Camphorsulfonyl oxaziridine

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... 

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. ... 

KN(SiMe3)2 / camphorsulfonyl oxaziridine Me,SO, (MesSihNH/m-aCtHtCCbH/n-BaiNF... 

Commercially available (7 y)(+)-(10-camphorsulfonyl)oxaziridine (CSO) and (/A),(+)-(8,8-dichlorocamphorsulfo-nyl)oxaziridine (DCSO) were used as oxidizing reagents rapidly furnishing quantitative silyl phosphite derivatives (Equation 11) <1998TL7123>. 

Reaction of 2-(benzisoxazol-3-yl)-7-[2-(3,3-tetramethylenegluta-rimido)ethyl]perhydropyrido[l,2- ]pyrazine with sodium bis(trimethyl-silyl)amide in THF at -70°, then with (+)-(camphorsulfonyl)oxaziridine gave a diastereomeric mixture of 7-[2-(2fl-hydroxy-3,3-tetramethyleneglu-trimido)ethyl] derivatives (93MIP1). 

KN(SiMe3)2/camphorsulfonyl oxaziridine Me3SiI, (Me3Si)2NH / m-ClC COaH / n-Bu4NF... 

Enantioselective Oxygenation of Prostereogenic Enolates with Enantiomerically Pure (Camphorsulfonyl)oxaziridines... 

Dihydro-2-hydroxy-2-methyl-l(2//)-naphthalenone (7), a model for many natural products, is obtained in >95% ee via oxidation of the sodium enolate of 3,4-dihydro-2-methyl-1 (2f/)-naphthalenone (5) with ( + )-[(8,8-dichlorocamphoi)sulfonyl]oxaziridine (6)88. This material was obtained in less than 16% ee using (j-)-(camphorsulfonyl)oxaziridine 26. 

Deprotonation of allyl ether 233 with t-BuLi in the presence of tetramethylethylenediamine (TMEDA) followed by reaction with (camphorsulfonyl)oxaziridine 202 produced alcohol 234. The product was isolated in 29% yield as the sole geometrical isomer <2005TL3465>. An advantage of using the (camphorylsulfonyl)oxaziridines for these hydroxyla-tions is that the T-sulfonylimine by-product is resistant to addition of the organometallic reagent <1987TL5115>. 

Hydroxylation of the lithium enolate of 264 with (-l-)-(camphorsulfonyl)oxaziridine 202 followed by trapping of the a-hydroxyl products with /-butyldiphenylsilyl chloride (TBDPSCl) gave mixtures of a-silyloxy products 265 and 266 in 84% yield in a ratio of 11 1 <2001JOC3338>. Use of (+)-(dichlorocamphorsulfonyl)oxaziridine 179 improved both the yield and diastereoselectivity. 

Treatment of cyclophenophrobide a-eno 269 with (—)-(camphorsulfonyl)oxaziridine 202 in the presence of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gave hydroxy chlorophyllone a-hydroxy diketone 270 in 94% yield and 90% de <1996JOC2501>. For hydroxylations of related /3-diketones, see <1995TA313> and <2006T3412>. 

Treatment of the laterally lithiated amide generated from lactam 273 with LDA with /ra r-2-phenylsulfonyl-3-phenyloxaziridine 33 afforded hydroxyl product 274 in 85% yield as a single isomer <1999JOC8627>. Use of (+)-(camphorsulfonyl)oxaziridine 202 gave similar results. The /ra t-stereoselectivity is consistent with the earlier finding that the hydroxylation stereochemistry is controlled by nonbonded steric interactions in the transition state such that the oxygen of the oxaziridine is delivered from the sterically least hindered direction. Treatment of 275 with LDA followed by (+)-(camphorsulfonyl)oxaziridine 202 afforded hydroxyl product 276 in 47% yield and 60% ee <1997T8881>. 

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. 

Convenient procedures for the preparation of substituted camphorsulfonyl oxaziridines have been reported <1997JOC6093, 19960S159>. 

The (camphorsulfonyl)oxaziridine derivatives, for example (74), generally give lower ees for the asymmetric oxidation of sulfides to sulfoxides than A-(phenylsulfonyl)(3,3-dichloro-camphoryl)oxaziridine (69) <88JA8477,88JCS(P1)1753,92JOC7274). 

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... 

Detailed studies by Davis et al. [63] demonstrated that enolate oxidation, using (camphorsulfonyl)oxaziridine (lS)-(+)-52 or (li<)-(-)-52 (Fig. 7) as a chiral oxidizing agent, was highly enantioselective. However, the yields and ees of the hydroxylated products are strongly dependent on the structure of the substrate and oxaziridine as well as the reaction conditions (enolate counter-ion, solvent, temperature) [64]. 

In the synthesis of a second-generation anticancer taxoid ortataxel, deprotonation of 13-oxobaccatin III by KHMDS (THF-HMPA, 83 17) followed by electrophilic oxidation with (IR)-(lO-camphorsulfonyl)oxaziridine afforded the C14-hydroxylated... 

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