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

Scheme 2.8
2-(Arylthi0)-2-nitrooxiranes 2.27 have been used as intermediates towards the
synthesis of various B-hydroxy-a-arnino acids." Either diastereomer can be accessed in
the case of P-alkyl-8-hydroxy-a-amino acids while only anti-p-aryl-p-hydroxy-a-amino
acids may be synthesized (Scheme 2.9). Various aldehydes were condensed with the
dianion of (4-methy1phenylthio)nitromethane to give the alkenes 2.27 in 5244% yield
(Sc heme 2.9). Subsequent deprotection, epoxidation and reprotection gave epoxides 2.28
in 40-55% yield over three steps. Syn epoxidation occurs with tBuOOLi with 12: 1
selectivity whereas anti epoxidation with Ph300K gives 15: 1 selectivity. Ring opening
of the epoxide with aqueous ammonia and trapping with benzyl chloroformate gave the
desired protected p-alkyl-f3-hydrox y thioester derivatives in 55-68 % yield. Treatment
with either mercuric acetate or MeOLi in methanol gave the methyl ester protected p-
alkyl-8-hydroxy-a-arnino acid.