Gold(I)-Catalyzed Hydroaminative Cyclization Leading to 2,5-Dihydroisoxazole

Abstract

Gold(I)-catalyzed intramolecular hydroamination of O-propargyl-N-Boc-hydroxylamine is reported. The present ­method provides a novel, catalytic, and mild approach to 2,5-dihydroisoxazole derivatives. In addition, a mechanism of N-O cleavage as a side reaction as well as desired cyclization is proposed.

Key words

2,5-dihydroisoxazoles - heterocycles - cyclizations - gold catalysis - carbenoids

References and Notes

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12

It is to be noted that the formed 2,5-dihydroisoxazole 2a was rather unstable and slowly decomposes into unidentified product upon keeping at r.t. for long period.

13

Representative Procedure for the Formation of 2d
To a solution of 1d (133 mg, 0.780 mmol) in CH₂Cl₂ (4 mL) was added Au(PPh₃)Cl (1.9 mg, 3.9 µmol, 0.5 mol%) and AgOTf (1 mg, 3.9 µmol), and the mixture was stirred for 2 h at r.t. After removal of solvent, the residual oil was purified by silica gel flash chromatography to afford 2d as colorless oil (90.3 mg, 68%). ¹H NMR (400 MHz, CDCl₃): δ = 6.60 (t, J = 2.2, 4.7 Hz, 1 H), 5.30 (td, J = 1.8, 4.7 Hz, 1 H), 5.03 (t, J = 2.2 Hz, 1 H), 1.54 (s, 9 H). ¹³C NMR (100 MHz, CDCl₃): δ = 151.0, 125.8, 103.2, 82.1, 75.0, 28.1. LRMS (EI): for C₈H₁₃NO₃ [M⁺]: m/z = 171; for C₇H₁₀NO₃ [M⁺ - Me]: m/z = 156; for C₇H₁₃NO [M⁺ - CO₂]: m/z = 127; for C₃H₅NO [M⁺ - t-BuOCO + H]: m/z = 71. Anal. Calcd for C₈H₁₃NO₃: C, 56.13; H, 7.65; N, 8.18. Found: C, 56.16; H, 7.47; N, 8.14.