Regiospecific Normal Diels–Alder Reaction of trans-1,2-Biscoumarinylethenes

Abstract

The reaction of different 7,8-substituted coumarin 4-acetic acids with 7-diethylaminocoumarin-3-carbaldehyde in the presence of piperidine in methanol gives 1,2-biscoumarinylethenes. These compounds undergo regiospecific Diels–Alder reactions at their electron-rich diene components C₃–C₄–C₁₀–C₉, with electron-deficient dienophiles. The feasibility of normal electron-demand Diels–Alder reactions could be explained on the basis of the HOMO–LUMO gap. All the compounds are new and are intensely colored.

Key words

coumarins - cycloaddition reaction - Diels–Alder reaction - 3,4-annulated coumarin - regiospecific reaction

• References and Notes

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• 17 Experimental Procedure for 3a Compound 2a (0.218 g, 1 mmol) and piperidine (0.085 g, 1 mmol) were stirred in MeOH (6 mL) for 15 min and 1 (0.245 g, 1 mmol) was added slowly at r.t. After complete consumption of 1, the precipitated solid was collected by filtration and washed with a small quantity of cold MeOH followed by H₂O to remove traces of piperidine. Solid 3a was washed with EtOAc. The second crop of 3a was obtained from the filtrate; the filtrate was evaporated to obtain a sticky mass which was purified by column chromatography on silica gel using toluene–EtOAc (70:30, v/v); orange solid; 64% yield; mp 231–233 °C. IR (neat): ν = 1698, 1609, 1575, 1513, 1352, 1252, 1192, 1132 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 1.25 (t, 6 H, 2 × CH₃, J = 7.5 Hz), 2.46 (s, 3 H, CH₃), 3.47 (q, 4 H, 2 × CH₂, J = 7.5 Hz), 6.49 (s, 1 H, C3H), 6.51 (d, 1 H, C8′H, J = 2.0 Hz), 6.65 (dd, 1 H, C6′H, J = 2.0, 9.0 Hz), 7.13–7.15 (m, 2 H, C5′H and C8H), 7.20 (d, 1 H, C10H, J = 15.5 Hz), 7.37 (d, 1 H, C6H, J = 8.5 Hz), 7.77 (d, 1 H, C5H, J = 8.5 Hz), 7.79 (s, 1 H, C4′H), 7.99 (d, 1 H, C9H, J = 15.5 Hz). MS: m/z = 402.57 [M + 1]. Anal. Calcd (%) for C₂₅H₂₃NO₄ (401.45): C, 74.79; H, 5.77; N, 3.49. Found: C, 74.58; H, 5.88; N, 3.52.
• 18 Experimental Procedure for 16a Compound 3a (0.401 g, 1 mmol) and 11 (0.256 g, 2 mmol) were refluxed in dioxane (6 mL) for 2 h. After complete consumption of 3a, the solution was cooled to r.t. and was evaporated to obtain a sticky mass which was purified by column chromatography on silica gel using toluene–EtOAc (80:20, v/v); buff solid; 85% yield; mp 269–270 °C. IR (neat): δ = 2975, 1702, 1594, 1519, 1251, 1128 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 1.23 (t, 6 H, 2 × CH₃, J = 6.9 Hz), 2.50 (s, 3 H, CH₃), 3.50 (q, 4 H, 2 × CH₂, J = 6.9 Hz), 3.59 (d, 2 H, C10H, J = 7.2 Hz), 4.29 (t, 1 H, C9H, J = 7.2 Hz), 6.44 (d, 1 H, C8′H, J = 2.1 Hz), 6.64 (dd, 1 H, C6′H, J = 2.1, 8.7 Hz), 7.22 (s, 1 H, C8H), 7.24 (d, 1 H, C6H, J = 7.8 Hz), 7.38 (d, 1 H, C5′H, J = 8.7 Hz), 7.58 (d, 1 H, C5H, J = 8.1 Hz), 7.93 (s, 1 H, C4′H). ¹³C NMR (300 MHz, CDCl₃): δ = 12.4 (2 × CH₃), 22.0 (ArCH₃), 28.3 (C10), 36.9 (C9), 42.3 (C8), 45.1 (2 × CH₂), 46.5 (C7), 97.0 (C8′), 107.5 (C3′), 108.9 (CN), 109.2 (CN), 109.4 (C10b), 109.7 (2 × CN), 109.8 (C6′), 111.6 (C4a′), 114.6 (C6a), 117.7 (C4), 124.8 (C2), 127.2 (C1), 130.1 (C5′), 142.8 (C4′), 147.4 (C3), 152.0 (C7′), 152.3 (C10a), 152.9 (C4a′), 156.5 (C8a′), 157.1 (C2′), 161.5 (C6). MS: m/z = 530.46 [M + 1]. Anal. Calcd (%) for C₃₁H₂₃N₅O₄ (529.55): C, 70.31; H, 4.38; N, 13.23. Found: C, 70.48; H, 4.47; N, 13.19.