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

Scheme 2.6
Epoxides are obvious intermediates in the synthesis of B-hydroxy-a-amino acids
since nucleophilic opening of the epoxide installs the desired p-hydroxy functionality.
Furthemore, Sharpless epoxidation generally provides a convenient route to the desired
epoxide chirality. in fact, many of the epoxide-derived methods described in section
1.3.2.1 are capable of the synthesis of p-hydroxy-a-arnino acids.
Genêt and coworkers have used the Sharpless epoxidation of crotyl alcohol 2.24
to establish the epoxide in the desired confoxmation for the synthesis of L-allu-
threonine? Oxidation to the acid 2.25 followed by ring opening with arnmonia gives L-
allo-threonine in 29% overail yieid (Scheme 2.7). However, the amination takes 10 days
limiting the applicability of the method.
Scheme 2.7
O iab
Sharpless , +#*..qCOpH
O .L+ 'O'J CH^
OH epoxidatiort 4'**4 CH2*" NdO,
HP C02H
2.24 2.25
Goodman has used a cyclic sulfate 2.26 (an epoxide equivalent) derived from
benzyl crotonate to synthesize allo-threonines and B-hydroxyvaiine in both gwd yield
and >99% ee (Scheme 2.8).37