A-Sulfonyloxaziridines

The tartrate (or TADDOL) derived approach to catalyst design has also been applied to the enantioselective a-hydroxylation of p-ketoesters. In this case, an enantiospecihc titanium(IV) complex combines with a sulfonyloxaziridine as the... 

When overoxidation takes place by using peracids, A-sulfonyloxaziridines have been proposed as selective and mild oxidizing agents411. Oxidation of Michler s thioketone with chlorine, resulting from the decomposition of chloroform, yielded a compound without sulfur in the molecule412. 

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

Oxidation of enethiolates derived from dithioesters 123 with A -sulfonyloxaziridine 117 gave the corresponding vinyl sulfenates 124. Subsequent alkylation with alkyl halides led to ketene dithioacetal A-oxides 125 in good to excellent yields (Equation 5) <2004JOC6916>. 

Oxidation of pentacarbonyl[amino(arylthioalkyl)carbine]chromium(0) complexes 131 with A -sulfonyloxaziridine 132 afforded the corresponding sulfoxides 133 in good to excellent yields <1995SL666>. The oxidation is chemoselective, affording only sulfoxide products whereas the use of dimethyloxirane resulted in overoxidation to sulfones. 

Rosse et al. described the oxidation of the resin-bound /3-sulfenyl hydroxamic acids A -sulfonyloxaziridines <2001SL538>. For example, oxidation of 143 with 33 afforded the corresponding sulfoxide 144 in 71% yield after cleavage of the resin with TFA. Apfel et al. in the preparation of some potent peptide deformylase inhibitors and antibacterial agents employed a similar transformation<2000JME2324>. 

Asymmetric sulfide oxidations are reported using oxaziridines other than A -sulfonyloxaziridines, but it is necessary to use a protic acid or Lewis acid to increase their reactivity. For example, -tolyl methyl sulfide 152 with bicyclic oxaziridine 153 in the presence of TFA gave the (A)-sulfoxide 154 in 50% yield and 42% ee in 24 h <1999T155>. It is interesting to note that use of MsOH resulted in much faster reaction with the oxidation complete in less than a minute. Similarly, sulfide 155 with chiral oxaziridine 156 in the presence of zinc chloride afforded sulfoxide 157 in 30% yield and 55% ee <2005JOC301>. 

Goto et al. reported that oxidation of the hindered, stable, sulfenic acid 165 with A -sulfonyloxaziridine 33 in methylene chloride at room temperature gave the corresponding sulfinic acid 166 in 70% yield <1997JA1460>. 

Epoxidation of alkene 216 with A-sulfonyloxaziridine 217 gave /ra r-epoxide 218 and m-epoxide 219 in 64% and 15% yield, respectively <1999TL8637>. Use of methyl(trifluoromethyl)dioxirane exhibited no selectivity. 

Cascade carbolithiation-hydroxylation of 242 with /-BuLi and A -sulfonyloxaziridine 33 gave bicyclic allylic alcohol 243. The alcohol was isolated in 49% yield and >20 1 d.r. <2004JA3434>. 

Many metal enolate a-hydroxylations have been performed with racemic fra r-2-phenylsulfonyl-3-phenyloxazir-idine 33 because it is more reactive than the enantiomerically pure camphor-derived A -sulfonyloxaziridines (Equation 11). Examples are given in Table 20. 

Treatment of a,/ -unsaturated lactam 252 with lithium hexamethyldisilazide (LiHMDS) followed by oxidation of the resulted dienolate with A -sulfonyloxaziridine 33 gave rise to the a-hydroxylated product 253 in 38% yield <2001TL7879>. The y-hydroxy product 354 was also isolated in 35% yield. 

The organocatalytic asymmetric a-hydroxylation of ketones with A -sulfonyloxaziridines was studied by Engqvist et al. <2005TL2053>. For example, the direct diamine-catalyzed enantioselective a-hydroxylation of ketones 255 with 7ra S -2-/>-tolylsulfonyl-3-phenyloxaziridine 33 in the presence of chiral diamine 256 afforded the corresponding a-hydroxylated products 257 in moderate yields with up to 63% ee. 

Hydroxylation of enolate generated from 7-TES-13-oxobaccatin III 267 with various A -sulfonyloxaziridines, 33, (+)-146, and (+)-202, were studied by Baldelli et al. <2003JOC9773>. 7-TES-13-oxo-14/30H-baccatin III 268 was obtained in 83-88% yield. The yield and diastereoselectivity of the hydroxylation were not affected by the choice of oxaziridine, indicating that the stereoselectivity was substrate controlled. 

Reaction of a mixture of unsaturated esters 271a and 271b with KHMDS followed by addition of A -sulfonyloxaziridine 33 gave a-hydroxy ester 272 in 72% yield <1995TL413>. The bromo analogue was prepared similarly in 78% yield. 

Indole 286 reacts A -sulfonyloxaziridine 33 to gave a mixture of diastereomeric adducts 288 and 289 in a combined yield of 71% <1997JA1159>. A plausible mechanism for this transformation is that C-3 of the indole system acts as a nucleophile to yield a zwitterionic intermediate 287 which cyclizes to the products. 

Garcia Ruano al. reported an important new procedure for the preparation of diverse A -sulfonyloxaziridines <2005OL5493>. This method involves the one-pot, two-step oxidation of readily available A -sulfmylimines 352 first with MCPBA followed by MCPBA/KOH to give the oxaziridines 353 in excellent yields (Table 29). 

The advantage that this procedure has over earlier methods is that A -sulfonyloxaziridines derived from aliphatic aldehydes and ketones can be prepared. 

Asymmetric Oxidation of Sulfides. Prochiral sulfides are oxidized by (camphorylsulfonyl)oxaziridine (1) to optically active sulfoxides. Over-oxidation to sulfones is not observed (eq 1 ). However, the best chiral A-sulfonyloxaziridines for the asymmetric oxidation of sulfides to sulfoxides are the (+)- and (Phenylsulfonyl )(3,3-dichlorocamphoryl )oxazi ridinesfi... 

V-Sulfonyl amides (ArS02NHC(0)Ph), isolated in the photolysis of A-sulfonyloxaziridines, are formed from the oxaziridine singlet state <77TL1721>. 

Thiones (Ar2C=S) are oxidized to thiones 5-oxides (Ar2C=S=0) by A-sulfonyloxaziridine (78) (Scheme 14) <87JCS(Pl)l 113). The oxidation is fast and quantitative, exhibiting second-order kinetics. Thiocamphor 5-oxide (79) and thiofenchone 5-oxide (80) were prepared in 91% and 76% yield, respectively by oxidation of the corresponding thiones with (78b). Earlier attempts to prepare these materials with other oxidants resulted in low yields and the products were difficult to purify. Phosphorothionates (R2P(S)OPh) are oxidized to phosphates (R2P(0)0Ph) by A-sulfonyl-oxaziridines <88MI 112-01). 

Of particular concern with a-hydroxy carbonyl compounds is the stereochemistry of the hydroxy group attached to the stereogenic carbon because biological activity is often critically dependent on its orientation. A-Sulfonyloxaziridines have played a prominent role in the stereoselective synthesis of this key structural element (Scheme 25). Enantiomerically and diastereomerically enriched materials have been prepared by (1) the hydroxylation of chiral nonracemic enolates with racemic A-sulfonyloxaziridines, for example (63a) (2) the asymmetric hydroxylation of prochiral enolates with enantiopure A-sulfonyloxaziridines and (3) a combination of the first two, double stereodifferentiation. 

---