Direct Preparation of Benzofurans from O-Arylhydroxylamines

Abstract

Reaction of O-arylhydroxylamine hydrochlorides with either cyclic or acyclic ketones in the presence of methanesulfonic acid leads directly to the benzofuran derivative via a proposed one-pot condensation-rearrangement-cyclisation reaction sequence in good to excellent yields.

Key words

benzofuran - hydroxylamine - sigmatropic rearrangement

References and Notes

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12

Typical Experimental Procedure; 1,2,3,4-Tetrahydrodibenzofuran (8):^¹³ O-Phenylhydroxylamine hydrochloride (0.146 g, 1 mmol) was dissolved in THF (2 mL) and warmed to 60 ˚C. After 5 min methanesulfonic acid (0.150 g, 2 mmol) and cyclohexanone (0.100 g, 1 mmol) were added and the reaction was monitored by TLC. On completion, the solvent was removed under reduced pressure. Purification by column chromatography (PE-EtOAc, 20:1) gave the title compound 8 (0.121 g, 70%) as a light yellow oil. ^¹H NMR (400 MHz, CDCl₃): δ = 7.28-7.32 (m, 2 H), 7.06-7.12 (m, 2 H), 2.62-2.66 (m, 2 H), 2.50-2.54 (m, 2 H), 1.80-1.86 (m, 2 H), 1.71-1.78 (m, 2 H). ^¹³C NMR (62.5 MHz, CDCl₃): δ = 154.4, 154.0, 128.9, 123.0, 122.1, 118.4, 112.9, 110.8, 23.5, 23.0, 22.7, 20.5. LRMS (EI⁺): m/z = 172.1 [M]⁺. HRMS (MALDI): m/z [M]⁺ calcd for C₁₂H₁₂O: 172.0883; found: 172.0880.