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

MH). Changing the reaction conditions and base altered the stereoselectivity from
1 : 1 1 anti:syn (2S,3R:2S,3S) to 20: 1 antisyn (2S,3R:2S,3S). The same conditions were
used to synthesize the p-azide 5.11 @=Pm, ~'=t~u,Me, R~=M~) using trisyl azide 4.105
and di-r-butylazodicarboxylate (DTBAD). Trisyl azide consistently gave a 1 : 1 mixture of
fkpimers whereas the use of DTBAD to trap the enolate generated by LiHMDS in
HMPA gave a 30: 1 ratio of anti:syn (2S,3R:2S,3S).Y
Hanessian's group used similar methodology in reacting 5.11 (P=Ts, R'=R~=ZBU)
with Davis* oxaziridine 4.92 to give the $-hydroxyaspartate derivative 5.12 (P=Ts,
R'=R~=ZBU, &OH) in 45: 1 anti:syn (2S,3R:2S,3S), al though this methodology suffers
from the relatively harsh conditions required for removal of the sulfonamide N-protecting
group." Hanessian et al. also reported the allylation of various protected derivatives of
5.11 (P=Cbz, R'=M~, TMSE, R~=M~, TMSE) in generally greater than 955
(îS,3 R:2S,3S) diastereoselectivity?
Chamberlin and coworken used the synthon 5.11 (P=PhFl, Bn, R'=~Bu, R'=M~)
to generate both p-akyI diastereomers as a hinction of enolate geometry? The
potassium enolate gave selectivities of 1:10 syn:anti (2R93S:2R,3R) whereas lithium
enolates gave ratios of 23:l syn:anti (2Rq3S:2R,3R) in the addition of methyl iodide.
Chelation and non-chelation controlled addition models were used to describe the
observed results.
Parr et al. reported in 1999 the first concise study of the numerous factors
influencing 1.2-asyrnmetnc induction in dianionic fùnctionalization of L-aspartic acid
dies ter^.'^ This study was based upon earlier findings in which the enolate of 5.11
(P=Cbz, R'=ZBU, R~=M~) was methylated in poor diastereoselectivity while allylation