75%), 459.2 (100%), 105.0 (32%); HR-MS (EI) m/z [M+] calcd for C31H24N2O4 488.1736,
found 488.1731.
3-Benzoyl-5-hydroxy-5,7-diphenyl-4-(p-tolyl)-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]pyridine-6,8
˝
(5H,7H)-dione (4f): Pale yellow solid from ethyl acetate; mp. 188–190 C;
ν
(KBr)/cm´1 3474,
max
1776, 1667; δH (600 MHz, DMSO-d6) 2.0 (s, 3H), 3.3 (s, 1H), 3.9 (d, 1H, J 8.4), 4.9 (d, 1H, J 8.4), 6.6–7.5
(m, 19H), 8.7 (s, 1H); δC (150 MHz, DMSO-d6) 21.1, 51.9, 59.9, 83.2, 125.7, 126.5, 126.9, 127.5, 127.9,
128.8, 128.9, 129.3, 129.9, 130.3, 131.1, 133.3, 134.2, 135.6, 136.9, 137.17, 139.6, 145.6, 147.3, 149.6, 159.5,
173.1, 173.7, 196.2; MS m/z 550.5 (M+, 100%), 521.2 (80%), 105.0 (63%); HR-MS (EI) m/z [M+] calcd for
C36H26N2O4 550.1892, found 550.1888.
3-Benzoyl-4-(4-chlorophenyl)-5-hydroxy-7-methyl-5-phenyl-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]
pyridine-6,8(5H,7H)-dione (4g): Colorless solid from ethyl acetate; mp. 196–198 ˝C; max (KBr)/cm´1
ν
3383, 1712, 1688; δH (400 MHz, CDCl3) 3.1 (s, 3H), 3.8 (s, 1H), 3.9 (d, 1H, J 8.4), 4.7 (d, 1H, J 8.4), 6.8–7.5
(m, 14H), 8.8 (s, 1H); δC (100 MHz, CDCl3) 25.6, 51.5, 56.3, 78.6, 83.2, 123.9, 127.1, 127.5, 128.4, 129.6,
131.4, 133.7, 134.1, 135.2, 136.7, 137.0, 138.4, 145.7, 147.3, 150.7, 157.1, 173.4, 175.6, 195.4; MS m/z 508.1
(M+, 3%), 510.2 (M++2, 5%), 288.2 (100%), 105.0 (56%); HR-MS (EI) m/z [M+] calcd for C30H21ClN2O4
508.1189, found 508.1184.
3-Benzoyl-4-(4-chlorophenyl)-5-hydroxy-5,7-diphenyl-5a,8a-dihydropyrrolo[3’,4’:4,5]cyclopenta[1,2-b]pyridine
-6,8(5H,7H)-dione (4h): Colorless solid from ethyl acetate; mp. 178–180 ˝C;
ν
(KBr)/cm´1 3561,
max
1711, 1668; δH (400 MHz, DMSO-d6) 3.3 (s, 1H), 3.9 (d, 1H, J 8.8), 4.8 (d, 1H, J 8.8), 6.9–7.5 (m, 19H), 8.7
(s, 1H); δC (150 MHz, DMSO-d6) 52.0, 59.9, 83.1, 125.9, 126.8, 127.1, 127.7, 127.9, 128.9, 129.1, 129.4,
130.1, 131.4, 132.9, 133.3, 134.5, 136.6, 137.1, 137.6, 139.7, 145.5, 145.9, 150.0, 159.9, 173.1, 173.7, 196.0;
MS m/z 570.2 (M+, 93%), 572.2 (M+2, 36%), 542.2 (95%), 105.0 (100%); HR-MS (EI) m/z [M+] calcd for
C35H23ClN2O4 570.1346, found 570.1341.
8,10-Dibenzoyl-9-phenyl-4,11-diazatetracyclo[5,3,1,02,6,08,10]undecane-3,5-dione (5a): Colorless solid from
ethyl acetate; mp. 275–277 ˝C; max (KBr)/cm´1 3469, 3367, 1773, 1708; δH (600 MHz, CDCl3) 3.6 (s,
ν
2H), 3.7 (s, 1H), 4.7 (s, 2H), 7.3 (s, 1H), 7.4–7.9 (m, 15H), 7.9 (t, 1H); δC (150 MHz, DMSO-d6) 39.7, 48.8,
51.3, 62.5, 127.5, 128.1, 128.4, 128.8, 130.3, 132.9, 134.9, 138.6, 177.0, 198.1; MS m/z 462.20 (M+, 11%),
288.1 (66%), 105.0 (100%); HR-MS (EI) m/z [M+] calcd for C29H22N2O4 462.1579, found 462.1574.
8,10-Dibenzoyl-4-methyl-9-phenyl-4,11-diazatetracyclo[5,3,1,02,6,08,10]undecane-3,5-dione (5b): Colorless
solid from ethyl acetate; mp. 248–250 ˝C;
ν
(KBr)/cm´1 3411, 1753, 1718; δH (400 MHz, CDCl3)
max
2.8 (s, 3H), 3.5 (d, 2H, J 2.4), 3.7 (s, 1H), 4.5 (d, 2H, J 1.2), 7.2–7.6 (m, 15H), 7.9 (t, 1H); δC (150 MHz,
CDCl3) 23.9, 38.8, 46.6, 50.6, 61.6, 126.6, 127.2, 127.5, 127.9, 129.4, 131.9, 134.1, 137.8, 176.4, 197.2; MS
m/z 476.4 (M+, 2%), 288.2 (12%), 105.0 (100%); HR-MS (EI) m/z [M+] calcd for C30H24N2O4 476.1736,
found 476.1346.
8,10-Dibenzoyl-4-methyl-9-p-tolyl-4,11-diazatetracyclo[5,3,´1,102,6,08,10]undecane-3,5-dione (5c): Colorless
˝
solid from ethyl acetate; mp. 268–270 C;
ν
(KBr)/cm 3342, 1773, 1701; δH (400 MHz, CDCl3) 2.4
max
(s, 3H), 2.8 (s, 3H), 3.5 (s, 2H), 3.6 (s, 1H), 4.5 (s, 2H), 7.3–7.6 (m, 14H), 7.9 (t, 1H); δC (100 MHz, CDCl3)
21.1, 24.7, 39.6, 47.5, 51.4, 62.5, 127.3, 128.1, 128.8, 130.9, 131.6, 132.9, 138.5, 138.6, 177.4, 198.2; MS
m/z 490.5 (M+, 8%), 288.5 (28%), 105.0 (100%); HR-MS (EI) m/z [M+] calcd for C31H26N2O4 490.1892,
found 490.1888.
9-p-Chlorophenyl-8,10-dibenzoyl-4,11-diazatetracyclo[5,3,1,02,6,08,10]undecane-3,5-dione (5d): Colorless solid
from ethyl acetate; mp. 268–270 ˝C; max (KBr)/cm´1 3399, 1741, 1726; δH (600 MHz, CDCl3) 3.6 (s,
ν
2H), 3.6 (s, 1H), 4.6 (s, 2H), 7.3–7.6 (m, 15H), 7.9 (t, 1H); δC (150 MHz, CDCl3) 29.9, 39.0, 51.6, 62.6,

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128.2, 129.1, 129.1, 130.8, 133.2, 133.7, 134.7, 138.7, 176.9, 197.9; MS m/z 496.3 (M+,12%), 294.2, (84%),
105.0 (100%); HR-MS (EI) m/z [M+] calcd for C29H21ClN2O4 496.1189, found 496.1184.
9-p-Chlorophenyl-8,10-dibenzoyl-4-methyl-4,11-diazatetracyclo[5,3,1,02,6,08,10]undecane-3,5-dione (5e): Colorless
˝
´1
(KBr)/cm 3330, 1777, 1703; δH (600 MHz, CDCl3) 2.8
max
solid from ethyl acetate; mp. 296–298 C;
ν
(s, 3H), 3.5 (s, 2H), 3.6 (s, 1H), 4.5 (s, 2H), 7.3–7.6 (m, 14H), 7.9 (t, 1H); δC (150 MHz, CDCl3) 24.7, 38.8,
51.6, 62.4, 128.0, 128.8, 128.8, 130.5, 132.9, 133.5, 134.4, 138.6, 177.2, 197.7; MS m/z 510.2 (M+, 10%), 513.2
(M++2, 6.2%), 105.0 (100%); HR-MS (EI) m/z [M+] calcd for C30H23ClN2O4 510.1346, found 510.1340.
3-Benzoyl-4,5-diphenyl-1,6a,9a,9b-tetrahydropyrrolo[3’,4’:5,6]pyrano[4,3-b]pyridine-7,9(4H,8H)-dione (6a):
˝
Colorless solid from ethyl acetate; mp. 223–225 C;
ν
(KBr)/cm´1 3334, 1721; δH (600 MHz,
max
DMSO-d6) 3.9 (t, 1H, J 7.8 ), 4.8 (d, 1H, J 8.4), 4.9 (s, 1H), 5.2 (d, 1H, J 8.4), 7.17 (s, 1H), 7.2–7.4 (m,
15H), 7.4 (s, 1H), 11.7 (s, 1H); δC (150 MHz, DMSO-d6) 36.4, 47.9, 49.1, 74.3, 110.8, 115.2, 126.5, 127.2,
128.3, 128.5, 128.6, 128.7, 128.9, 129.4, 130.1, 133.9, 140.9, 146.3, 147.8, 152.1, 175.7, 176.1, 190.5; MS
m/z 462.1 (M+, 6%), 365.2 (85%), 288.0 (100%); HR-MS (EI) m/z [M+] calcd for C29H22N2O4 462.1579,
found 462.1574.
4. Conclusions
The present investigation presents a novel photochemical behavior of 4-aryl-3,5-dibenzoyl
-1,4-dihydropyridine derivatives 1a
at > 290 nm and afforded three different cycloadducts
compounds undergo intermolecular cycloaddition reactions through three biradical intermediates
–
c
with maleimides 2a
–
c
using medium pressure Hg-arc lamp
λ
4, 5, 6.
These results indicate that
1
and behave photochemically different than those reported previously for the analogous 3,5-diacetyl
and 3,5-dicarboxylic acid derivatives. The present work also offers interesting simple access
to novel tricyclic and tetracyclic nitrogen heterocyclic ring systems of potential biological and
synthetic applications.
Supplementary Materials: Crystallographic data of (excluding structure factors) for the structure in this paper
have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC
1475111 (5a), CCDC 1475112 (5b), CCDC 1475113 (6a), CCDC 1475114 (4d) and CCDC 1475115 (8). Copies
of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK, (fax: +44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk. Supplementary materials can be accessed at:
http://www.mdpi.com/1420-3049/21/7/866/s1.
Acknowledgments: The support of the University of Kuwait received through research grant # SC03/12 and the
facilities of ANALAB/SAF (grants no. GS01/01, GS02/01, GS03/08) are gratefully acknowledged.
Author Contributions: Nader A Aljalal, Nouria A AlAwadi and Yehia A Ibrahim conceived and designed
the experiments; Maher R Ibrahim and Osama M Sayed performed the experiments and analyzed the data;
Nader A Aljalal contributed reagents/materials/analysis tools; Nader A Aljalal an Yehia A Ibrahim wrote
the paper.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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Sample Availability: Samples of the compounds 1a–c, 3a–c, 5a–c and 4a are available from the authors.
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2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
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