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Synlett 2010(5): 741-744  
DOI: 10.1055/s-0029-1219399
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Brønsted Acidic Ionic Liquid as an Efficient and Reusable Catalyst for Synthesis of 14-Aryl- or 14-Alkyl-14H-dibenzo[a,j]xanthenes under Solvent-Free Conditions

Abdol R. Hajipour*a,b, Yosof Ghayebb, Nafisehsadat Sheikhanb, Arnold E. Ruohoa
a Department of Pharmacology, University of Wisconsin, Medical School, 1300 University Avenue, Madison 53706-1532, WI, USA
b Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan 84156, Iran
Fax: +98(311)3912350; e-Mail: haji@cc.iut.ac.ir;
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Publication History

Received 8 June 2009
Publication Date:
17 February 2010 (online)

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Abstract

A mild and efficient method has been developed for the preparation of 14-aryl- or 14-alkyl-14H-dibenzo[a,j]xanthenes from one-pot condensation of aldehydes with 2-naphthol using catalytic amount of Brønsted acidic ionic liquid ([TEBSA][HSO4]) under thermal solvent-free conditions. Excellent yields, short reaction times, easy workup and reusability of the catalyst as well as solvent-free conditions are advantages of this procedure.

Keywords

[TEBSA][HSO4] - Brønsted acidic ionic liquid - xanthenes - solvent-free - one-pot

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All reagents were purchased from Merck and Aldrich and used without further purification. All yields refer to isolated products after purification. [TEBSA][HSO4] was synthesized according to reported procedure.²9 Products were characterized by spectroscopy data (IR, ¹H NMR, ¹³C NMR spectra) and melting point. ¹H NMR (300, 400 and 500 MHz) and ¹³C NMR (75, 100 and 125 MHz) spectra were run in DMSO-d 6 and CDCl3 solvents relative to TMS (δ = 0.00 ppm). IR spectra were recorded on a Shimadzu 435 IR spectrophotometer and performed using KBr pellets. All melting points were taken on a Gallenkamp melting apparatus and are uncorrected.
Preparations of Brønsted Acidic Ionic Liquid { N -(4-Sulfonic Acid) Butyl Triethyl Ammonium Hydrogen Sulfate ([TEBSA][HSO 4 ])}: 1,4-Butane sultone (10 mmol, 1.0 mL), triethylamine (10 mmol, 1.4 mL) and MeCN (5 mL) were charged into a 100-mL round-bottom flask. Then, the mixture was refluxed for 10 h. The white solid zwitterion was filtered and washed with EtOAc to remove nonionic residues and dried in vacuum (2 g, 85% yield). Then, a stoichiometric amount of concentrated sulfuric acid (96%, 0.5 mL) was added dropwise to zwitterions and the mixture was stirred for 6 h at 80 ˚C to produce the Brønsted acidic ionic liquid. ¹H NMR (400 MHz, DMSO-d 6): δ = 1.16 (br s, 9 H), 1.59-1.73 (m, 4 H), 2.53 (t, J = 7.6 Hz, 2 H), 3.09-3.25 (m, 8 H), 6.46 (s, 2 H). ¹³C NMR (100 MHz, DMSO-d 6):
δ = 7.60, 20.27, 22.23, 50.67, 52.48, 56.15.
General Procedure for the Preparation of 14-Aryl- or 14-Alkyl-14 H -dibenzo[a,j]xanthenes: A mixture of aldehyde (1 mmol), 2-naphthol (2 mmol) and [TEBSA][HSO4] (0.15 mmol) was stirred at 120 ˚C in an oil bath. The completion of the reaction was monitored with TLC (EtOAc-cyclo-hexane, 1:3). After the appropriate time (as shown in Table  [²] ), the mixture was cooled to r.t. and H2O (10 mL) was added and the product was filtered and then recrystallized from EtOAc. The products were characterized by spectral data (IR, ¹H NMR and ¹³C NMR) and comparison of their physical data with the literature data. The spectral data of some synthesized compounds are given below.
Compound 3d: ¹H NMR (400 MHz, DMSO-d 6): δ = 7.03 (s, 1 H), 7.11-7.21 (m, 2 H), 7.39-7.47 (m, 4 H), 7.55 (t, J = 8.0 Hz, 2 H), 7.65-7.69 (m, 1 H), 7.95 (t, J = 10.0 Hz, 4 H), 8.48 (d, J = 8.0 Hz, 2 H). ¹³C NMR (125 MHz, CDCl3): δ = 36.79, 112.77, 118.16, 124.26, 126.33, 126.95, 128.81, 129.19, 130.23, 131.34, 132.79, 138.40, 150.31. IR (KBr): 3057, 1622, 1598, 1515, 1463, 1429, 1404, 1353, 1252 cm-¹. HRMS: m/z [M + H+] calcd for C27H16Cl2O: 427.0578; found: 426.8718.
Compound 3i: ¹H NMR (500 MHz, CDCl3): δ = 3.82 (s, 3 H), 6.56 (s, 1 H), 7.47 (dt, J = 0.9, 7.9 Hz, 2 H), 7.55 (d, J = 8.9 Hz, 2 H), 7.61-7.67 (m, 4 H), 7.83-7.89 (m, 6 H), 8.38 (d, J = 8.5 Hz, 2 H). ¹³C NMR (125 MHz, CDCl3): δ = 38.54, 52.36, 116.88, 118.45, 122.86, 124.82, 127.37, 128.74, 129.33, 129.65, 130.32, 131.48, 131.73, 149.12, 150.42, 167.05. IR (KBr): 3060, 2998, 2950, 1709, 1621, 1607, 1591, 1515, 1459, 1433, 1402, 1287, 1251, 1241, 1190, 1116 cm-¹. HRMS: m/z [M + H+] calcd for C29H20O3: 417.1412; found: 417.1561.
Compound 3j: ¹H NMR (300 MHz, CDCl3): δ = 3.59 (s, 3 H), 6.42 (s, 1 H), 6.48 (d, J = 8.2 Hz, 1 H), 7.04 (t, J = 8.4 Hz, 2 H), 7.13 (d, J = 7.5 Hz, 1 H), 7.37 (t, J = 7.2 Hz, 2 H), 7.44 (d, J = 9.0 Hz, 2 H), 7.55 (t, J = 7.6 Hz, 2 H), 7.71-7.82 (m, 4 H), 8.36 (d, J = 8.7 Hz, 2 H). ¹³C NMR (75 MHz, CDCl3): δ = 38.17, 55.23, 111.19, 115.16, 117.40, 118.23, 121.00, 122.94, 124.45, 127.00, 128.99, 129.07, 129.50, 131.27, 131.68, 145.01, 148.96, 159.97. IR (KBr): 3070, 3015, 2961, 2934, 2899, 1603, 1593, 1581, 1514, 1486, 1457, 1431, 1401, 1271, 1252, 1241 cm-¹.
Compound 3l: ¹H NMR (500 MHz, CDCl3): δ = 2.18 (s, 3 H), 6.51 (s, 1 H), 7.00 (d, J = 8.0 Hz, 2 H), 7.44-7.48 (m, 4 H), 7.53 (d, J = 8.9 Hz, 2 H), 7.63 (t, J = 8.3 Hz, 2 H), 7.83 (d, J = 8.9 Hz, 2 H), 7.87 (d, J = 8.0 Hz, 2 H), 8.45 (d, J = 8.5 Hz, 2 H). IR (KBr): 3070, 3020, 2902, 1620, 1591, 1509, 1458, 1431, 1401, 1247 cm-¹.
Compound 3m: ¹H NMR (500 MHz, CDCl3): δ = 0.88 (d, J = 6.9 Hz, 6 H), 2.31-2.35 (m, 1 H), 5.50 (d, J = 3.8 Hz, 1 H), 7.47 (d, J = 8.8 Hz, 2 H), 7.50 (d, J = 7.1 Hz, 2 H), 7.65 (t, J = 7.0 Hz, 2 H), 7.83 (d, J = 8.8 Hz, 2 H), 7.92 (d, J = 8.1 Hz, 2 H), 8.35 (d, J = 8.5 Hz, 2 H). IR (KBr): 3068, 2959, 2925, 2876, 1631, 1620, 1590, 1516, 1457, 1434, 1398, 1251, 1237 cm-¹.