Asymmetric oxidation, Davis oxaziridine reagents

Davis et al.111 developed another method for reagent-controlled asymmetric oxidation of enolates to a-hydroxy carbonyl compounds using (+)-camphor-sulfonyl oxaziridine (147) as the oxidant. This method afforded synthetically useful ee (60-95%) for most carbonyl compounds such as acyclic keto esters, amides, and a-oxo ester enolates (Table 4-20). 

Chiral Davis oxaziridines allow the oxidation of phosphonates to a-hydroxy-phosphonates in good ee with apparently wide generality and with a sense of induction that is well controlled by the chirality of the reagent used.109 mCPBA oxidation of a bi-cyclic e do-camphorylsulfonylimine surprisingly resulted in an exo-camphorylsulfonyl-oxaziridine, whereas all other camphorylsulfonylimines resulted only in endo-oxaziiidines.110 Asymmetric oxidation of sulfides to sulfoxides and the a-hydroxylation of enolates were predicted by models in which steric interactions are minimized. 

Oxaziridines. Davis has developed the use of chiral 2-sulfonyloxaziridines derived from camphorsulfonic acid as chiral auxiliaries in the asymmetric oxidation reactions. Although other oxaziridines may be preferable, the camphor-derived oxaziridines can be used for the oxidation of sulfides and disulfides to sulfoxides and thiosulfinates as well as for the epoxidation of alkenes. On the other hand, the camphoryloxaziridines are the preferred reagents for hydroxylation of lithium enolates of esters, amides, and ketones, as utilized in the synthesis of kjellmanianone (eq 17). ... 

Davis and coworkers have shown that A -sulfonyl oxaziridines are potent oxidants [741], In order to induce asymmetry, these authors prepared rigid, bulky oxaziridines bearing the bomane skeleton. Oxaziridines 2.82 (X = H, Cl or Me) are very interesting reagents for the asymmetric oxidation of alkali metal enolates to a-hydroxyketones [147, 742-746], In some cases, the presence of chlorine or methoxy substituents on reagents 2.82 is necessary to observe a high asymmetric induction [747, 748], Both enantiomers are available, and these chiral reagents may be recycled. 

Davis and coworkers [506, 744] performed the asymmetric oxidation of pro-chiral sulfides with chiral oxaziridines, and reagents bearing the bomane skeleton 2.82 or 2.83 (X = Cl) were the most efficient. The chiral oxaziridine oxidations are... 

Sulfenimines undergo asymmetric oxidations to form sulfinimines via a reaction with Davis reagents. The sulfenimine 77 was oxidized by 9 to yield the sulfmimine 78 in 82% yield and 97% ee. Yields and enantiomeric excess varied based on the oxaziridine reagent used.64... 

One can oxidize N-sulfonyl imines to the corresponding oxaziridines using oxidants such as a peracid or oxone [5 b, 93,94], Thus, Davis and coworkers have developed a biphasic procedure for converting imine 278 to frans-oxaziridine 279, in high yield [5b] [Eq. (65)]. Similarly, camphor-derived N-sulfonyl imines 277 and 268 can be oxidized to the endo oxaziridines 280 and 281, respectively [92,93] [Eq. (66), (67)]. Davis and others have now demonstrated the exceptional utility of oxaziridines as oxidants in organic synthesis and, in particular, the value of camphor-derived reagents, such as 280 and 281, in asymmetric synthesis [94,95],... 

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