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

and during acid hydrolysis. HPLC analysis conditions for al1 the y-akyl glutamates were
identical to those describeci previously (section 2.4.lSa) and gave excellent separation.
HPLC analysis of the diarninoglutamic acids was unsuccessful due to the partial reaction
of both amino groups with the derivatizing agent to give two peaks for meso-
diaminoglutamic acid 4.111 and three for 2S,4S-diarninoglutamic 4.11 acid.
It was also difficult to obtain samples that were analytically pure by elemental
analysis although spectroscopic techniques al1 indicated high purity. This rnight be
caused by smdl amounts of residual salts and the fact that recrystallizations were
generally unsuccessful. However, optical activity for ail the isolated y-substituted
glutamic acids agreed with literahire results.
4.2.10 Discussion of the Stereoselectivity of Electrophilic addition to the Cbz-
Glu(0Me)OBO ester 4.69 Enolate
The model depicted in figure 4.10 has been previously proposed to explain the
1,3-induction occumng in the addition of various electrophiles to the enolates of
protected glutamates.lq Gu et al. have also explained their high diastereoselectivities by
way of a chelated dianion, similar to those reported for the alkylation of suc ci ni mi de^.^^
Indeed, Our results further support these models since the increased steric buk of the
OB0 ester comlates well with the increase in diastereoselectivity observed as predicted
by the model (Figure 4.10). For example, the hydroxylation of protected glutamic acid
4-45 (P=Cbz, R'=R~=M~), where a methyl ester replaces the OB0 ester proceeds with
9: 1 2S94S;2S,4R selectiviw compared to >95:5 in the case of Cbz-Glu(0Me)OBO ester
4.69 to give 4-96.
183