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Molecules 2016, 21(7), 866; https://doi.org/10.3390/molecules21070866

Article
Photochemistry of 1,4-Dihydropyridine Derivatives: Diradical Formation, Delocalization and Trapping as a Route to Novel Tricyclic and Tetracyclic Nitrogen Heterocyclic Ring Systems
Department of Chemistry, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
*
Author to whom correspondence should be addressed.
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 1ac and maleimides 2ac using medium pressure Hg-arc lamp (λ > 290) nm afforded three different cycloadducts 4, 5, 6 in addition to the oxidation products 3. These results indicate that compounds 1ac undergoes intermolecular cycloaddition reaction through three biradical 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 [1,2,3,4,5,6]. 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 [7,8,9]. Moreover, Ponticelli reported that [10,11,12,13] 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) [14,15,16,17].
In this work, we describe the photochemical behavior of 4-aryl-3,5-dibenzoyl-1,4-dihydropyridine derivatives 1ac and attempts to trap the possible intermediate diradicals with maleimides 2ac.

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 1ac with maleimides 2ac using 400 W medium pressure mercury arc-lamp produced novel fused heterocyclic ring systems 4ah, 5ae and 6a, together with the photooxidation products, the 3,5-dibenzoyl-4-aryl-pyridine derivatives 3ac (Scheme 2, Table 1).
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 5ae showed interesting small coupling constants (J 0-2) for the vicinal ring protons (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.
The formation of the different photo-adducts 4ah, 5ae, and 6a can be satisfactorily interpreted by the formation of the diradical intermediates AD. Diradical A, which is formed from the initial excitation of the (n, π*), undergoes a hydrogen shift from the β-carbon to form diradical B, which then undergoes cycloaddition with 2 followed by dehydrogenation to give 4. Diradical A can also resonate to diradicals C, D and E. Trapping of the diradical C with 2a yields 6a, diradical C which is in resonance with D may also go through loss of H2 to give the pyridine derivatives 3ac. Finally, trapping of diradical E with the appropriate maleimide gives the corresponding interesting tetracyclic system 5 (Scheme 3).
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 (7) produced the cage compound 8 as shown in Scheme 4 and Figure 2. These facts showed different photochemical behavior for compounds 1ac than those 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.

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 FTIR 6300 spectrophotometer (JASCO, Easton, MD, USA). 1H-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 pressure mercury arc-lamp was used for the irradiation. The starting 1,4-dihydropyridine derivatives 1ac were synthesized as described recently [18].

3.2. General procedure for the photoreaction of 1,4-dihydropyridines 1ac with maleimides 2ac

A solution of each of 1ac (1 mmol) and the appropriate 2ac (3 mmol) in acetonitrile (100 mL) 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 (λ > 290 nm) medium pressure mercury arc-lamp. The 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; νmax (KBr)/cm−1 3258, 1723, 1692, 1583; δH (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(5H,7H)-dione (4b): Colorless solid from ethyl acetate; mp. 260–262 °C; νmax (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; νmax (KBr)/cm−1 3062, 1717, 1669; δH (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; νmax (KBr)/cm−1 3520, 1716, 1653; 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-6,8(5H,7H)-dione (4e): Colorless solid from ethyl acetate; mp. 236–238 °C; νmax (KBr)/cm−1 3504, 1703, 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 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; νmax (KBr)/cm−1 3474, 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; νmax (KBr)/cm−1 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; νmax (KBr)/cm−1 3561, 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; νmax (KBr)/cm−1 3469, 3367, 1773, 1708; δH (600 MHz, CDCl3) 3.6 (s, 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; νmax (KBr)/cm−1 3411, 1753, 1718; δH (400 MHz, CDCl3) 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[5,3,1,02,6,08,10]undecane-3,5-dione (5c): Colorless solid from ethyl acetate; mp. 268–270 °C; νmax (KBr)/cm−1 3342, 1773, 1701; δH (400 MHz, CDCl3) 2.4 (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; νmax (KBr)/cm−1 3399, 1741, 1726; δH (600 MHz, CDCl3) 3.6 (s, 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, 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 solid from ethyl acetate; mp. 296–298 °C; νmax (KBr)/cm−1 3330, 1777, 1703; δH (600 MHz, CDCl3) 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 (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; νmax (KBr)/cm−1 3334, 1721; δH (600 MHz, 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 1ac with maleimides 2ac using medium pressure Hg-arc lamp at λ > 290 nm and afforded three different cycloadducts 4, 5, 6. These results indicate that compounds 1 undergo intermolecular cycloaddition reactions through three biradical 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 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: [email protected]. Supplementary materials can be accessed at: https://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 1ac, 3ac, 5ac and 4a are available from the authors.
Scheme 1. Reported photochemistry of 1,4-dihydropyridines.
Scheme 1. Reported photochemistry of 1,4-dihydropyridines.
Molecules 21 00866 sch001
Scheme 2. Photoproducts upon irradiation of 1,4-dihydropyridines 1a–c with maleimides 2a–c.
Scheme 2. Photoproducts upon irradiation of 1,4-dihydropyridines 1a–c with maleimides 2a–c.
Molecules 21 00866 sch002
Figure 1. X-ray crystal structures of (4d), (5a), (5b) and (6a).
Figure 1. X-ray crystal structures of (4d), (5a), (5b) and (6a).
Molecules 21 00866 g001aMolecules 21 00866 g001b
Scheme 3. The diradical photoreaction mechanism.
Scheme 3. The diradical photoreaction mechanism.
Molecules 21 00866 sch003
Scheme 4. Formation of cage structure 8.
Scheme 4. Formation of cage structure 8.
Molecules 21 00866 sch004
Figure 2. X-ray crystal structure of cage compound 8.
Figure 2. X-ray crystal structure of cage compound 8.
Molecules 21 00866 g002
Table 1. Photoproducts from irradiation of 1,4-DHPs 1ac with maleimides 2ac.
Table 1. Photoproducts from irradiation of 1,4-DHPs 1ac with maleimides 2ac.
Entry *ReactantsPhotoproducts (Yield %)
1a–c2a–c3a–c4a–h5a–e6a
11a2a3a (48)4a (23)5a (15)6a (13)
21b2a3b (45)4b (28)5b (12)-
31c2a3c (44)4c (29)--
41a2b3a (41)4d (32)--
51b2b3b (41)4e (27)5c (13)-
61c2b3c (39)4f (34)--
71a2c3a (37)-5d (23)-
81b2c3b (39)4g (29)5e (10)-
91c2c3c (42)4h (27)--
* Reaction conditions: a solution of 1ac (1 mmol) with 2ac (3 mmol) was irradiated at λ > 290 nm in acetonitrile (100 mL) for 3 h under N2.
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