Oxaziridines applications

The use of chiral solvents in this photorearrangment has been shown to promote asymmetric synthesis of oxaziridines,54 and application of the cyclization to highly substituted azoxy compounds provides a route to oxadiaziridines.55... 

Also, nitrones can be formed by photochemical oxidation (X350 nm) of aldimines in acetonitrile, in the presence of O2 over a TiC>2 suspension (20, 21). Air oxidation of imines into oxaziridines with their subsequent transformation into nitrones, using cobalt catalysts, provides good yields. Utilization of molecular oxygen in the oxidation process seems highly promising due to its cost-effectiveness, availability, and the possibility of industrial application (22). 

The first CEM system described by Griffing and West (84) consisted of an organic dye dispersed in an inert polymer film that is spin coated onto the surface of a resist and subsequently removed following exposure but prior to resist development. The chemistry of this system is based on the photoisomerization of an aromatic dye to an oxaziridine (87) (Figure 10). Other workers have evaluated polysilanes (88) and diazonium salt chemistry (89,90) for CEM applications. 

The first authentic three-membered rings with two heteroatoms were synthesized only after 1950. Because they became available at once by simple and generally applicable procedures, oxaziridines (1) (64HC(19-1)624,68HOU(10-4)449), diaziridines (2) (67HOU(10-2)7l) and diazirines (3) (68HOU(10-4)895) are now extensive, well investigated classes of compound. 

Applications of oxaziridine rearrangements in asymmetric syntheses have been reviewed,596 and the formation of A,A-disubstituted formamides (462) on sodium perborate oxidation of alkyl A-arylaldimines (460) has been rationalized597 in terms of an intermediate oxaziridine (461) that rearranges via acid-catalysed O—N cleavage. 

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

Another substrate class, for which the outcomes of a radical and a carbocationic process are opposite, are indoles (Fig. 85) [418], Indeed, when oxaziridines 315a or 315c were treated with indoles 314c in the presence of 2 or 10 mol% of C11CI2/ TBAC oxazolidinoindolines 316c were obtained as the exclusive products in 53-90% yield. The reaction is applicable to 2-, 3-, and 2,3-disubstituted indoles. Chiral indole derivatives acylated with (S)-proline units at nitrogen underwent asymmetric diastereoselective aminohydroxylation reactions with 86-91% de. Tricyclic hemiaminals derived from tryptamine derivatives could be transformed to pyrrolidinoindolines, which are core structures of a number of alkaloids. 

A review of nonenzymatic asymmetric epoxidations covering the literature through 1983 has been published elsewhere. Improved enantioselectivity (to as high as 64% ee) for epoxidations of some al-kenes with chiral oxaziridines has been described and results are included in a review of synthetic applications of oxaziridines. A summary of catalytic asymmetric epoxidations of alkenes is present in Table 12, together with brief comments on each method. 

By far the most widely used synthetic application of the oxaziridines is oxidation. The aptotic A -sulfonyl- and peroxaziridines transfer oxygen at rates comparable to peracids via a mechanism where the nucleophilic species attacks the electrophilic oxaziridine oxygen atom in an SN2-type fashion <1996CHEC-II(1A)365>. 

A potentially important industrial application of this reaction involves the thermal rearrangement of 3,3-pentamethylene-oxaziridine 67 to e-caprolactam, a reaction that is strongly catalyzed by transition-metal ions. ... 

The rapid development of chiral phosphine derivatives of ferrocene was undoubtedly due to their application in catalysis. In contrast, chiral sulfur compounds from lithiated iV,iV-dimethyl-l-ferrocenylethylamine were only prepared about 15 years later [140], For the synthesis of such derivatives, the lithiated amine is treated with disulfides as shown in Fig. 4-24, top (and analogously, diselenides [141]). The sulfides obtained are easily oxidized by peracids or NaI04 to the corresponding sulfoxides. As sulfur becomes a new center of chirality by the oxidation, diastereoisomeric sulfoxides are formed in ratios depending on the oxidant [140]. If chiral oxaziridines [106, 142] are used as oxidizing agents, the diastereoisomeric ratio is appreciably... 

Compounds 11-16 arise from an analogous but intramolecular reaction. A special application of the chiral imine procedure, i.e., the use of an oxaziridine rather than an electrophilic olefin, yielding compound 17, has been reported. 

The utility of oxaziridines in asymmetric a-hydroxylation also extends to reactions with achiral enolates. This has been made possible by the discovery that certain chiral A -sulfonyl oxaziridines can react with enolates to afford a-hydroxy carbon compounds in excellent yield and enantioselectivity. An application of a highly selective sulfonyloxaziridine derived from camphor to the synthesis of daunomycin is shown in Scheme 8.23. Attack of the oxaziridine presumably occurs such that the enolate ester avoids nonbonded interactions with the exo methoxy group on the bicyclic ring system (cf. Schemes 8.23c and d). This is a very useful reaction of wide scope, and can be carried out on both stabilized enolates derived from keto esters (shown) and simple ketone enolates [99]. 

Scheme 8.23. (a) Application of enolate oxidation reactions of a chiral oxaziridine to the synthesis of an AB ring synthon of daunomycin [100]. ffc) Structure of the oxaziridine used. Proposed (c) favored and (d) disfavored transition structures (see also ref. [101]). 

Although few examples of the oxidation of a,p-unsaturated ketones widi these recently established reagents (Section 2.3.2.1.2.iii) have been reported, the application is clearly plausible. The use of a chiral camphOT-deiived oxaziridine to effect this process has been reported. Ihus ( t-)-lgellmanionone (123) was prepared by treatment of the precursor (122) widi oxaziridine (45) in THP at -78 C. Although die yield in this case was only moderate, it would be unwise to generalize at this stage. 

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