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Synlett 2015; 26(08): 1101-1105
DOI: 10.1055/s-0034-1379998
letter
© Georg Thieme Verlag Stuttgart · New York

An Efficient Aromatization of 2-Amino-4,5,6,7-tetrahydrobenzo-[b]thiophene-3-carboxylates in Dimethyl Sulfoxide Catalyzed by p-Toluenesulfonic Acid

Mehdi Adib*
a   School of Chemistry, College of Science, University of Tehran, P. O. Box 14155-6455, Tehran, Iran   eMail: madib@khayam.ut.ac.ir
,
Mehd Soheilizad
a   School of Chemistry, College of Science, University of Tehran, P. O. Box 14155-6455, Tehran, Iran   eMail: madib@khayam.ut.ac.ir
,
Saideh Rajai-daryasaraei
a   School of Chemistry, College of Science, University of Tehran, P. O. Box 14155-6455, Tehran, Iran   eMail: madib@khayam.ut.ac.ir
,
Peiman Mirzaei
b   Department of Chemistry, Shahid Beheshti University, Tehran, Iran
› Institutsangaben
Weitere Informationen

Publikationsverlauf

Received: 16. Dezember 2014

Accepted 12. Januar 2015

Publikationsdatum:
05. März 2015 (online)

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Abstract

A novel oxidation–aromatization of alkyl 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylates is reported. The Gewald product, obtained from three-component condensation reaction between a cyclohexanone, an alkyl cyanoacetate, and sulfur, underwent an oxidation reaction in dimethyl sulfoxide in the presence of a catalytic amount of p-toluenesulfonic acid to give the corresponding alkyl 2-aminobenzo[b]thiophene-3-carboxylate in excellent yield.

Key words

2-aminobenzo[b]thiophene-3-carboxylates - Gewald reaction - oxidation–aromatization - dimethyl sulfoxide - p-toluenesulfonic acid
 

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  • 22 General Procedure for the Preparation of Methyl 2-Aminobenzo[b]thiophene-3-carboxylate (7a) A mixture of cyclohexanone (0.196 g, 2.0 mmol), methyl cyanoacetate (0.198 g, 2.0 mmol), elemental sulfur (0.064 g, 2.0 mmol), and morpholine (0.174 g, 2.0 mmol) was heated at 75 °C for 1 h. After nearly complete conversion into the corresponding 2-aminothiophene, as was indicated by TLC monitoring, the reaction mixture was cooled to r.t., and the solid residue was recrystallized from EtOH to afford methyl 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate (6a ). Then a solution of 6a (0.211 g, 1 mmol) and TsOH (0.052 g, 0.3 mmol) in DMSO (1 mL) was heated at 190 °C for 45 min in a silicone oil bath. Progress of the reaction was monitored by TLC. After completion of the reaction, the mixture was cooled to r.t., H2O (4 mL) was added to the mixture, which then was extracted with CH2Cl2 (3 × 5 mL). The extract was dried with Na2SO4, and the solvent was removed. The residue was crystallized from n-hexane–EtOAc (5:1) to afford the pure product as pale yellow crystals, mp 126–128 °C, yield 0.203 g, 98% (on the basis of 6a). IR (KBr): νmax = 3395, 3280 and 3167 (NH), 1647 (C=O), 1591, 1493, 1464, 1431, 1372, 1275, 1231, 1146, 1011, 779, 741, 662 cm–1. MS: m/z (%) = 207 (95) [M+], 175 (100), 147 (40), 121 (37), 103 (8), 77 (9), 69 (6). 1H NMR (500.1 MHz, CDCl3): δ = 3.97 (s, 3 H, CH3), 6.55 (br s, 2 H, NH2), 7.16 (td, J = 7.6, 1.2 Hz, 1 H, CH), 7.34 (td, J = 7.8, 1.2 Hz, 1 H, CH), 7.52 (d, J = 8.0 Hz, 1 H, CH), 8.10 (d, J = 8.0 Hz, 1 H, CH). 13C NMR (100.6 MHz, CDCl3): δ = 51.1 (OCH3), 99.7 (CCO2), 121.4, 122.4, 122.5 and 125.5 (4 × CH), 128.8 and 137.3 (2 × C), 164.5 (SCN), 166.7 (C=O).