3.1. Chemistry
The starting material 4-hydroxy-6-methylcoumarin (
1b) was synthesised according to the method reported by Shah and colleagues [
42]. All other reagents and solvents were obtained from commercial suppliers and used without further purification. Melting points (MP; °C, uncorrected) were recorded using a melting point apparatus (Electrothermal, Staffordshire, UK). The infrared (IR) spectra (expressed in wave number υ [cm
−1]) were recorded by the SHIMADZU FT/IR S1 Plus Spectrometer (Shimadzu Europa GmbH, Duisburg, Germany) using potassium bromide (KBr) discs. The Agilent Technologies 600 Ultra Shield NMR Spectrometer (Santa Clara, CA, USA) was utilised to obtain NMR spectra at 600 and 154 MHz for
1H and
13C, respectively. Chemical shifts are expressed in δ (ppm); coupling constants are reported in Hz. CDCl
3 and DMSO-
d6 were used as solvents. The splitting patterns were designated as: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet), m (multiplet), br (broad), and br s (broad singlet). The Bruker Ascend 700 NMR Spectrometer (Bruker, Fällanden, Switzerland) was utilised to obtain NMR spectra of compounds
2a,
4a,
5d, and
9c,
d,f at 700.17 and 176.08 MHz for
1H and
13C, respectively. The
13C-NMR spectrum of compound
5c was obtained at 176.08 MHz. The
1H and
13C-NMR spectra for three compounds in each scheme were presented in the
Supplementary Materials including one intermediate and two target compounds (
2a,
4a,
5c;
6b,
8e,
9f;
Figures S1–S16). These data include two dimensional NMR spectra of
4a and
8e. The Shimadzu GC-MS-QP 2010 instrument (Shimadzu Europa GmbH, Duisburg, Germany) was used to record the electron impact mass spectra (EI-MS) at 70 eV. Elemental analyses (C, H, and N) were carried out at the Microanalytical Center of the Faculty of Science of Cairo University, Cairo, Egypt. They aligned with the proposed structures within ± 0.1–0.3% of the theoretical values. The microwave reaction was conducted using a microwave reactor (Biotage Initiator+, EXP EU, 400 W, 2450 MHz; Biotage, Uppsala, Sweden). The reactions were monitored by thin-layer chromatography (TLC) using silica gel-precoated aluminium sheets (60 F254, Merck, Kenilworth, NJ, USA) and visualised with UV at 365 and 254 nm.
3.1.1. 4-Hydroxy-6-methylcoumarin (1b)
This compound was synthesised following the reported procedure [
42]. Yield: 75.67%. MP: 259–261 °C. EI-MS,
m/z (Rel. Int.%): 176 (M
+, 40.7), 147.
3.1.2. 2-Amino-9-methyl-5-oxo-4-aryl-4H,5H-pyrano[3,2-c]chromene-3-carbonitriles (2a–b)
These compounds were synthesised according to the reported procedure [
22]. A mixture of arylaldehyde namely,
p-nitrobenzaldehyde or furfuraldehyde, (1mmol), 4-hydroxy-6-methylcoumarin (0.176 g 1mmol), malononitrile (0.066 g, 1mmol) and KHP (0.2 g) in distilled water (5 mL) was heated at 50
oC for 5–6 h. After the completion of the reaction, the reaction mixture was cooled to room temperature and the solid was filtered off, washed with cold distilled water, dried, and recrystallised with CHCL
3/ethanol to give compounds
2a,b.
(±)-2-Amino-9-methyl-4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (2a): Yield: 98%; yellow solid. MP: 259–261 °C. IR (υmax, cm−1): 3460, 3346 (NH2), 2204 (CN), 1716 (C=O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.44 (s, 3H, Ar-CH3), 4.6 (s, 1H, CH-4), 7.3 (d, J = 8.4, 1H, Ar-H), 7.55–7.59 (m, 5H, Ar-H and NH2), 7.73 (s, 1H, Ar-H-10), 8.2 (d, J = 9, 2H, Ar-H-3′, 5′). 13C-NMR (DMSO-d6, 154 MHz): δ 20.9 (CH3), 37.2 (C-4), 57.1 (C-3), 103.1 (C-4’a), 113.05, 116.9, 119.3, 122.6, 124.2, 129.6, 134.4, 134.6, 147.0, 150.9, 151.3, 154.3, 158.5, 160.1 (Ar-C, CN and C=O). EI-MS, m/z (%): 377 (M + 2, 0.13), 73, consistent with the molecular formula C20H13N3O5 (375.33).
(±)-2-Amino-4-(furan-2-yl)-9-methyl-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (2b): Yield: 93.7%; grey solid. MP: 251–253 °C. IR (υmax, cm−1): 3392, 3323 (NH2), 2198 (CN), 1701 (C=O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.38 (s, 3H, Ar-CH3), 4.57 (s, 1H, CH-4), 6.2 (d, J = 2.4, 1H, Ar-H), 6.3 (d, J = 3.2, 1H, Ar-H), 7.3 (d, J = 8.4, 1H, Ar-H), 7.44 (s, 2H, NH2), 7.49 (m, 2H, Ar-H), 7.6 (s,1H, Ar-H, CH-10). 13C-NMR (DMSO-d6, 154 MHz): δ 20.95 (CH3), 37.3 (C-4), 55.6 (C-3), 102.3 (C-4’a), 106.8, 110.6, 113.0, 116.8, 119.1, 122.3, 133.8, 134.4, 142.8, 150.8, 152.9, 153.9, 158.7, 160.3 (Ar-C, CN and C=O). EI-MS, m/z (%): 321 (M + 1, 0.15), 256, consistent with the molecular formula C18H12N2O4 (320.3).
3.1.3. 2-Amino-9-methyl-5-oxo-4-aryl-4H,5H-pyrano[3,2-c]chromene-3-carboxamides (3a,b)
A mixture of 2a or 2b (0.01 mol) and concentrated H2SO4 (15 mL) was kept at room temperature (25 ± 2 °C) with continuous stirring for 10–14 h and then poured on cold water. The solid was filtered, washed with distilled water, dried, and crystallised with the indicated solvent.
(±)-2-Amino-9-methyl-4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carboxamide (3a): Crystallisation solvent: ethanol (EtOH)/CHCl3. Yield: 92.9%; yellow solid. MP: 142–144 °C. IR (υmax, cm−1): 3446, 3408 (NH2), 1734 (C=O ester), 1676 (C=O amide). 1H-NMR (DMSO-d6,, 600 MHz): δ 2.34 (s, 3H, CH3), 4.86 (s, 1H, CH-4), 7.0 (s, 2H, NH2), 7.1 (d, J = 9, 2H, Ar-H, CH-2′,6′), 7.3 (d, J = 8.4, 1H, Ar-H), 7.5 (s, 2H, NH2), 7.6 (d, J = 7.2, 1H, Ar-H), 7.7 (s, 1H, Ar-H, CH-10), 8.1 (d, J = 7.8, 2H, Ar-H, CH-3′,5′). 13C-NMR (DMSO-d6, 154 MHz): 20.9 (CH3), 40.4 (C-4) 54.1 (C-3), 103.4 (C-4’a), 113.0, 116.2, 123.32, 124.6, 129.0, 129.7, 133.2, 146.2, 150.1, 150.7, 151.5, 160.6, 162.4, 169.9 (Ar-C, CN and C=O). EI-MS, m/z (%): 393 (M+, 1.1), 204, consistent with the molecular formula C20H15N3O6 (393.36).
(±)-2-Amino-4-(furan-2-yl)-9-methyl-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carboxamide (3b): Crystallisation solvent: EtOH. Yield: 62.5%; brown solid. MP: 202–204 °C. IR (υmax, cm−1): 3446, 3425 (NH2), 1724 (C=O ester), 1685 (C=O amide). 1H-NMR (DMSO-d6,, 600 MHz): δ 2.22 (s, 3H, CH3), 4.83 (s, 1H, H-4), 6.49 (d, J = 2.4, 1H, Ar-H), 6.78 (d, J = 3.2, 1H, Ar-H), 7.04–7.51 (m, 7H, Ar-H and NH2), 7.79 (s, 1H, Ar-H-10). 13C-NMR (DMSO-d6, 154 MHz): 20.7 (CH3), 38.6 (C-4), 59.4 (C-3), 103. (C-4’a), 109.0, 115.6, 116.5, 121.5, 123.0, 123.4, 133.1, 134.1, 143.6, 151.7, 152.7, 162.7, 163.5, 166.5 (Ar-C, CN and C=O). EI-MS, m/z (%): 339 (M + 1, 0.1), 338 (M+, 0.1), 296 (25.82), 73, consistent with the molecular formula C18H14N2O5 (338.31).
3.1.4. N′-(3-cyano-9-methyl-5-oxo-4-aryl-4H,5H-pyrano[3,2-c]chromen-2-yl)-N,N-dimethyl formimidamides (4a,b)
A mixture of 2a or 2b (2.7 mmol) and DMF-DMA (25 mL) was heated at 100 °C for 15–40 min. The solid was obtained by filtration, washed with EtOH, dried, and then crystallised with CHCl3/EtOH.
(±)-N′-(3-cyano-9-methyl-4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromen-2-yl)-N,N-dimethylformimidamide (4a): Yield: 66.3%; yellow solid. MP: 279–281 °C. IR (υmax, cm−1): 2204 (CN), 1722 (C=O), 1664 (C=N). 1H-NMR (DMSO-d6, 600 MHz): δ 2.4 (s, 3H, Ar-CH3), 3.06 (s, 3H, N-CH3), 3.2 (s, 3H, N-CH3), 4.7 (s,1H, CH-4), 7.3 (d, J = 8.4, 1H, Ar-H), 7.5 (dd, J = 1.4 and J = 8.4, 1H, Ar-H), 7.6 (dd, J = 2.1 and J = 7, 2H, Ar-H), 7.9 (d, J = 1.4, 1H, Ar-H), 8.2 (dd, J = 2.1 and J = 7, 2H, Ar-H), 8.5 (s,1H, N=CH). 13C-NMR (DMSO-d6, 154 MHz): 20.8 (Ar-CH3), 34.9 (N-CH3), 38.7 (C-4), 41.2 (N-CH3), 73.0 (C-3), 102.6 (C-4’a), 113.3, 116.7, 119.2, 123.3, 124.2, 129.8, 134.5, 134.8, 147.1, 150.7, 150.9, 154.9, 155.3, 157.9, 160.3 (Ar-C, CN and C=O). EI-MS, m/z (%): 431 (M + 1, 0.1), 430 (M+, 0.14), 415 (5.77), 73, consistent with the molecular formula C23H18N4O5 (430.41).
(±)-N′-(3-cyano-4-(furan-2-yl)-9-methyl-5-oxo-4H,5H-pyrano[3,2-c]chromen-2-yl)-N,N-dimethylformimidamide (4b): Yield: 39%; white solid. MP: 273–275 °C. IR (υmax, cm−1): 2198 (CN), 1724 (C=O), 1654 (C=N). 1H-NMR (CDCl3, 600 MHz): δ 2.4 (s, 3H, Ar-CH3), 3.1 (s, 3H, N-CH3), 3.2 (s, 3H, N-CH3), 4.8 (s, 1H, CH-4), 6.31–6.36 (m, 2H, Ar-H), 7.23–7.26 (m, 2H, Ar-H), 7.37 (d, J = 8.4, 1H, Ar-H), 7.5 (s, 1H, Ar-H, CH-10), 8.2 (s, 1H, N=CH). 13C-NMR (CDCl3, 154 MHz): 21.0 (Ar-CH3), 32.6 (C-4), 35.0 (N-CH3), 41.2 (N-CH3), 72.9 (C-3), 101.4 (C-4’a), 107.6, 110.8, 113.4, 116.8, 118.7, 121.8, 133.6, 134.1, 142.2, 150.9, 152.7, 153.5, 154.8, 159.1, 160.5 (Ar-C, CN and C=O). EI-MS, m/z (%): 376 (M + 1, 1.9), 99, consistent with the molecular formula C21H17N3O4 (375.38).
3.1.5. 2-(Arylideneamino)-9-methyl-4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitriles (5a–f)
An aromatic aldehyde (benzaldehyde, 2,4-dichlorobenzaldehyde, or 2,3,4-trimethoxybenzaldehyde, 1 mmol) was added to a mixture of 2a or 2b (1mmol) in 1,4-dioxane and then irradiated in a Biotage Initiator+ microwave for 90–110 min (TLC monitoring). After solvent evaporation, the residue was washed with toluene to obtain pure product and dried.
(±)-2-(benzylideneamino)-9-methyl-4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (5a): Yield: 23.5%; buff solid. MP: 208–209 °C. IR (υmax, cm−1): 2198 (CN), 1701 (C=O), 1670 (C=N). 1H-NMR (DMSO-d6, 600 MHz): 2.39 (s, 3H, CH3), 4.65 (s, 1H, CH-4), 7.2–7.7 (m, 9H, Ar-H), 8.1–8.3 (m, 4H, Ar-H and N=CH). 13C-NMR (DMSO-d6, 154 MHz): 20.9 (CH3), 37.2 (C-4), 57.1 (C-3), 103.1 (C-4’a), 112.9, 116.8, 119.3, 122.6, 124.2, 128.0, 128.6, 129.0, 129.3, 129.5, 134.2, 134.4, 134.6, 147.0, 150.9, 151.30, 158.4, 160.1 (Ar-C, CN and C=O). EI-MS, m/z (%): 465 (M + 2, 3.7), 57, consistent with the molecular formula C27H17N3O5 (463.44).
(±)-2-(benzylideneamino)-4-(furan-2-yl)-9-methyl-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (5b): Yield: 5.8%; brown solid. MP: 250–252 °C. IR (υmax, cm−1): 2194 (CN), 1703 (C=O), 1670 (C=N). 1H-NMR (DMSO-d6, 600 MHz): 2.4 (s, 3H, CH3), 4.4 (s, 1H, CH-4), 7.2–7.5 (m, 11H, Ar-H), 7.7 (s, 1H, N=CH). 13C-NMR (DMSO-d6, 154 MHz): 20.9 (CH3), 37.3 (C-4), 58.4 (C-3), 104.3 (C-4’a), 107.7, 110.5, 113.0, 116.8, 119.7, 122.5, 127.6, 128.0, 128.6, 129.0, 129.3, 134.2, 134.5, 143.8, 150.7, 153.8, 158.4, 160.1 (Ar-C, CN and C=O). EI-MS, m/z (%): 410 (M + 2, 4), 409 (M + 1, 3), 408 (M+, 18), 56, consistent with the molecular formula C25H16N2O4 (408.41).
(±)-9-methyl-4-(4-nitrophenyl)-5-oxo-2-((3,4,5-trimethoxybenzylidene)amino)-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (5c): Yield: 30.5%; buff solid. MP: 221–223 °C. IR (υmax, cm−1): 2200 (CN), 1702 (C=O), 1672 (C=N). 1H-NMR (DMSO-d6, 600 MHz): 2.3 (s, 3H, CH3), 3.6 (s, 3H, OCH3), 3.9 (s, 6H, 2OCH3), 4.6 (s, 1H, CH-4), 6.4 (s, 1H, Ar-H), 6.5 (s, 1H, Ar-H), 7.2–7.7 (m, 5H, Ar-H), 8.0–8.1 (m, 3H, Ar-H and N=CH). 13C-NMR (DMSO-d6, 154 MHz): 20.8 (CH3), 37.2 (C-4), 56.2 (2OCH3, 3′, 5′-trimethoxyphenyl), 57.2 (C-3), 60.7 (OCH3), 102.9 (C-4’a), 107.1, 112.8, 116.8, 119.4, 122.7, 123.8, 124.2, 129.5, 131.1, 134.6, 136.8, 139.5, 147.0, 151.2, 153.2, 158.5, 160.4, 160.5 (Ar-C, CN and C=O). EI-MS, m/z (%): 555 (M + 2, 0.1), 554 (M + 1, 0.23), 204, consistent with the molecular formula C30H23N3O8 (553.52).
(±)-4-(furan-2-yl)-9-methyl-5-oxo-2-((3,4,5-trimethoxybenzylidene)amino)-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (5d): Yield: 17.5%; brown solid. MP: 248–250 °C. IR (υmax, cm−1): 2196 (CN), 1701 (C=O), 1664 (C=N). 1H-NMR (DMSO-d6, 600 MHz): δ 2.4 (s, 3H, CH3), 3.3 (s, 3H, OCH3), 3.6–3.7 (m, 6H, 2OCH3), 4.4 (s, 1H, CH-4), 6.42–6.49 (m, 4H, Ar-H), 7.2–7.7 (m, 4H, Ar-H), 9.4 (s, 1H, N=CH). 13C-NMR (DMSO-d6, 154 MHz): 20.9 (CH3), 37.6 (C-4), 56.3 (2OCH3, 3′, 5′-trimethoxyphenyl), 58.3 (C-3), 60.3 (OCH3), 104.0 (C-4’a), 105.2, 113.1, 116.8, 119.7, 122.6, 134.1, 134.4, 137.0, 139.5, 150.7, 153.2, 153.9, 158.4, 160.2, 160.3 (Ar-C, CN and C=O). EI-MS, m/z (%): 496 (M − 2, 1.18), 135, consistent with the molecular formula C28H22N2O7 (498.48).
(±)-2-((2,4-dichlorobenzylidene)amino)-9-methyl-4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (5e): Yield: 37.2%; buff solid. MP: 236–238 °C. IR (υmax, cm−1): 2198 (CN), 1718 (C=O), 1676 (C=N). 1H-NMR (DMSO-d6, 600 MHz): δ 2.4 (s, 3H, CH3), 4.6 (s, 1H, CH-4), 7.03–8.4 (m, 10H, Ar-H), 10.2 (s, 1H, N=CH). 13C-NMR (DMSO-d6, 154 MHz): 20.96 (CH3), 37.2 (C-4), 57.1 (C-3), 103.1 (C-4’a), 113.0, 116.9, 119.3, 122.6, 123.7, 124.2, 128.6, 129.1, 129.6, 129.7, 130.7, 131.1, 131.9, 134.4, 134.6, 147.0, 150.9, 151.3, 158.5, 160.1(Ar-C, CN and C=O). EI-MS, m/z (%): 533 (M + 1, 0.2), 532 (M+, 3.7), 531 (M − 1, 8.7), 530 (M − 2, 27.8), 291, consistent with the molecular formula C27H15Cl2N3O5 (532.33).
(±)-2-((2,4-dichlorobenzylidene)amino)-4-(furan-2-yl)-9-methyl-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (5f): Yield: 17.8%; brown solid. MP: >300 °C. IR (υmax, cm−1): 2220 (CN), 1718 (C=O), 1647 (C=N). 1H-NMR (DMSO-d6, 600 MHz): δ 2.38 (s, 3H, CH3), 4.4 (s, 1H, CH-4), 7.2–8.1 (m, 9H, Ar-H), 10.1 (s, 1H, N=CH). 13C-NMR (DMSO-d6, 154 MHz): 20.8 (CH3), 37.4 (C-4), 58.5 (C-3), 100.1 (C-4’a), 107.2, 113.5, 116.68, 117.1, 119.7, 122.2, 122.8, 125.6, 126.2, 128.6, 129.3, 130.9, 131.7, 133.7, 134.3, 148.2, 149.5, 152.2, 158.9, 160.3 (Ar-C, CN and C=O). EI-MS, m/z (%): 478 (M + 1, 1.12), 476 (M − 1, 2.72), 475 (M − 2, 6), 474 (M − 3, 8), 91, 73, consistent with the molecular formula C25H14Cl2N2O4 (477.3).
3.1.6. 6-substituted-4-hydroxyl-2-oxo-2H-chromene-3-sulfonyl chloride (6a,b)
Chlorosulfonic acid (13.5 mmol) was added dropwise to a solution of 1a or 1b (11.3 mmol) in dichloromethane (22.7 mL) cooled to 0 °C. The reaction mixture was continuously stirred at ambient temperature overnight. The precipitates were filtered, washed with dichloromethane and petroleum ether, and then dried.
6-Chloro-4-hydroxy-2-oxo-2H-chromene-3-sulfonyl chloride (6a): Yield: 97%; white solid. MP: 118–120 °C. IR (υmax, cm−1):3317 (br., OH), 1685 (C=O), 1375 (asymmetric, S-O), 1176 (symmetric, S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 7.3–7.6 (m, 2H, Ar-H), 7.7 (s, 1H, Ar-H-5), 14.0 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 108.54 (C-3), 116.7, 118.9, 123.8, 128.6, 133.7, 151.6, 156.9, 161.6 (Ar-C, CN and C=O). EI-MS, m/z (%): 299 (M + 4, 0.2), 298 (M + 3, 0.4), 297 (M + 2, 2.1), 296 (M + 1, 3.5), 295 (M+, 13.21), 294 (M − 1, 0.5), 293 (M − 2, 0.1), 207, consistent with the molecular formula C9H4Cl2O5S (295.1).
4-Hydroxy-6-methyl-2-oxo-2H-chromene-3-sulfonyl chloride (6b): Yield: 98%, white solid. MP: 116–117 °C. IR (υmax, cm−1): 3381 (br., OH), 1695 (C=O), 1328 (asymmetric, S-O), 1131 (symmetric, S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.34 (s, 3H, CH3), 7.2 (d, J = 7.8, 1H, Ar-H-7), 7.4 (d, J = 8.4, 1H, Ar-H-8), 7.6 (s, 1H, Ar-H-5), 13.9 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHZ): 20.7 (CH3), 107.9 (C-3), 114.9, 116.4, 124.2, 133.9, 134.8, 151.1, 157.5, 162.7(Ar-C, CN and C=O). EI-MS, m/z (%): 278 (M + 4, 0.07), 277 (M + 3, 0.52), 276 (M + 2, 2.72), 275 (M + 1, 0.23), 274 (M+, 0.05), 44, consistent with the molecular formula C10H7ClO5S (274.68).
3.1.7. 4-Hydroxy-6-(substituted) coumarin-3-sulfonamides (7a–f)
A mixture of 6a or 6b (0.002 mol), sulfa compounds (sulfanilamide, sulfadiazine, or sulfathiazole, 0.002 mol), and absolute EtoH (15 mL) was heated under reflux for 3–6 h (TLC monitoring). After cooling, the solid was filtered out, washed with absolute EtOH and petroleum ether, and then dried to obtain the pure products.
6-Chloro-4-hydroxy-2-oxo-N-(4-sulfamoylphenyl)-2H-chromene-3-sulfonamide (7a): Yield: 27%, white solid. MP: 245–247 °C. IR (υmax, cm−1):3346–3257 (NH2, NH), 3101 (br, OH), 1726 (C=O), 1338 (asymmetric S-O), 1165 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 5.62 (br. s, 2H, NH2), 6.9–7.0 (m, 3H, Ar-H-2’, 6’ and NH), 7.4 (s, 1H, Ar-H-5), 7.6–7.8 (m, 4H, Ar-H). 13C-NMR (DMSO-d6, 154 MHz): 108.5 (C-3), 116.8, 118.9, 119.0, 122.8, 123.8, 127.8, 128.6, 132.8, 133.7, 151.7, 157.0, 161.6 (Ar-C, CN and C=O) EI-MS, m/z (%): 433 (M + 3, 0.1), 432(M + 2, 0.22), 431 (M +1, 0.85), 430 (M+, 1.9), 73, consistent with the molecular formula C15H11ClN2O7S2 (430.84).
4-Hydroxy-6-methyl-2-oxo-N-(4-sulfamoylphenyl)-2H-chromene-3-sulfonamide (7b): Yield: 56%; white solid. MP: 241–243 °C. IR (υmax, cm−1): 3352, 3250 (NH2, NH), 3142 (br., OH), 1680 (C=O), 1355 (asymmetric S-O), 1157 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.36 (s, 3H, CH3), 2.49 (s, 1H, NH), 4.55 (br. s, 2H, NH2), 6.9 (d, J = 8.4, 2H, Ar-H-2’, 6’), 7.2 (d, J = 8.4, 1H, Ar-H-7), 7.4 (d, J = 7.8, 1H, Ar-H-8), 7.6 (m, 3H, Ar-H- 3’, 5’ and 5), 13.9 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 20.78 (CH3), 107.9 (C-3), 114.9, 116.5, 117.8, 124.2, 127.8, 133.9, 134.9, 151.1, 157.6, 162.7 (Ar-C, CN and C=O). EI-MS, m/z (%): 411 (M + 1, 0.51), 410 (M+, 2.62), 408 (M − 2, 12.76), 133, consistent with the molecular formula C16H14N2O7S2 (410.42).
6-Chloro-4-hydroxy-2-oxo-N-(4-(N-(pyrimidin-2-yl)sulfamoyl)phenyl)-2H-chromene-3-sulfonamide (7c): Yield: 78.2%; yellow solid. MP: 230–232 °C. IR (υmax, cm−1): 3373 (2NH), 3084 (br., OH), 1691 (C=O), 1346 (asymmetric S-O), 1151 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 4.7 (br. s, 1H, NH), 6.6 (d, J = 9.6, 2H, Ar-H-2’, 6’), 7.0 (m, 1H, diazine-H-4), 7.4–7.7 (m, 5H, Ar-H and NH), 7.8 (s, 1H, Ar-H-5), 8.4 (m, 2H, diazine-H-3, 5). 13C-NMR (DMSO-d6, 154 MHz): 108.6 (C-3), 113.7, 116.0, 116.8, 118.9, 123.8, 126.8, 128.6, 130.2, 133.6, 151.7, 156.9, 157.6, 158.7, 161.6 (Ar-C, CN and C=O). EI-MS, m/z (%): 511 (M + 3, 0.6), 315, consistent with the molecular formula C19H13ClN4O7S2 (508.9).
4-Hydroxy-6-methyl-2-oxo-N-(4-(N-(pyrimidin-2-yl)sulfamoyl)phenyl)-2H-chromene-3-sulfonamide (7d): Yield: 86.3%.; yellow solid. MP: 207–209 °C. IR (υmax, cm−1): 3620.39 (OH), 3423, 3356 (2NH), 1691 (C=O), 1334 (asymmetric S-O), 1166 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.39 (s, 3H, CH3), 6.6 (d, J = 9.6, 2H, Ar-H-2’, 6’), 7.0 (m, 1H, diazine-H-4), 7.2 (d, J = 10.8, 1H, Ar-H-8), 7.4 (d, J = 9.6, 1H, Ar-H-7), 7.64–7.67 (m, 3H, Ar-H-5, 3’, 5’), 8.4 (m, 2H, diazine-H-3,5). 13C-NMR (DMSO-d6, 154 MHz): 20.78 (CH3), 108.0 (C-3), 113.3, 114.9, 116.0, 116.5, 124.2, 126.2, 128.2, 128.4, 130.2, 133.9, 134.8, 151.1, 157.6, 158.7, 162.7 (Ar-C, CN and C=O) EI-MS, m/z (%): 490 (M + 2, 2.54), 489 (M + 1, 5.08), 355, consistent with the molecular formula C20H16N4O7S2 (488.4).
6-Chloro-4-hydroxy-2-oxo-N-(4-(N-(thiazol-2-yl)sulfamoyl)phenyl)-2H-chromene-3-sulfonamide (7e): Yield: 74.13%; white solid. MP: 215–217 °C. IR (υmax, cm−1): 3448 (2NH), 3140 (br., OH), 1676 (C=O), 1338 (asymmetric S-O), 1147 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 5.3 (br. s, 1H, NH), 5.4 (br. s, 1H, NH), 6.7 (d, J = 3.6, 1H, thiazole-H), 6.8 (d, J = 8.4, 2H, Ar-H-2’, 6’), 7.2 (d, J = 3.6, 1H, thiazole-H), 7.42 (d, J = 8.4, 1H, Ar-H-8), 7.58 (d, J = 9.6, 2H, Ar-H-3’ and 5’), 7.72 (d, J = 7.8, 1H, Ar-H-7), 7.8 (s, 1H, Ar, CH-5). 13C-NMR (DMSO-d6, 154 MHz): 108.3 (C-3), 108.6, 116.6, 116.8, 118.9, 123.8, 124.8, 128.1, 128.6, 132.8, 133.7, 147.1, 151.7, 157.0, 161.6, 168.7, (Ar-C, CN and C=O). EI-MS, m/z (%): 514 (M + 1, 0.2), 513 (M+, 1.2), 73, consistent with the molecular formula C18H12ClN3O7S3 (513.95).
4-Hydroxy-6-methyl-2-oxo-N-(4-(N-(thiazol-2-yl)sulfamoyl)phenyl)-2H-chromene-3-sulfonamide (7f): Yield: 84.7%; white solid. MP: 167–169 °C. IR (υmax, cm−1): 3421(OH) overlapped with 3338 (2NH), 1683 (C=O), 1350 (asymmetric S-O), 1153 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.37 (s, 3H, CH3), 6.7 (m, 3H, Ar, CH-2’, 6’ and thiazole-H), 7.1 (d, J = 3.6, 1H, thiazole-H), 7.2 (d, J = 8.4, 1H, Ar-H-8), 7.4 (d, J = 7.8, 1H, Ar-H-7), 7.5 (d, J = 8.4, 2H, Ar-H-3’, 5’), 7.6 (s, 1H, Ar-H-5), 13.9 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 20.78 (CH3), 107.9 (C-3), 108.1, 114.9, 115.3, 116.5, 124.2, 124.8, 128.1, 131.2, 133.9, 134.9, 145.9, 151.1, 157.5, 168.6, 162.7 (Ar-C, CN and C=O). EI-MS, m/z (%): 493 (M+, 0.1), 491(M − 2, 3.2), 490 (M − 3, 3.2), 73, consistent with the molecular formula C19H15N3O7S3 (493.53).
3.1.8. 4-Hydroxy-6-(substituted)coumarin-3-sulfonamides (8a–f)
A mixture of 6a or 6b (0.002 mol), 4-substituted-aniline (aniline, 4-hydroxyaniline, or 4-acetylaniline, 0.004 mol) and absolute EtOH (15 mL) was refluxed for 6–8 h. While warm, the solid was collected by filtration and then washed with hot absolute EtOH and petroleum ether, respectively. After drying, the solid was recrystallised if necessary.
6-Chloro-4-hydroxy-2-oxo-N-phenyl-2H-chromene-3-sulfonamide (8a): Yield: 92.65%. MP: 337–339 °C. IR (υmax, cm−1): 3367 (NH), 3169 (br., OH), 1683 (C=O), 1348 (asymmetric S-O), 1168 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 7.2–7.4 (m, 6H, Ar-H), 7.72 (d, J = 8.4, 1H, Ar-H), 7.8 (s, 1H, Ar-H-5). 13C-NMR (DMSO-d6, 154 MHz): 108.6 (C-3), 116.8, 117.6, 118.9, 122.7, 123.8, 127.5, 128.6, 130.2, 133.7, 151.7, 157.0, 161.6 (Ar-C, CN and C=O). EI-MS, m/z (%): 350 (M − 1, 1.18), 57, consistent with the molecular formula C15H10ClNO5S (351.76).
4-Hydroxy-6-methyl-2-oxo-N-phenyl-2H-chromene-3-sulfonamide (8b): Yield: 84.5%; white solid. MP: 324–325 °C. IR (υmax/cm−1): 3356 (NH), 3111 (br., OH), 1685 (C=O), 1354 (asymmetric S-O), 1159 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.37 (s, 3H, CH3), 7.3–7.6 (m, 8H, Ar-H), 9.6 (br. s, 1H, NH), 13.9 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 20.79 (CH3), 107.9 (C-3), 114.9, 116.5, 123.0, 124.2, 127.9, 130.2, 133.1, 133.9, 134.9, 151.1, 157.6, 162.7 (Ar-C, CN and C=O). EI-MS, m/z (%): 434 (M + 3, 1.65), 333 (M+2, 3.92), 332 (M+1, 26.25), 207, consistent with the molecular formula C16H13NO5S (331.34).
6-Chloro-4-hydroxy-N-(4-hydroxyphenyl)-2-oxo-2H-chromene-3-sulfonamide (8c): Crystallisation solvent: CHCl3. Yield: 48.3%; white solid. MP: 250–251 °C. IR (υmax, cm−1): 3278 (br., OH overlapped with NH), 1681 (C=O), 1346 (asymmetric S-O), 1168 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 6.8 (m, 2H, Ar-H-2’, 6’), 7.1 (m, 2H, Ar-H-3’, 5’), 7.4 (d, J = 8.4, 1H, Ar,CH-8), 7.7 (d, J = 7.8, 1H, Ar,CH-7), 7.8 (s, 1H, Ar,CH-5), 9.7 (s, 1H, NH), 10.0 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 108.5 (C-3), 116.5, 116.8, 118.9, 123.8, 124.2, 128.6, 133.7, 151.7, 157.0, 160.1, 161.7 (Ar-C, CN and C=O). EI-MS, m/z (%): 366 (M − 1, 0.1), 363 (M − 4, 28.19), 332 (5.86), 196, consistent with the molecular formula C15H10ClNO6S (367.76).
4-Hydroxy-N-(4-hydroxyphenyl)-6-methyl-2-oxo-2H-chromene-3-sulfonamide (8d): Crystallisation solvent: CHCl3. Yield: 63.13%; white solid. MP: 253–254 °C. IR (υmax, cm−1): 3298 (br., OH overlapped with NH), 1662 (C=O), 1354 (asymmetric S-O), 1166 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.38 (s, 3H, CH3), 6.8 (m, 2H, Ar, CH-2’, 6’), 7.1–7.2 (m, 3H, Ar, CH-3’, 5’ and 8), 7.4–7.6 (m, 2H, Ar, CH-7 and 5), 9.7 (br. s, 1H, NH). 13C-NMR (DMSO-d6, 154 MHz): 20.79 (CH3), 107.9 (C-3), 114.9, 116.5, 123.4, 124.2, 124.3, 133.9, 134.9, 151.1, 157.2, 157.6, 162.7, 173.7 (Ar-C, CN and C=O). EI-MS, m/z (%): 345 (M − 2, 0.04), 344 (M − 3, 0.48), 343 (M − 4, 3.31), 342 (M − 5, 9.7), 341 (M − 6, 52.35), 209, consistent with the molecular formula C16H13NO6S (347.34).
N-(4-acetylphenyl)-6-chloro-4-hydroxy-2-oxo-2H-chromene-3-sulfonamide (8e): Crystallisation solvent: CHCl3/isopropanol. Yield: 81.25%; white solid. MP: 238–240 °C. IR (υmax, cm−1): 3344 (NH), 3088 (br., OH), 1718 (C=O ketone), 1678 (C=O ester), 1357 (asymmetric S-O), 1166 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.38 (s, 3H, CH3C=O), 6.6 (d, J = 8.4 2H, Ar, CH-2’, 6’), 7.3 (d, J = 7.8, 1H, Ar,CH-8), 7.6 (m, 3H, Ar,CH-3’, 5’ and 7), 7.7 (s, 1H, Ar,CH-5), 13.9 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 26.4 (CH3), 108.5 (C-3), 115.0, 116.7, 118.8, 123.8, 127.4, 128.5, 130.8, 133.6, 150.6, 151.6, 156.9, 161.6 (Ar-C, CN and C=O). 195.8 (C=O ketone). EI-MS, m/z (%): 397 (M + 4, 0.13), 391 (M − 2, 0.13), 390 (M − 3, 0.83), 54, consistent with the molecular formula C17H12ClNO6S (393.8).
N-(4-acetylphenyl)-4-hydroxy-6-methyl-2-oxo-2H-chromene-3-sulfonamide (8f): Crystallisation solvent: CHCl3/isopropanol. Yield: 74.5%; white solid. MP: 231–232 °C. IR (υmax, cm−1): 3342 (NH), 3068 (br., OH), 1728 (C=O ketone), 1707 (C=O ester), 1359 (asymmetric S-O), 1163 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.37 (s, 3H, CH3), 2.38 (s, 3H, CH3C=O), 6.6 (d, J = 7.8, 2H, Ar,CH-2’, 6’), 7.2 (d, 1H, J = 9, Ar,CH-8), 7.4 (d, J = 8.4, 1H, Ar,CH-7), 7.66–7.69 (m, 3H, Ar,CH-5, 3’, 5’), 13.9 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 20.7 (CH3), 26.6 (CH3-C=O), 107.9 (C-3) 114.9, 116.4, 116.9, 124.2, 129.4, 130.7, 133.9, 134.9, 147.5, 151.1, 157.6, 162.77 (Ar-C, CN and C=O). 196.18 (C=O ketone). EI-MS, m/z (%): 377 (M + 4, 0.1), 476 (M + 3, 0.5), 375 (M + 2, 2.4), 374 (M + 1, 4), 373 (M+, 15.3), 73, consistent with the molecular formula C18H15NO6S (373.38).
3.1.9. Coumarin-Sulfonamide Chalcones (9a–f)
Equimolar quantities of 8e or 8f (0.01 mol) and an aromatic aldehyde (4-nitrobenzaldehyde, 4-chlorobenzaldehyde, or 4-methoxybenzaldehyde, 0.01 mol) were dissolved in a minimal amount of EtOH. Sodium hydroxide solution (2 mL, 0.02 M) was added slowly and stirred at ambient temperature for 24 h. The mixture was poured slowly into 400 mL of ice water with constant stirring and then refrigerated for 24 h. The formed precipitates were filtered, washed with distilled water, dried, and recrystallised with CHCl3 if necessary.
(E)-6-Chloro-4-hydroxy-N-(4-(3-(4-nitrophenyl)acryloyl)phenyl)-2-oxo-2H-chromene-3-sulfonamide (9a): Yield: 35.34%; orange solid. MP: 205–207 °C. IR (υmax, cm−1): 3487 (OH), 3388 (NH), 1703 (C=O ester), 1637 (C=O ketone), 1342 (asymmetric S-O), 1180 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 6.3–6.6 (m, 4H, Ar-H), 7.7 (d, J = 9, 1H, Ar-H), 7.9–8.3 (m, 9H, CHα=CHβ, Ar-H and NH). 13C-NMR (DMSO-d6, 154 MHz): 101.1 (C-3), 113.2, 115.9, 122.0, 124.4, 124.6, 125.4, 127.2, 130.0, 130.4, 130.5, 130.8, 131.9, 139.1, 142.34, 148.2, 151.20, 157.7, 161.9 (Ar-C, CN and C=O), 185.92 (C=O ketone). EI-MS, m/z (%): 527 (M + 1, 8), 526 (M+, 11), 373, consistent with the molecular formula C24H15ClN2O8S (526.9).
(E)-4-Hydroxy-6-methyl-N-(4-(3-(4-nitrophenyl)acryloyl)phenyl)-2-oxo-2H-chromene-3-sulfonamide (9b): Yield: 30%; orange solid. MP: 220–220 °C. IR (υmax, cm−1): 3487 (OH), 3388 (NH), 1707 (C=O ester), 1637 (C=O ketone), 1340 (asymmetric S-O), 1182 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.38 (s, 3H, CH3), 6.67–6.7 (m, 4H, Ar-H), 7.25 (s, 1H, Ar, CH-5), 7.4–8.3 (m, 9 H, CHα=CHβ, Ar-H, and NH), 13.9 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 20.82 (CH3), 107.9 (C-3) 114.9, 116.4, 116.8, 121.67, 123.5, 124.3, 128.4, 129.3, 130.7, 133.9, 134.9, 138.2, 140.7, 141.94, 145.9, 151.1, 157.6, 162.32 (Ar-C, CN and C=O), 186.10 (C=O ketone). EI-MS, m/z (%): 506 (M+, 0.24), 503 (M − 3, 1.61), 73, consistent with the molecular formula C25H18N2O8S (506.48).
(E)-6-Chloro-N-(4-(3-(4-chlorophenyl)acryloyl)phenyl)-4-hydroxy-2-oxo-2H-chromene-3-sulfonamide (9c): Yield: 22.32%; yellow solid. MP: 157–159 °C. IR (υmax, cm−1): 3460 (OH), 3340 (NH), 1703 (C=O ester), 1629 (C=O ketone), 1346 (asymmetric S-O), 1176 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): 5.6 (s, 1H, NH), 6.6 (m, 2H, Ar-H), 7.4 (d, J = 8.4, 1H, Ar-H), 7.51 (d, J = 8.4, 1H, Ar-H), 7.61 (d, J = 16.1, 1H, C=CHα), 7.7 (dd.,J = 2.5, J = 6.3, 2H, Ar-H), 7.8 (d, J = 8.4, 1H, Ar-H), 7.89–7.92 (m, 3H, Ar-H and C=CHβ), 7.95 (d, J = 8.4, 2H, Ar-H), 14.1 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 108.6, 113.4, 116.8, 118.9, 122.8, 123.6, 123.8, 128.6, 129.3, 130.6, 131.6, 132.8, 133.7, 134.6, 134.8, 140.4, 151.7, 156.9, 161.6 (Ar-C, CN and C=O). 186.1 (C=O ketone). EI-MS, m/z (%): 519 (M + 3, 0.3), 518 (M + 2, 0.6), 517 (M + 1, 1.6), 221, consistent with the molecular formula C24H15Cl2NO6S (516.35).
(E)-N-(4-(3-(4-chlorophenyl)acryloyl)phenyl)-4-hydroxy-6-methyl-2-oxo-2H-chromene-3-sulfonamide (9d): Yield: 22.41%; yellow solid. MP: 141–143 °C. IR (υmax, cm−1): 3462 (OH), 3342 (NH), 1701 (C=O ester), 1647 (C=O ketone), 1346 (asymmetric S-O), 1178 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 2.39 (s, 3H, CH3), 6.7 (d, J = 8.4, 2H, Ar-H), 7.2 (d, J = 8.4, 1H, Ar-H), 7.4–7.5 (m, 3H, Ar-H), 7.6 (d, J = 15.4, 1H, C=CHα), 7.67 (s, 1H, Ar-H), 7.8–7.9 (m, 3H, Ar-H and C=CHβ), 7.99 (d, J = 9.1, 2H, Ar-H), 14.0 (s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 20.80 (CH3), 108.03 (C-3), 114.5, 114.9, 116.5, 123.5, 124.3, 129.3, 129.8, 130.7, 131.5, 131.6, 133.9, 134.5, 134.8, 134.9, 140.7, 151.2, 157.5, 162.7 (Ar-C, CN and C=O). 186.44 (C=O ketone). EI-MS, m/z (%): 497 (M + 2, 1.1), 496 (M + 1, 10.4), 495 (M+, 60.46), 55, consistent with the molecular formula C25H18ClNO6S (495.93).
(E)-6-Chloro-4-hydroxy-N-(4-(3-(4-methoxyphenyl)acryloyl)phenyl)-2-oxo-2H-chromene-3-sulfonamide (9e): Yield: 30.23%; yellow solid. MP: >300 °C. IR (υmax, cm−1): 3468 (OH), 3329 (NH), 1695 (C=O ester), 1626 (C=O ketone), 1350 (asymmetric S-O), 1163 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): δ 3.8 (s, 3H, OCH3), 6.6 (d, J = 7.7, 2H, Ar-H), 6.9 (m, 2H, Ar-H), 7.09 (m, 2H, Ar-H), 7.35 (d, J = 8.4, 1H, Ar-H), 7.57 (d, J = 8.4, 1H, Ar-H), 7.7–7.9 (m, 3H, Ar-H, CHα=C and C=CHβ), 8.2 (d, J = 8.4, 2H, Ar-H), 8.5 (s, 1H, NH). 13C-NMR (DMSO-d6, 154 MHz): 55.7 (OCH3), 108.3 (C-3), 113.1, 114.7, 119.9, 120.3, 121.6, 125.9, 128.2, 129.1, 130.4, 130.7, 131.2, 131.4, 135.2, 144.1, 141.8, 161.3, 157.1, 162.0 (Ar-C, CN and C=O). 186.3 (C=O ketone). EI-MS, m/z (%): 513 (M + 2, 0.12), 73, consistent with the molecular formula C25H18ClNO7S (511.93).
(E)-4-Hydroxy-N-(4-(3-(4-methoxyphenyl)acryloyl)phenyl)-6-methyl-2-oxo-2H-chromene-3-sulfonamide (9f): Yield: 15.2%; yellow solid. MP: >300 °C. IR (υmax, cm−1):3468 (OH), 3331 (NH), 1701 (C=O ester), 1626 (C=O ketone), 1342 (asymmetric S-O), 1163 (symmetric S-O). 1H-NMR (DMSO-d6, 600 MHz): 2.39 (s, 3H, CH3), 3.81 (s, 3H, OCH3), 6.8 (d, J = 7.7, 2H, Ar-H), 7.01 (d, J = 7.7, 2H, Ar-H), 7.2 (d, J = 8.4, 1H, Ar-H), 7.5 (d, J = 8.4, 1H, Ar-H), 7.6 (d, J = 15.4, 1H, C=CHα), 7.67 (s, 1H, Ar-H-5), 7.7 (d, J = 15.4, 1H, C=CHβ), 7.8 (d, J = 7.7, 2H, Ar-H), 8.0 (d, J = 8.2, 2H, Ar-H), 14.04 (br. s, 1H, OH). 13C-NMR (DMSO-d6, 154 MHz): 20.79 (CH3), 55.8 (OCH3), 108.0 (C-3), 114.8, 114.9, 115.5, 116.5, 120.1, 124.3, 128.5, 130.8, 130.9, 131.2, 132.3, 133.9, 134.9, 142.5, 151.2, 157.6, 161.4, 162.7 (Ar-C, CN and C=O). 186.8 (C=O ketone). EI-MS, m/z (%): 491 (M+, 0.8), 490 (M − 1, 5.29), 78, consistent with the molecular formula C26H21NO7S (491.51).
3.1.10. X-Ray Crystallographic Study of Compound 2a
The Bruker-Nonius Apex X8 CCD Diffractometer was used to analyse the X-ray single-crystal structure. Yellow needles of compound 2a were obtained by crystallisation from EtOH by allowing the solvent to slowly evaporate. Crystallographic data for the structure of compound 2a have been deposited at the Cambridge Crystallographic Data Center (deposit CCDC 1985063).