N-Sulfonyloxaziridine

The N-sulfonyloxaziridines are an important class of selective, aprotic oxidizing reagents.12 Enantiomerically pure N-sulfonyloxaziridines have been used in the asymmetric oxidation of sulfides to sulfoxides (30-91% ee),13 selenides to selenoxides (8-9% ee),14 disulfides to thiosulfinates (2-13% ee),5 and in the asymmetric epoxidation of alkenes (19-65% ee).15-16 Oxidation of optically active sulfonimines (R S02N=CHAr) affords mixtures of N-sulfonyloxaziridine diastereoisomers requiring separation by crystallization and/or chromatography.13... 

Complete details for synthesis of ( + )- or ( —)-l from (IS)- or (1R)-10-camphor-sulfonic acid in 77% yield are now available. In general, this oxaziridine is less active than other N-sulfonyloxaziridines, but it is the preferred reagent for hydrox-ylation of lithium enolates of esters, amides, and ketones in 30-95% ee.1... 

Table 29 Preparation of N-sulfonyloxaziridines 353 from A/-sulfinylimines 352 <2005OL5493>...

The asymmetric hydroxylation of ester enolates with N-sulfonyloxaziridines has been less fully studied. Stereoselectivities are generally modest and less is known about the factors influencing the molecular recognition. For example, (/J)-methyl 2-hydroxy-3-phenylpropionate (10) is prepared in 85.5% ee by oxidizing the lithium enolate of methyl 3-phenylpropionate with (+)-( ) in the presence of HMPA (eq 13). Like esters, the hydroxylation of prochiral amide enolates with N-sulfonyloxaziridines affords the corresponding enantiomerically enriched a-hydroxy amides. Thus treatment of amide (11) with LDA followed by addition of (+)-( ) produces a-hydroxy amide (12) in 60% ee (eq 14). Improved stereoselectivities were achieved using double stereodifferentiation, e.g., the asymmetric oxidation of a chiral enolate. For example, oxidation of the lithium enolate of (13) with (—)-(1) (the matched pair) affords the a-hydroxy amide in 88-91% de (eq 15). (+)-(Camphorsulfonyl)oxaziridine (1) mediated hydroxylation of the enolate dianion of (/J)-(14) at —100 to —78 °C in the presence of 1.6 equiv of LiCl gave an 86 14 mixture of syn/anti-(15) (eq 16). The syn product is an intermediate for the C-13 side chain of taxol. 

The nitrogen atom in oxaziridines usually constitutes a stable stereogenic center. The nitrogen atoms in A -alkyloxaziridines and N-aryloxaziridines have barriers to inversion of the order of ca. 30-33 kcal mol" (1 cal =4.18 J), rendering these compounds configurationally stable at ambient temperature. Alternatively, the various N-sulfonyloxaziridines appear to be more readily epimerized, with inversion barriers of ca. 19-21 kcal mol". 3-Alkoxyoxaziridines readily epimerize at room temperature, possibly through zwitterionic intermediates. ... 

Maccagnani, G., Innocent , A., Zani, P., Battaglia, A. Oxidation of thiones to sulfines (thione S-oxides) with N-sulfonyloxaziridines synthetic and mechanistic aspects. J. Chem. Soc., Perkin Trans. 21987,1113-1116. 

The product stereochemistry for reagent-induced hydroxylations are under the control of a noncovalently bound chiral reagent which avoids the introduction and eventual removal of the chiral auxiliary as discussed in the preceding section. This method requires an enantiopure N-sulfonyloxaziridine of which (camphorylsulfonyl)oxaziridines (74), (114), and (158) have proven the most useful <92CRV919>. Both epimeric a-hydroxy carbonyl compounds are readily available because the antipodal oxidant controls the absolute stereochemistry of the product (Scheme 25). Oxaziridines (74) and (114) are commercially available. 

Chiral N-sulfonyloxaziridines are useful reagents for the asymmetric synthesis of sulfoxides, selenoxides, and other substrates. Davis and co-workers <97JOC3625> have reported the first example of an e.vtt-camphorylsulfonyloxaziridine 175. prepared by the MCPBA oxidation of camphor inline 174 in the presence of potassium hexamethyldisilazide. This compound was then studied in various asymmetric oxidation applications. 

Davis et al. have developed a useful class of chiral oxidants based on N-sulfonyloxaziridines derived from camphor (for a review, see ref. [40]). Compounds (28) and (29) are the most efficient reagents [41,42]. Representative results are listed in Table 1.1. 

N- Sulfonyloxaziridines Amine oxides from tert. amines R3N - R3N-.0... 

---