Asymmetric oxaziridination

The derivative (72) (TBS = t-butyldimethylsilyl) from a series of synthesized cinchona alkaloid derivatives is used as a catalyst for the asymmetric oxaziridination of aryl aldimines with m-CPBA in toluene to give tra 5 -(R,R)-oxaziridines with up to 95% ee.438... 

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... 

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... 

The enantioenriched sulfoxide intermediate 72 (R = CH2OH), obtained by asymmetric 5-oxidation with a chiral oxaziridine (89 11 enantiomeric ratio), has provided a highly enantioselective synthesis of the benzothiepin derivative 71 (4R, 5R). The aldehyde intermediate 72 (R = CHO) was cyclized asymmetrically to 71 (4R, 5R) with >98 2 enantiomeric ratio. Base treatment (f-BuOK, -10°C, THF) of the racemic benzothiepin 73... 

Enantioselective synthesis of a-hydroxy phosphonates can also be achieved by asymmetric oxidation with camphorsulfonyl oxaziridines (Scheme 2-60).156 Reasonable yields can usually be obtained. (+)-147a or (+)-147b favors formation of the (S )-product, as would be expected, because these oxidations proceed via a transition state that parallels that previously discussed for the stereoselectivity observed with ketones.157... 

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). 

Fluorination of the sodium enolate of 2-methyl-1-tetralone by (-)-A-tluoro-2,10-(3,3-dichlorocamphorsultam) gives (5 )-(- -)-2-iluoro-2-methyl-1-tetralone in 70% ee, which corresponds to the opposite asymmetric induction to that achieved using non-racemic (camphorsulfonyl)oxaziridines as closely related hydroxylation reagents. ... 

ASYMMETRIC OXIDATION OF LITHIUM ENOLATES OF ESTERS AND AMIDES USING (+)-(2R.8aS)-10-(CAMPHORYLSULFONYL)OXAZIRIDINE... 

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. 

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. 

The research work of recent years includes predominantly the epoxidation of alkenes9,200, asymmetric hydroxylations209,224-228 and the asymmetric oxidation of sulfides to sulfoxides205,209,229,230. Optical yields of practical significance were obtained (>90%). A detailed review published in 1991231 reports about the versatile use of oxaziridines in the field of the electrophilic amination. 

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]. 

Whereas several X-ray structural analyses prove the endo orientation of the oxygen atom of the oxaziridine ring, the assumption of an exo oriented hydroperoxy group in 83 is based solely on the opposite sense of asymmetric induction observed for 82 and 83. 

An asymmetric synthesis of benzenesulfinates bearing a phosphonate group at the ortho -position has been achieved by a diastereoselective oxidation of the correspond- (g) ing sulfenates with chiral oxaziridines.210... 

V-Alkoxycarbonyl- and /V-carboxamido-oxaziridines (35) have been developed as reagents capable of converting aromatic alkenes into epoxide, aziridine, or hydrooxidation products, in ratios depending on the oxaziridine structure. Chiral oxaziridines can effect epoxidation and hydrooxidation with promising levels of asymmetric induction.48... 

The vast majority of organocatalytic reactions proceeds via covalent formation of the catalyst-substrate adduct to form an activated complex. Amine-based reactions are typical examples, in which amino acids, peptides, alkaloids and synthetic nitrogen-containing molecules are used as chiral catalysts. The main body of reactions includes reactions of the so-called generalized enamine cycle and charge accelerated reactions via the formation of iminium intermediates (see Chapters 2 and 3). Also, Morita-Baylis-Hillman reactions (see Chapter 5), carbene-mediated reactions (see Chapter 9), as well as asymmetric ylide reactions including epoxidation, cyclopropanation, and aziridination (see Chapter 10), and oxidation with the in situ generation of chiral dioxirane or oxaziridine catalysts (see Chapter 12), are typical examples. 

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. 

Electrophilic amination with oxaziridines 91S327. A-Sulfonyloxaziridines as reagents in asymmetric hydroxylation of eno-lates 92CRV919. 

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