[4-Iodo-3-(isopropylcarbamoyl)phenoxy]acetic Acid as a Highly Reactive and Easily Separable Catalyst for the Oxidative Cleavage of Tetrahydrofuran-2-methanols to γ-Lactones

Abstract

[4-Iodo-3-(isopropylcarbamoyl)phenoxy]acetic acid was developed as a highly reactive and easily separable catalyst for the oxidative cleavage of tetrahydrofuran-2-methanols to γ-lactones in the presence of Oxone^® (2KHSO₅·KHSO₄·K₂SO₄) as the co-oxidant. The reactivity of this new catalyst was considerably greater than that of our previously reported catalyst, 2-iodo-N-isopropylbenzamide. The new catalyst and product were easily separated by only liquid–liquid separation without chromatography. In addition, using a mixture of nitromethane and N,N-dimethylformamide as the solvent and heating enabled a low catalyst loading, a short reaction time, and high product yield. Oxidative cleavage using the new catalyst can be used as a practical and efficient method for synthesizing γ-lactones.

Key words

hypervalent iodine - iodobenzamide - organic catalysis - oxidative cleavage - Oxone^®

• References and Notes

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• 6 Synthesis of [4-Iodo-3-(isopropylcarbamoyl)phenoxy]acetic Acid (4) Lithium hydroxide monohydrate (201 mg, 4.78 mmol) was added to a solution of 8 (1.20 g, 3.19 mmol) in a mixture of MeOH (30 mL) and water (10 mL) at room temperature. After stirring for 1.5 h, MeOH was removed under reduced pressure, and sat. aq. NaHCO₃ was added to the resulting mixture. After washing with Et₂O, the aqueous solution was acidified with 10% HCl. The mixture was saturated with NaCl and subsequently extracted with EtOAc. The organic layer was washed with brine, dried over Na₂SO₄, filtered, and concentrated under reduced pressure. The residue was purified by recrystallization from hexane and EtOAc to give 4 (923 mg, 80%) as colorless needles; mp 187–189 °C (hexane/EtOAc). IR (KBr): 3423, 3322, 3258, 3072, 2975, 2941, 2879, 1747, 1640, 1602, 1583, 1547, 1469, 1438, 1402, 1367, 1329, 1311, 1283, 1259, 1232, 1204, 1172, 1131, 1080, 1013, 867, 822, 800 cm^–1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 13.06 (br s, 1 H), 8.20 (br d, J = 7.6 Hz, 1 H), 7.69 (d, J = 9.2 Hz, 1 H), 6.84 (d, J = 3.1 Hz, 1 H), 6.74 (dd, J = 9.2, 3.1 Hz, 1 H), 4.70 (s, 2 H), 4.03–3.94 (m, 1 H), 1.14 (d, J = 6.9 Hz, 6 H). ¹³C NMR (126 MHz, DMSO-d ₆): δ = 170.0, 167.7, 157.8, 144.4, 139.9, 117.1, 114.8, 82.9, 64.8, 41.1, 22.3. HRMS (FAB): m/z calcd for C₁₂H₁₅INO₄ [M + H]⁺: 364.0046; found: 364.0026.
• 7 Typical Experimental Procedure for the Oxidative Cleavage of Tetrahydrofuran-2-methanols 2 with 4 and Oxone^® Catalyst 4 (7.3 mg, 0.02 mmol) was added to a solution of 2 (0.40 mmol) in MeNO₂–DMF (10:1, 1.8 mL) at room temperature. Oxone^® (984 mg, 1.6 mmol) was then added to the mixture at 50 °C. After stirring until the reaction was completed (checked by TLC), the resulting mixture was diluted with Et₂O. The ethereal solution was washed with water, sat. aq. NaHCO₃, and brine, dried over MgSO₄, filtered, and concentrated under reduced pressure to give 3, which was pure in a majority of cases. Further purification was conducted by silica gel column chromatography if required.
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• 9 For example, catalyst 4 was recovered in 88% yield for the reaction of 2b with 4 and Oxone^® (Table 3, entry 1).