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Synlett 2008(20): 3125-3128  
DOI: 10.1055/s-0028-1087273
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Microwave-Promoted and Lewis Acid Catalysed Synthesis of 2,4,6-Triarylpyridines Using Urea as Benign Source of Ammonia

Moyurima Borthakur, Mandakini Dutta, Shyamalee Gogoi, Romesh C. Boruah*
Medicinal Chemistry Division, North-East Institute of Science and Technology, Jorhat 785006, India
e-Mail: rc_boruah@yahoo.co.in;
Further Information

Publication History

Received 10 July 2008
Publication Date:
26 November 2008 (online)

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Abstract

An efficient method for the synthesis of 2,4,6-triaryl­pyridines via microwave-promoted and BF3˙OEt2-catalysed one-pot reaction of ω-pyrrolidinoacetophenone with chalcone is reported. This method illustrates urea as an environmentally benign source of ammonia for the synthesis of 2,4,6-triarylpyridines.

Key words

2,4,6-triarylpyridine - urea - microwave - Lewis acid

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24

Microwave experiments were conducted in open reaction vessels of a Synthwave 402 reactor manufactured by M/s Prolabo, 54 rue Roger Salengro, Cedex, France. The temperature of the reaction mixture was set at 140 ˚C and measured by a computer controlled sensor using 80% power (maximum output 300 Watts) with an operating frequency of 2.45 GHz. The reaction time specified is the total irradiation time. The hold time at final temperature is 25% of the total time.

25

Spectral and Analytical Data of Selected Compounds:
Compound B: mp 97-99 ˚C; R f = 0.7 (EtOAc-hexane, 10:90). IR (KBr): 3059, 3028, 2922, 1686, 1653, 1606, 1448, 1217, 1180, 1010, 755 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 8.01 (d, 2 H, J = 7.4 Hz), 7.92 (d, 2 H, J = 7.4 Hz), 7.88 (d, 2 H, J = 8.0 Hz), 7.18-7.54 (m, 6 H), 5.25 (s, 1 H), 4.84 (s, 2 H), 2.40 (s, 3 H). ¹³C NMR (75 MHz, CDCl3): δ = 196.0, 190.5, 152.7, 152.0, 143.3, 141.7, 136.9, 136.2, 134.2, 132.9, 132.4, 129.1, 129.0, 128.5 (2 × C), 128.4, 128.3, 128.2, 128.2, 128.0, 126.5, 123.3, 42.7, 21.4. MS (ESI): m/z = 341 [M+ + 1].
Compound 3a: mp 135-36 ˚C; R f = 0.8 (EtOAc-hexane, 10:90). IR (KBr): 3035, 2924, 1595, 1550, 1495, 1449, 1180, 756 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 8.21 (m, 4 H), 7.89 (s, 2 H), 7.75 (d, 2 H, J = 7.0), 7.03-7.53 (m, 9 H). ¹³C NMR (75 MHz, CDCl3): δ = 157.2 (2 × C), 149.9, 139.3 (2 × C), 138.8, 128.9 (2 × C), 128.8, 128.7 (3 × C), 128.4 (3 × C), 126.9 (3 × C), 126.9 (3 × C), 116.9 (2 × C). MS (ESI): m/z = 308 [M+ + 1].
Compound 3k: mp 152-54 ˚C; R f = 0.8 (EtOAc-hexane, 10:90). IR (KBr): 3056, 2922, 1596, 1544, 1492, 1184 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 7.32-8.14 (m, 13 H), 7.82 (s, 2 H), 2.42 (s, 3 H). ¹³C NMR (75 MHz, CDCl3): δ = 157.3, 157.1, 155.8, 150.0, 149.7, 138.9, 138.7, 137.8, 136.6, 136.3, 134.8, 129.2, 129.1, 128.9, 128.8, 128.6, 128.1, 126.9, 126.7 (2 × C), 125.6, 116.8, 116.3, 21.1. MS (ESI): m/z = 356 [M+ + 1].
Compound 3n: mp 133-35 ˚C; R f = 0.7 (EtOAc-hexane, 10:90). IR (KBr): 3034, 2924, 1597, 1546, 1496, 1417, 1185 cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 8.20 (m, 4 H), 7.87 (s, 2 H), 7.64 (d, 2 H, J = 5.8 Hz), 7.14-7.51 (m, 7 H), 2.42 (s, 3 H), 1.34 (s, 3 H). ¹³C NMR (75 MHz, CDCl3): δ = 157.2 (2 × C), 139.4 (2 × C), 129.6 (3 × C), 128.7 (3 × C), 128.4 (3 × C), 126.9 (4 × C), 126.7 (4 × C), 116.6 (2 × C), 29.5, 21.0. MS (ESI): m/z = 336 [M+ + 1].