Solution and Solid Phase Synthesis of Unusual a-Amino Acids From

Scheme 4.22
M'2aL
O
2) 1) LiHMDS, PhCHO
CbzH
4.66 4.91
The 2-sulfonyloxaziridine 4.92 of Davis and coworkersU has been used
extensively to a-hydroxylate ketones and enolates due to its convenient preparation.
However, only one account of its use to synthesize y-hydroxyglutamic acids has been
reported giving 2S,4S-Cbz-Glu(y-OH)(OMe)OMe d46 (P=Cbz, R'=R~=M~, E-OH) in
70% yield as a 9: 1 mixture of inseparable diastere~mers.~ Using identical conditions as
those optimized for the alkylation of Cbz-Glu(0Me)OBO 4.69, Davis' oxaziridine 4.92
was added to the lithium enolate resulting in a cornplex mixture of products that were not
isolated. However, when 3 equivalents of LNMDS was added to a stimng mixture of
the oxaziridine 4.92 and Cbz-LGIu(0Me)OBO ester 4.69 at -78"C, three distinct
products were isolated. The suifonamide 4.94, an adduct of the sulfonimine 4.93 and
enolate of 4.69, the silyl ether 4.95, presumably generated through trimethylsilylation of
the oxyanion by hexamethyldisilazane, and the desired produa 4.96 (Scheme 4.24).