Hydroxylation of ester enolates

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. 

Diastereoselective hydroxylation of ester enolates The (E)-enolate of the chiral ester 1, derived from ( + )-camphor, generated with lithium N-isopropyl-cyclohexylamide (LICA) and HMPT (7, 209-210), undergoes hydroxylation with this reagent (11, 382) from the less hindered side with excellent diastereoselection. The (Z)-enolate as expected shows opposite, but modest, diastereoselectivity. 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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