Several structurally differentiated N-alkyl imines were oxidized to their corresponding oxaziridines using UHP/maleic anhydride system. Oxidation reaction was performed under mild conditions and oxaziridines were obtained in good to excellent yields.Oxaziridines are useful aprotic oxidizing reagents for a large variety of substrates. The aprotic nature of these reagents has made possible the facile synthesis of extremely acid-sensitive species such as a-siloxy epoxides 1 and sulfenic acids. 2 In recent years, the asymmetric oxidation of prochiral centers to the desired optically active products by using chiral oxaziridines have received considerable attention. 3 Several methods have been employed for the oxidation of imines into the corresponding oxaziridines. The most general reagent is m-chloroperbenzoic acid. 4,5 Other oxidizing agents such as buffered oxone ® (2KHSO 5 ×KHSO 4 ×K 2 SO 4 ) 6 , in situ generation of sulfonic or carboxylic peracids, 7 and molecular oxygen in the presence of transition-metal complexes 8 have been also used.Although a variety of reagents are capable of affecting these oxidations, 1-8 as far as we know this transformation is not so easy and is a tricky step because these compounds are very sensitive to the oxidizing agents and reaction conditions. However, most of the reported reagents produce by-products, which either destroy, or are difficult to remove from the sensitive oxaziridines. For example, m-chloroperbenzoic acid is expensive and large-scale oxidations are sometimes contaminated with bis-(m-chloroperbenzoyl) peroxide, which complicates product purification. Meanwhile, we also know that imines are susceptible to hydrolysis in aqueous media. 1-8 For these reasons an oxidizing reagent that would avoid these difficulties was sought. Therefore, we were interested in finding a new reagent or reagent system to overcome the above limitations and we have investigated a number of different reaction conditions based upon the in situ generation of H 2 O 2 . In continuing our studies on the application of oxone ® (2KHSO 5 ×KHSO 4 ×K 2 SO 4 ) 9-11 and H 2 O 2 we found that UHP 12 [I an addition compound of hydrogen peroxide and urea] is the best candidate for this purpose. A survey of the literature revealed that UHP I is an inexpensive, stable, and easy to handle source of pure H 2 O 2 . 13 In the last few years, several reports appeared on the use of UHP in the oxidations, namely for the conversion of amines to nitroalkane, 14 Baeyer-Villiger oxidations of ketones to lactones, 15 oxidations of sulfides to sulfones, 16 oxidation of aromatic aldehydes, 17 aromatization of 1,4-dihydropyridines, 18 selective oxidation of uracil and adenine derivatives, 19 epoxidation of alkenes, 20 and conversion of pyridines to pyridine N-oxides under different conditions. 21,22 Thus, in this article we wish to report a simple, cheap and convenient method for the effective conversion of N-alkyl imines 1 to their corresponding oxaziridines 2 under mild conditions (Scheme).A good range of N-alkyl imines...
