CC BY 4.0 · SynOpen 2018; 02(03): 0246-0250
DOI: 10.1055/s-0037-1609936
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Synthesis of 2,3-Disubstituted Carbazoles, Benzo[c]carbazoles, and Phenanthrenes Through FeCl3-Mediated Cyclization of Triene Frameworks

Potharaju Raju
,
Thiyagarajan Mageshwaran
,
Bose Muthu Ramalingam
,
Department of Organic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai-600 025, Tamil Nadu, India   eMail: mohan_67@hotmail.com
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Publikationsverlauf

Received: 11. Juli 2018

Accepted after revision: 02. August 2018

Publikationsdatum:
27. August 2018 (online)


Abstract

A facile synthesis of 2,3-disubstituted carbazoles through electrocyclization of 2,3-divinylindoles using FeCl3 in DMF at reflux is reported. The methodology was found to be applicable for smooth transformation of 3-aryl-2-vinylindole as well as 2-styrylbiphenyl into the respective benzo[c]carbazole and phenanthrene.

Supporting Information

 
  • References and Notes

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  • 7 Representative procedure for 2a : To a stirred solution of 2,3-divinyl compound 1a (0.2 mmol) in anhydrous DMF (10 mL) at r.t., anhydrous FeCl3 (0.4 mmol) was added and the reaction mixture was heated at reflux for 6 h. After completion of the reaction (monitored by TLC), the mixture was poured over crushed ice (50 g) containing conc. HCl (3 mL). The crude product was extracted with ethyl acetate (3 × 20 mL) and the combined organic layer was dried (Na2SO4). Removal of solvent followed by column chromatographic purification (1% EtOAc/hexane) afforded carbazole 2a (74 mg, 74%) as a colorless solid. Mp 220–222 °C; 1H NMR (300 MHz, CDCl3): δ = 8.26 (s, 1 H), 8.23 (d, J = 7.2 Hz, 1 H), 8.15 (s, 1 H), 8.07 (d, J = 8.1 Hz, 1 H), 7.89 (d, J = 7.5 Hz, 1 H), 7.69 (d, J = 7.8 Hz, 2 H), 7.61 (m, 8 H), 4.05 (q, J = 6.5 Hz, 2 H), 1.01 (t, J = 7.1 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 166.1, 148.4, 140.1, 139.1, 138.9, 137.6, 137.1, 134.2, 132.6, 131.5, 129.2, 128.4, 128.3, 126.4, 126.1, 125.7, 125.1, 124.7, 124.0, 123.1, 120.5, 116.5, 115.3, 61.1, 13.8 ppm. Representative procedure for 3a : To a stirred solution of 3-aryl-2-vinylindole 6a (0.1 g, 0.21 mmol) in anhydrous 1,2-DCE (8 mL) at r.t., anhydrous FeCl3 (18 mg, 0.10 mmol) was added and the reaction mixture was stirred at reflux for 3 h. After completion of the reaction (monitored by TLC), the mixture was poured over ice water (20 mL) and acidified with conc. HCl (2 mL). It was then extracted with CH2Cl2 (2 × 10 mL) and the combined organic layer was dried (Na2SO4). Removal of solvent followed by column chromatographic purification (Silica gel; EtOAc–hexane, 2:8) gave benzo[c]carbazole 3a as a colorless solid (79 mg, 79%). Mp 168–170 °C; 1H NMR (300 MHz, CDCl3): δ = 8.69 (d, J = 8.4 Hz, 1 H), 8.46–8.40 (m, 2 H), 7.97 (d, J = 8.4 Hz, 1 H), 7.73 (d, J = 7.8 Hz, 2 H), 7.60 (t, J = 7.2 Hz, 1 H), 7.42–7.32 (m, 7 H), 7.24–7.17 (m, 2 H), 7.00 (d, J = 8.4 Hz, 2 H), 3.84 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 159.2, 140.7, 138.2, 137.9, 136.1, 133.8, 133.0, 131.3, 129.8, 129.1, 129.0, 127.6, 127.1, 127.0, 126.3, 126.1, 124.8, 124.4, 123.7, 122.1, 119.0, 115.6, 115.3, 113.8, 55.4 ppm. Dept-135 (75 MHz, CDCl3): δ = 133.8, 131.3, 129.0, 127.5, 127.1, 126.3, 126.0, 124.8, 124.4, 123.7, 122.0, 115.6, 115.3, 113.7, 55.3 ppm. HRMS (EI): m/z [M+] calcd for C29H21NO3S: 463.1242; found: 463.1220. Representative procedure for 15a : To a stirred solution of 2-styrylbiphenyl compound biphenyl vinylene 14a (0.1 g, 0.26 mmol) in anhydrous 1,2-DCE (10 mL) at r.t., anhydrous FeCl3 (21 mg, 0.13 mmol) was added and the reaction mixture was stirred at reflux for 3 h. Following a similar work up procedure to that for 6a afforded 2,3-dimethoxyphenanthrene 15a (62 mg, 62%) as a colorless solid. Mp 170–172 °C; 1H NMR (300 MHz, CDCl3): δ = 8.63 (d, J = 8.4 Hz, 1 H), 8.06 (s, 1 H), 7.97 (d, J = 8.4 Hz, 1 H), 7.68–7.62 (m, 2 H), 7.50 (t, J = 7.2 Hz, 1 H), 7.27 (d, J = 3.3 Hz, 1 H), 7.12–7.02 (m, 3 H), 4.17 (s, 3 H), 4.06 (s, 3 H), 4.00 (s, 3 H), 3.93 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 149.6, 149.3, 148.6, 148.3, 136.9, 133.7, 130.5, 130.0, 127.0, 126.7, 126.5, 126.0, 125.5, 124.4, 122.4, 122.3, 113.5, 111.0, 108.2, 103.2, 56.1, 56.0, 55.9, 55.8 ppm. HRMS (EI): m/z [M+] cacld for C24H22O4: 374.1518; found: 374.1515.
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