An Efficient Strategy for Protecting Dihydroxyl Groups of Catechols

Abstract

A novel strategy for protecting dihydroxyl groups of catechols has been developed. Base-mediated cyclizations of catechols with 1,3-dibromopropane provided the corresponding benzo[b]1,4-dioxepans, and herefrom the protecting group was easily cleaved by aluminum chloride. The preparation of the antibacterial and antifungal agent 4-(2-aminothiazol-4-yl)benzene-1,2-diol from catechol reliably verified its availability amenable to various harsh reaction conditions.

Key words

catechols - protection - ortho-dihydroxyl - 1,3-dibromopropane - benzo[b]1,4-dioxepans - deprotection - aluminum chloride

• References and Notes

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• 24 General Cyclization Procedure To EtOH (15 mL) were added in turn catechols 1 (5.0 mmol), K₂CO₃ (2.42 g, 17.5 mmol) and 1,3-dibromopropane (0.66 mL, 6.5 mmol), then the mixture was heated at reflux for 5 h. The resulting mixture was filtered and concentrated to acquire the crude product, the purification of which by column chromatography afforded the corresponding product 2 with PE–EtOAc (30:1, v/v) as eluents. Representative Compound 2a Colorless liquid, 0.69 g (92% yield). ¹H NMR (400 MHz, CDCl₃): δ = 2.19 (quint, J = 5.6 Hz, 2 H, CH₂), 4.22 (t, J = 5.6 Hz, 4 H, CH₂), 6.90–6.95 (m, 2 H, Ar), 6.96–7.01 (m, 2 H, Ar) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 31.9, 70.5 (2 C), 121.6 (2 C), 123.3 (2 C), 151.2 (2 C) ppm. ESI-HRMS: m/z [M + H⁺] calcd for C₉H₁₁O₂: 151.0759; found: 151.0757
• 25 General Deprotection Procedure A solution of benzo[b]1,4-dioxepans 2 (3.0 mmol) in benzene (10 mL) was treated by anhyd AlCl₃ (1.20 g, 9.0 mmol), and the mixture was heated to reflux for specified time. Then, the reaction mixture was quenched by sat. aq NH₄Cl (20 mL), and the aqueous phase was extracted with EtOAc (3 × 10 mL). The combined organic phase was washed by sat. brine (2 × 20 mL) and concentrated to provide the crude product, the purification of which by column chromatography afforded the corresponding product 1 with PE–EtOAc (10:1, v/v) as eluents.
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• 27 Synthetic Procedures for Compounds 4–6 Compound 4 Bromine (0.17 mL, 3.4 mmol, dissolved in 2 mL EtOH) was added dropwise to a stirred solution of 3 (0.50 g, 2.6 mmol) in EtOH (8 mL), and the mixture was stirred at r.t. for 1 h. Then, the solvent was removed to get the crude product, the purification of which by column chromatography afforded the pure product 4 with PE–CH₂Cl₂ (15:1, v/v) as eluents; white solid; mp 72–74 °C; 0.62 g (88% yield). ¹H NMR (400 MHz, CDCl₃): δ = 2.25 (quint, J = 5.6 Hz, 2 H, CH₂), 4.28 (t, J = 5.6 Hz, 2 H, CH₂), 4.34 (t, J = 5.6 Hz, 2 H, CH₂), 4.37 (s, 2 H, CH₂), 7.00 (d, J = 8.4 Hz, 1 H, Ar), 7.57 (d, J = 8.4 Hz, 1 H, Ar), 7.60 (s, 1 H, Ar) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 30.79, 30.81, 70.2, 70.4, 121.6, 122.7, 124.7, 129.1, 150.6, 156.0, 189.8 ppm. ESI-HRMS: m/z [M + H⁺] calcd for C₁₁H₁₂O₃Br: 270.9970; found: 270.9975. Compound 5 A solution of 4 (0.54 g, 2.0 mmol) and thiourea (0.18 g, 2.4 mmol) in absolute EtOH (12 mL) was refluxed for 2 h. After removal of the solvent, the residue was treated with aq NaOH (1 mol/L, 10 mL), and extracted with EtOAc (3 × 10 mL). The combined organic phase was concentrated to provide the crude product, the purification of which by column chromatography afforded the pure product 5 with PE–EtOAc (4:1, v/v) as eluents; light yellow solid; mp 174–177 °C, 0.46 g (92% yield). ¹H NMR (400 MHz, CDCl₃): δ = 2.20 (quint, J = 5.2 Hz, 2 H, CH₂), 4.22 (t, J = 5.2 Hz, 4 H, CH₂), 5.05 (s, 2 H, NH₂), 6.60 (s, 1 H, thiazole), 6.97 (d, J = 8.4 Hz, 1 H, Ar), 7.34 (d, J = 8.4 Hz, 1 H, Ar), 7.40 (s, 1 H, Ar) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 31.8, 70.6 (2 C), 102.1, 119.3, 121.1, 121.7, 130.3, 150.5, 151.0, 151.2, 167.1. ESI-HRMS: m/z [M + H⁺] calcd for C₁₂H₁₃N₂O₂S: 249.0698; found: 209.0693. Compound 6 A solution of 5 (0.40 g, 1.6 mmol) in benzene (8 mL) was treated by anhyd AlCl₃ (0.64 g, 4.8 mmol), and the mixture was heated to reflux for 10 h. Then, the mixture was quenched by sat. aq NH₄Cl (20 mL). Then aq NaOH (1 mol/L) was added to keep pH 7. The aqueous phase was extracted with EtOAc (3 × 20 mL). The combined organic phase was washed by sat. brine (2 × 20 mL), and then concentrated to provide the crude product, the purification of which by column chromatography afforded the pure product 6 with PE–EtOAc (2:1, v/v) as eluents; yellow solid; mp 228–231 °C; 0.28 g (84% yield). ¹H NMR (400 MHz, DMSO-d ₆): δ = 6.64 (s, 1 H, thiazole), 6.69 (d, J = 8.0 Hz, 1 H, Ar), 6.92 (s, 2 H, OH), 7.06 (dd, J ₁ = 8.0 Hz, J ₂ = 2.0 Hz, 1 H, Ar), 7.19 (d, J = 2.0 Hz, 1 H, Ar), 8.90 (s, 2 H, NH₂) ppm. ¹³C NMR (100 MHz, DMSO-d ₆): δ = 98.3, 113.3, 115.4, 116.9, 126.8, 144.8, 144.9, 150.3, 167.7 ppm. ESI-HRMS: m/z [M + H⁺] calcd for C₉H₉N₂O₂S: 209.0385; found: 209.0381.