Desulfonylation of N-Sulfonyl Tetrahydroisoquinoline Derivatives by Potassium Fluoride on Alumina Under Microwave Irradiation: Selective Synthesis of 3,4-Dihydroisoquinolines and Isoquinolines

Abstract

In a solvent-free system, the microwave irradiation of mixtures of N-sulfonyl tetrahydroisoquinolines and 37% potassium fluoride supported on alumina selectively furnished 3,4-dihydroisoquinolines or isoquinolines, depending upon the reaction time.

Key words

isoquinolines - 3,4-dihydroisoquinolines - KF on alumina - microwave-assisted desulfonylation

References

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22

The selective elaboration of 1-benzoyl-3,4-dihydroiso-quinoline and the related 1-benzoylisoquinoline depicted in entry 1 of Table [1] are representative of a typical experimental procedure. The sulfonamide and 37% KF/alumina (10 weight equivalents, prepared as reported by Schmittling and Sawyer⁶) were mixed in a Pyrex test tube and subjected to microwave irradiation (490 W) during 10 s in a domestic microwave oven (Panasonic NN 6556). The solid was allowed to cool to room temperature and washed with CH₂Cl₂ (3 × 10 mL). The combined organic washings were concentrated under reduced pressure and purified by flash-chromatography, affording 1-benzoyl-3,4-dihydro-isoquinoline as an oil in 60% yield; IR (film, cm^-1): 3056, 2924, 1670, 1448, 1248, 923, 839, 716; ¹H NMR (200 MHz, CDCl₃): δ 2.89 (2 H, t, J = 7 Hz), 3.98 (2 H, t, J = 7 Hz), 7.27-7.60 (7 H, m) and 8.02-8.05 (2 H, m); ¹³C NMR (50 MHz, CDCl₃): δ 25.57, 47.28, 126.59, 127.18, 127.82, 128.55, 130.34, 131.64, 133.87 (2C), 135.50, 137.17, 165.34 and 193.81. Increasing the reaction time to 20 s, followed by the same work-up procedure and flash-chromatography furnished 1-benzoylisoquinoline in 56% yield, as a solid mp 74-76 ºC (ref. [14a] 78-79 ºC; ref. [14b] 76-77 ºC; ref. [14c] 76-77 ºC); IR (film, cm^-1): 3054, 1669, 1580, 1498, 1448, 1352, 1227, 1247, 1155, 923, 829, 715, 686; ¹H NMR (200 MHz, CDCl₃): δ 7.43-7.51 (2 H, m), 7.57-7.65 (2 H, m), 7.71-7.83 (2 H, m), 7.91-7.93 (3 H, m), 8.22 (1 H, d, J = 8 Hz) and 8.66 (1 H, d, J = 8 Hz); ¹³C NMR (50 MHz, CDCl₃): δ 122.57, 126.20, 126.41, 127.08, 128.31, 128.46, 130.74 (2C), 133.65, 136.62, 136.70, 141.14, 156.43 and 194.71.