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

Scheme 2.16
0 0
L-Selectridû
9:1t099A
L-Ser- HO +OH #&k4O-f&
NH2S02Ph
/ NHSO2Ph
NHfi02Ph \
1.50
R = CH, CH=Cb,
2.32
LiBH,
6:1 HO-R
OH
Nucleophilic additions4' to Garner's aidehyde 1.53 introduces the f3-hydroxy
motif in the correct position with good selectivity although the method suffers from the
limitation that an oxidation step is required to generate the a-carboxylate necessitating a
tedious protec tion/oxidation/deprotec tion scheme.
Joullié and coworkers have demonstrated good to high selectivity in the Grignard
addition of various nucleophiles to Garner's aldehyde 1.53. Generally the erythro
diastereomer predominated (9:l to 15: 1) dthough in the case of PhMgBr the threo
diastereomer was generated in excess (8:l) (Scherne 2.17).* Deprotection gave the
amino alcohol2.33 in 10-22% yield. The two-step oxidation to the protected b-hydroxy-
a-amino acids proved to be difficult, proceeding in 5-22% yield under a number of
conditions and demonstrating one of the shortcomings of Garner's aldehyde.
OH
1