Regio- and Stereoselective Synthesis of Pterocarpan Derivatives on a ­Carbazole Scaffold by Cascade Sigmatropic Rearrangements

Abstract

Thermal rearrangement of 1-aryloxy-4-carbazolyloxybut-2-ynes, regio- and stereoselectively led to the formation of benzofuropyranocarbazoles in a single step in good yields. An investigation into the mechanism of the formation of these heteroannulated pterocarpan derivatives led to the serendipitous formation of isomeric benzofurofurocarbazole ring system.

sigmatropic rearrangements - heterocycles - regioselectivity - ring contractions

References and Notes

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13

All new compounds reported here gave satisfactory spectroscopic and/or analytical data. General Procedure for the Rearrangement of 3 to 4.
A solution of compound 3 (0.6 mmol) in N,N-diethylaniline (6 mL) was heated to reflux for 12 h under nitrogen atmosphere. It was then allowed to come to r.t. and then poured into ice-cold 2 N HCl (50 mL) while stirring with a glass rod. The aqueous solution was extracted with EtOAc (3 × 25 mL) and the combined organic extract was washed sequentially with sat. aq NaHCO₃ (2 × 20 mL), H₂O (20 mL), brine (25 mL) and then dried (Na₂SO₄). It was then filtered, and the filtrate was concentrated in vacuo to leave a solid mass, which was chromatographed over silica gel using mixture of EtOAc and PE as eluent.
Analytical Data.
Compound 4a: mp 184-185 °C. IR (KBr): 3392, 1595, 1493, 1211, 1177 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 8.65 (s, 1 H), 7.98 (d, 1 H, J = 7.6 Hz), 7.86 (d, 1 H, J = 8.4 Hz), 7.52 (d, 1 H, J = 8.0 Hz), 7.38 (t, 1 H, J = 8.0 Hz), 7.30-7.17 (m, 3 H), 6.93-6.80 (m, 3 H), 4.49 (dd, 1 H, J = 10.8, 5.0 Hz), 3.83 (t, 1 H, J = 10.4 Hz), 3.71 (dd, 1 H, J = 10.0, 4.9 Hz), 1.90 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 158.1 (s), 153.7 (s), 139.8 (s), 138.5 (s), 129.2 (d), 125.8 (s), 124.9 (d), 124.7 (d), 123.5 (s), 120.9 (d), 120.7 (d), 119.7 (d), 119.5 (d), 118.5 (s), 110.8 (d), 110.4 (d), 109.9 (d), 109.0 (s), 83.8 (s), 67.6 (t), 49.5 (d), 26.2 (q). Anal. Calcd for C₂₂H₁₇NO₂: C, 80.71; H, 5.23; N, 4.28. Found: C, 80.94; H, 5.37; N, 4.40. MS (TOFMS ES⁺): m/z = 350 [M⁺ + Na].
Compound 4b: mp 160-161 °C. IR (KBr): 3457, 1610, 1463, 1427, 1302, 1219 cm^-1.¹H NMR (300 MHz, CDCl₃): δ = 8.54 (s, 1 H), 7.91 (d, 1 H, J = 7.5 Hz), 7.78 (d, 1 H, J = 8.4 Hz), 7.45 (d, 1 H, J = 8.1 Hz), 7.34 (dt, 1 H, J = 7.2, 1.0 Hz), 7.15 (d, 1 H, J = 7.2 Hz), 7.04 (d, 1 H, J = 7.5 Hz), 6.93 (d, 1 H, J = 7.5 Hz), 6.79-6.72 (m, 2 H), 4.41 (dd, 1 H, J = 11.1, 5.1 Hz), 3.81 (t, 1 H, J = 9.6 Hz), 3.64 (dd, 1 H, J = 9.6, 5.1 Hz), 2.17 (s, 3 H), 1.84 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 157.0 (s), 154.1 (s), 140.2 (s), 139.1 (s), 130.8 (d), 125.5 (s), 125.1 (d), 124.0 (s), 122.6 (d), 121.3 (d), 121.1 (d), 120.8 (s), 120.1 (d), 119.9 (d), 118.9 (s), 111.2 (d), 110.4 (d), 109.7 (s), 83.8 (s), 67.9 (t), 50.1 (d), 26.8 (q), 15.8 (q). Anal. Calcd for C₂₃H₁₉NO₂: C, 80.92; H, 5.61; N, 4.10. Found: C, 81.23; H, 5.77; N, 4.28. MS (TOFMS ES⁺):
m/z = 342 [M⁺ + H].
Compound 4c: mp 261-262 °C. IR (KBr): 3369, 1610, 1481, 1464, 1302, 1208, 1083 cm^-1.¹H NMR (300 MHz, CDCl₃): δ = 8.65 (s, 1 H), 7.97 (d, 1 H, J = 7.8 Hz), 7.85 (d, 1 H, J = 8.4 Hz), 7.51 (d, 1 H, J = 8.1 Hz), 7.37 (dt, 1 H, J = 7.1, 1.0 Hz), 7.22 (t, 1 H, J = 7.2 Hz), 7.09 (s, 1 H), 6.98 (d, 1 H, J = 7.2 Hz), 6.81 (d, 1 H, J = 7.2 Hz), 6.75 (d, 1 H, J = 8.1 Hz), 4.46 (dd, 1 H, J = 11.1, 5.3 Hz), 3.82 (dd, 1 H, J = 11.0, 10.1 Hz), 3.66 (dd, 1 H, J = 10.0, 5.2 Hz), 2.30 (s, 3 H), 1.88 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 156.0 (s), 153.6 (s), 139.8 (s), 138.6 (s), 130.3 (s), 129.6 (d), 125.7 (s), 125.5 (d), 124.7 (d), 123.5 (s), 120.7 (d), 119.7 (d), 119.5 (d), 118.5 (s), 110.8 (d), 110.9 (d), 109.8 (d), 109.1 (s), 83.7 (s), 67.6 (t), 49.5 (d), 26.1 (q), 20.8 (q). Anal. Calcd for C₂₃H₁₉NO₂: C, 80.92; H, 5.61; N, 4.10. Found: C, 81.06; H, 5.79; N, 4.31. MS (TOFMS ES⁺): m/z = 342 [M + H].
Compound 4d: mp 238-240 °C. IR (KBr): 3382, 1609, 1465, 1427, 1300, 1224 cm^-1.¹H NMR (300 MHz, CDCl₃): δ = 8.58 (s, 1 H), 7.98 (d, 1 H, J = 7.7 Hz), 7.87 (d, 1 H, J = 8.5 Hz), 7.52 (d, 1 H, J = 8.0 Hz), 7.38 (dt, 1 H, J = 7.2, 1.0 Hz), 7.23-7.20 (m, 2 H), 7.14 (dd, 1 H, J = 8.5, 2.1 Hz), 6.80 (t, 2 H, J = 8.4 Hz), 4.46 (dd, 1 H, J = 11.1, 5.1 Hz), 3.86 (t, 1 H, J = 9.8 Hz), 3.69 (dd, 1 H, J = 9.5, 5.3 Hz), 1.91 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 156.8 (s), 153.5 (s), 139.7 (s), 138.4 (s), 129.1 (d), 127.7 (s), 125.6 (s), 125.1 (d), 124.8 (d), 123.5 (s), 121.0 (d), 119.8 (d), 119.5 (d), 118.6 (s), 111.4 (d), 110.8 (d), 109.9 (d), 108.5 (s), 84.8 (s), 67.0 (t), 49.3 (d), 26.0 (q). Anal. Calcd for C₂₂H₁₆ClNO₂: C, 73.03; H, 4.46; N, 3.87. Found: C, 73.27; H, 4.63; N, 4.11. MS (TOFMS ES⁺): m/z = 362, 364 [M⁺ + H].
Compound 12: mp 288-290 °C. IR (KBr): 3348, 1639, 1460, 1226 cm^-1.¹H NMR (300 MHz, CDCl₃): δ = 8.20 (s, 1 H), 7.94-7.86 (m, 2 H), 7.48-7.43 (m, 2 H), 7.34 (t, 1 H, J = 7.4 Hz), 7.23-7.18 (m, 2 H), 6.96 (t, 1 H, J = 7.4 Hz), 6.78 (d, 1 H, J = 8.0 Hz), 6.70 (d, 1 H, J = 8.4 Hz), 2.01 (s, 3 H), 1.85 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 158.0 (s), 157.5 (s), 139.6 (s), 136.2 (s), 130.7 (d), 129.3 (s), 124.7 (d, two signals), 123.8 (s), 122.5 (d), 121.3 (d), 120.0 (d), 119.3 (d), 118.6 (s), 110.7 (d, two signals), 110.1 (s), 103.5 (d), 96.1 (s), 96.0 (s), 20.8 (q), 20.0 (q). Anal. Calcd for C₂₂H₁₇NO₂: C, 80.71; H, 5.23; N, 4.28. Found: C, 80.99; H, 5.31; N, 4.45. MS (TOFMS ES⁺): m/z = 328 [M⁺ + H].

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