Oxidative Aromatization of 3,5-Disubstituted Isoxazolines to the Corresponding Isoxazoles with N,N,N’,N’-Tetrabromobenzene-1,3-disolfonamide (TBBDS) and 1,3-Dibromo-5,5-dimetylhydantoin (DBH) as Efficient Reagents under Mild Reaction Conditions

The oxidation of 3,5-disubstituted isoxazolines to the corresponding isoxazoles has been carried out using N,N,N′,N′-tetrabromobenzene-1,3disolphonamide and 1,3-dibromo-5,5-dimethylhydantoin (DBH) as effective oxidizing agents under mild conditions at rt with good yields.Among five-membered heterocycles, isoxazolines and their aromatic derivatives are biologically active natural products and synthetic compounds of medicinal interest. 1 The biological and medicinal importance of these heterocycles stem from their various chemotherapeutic properties. 2 Isoxazoles are also versatile intermediates in organic synthesis, 3 used in the synthesis of a wide variety of natural products and pharmacopheres in medicinal chemistry. 4Isoxazolines and isoxazoles are biologically active as fungicides, herbicides, antibacterial, ant-tubercular, antiviral and nicotinic acetylene receptor ligands. 5,6oxazolines, easily prepared from the cyclization of related chalcones with hydroxylamine, can be conveniently oxidized to isoxazoles. 7,8Oxidative aromatization of some five-membered heterocycles has been accomplished by various reagents including Pd/C/AcOH, 9 activated carbon-O 2 system, 10 cobalt soap of fatty acids, 11 Pb(OAc) 4, 12 mercury or lead oxide, 13 MnO 2 , 7 potassium permanganate, 14,15 silver nitrate, 16 iodobenzene diacetate, 17 zirconium nitrate, 18 nickel peroxide, 19 cromite, 20 NBS, 21 Mn(OAc) 3 , 22 NaHCO 3 /DMF, 23 DDQ, 24 Na 2 CO 3 /CH 3 OH, 25 and tetrakispyridinenickel(II) dichromate. 26However, most of these methods suffer from certain limitations such as long reaction times, high temperatures, side