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

2.1.3.3 Electrophilic%Nucfeophilic Aminotion
Evans' methodology (Scheme 1.13) is capable of producing both 2S.3RnR.3S
(syn or rhreo) and 2S,3S/2R,3R (anti or erythro) diastereomers. Addition of the enolate
of 2.21 to acetaldehyde gives a 955 selectivity of the syn diastereomer although 10%
epimenzation occurs at the a-center upon azide displacement (Scheme 2.5).33 This
methodology has been used to synthesize numerous p-hydroxy-a-amino acids including
MeBmt 2.11Y
Scheme 2.5
R = Me, Ph
67%. 95:s
L-allet hreonine
38% overall yield from 2.21 (R=Me)
Electrophilic amination has been used to synthesize D-allo-threonine from chiral
3-hydroxybutanoate 2.22 protected as a dioxanone 2.23 in 42% yield from the dioxanone
and with 99% diastereoselectivity (Scheme 2.6) using di-tert-butyl azodicarboxylate
@BAD) as the aminating agent?
Seebach and coworken have used a similar methodology in the synthesis of
trifluorothreonine in both excellent yield and diastereoselectivity using 4.4.4-uifluoro-3-
hydroxybutanoate instead of 2.22.''
49