Triflic Acid Promoted Tandem
Ring-Closure-Aryl-Migration of 2′-Amino Chalcone
Epoxide: A New Synthetic Route to Azaisoflavones

Chandrasekaran Praveen; Kannabiran Parthasarathy; Paramasivan T. Perumal

doi:10.1055/s-0030-1258091

LETTER

Triflic Acid Promoted Tandem Ring-Closure-Aryl-Migration of 2′-Amino Chalcone Epoxide: A New Synthetic Route to Azaisoflavones

Chandrasekaran Praveen^a, Kannabiran Parthasarathy^b, Paramasivan T. Perumal*^a
^a Organic Chemistry Division, Central Leather Research Institute, Adyar, Chennai 600 020, India
^b Analytical Research & Development, Orchid Chemicals & Pharmaceuticals Ltd., Research & Development Centre, Sholinganallur, Chennai 600 119, India
Fax: +91(44)24911589; e-Mail: ptperumal@gmail.com;

Abstract

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Abstract

An unprecedented TfOH-promoted tandem ring-closure-aryl-migration of 2′-amino chalcone epoxide leading to 3-aryl-4(1H)-quinolones (azaisoflavones) was achieved. The outcome of the reaction was confirmed by NMR analysis and rationalized through the intermediacy of a phenonium ion. This synthetic protocol furnishes azaisoflavones straightforwardly from simple starting materials under mild conditions.

Key words

2′-amino chalcone epoxide - triflic acid - sequential ring-opening aryl-migration - phenonium ion - azaisoflavones

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References

References and Notes

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Representative procedure for the synthesis of azaisoflavones (3a-k): To a stirred solution of 2′-amino chalcone epoxide 1a (239 mg, 1.0 mmol) in anhydrous CH₂Cl₂ (1 mL) were added trifluoromethanesulphonic acid (452 mg, 3.0 mmol), dropwise at 0 ˚C. After completion of the reaction as indicated by TLC, the reaction mixture was quenched with ice-cold water and adjusted the pH to 7.5 with NaHCO₃, extracted with CH₂Cl₂ (3 × 15 mL), and the organic extract was dried with anhydrous sodium sulphate. Removal of the solvent under reduced pressure gave the crude product, which was purified by column chromatog-raphy (EtOAc-petroleum ether, 6:4) to afford the pure product 3a as a brown solid. Mp 258-260 ˚C (Lit.^¹9 257 ˚C). IR (KBr): 3454, 1621, 1561, 1515, 1356, 1292, 753, 697 cm^-¹. ^¹H NMR (400 MHz, DMSO-d ₆): δ = 7.27 (t, J = 7.4 Hz, 1 H), 7.34-7.42 (m, 3 H), 7.59 (d, J = 8.0 Hz, 1 H), 7.65 (t, J = 8.0 Hz, 1 H), 7.73 (d, J = 7.4 Hz, 2 H), 8.16 (d, J = 6.1 Hz, 1 H), 8.21 (d, J = 7.9 Hz, 1 H), 12.06 (d, J = 5.7 Hz, 1 H, D₂O exchangeable). ^¹³C NMR (100 MHz, DMSO-d ₆): δ = 118.2, 119.7, 123.3, 125.6, 125.8, 126.4, 127.9, 128.5, 131.5, 136.2, 138.1, 139.3, 174.7. MS (ESI): m/z = 222 [M + H]⁺. Anal. Calcd for C₁₅H₁₁NO: C, 81.43; H, 5.01; N, 6.33. Found: C, 82.01; H, 5.03; N, 6.35

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