Asymmetric oxidation, Davis oxaziridine

Davis and coworkers40 have developed use of diastereomerically pure 2-sulfonyl and 2-sulfamyloxaziridines for asymmetric oxidation of sulfides into sulfoxides (equation 7). The best results (using the sulfamyloxaziridines) range from 38 to 68% enantiomeric purity of the resultant sulfoxides. The structural diversity of such substituted oxaziridines, their... 

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. 

Oxidation by chiral oxaziridines. For more than a decade, Davis s group49,71 76 has been working on the stoichiometric asymmetric oxidation of prochiral sulfides. In a series of elegant and important papers, they have demonstrated that their approach is one of the best methods in the synthesis of chiral sulfoxides. This research has yielded four generations of chiral oxaziridines 41- 44 exhibiting different stereoselectivities as a result of their dissimilar active-site structures (Fig. 5). 

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, F. A., Sheppard, A. C., Chen, B. C., Hague, M. S. Chemistry of oxaziridines. 14. Asymmetric oxidation of ketone enolates using enantiomerically pure (camphorylsulfonyl)oxaziridine. J. Am. Chem. Soc. 1990,112, 6679-6690. 

Related reactions Davis oxaziridine oxidation, Prilezhaev reaction, Sharpless asymmetric epoxidation, Shi asymmetric epoxidation ... 

Quite recently, new methods for the synthesis of chiral sulfoxides have been discovered. Asymmetric oxidation of sulfides by peroxides in the presence of chiral titanium catalysts has been studied by Kagan and Modena [S02-S0S] ( 3.2.3), and the use of chiral oxaziridines ( 2.8) as oxidizing agents has been introduced by Davis [506]. 

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... 

Davis and coworkers have recently found that asymmetric oxidation of sulfides with (+ )-(camphorsulfonyl)oxaziridine (58) affords sulfoxides with 8-73% enantiomeric excess. In contrast, (— )-a,a-(dichlorocamphorsulfonyl)oxaziridine (59) in CHC13 affords uniformly high stereoselectivity (66-95% eeof(S)-configuration of sulfoxides). The solvent... 

Davis et al. used a chiral oxaziridine for the asymmetric oxidation of sulfides to sulfoxides.Oxidation of isopropyl- -tolyl sulfide (427) with oxaziridine 428, for example, gave 60.3% ee (S) of 429 (at -78°C in chloroform).5 0 The absolute configuration of the sulfide is determined by approach of the sulfide to the oxaziridine oxygen, as illustrated by 430. Steric factors appear to be the primarily reason for the chiral recognition. 5 1 In this model, attack by sulfur minimizes the Rl and Rs interactions with the oxaziridine... 

The asymmetric hydroxylation of A-non-substituted ethyl 2-carbamoyl-2-phenylacetate in THF was mediated by Davis oxaziridine, 2-(benzenesulfonyl)-3-phenyl-oxaziridine as oxidant in the presence of a combination of rare-earth metal alkoxide Pr(OPr )3 and the ligand A-((S)-l-(2-hydroxyphenylcarbamoyl)-2-methylpropyl)-2-hydroxybenzamide as catalyst afforded enantioenriched (68-92% ee) (R)-ethyl 2-carbamoyl-2-hydroxy-2-phenylacetate (yield 39-96%) with a densely... 

A large proportion of Davis s work has been involved in the elucidation of the transition state employed in the transfer of the oxygen from the oxaziridine to the olefin substrate. Davis favoured the planar transition state and was at the time supported by theoretical calculations however, more recent calculations favour the spiro transition state [6]. Davis has also described the asymmetric oxidation of enolate anions by chiral oxaziridines, which led to a-hydroxyketones with enanti-oselectivities of up to 95% ee [56, 57], Silyl enol ethers have also been reported to give epoxides when treated with oxaziridines, but the instability of these compounds is too great to allow isolation [37,58,59], To date, only Davis has reported successful isolation of a-silyloxy epoxides [60],... 

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],... 

Davis has described many examples of asymmetric epoxidation mediated by oxaziridines. The first chiral oxaziildine reported was derived from bromocamphor 17, but the observed enautioselectivities were low, the best being 40% ee for 1-phenylcyclohexene oxide (Scheme 1.21) [54]. 

---