Received date: 29/10/2012; Revised date: 03/02/2013; Accepted date: 09/02/2013
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C3- and O-alkylated 4-hydroxycoumarin derivatives synthesized from reusable solid superacid catalyst, namely, sulfated tin oxide (STO) in acetic acid under reflux conditions with good yields and well characterized by IR, 1H/13C NMR and mass spectrometry. The synthesized compounds were evaluated for anticoagulant activity. Among the compounds tested for anti-coagulant activity, compounds 3b, 3c, 3d and 3g showed significant activity compared to that of standard.
Sulfated tin oxide (STO), 4-hydroxy coumarin, secondary benzyl alcohol, secondary benzyl O-acetate, anti-coagulant activity.
Coumarin is a privileged scaffold among heterocycles and is known to possess a wide range of biological activities including antibiotic, anti-malarial, antifungal, anti-viral, and cytotoxic [1,2,3,4,5,6,7,8].
In particular, the 4-hydroxycoumarins and its derivatives (3-alkylated) have evoked a great deal of interest due to their utility as ‘anticoagulant rodenticides as well as antithrombotic agents’ such as warfarin, brodifacoum, difethialone, bromadiolone, coumatetralone, and flocoumafen  (Figure 1) and also as nonpeptide human immunodeficiency virus (HIV) protease inhibitors . The C3 or O-alkylation of 4-hydroxycoumarin is undoubtedly one of the most important and challenging reactions in synthetic chemistry due to its pharmaceutical utility as mentioned above and also can be diversified to synthesize 3,4-substitued compounds [11,12,13,14].
In continuation of our interest in developing novel synthetic methodologies, particularly carbon-carbon, carbon-heteroatom bond formations to synthesize pharmaceutically relevant heterocycles , we have very recently reported SO42-/SnO2-catalyzed C3-alkylation of 4-hydroxycoumarin with secondary benzyl alcohols and O-alkylation with O-acetyl compounds .
With the optimized reaction conditions in hand (10 mol% STO, AcOH, reflux), we then evaluated the scope of the benzylation of 4-hydroxycoumarin 1 using a variety of structurally divergent reactants (Scheme 1), When primary benzyl alcohols were used, they failed to give the expected product. These results clearly demonstrate that the direct C3-alkylation of 4-hydroxycoumarin was successful only with secondary benzylic alcohols. We obtained the corresponding C3-alkylated products in 70-81% yields, after 6 h (entries 1-3, Table 1). When nitro group containing benzylic alcohol was introduced instead of bromo- or methoxy, the reaction did not proceed at all.
After successful C3-alkylation of 4-hydroxycoumarin with secondary benzyl alcohols, we turned our attention to test the feasibility of O-alkylation using secondary benzyl acetates (prepared instantly for the purpose) reacting with 4-hydroxycoumarin under the above optimized conditions to generate novel compounds (entries 4-7, Table 1) moderate to good yields (72-85%) in the specified time. Enolic hydroxyl group is activated by Sn metal via Lewis acid catalysis to make the 3-position more nucleophilic. Where as, in case of O-alkylation, activation of carbonyl functionality of acetate making it as leaving group by Sn via Lewis acid catalysis and then the formed stabilized carbocation reacts with enolic hydroxide leaving AcOH as byproduct.
STO was prepared according to the literature report. All melting points were determined on an Electrothermal Gallenkamp apparatus. 1H and 13C NMR spectra were recorded on a Varian Gemini Spectrometer 300 MHz. IR spectra were recorded on Nicolet Fourier Transform spectrometer. Mass spectra were obtained on a 7070H or VG Autospec Mass spectrometer using LSIMS technique. Thin-layer chromatography (TLC) was performed on GF-25U (Anal. Tech) plates and silica gel glass-backed plates. Routine column chromatography was conducted using silica gel 100-200 mesh.
General experimental procedure for the C3-alkylation and O-alkylation of 4-hydroxycoumarins
To a mixture of 4-hydroxycoumarin (1, 1.0 mmol) and corresponding secondary benzyl alcohol/ secondary O-acetyl compound (2a-g, 1.1 mmol) in acetic acid (10 mL), STO (0.1 mmol) was added and the reaction mixture was stirred for the given time (see Table 1) at reflux temperature. After completion of the reaction (monitored by TLC), the reaction mixture was added into water. Adjusted to pH neutral with sodium carbonate and extracted in ethyl acetate. The organic phase was dried over anhydrous Na2SO4 and evaporated under vacuum. The residue was purified by silica gel column with petroleum ether/ethyl acetate (1:3) as eluent to afford the corresponding C3-alkylated/O-alkylated 4-hydroxycoumarin (3a-g).
Characterization Data for New Compounds
3-((E)-3-(4-chlorophenyl)-1-phenylallyl)-4-hydroxy-2H-chromen-2-one (3b): Pale yellow solid, mp: 168-171°C. IR (KBr): ν 3327, 1674, 1626, 1611, 1494, 1393, 1200, 756 cm-1. 1H NMR (300 MHz, CDCl3): δ 7.81-7.72 (m, 2H), 7.57-7.28 (m, 11H), 6.78-6.68 (m, 1H), 6.48 (d, J=16.4 Hz, 1H), 5.46 (d, J=6 Hz, 1H) ppm. 13C NMR (100 MHz, CDCl3): δ 162.9, 160.9, 152.5, 140.1, 136.2, 133.7, 132.2, 1130.0, 128.7, 128.2, 127.9, 127.6, 126.6, 124.1, 123.2, 116.5, 115.8, 106.5, 44.0 ppm. MS (ESI): m/z (rel. abund.%) 389 (M+, 100), 391 (M+, 30) ([M+1]+).
3-[(E)-1,3-diphenyl-2-propenyl]-4-hydroxy-2H-2-chromenone (3c). White solid. mp: 155-157 °C. IR (KBr): 3330, 1671, 1624, 1610, 1494, 1392, 1201, 754 cm-1. 1H NMR (CDCl3, 400 MHz): δ 5.50 (d, J=6.5 Hz, 1H), 6.55 (d, J=16.0 Hz, 1H), 6.81(dd, J=6.0, 16.0 Hz, 1H), 7.22-7.44 (m, 12H), 7.50-7.55 (m, 1H), 7.86 (dd, J=6.8, 8.0 Hz, 1H). 13C NMR (CDCl3, 100 MHz): δ 163.59, 161.26, 152.78, 140.00, 136.42, 133.83, 132.29, 129.29, 128.77, 128.43, 128.14, 128.11, 127.63, 126.67, 124.18, 123.33, 116.65, 116.09, 106.75, 44.14. MS (ESI): m/z (rel. abund.%) 355 (M+, 100).
4-(1-(4-methoxyphenyl) ethoxy)-2H-chromen-2-one (3d): Off white solid. mp: 180-184 °C. IR (KBr): 3342, 1673, 1628, 1514, 1249 cm-1. 1H NMR (300 MHz, CDCl3): δ 7.70 (dd, J = 8.8Hz, 1H), 7.48-7.52 (m, 2H), 7.41 (d, J =11.2 Hz, 1H), 7.26-7.22 (m, 2H), 6.99 (d, J =4 Hz, 2H), 6.04 (s, 1H), 4.65 (q, J =10 Hz, 1H), 3.79 (s, 3H), 1.60 (d, J = 9.6 Hz, 3H). ppm. 13C NMR (100 MHz, CDCl3): δ 163.6, 159.8, 159.1, 152.3, 133.1, 131.7, 128.5, 123.8, 122.6, 116.2, 116.1, 114.9, 110.0, 55.3, 33.6, 16.8 ppm. MS (ESI): m/z (rel. abund.%) 297.2 ([M+1]+ , 100).
4-((E)-1,3-diphenylallyloxy)-2H-chromen-2-one (3e): Pale yellow solid, mp: 132-136 °C. IR (KBr): ν 1678, 1626, 1613, 1501, 1394, 1203, 757 cm-1. 1H NMR (300 MHz, CDCl3): δ 7.78 (d, J =7.2 Hz, 1H), 7.55 (t, J =8 Hz, 1H), 7.24-7.64 (m, 12H), 6.94 (br, s 1H), 6.67 (dd, J =6.4, 9.6 Hz, 1H), 6.52 (d, J =16.4 Hz, 1H), 5.47 (d, J = 5.6 Hz, 1H) ppm. 13C NMR (100 MHz, CDCl3): δ 163.3, 161.5, 152.4, 139.7, 136.3, 133.9, 132.4, 129.2, 128.7, 128.2, 128.7, 127.7, 126.4, 124.4, 123.1, 116.5, 115.7, 106.4, 43.5 ppm. MS (ESI): m/z (rel. abund.%) 355.0 ([M+1]+ ,100).
4-(1-Phenylethoxy)-2H-chromen-2-one (3f). Off white solid; mp: 214-218 °C. IR (KBr): ν 1669, 1621, 1492, 1401, 1218, 1168, 741 cm-1. 1H NMR (300 MHz, CDCl3): δ 7.64 (d, J =12.4 Hz, 1H), 7.64-7.43 (m, 5H), 7.64-7.42 (m, 2H), 7.23 (dd, J =10.8 Hz, 1H), 5.99 (br s, 1H), 4.74 (q, J=9.6 Hz, 1H), 1.68 (d, J= 9.6 Hz, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ 163.8, 160.0, 152.6, 141.8, 132.1, 129.8, 127.7, 127.5, 123.9, 123.0, 116.3, 116.2, 110.3, 34.8, 16.8 ppm. MS (ESI): m/z (rel. abund.%) 267.3 ([M+1]+, 100).
4-(1, 2, 3, 4-tetrahydronaphthalen-4-yloxy)-2H-chromen-2-one (3g): Off white solid, mp: 178-180 °C. IR (KBr): 2938, 1674, 1628, 1389, 1214, 1148, 751 cm-1. 1H NMR (300 MHz, CDCl3): δ 7.65 (dd, J =12.8 Hz, 1H), 7.52 (t, 1H), 7.34-7.21 (m, 6H), 5.78 (s, 1H), 4.60 (t, J =10 Hz, 1H), 2.93 (t, J =8.8 Hz, 2H), 2.25-2.20 (m, 1H), 1.94-1.80 (m, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ 164.0, 160.3, 152.6, 138.1, 134.7, 132.0, 130.7, 129.4, 128.3, 127.8, 124.1, 123.3, 116.4, 116.1, 109.4, 36.5, 30.3, 29.8, 22.1 ppm. MS (ESI): m/z (rel. abund.%) 293 ([M+1]+ ,100).
Anticoagulant activity: Anticoagulant activity of the synthesized compounds (3a-3g) was assessed against sodium citrate (standard) in albino rats. The blood samples were collected from albino rats by puncturing carotid artery. Compounds and sodium citrate were added to the blood separately (5 ng/ml, 10 ng/ml, 100 ng/ml). The time required for the formation of the clot was measured, and the results of anticoagulant activity of are shown in Table 2.
Compounds 3b and 3d have shown highest anticoagulant activity when compared to the standard sodium citrate. Compounds 3c and 3g have shown almost similar activity compared to the standard. Almost all the compounds were found to be more potent than standard at 5 ng/ml of concentration.
C3- and O-alkylated 4-hydroxycoumarin derivatives were synthesized using sulfated tin oxide and screened for anti-coagulant activity. From the observation of the results of anticoagulant activity, it was concluded that, compound 3b with -Cl group and 3d with –OCH3 group have shown potent activity when compared to all the other compounds including standard sodium citrate. Compounds 3c and 3g have shown almost similar activity compared to the standard.
Dr. Ravi Varala thanks Prof. Rajendra Sahu (Director, AP-IIIT Basar) and Prof. R. V. Raja Kumar (Vice Chancellor, RGUKT) for their encouragement. Vijay Kotra thanks Prof. K.V Rao (Vice chancellor, ANU) for his encouragement.