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

and excellent &astereoselectivïty with no observable racemization. Only one
stereoisomer was visible within the detection limits of TLC, 'H-NMR and 1 3 ~ and - ~
was tentatively assigned as the isomer due to the observed results for the y-methyl
denvative. This was contmed upon deprotection, derivatization and HPLC analysis
(vide infra) and cornparison to literature results. The major by-product of the additions
was unreacted Cbz-Glu(0Me)OBO ester 4.69. Care was taken to ensure diastereomeric
e~chment did not occur der purification.
Ethyl iodide gave the y-ethyl derivative 4.78 in 75% yield, allyl bromide the y-
allyl denvative 4.79 in 83% yield and benzyl brornide the ybenzyl derivative 4.80 in
76% yield. The reactivity, leaving group and bulk of the electrophile had little effect on
the yield and selectivity.
4.2.3 Aldol Reaction of Cbz-Glu(OMe)OBO 4.69.
The aldol reaction of benzaldehyde with the enolate of 4.69 gave only two of the
four possible stereoisomers of 4.91 in 71% yield and ratio of 3: 1 (determined by 'H-
NMR and 13c-NMR) compared to the aldol reaction of 4.45 (P=Cbz, R'=~Bu, R ~=M~) in
which a 1:1 ratio of diastereomea was reported (Scheme 4.23)? The ratio was
determined by the integration of well resolved paaks in the 'H-NMR spectmm. The
stereochernistry of the major diastereomer was not identifid although, based on previous
results. presumably the 2S,U stereoisomer predominates.