摘要或動機

Synthesis of halooximes has attracted much attention given their importance as precursors to proline derivatives1, unnatural amino acids2 and a host of alkaloids3. Halooximes have numerous industrial and biological applications 4, for example oxime ethers increases specific channel opening activities, acting as potential targets in drug treatment for various diseases 5, most adrenergic β-receptor blocking compounds also conform to the structure of oxime ethers 6. They also have vitro antifungal activities against certain plants7. A known method for the synthesis of 3 using Me3SiX as a Lewis acid, via intermediate enamines with acyclic/ cyclic bis(oxy)enamines, gives yields between 21-68%. Low yield stems from lack of reaction specificity2. In this work, we investigate coupling nitrogen-oxides with CoBr2 to synthesise halooximes in high yield. Initial cyclic/ acyclic bis(oxy)enamines were prepared by reacting 1 equiv. nitro compound with 2.4 equiv. in excess of dichloromethane under argon atmosphere at a temperature of 0ºC. 2.2 equiv. of TMS-Br was further added and left to stir till thermal equilibrium with room conditions. The cyclic/ acyclic bis(oxy)enamine was isolated upon aqueous work-up, filtration and evaporation. Synthesis of halooximes was carried out by reacting 8mL to 1mmol equiv. of THF and 2 equiv. of CoBr2 under argon atmosphere and left to stir till CoBr2 dissolves. 2mL to 1mmol equiv. of CH2Cl2 was added to 1 equiv. of bis(oxy)enamines. The reaction mixture was left to stir at room temperature for 2 hours, after which aqueous work-up, filtration and evaporation were carried out. Our results are summarized in Table 1. We also obtained relatively high percentage yields for the direct synthesis of bromooximes from cyclic nitronates via a one-pot procedure: Low yield stems from lack of reaction specificity2. In this work, we investigate coupling nitrogen-oxides with CoBr2 to synthesise halooximes in high yield. Initial cyclic/ acyclic bis(oxy)enamines were prepared by reacting 1 equiv. nitro compound with 2.4 equiv. in excess of dichloromethane under argon atmosphere at a temperature of 0ºC. 2.2 equiv. of TMS-Br was further added and left to stir till thermal equilibrium with room conditions. The cyclic/ acyclic bis(oxy)enamine was isolated upon aqueous work-up, filtration and evaporation. Synthesis of halooximes was carried out by reacting 8mL to 1mmol equiv. of THF and 2 equiv. of CoBr2 under argon atmosphere and left to stir till CoBr2 dissolves. 2mL to 1mmol equiv. of CH2Cl2 was added to 1 equiv. of bis(oxy)enamines. The reaction mixture was left to stir at room temperature for 2 hours, after which aqueous work-up, filtration and evaporation were carried out. Our results are summarized in Table 1. We also obtained relatively high percentage yields for the direct synthesis of bromooximes from cyclic nitronates via a one-pot procedure: