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

Ser(a1d)-OB0 gave the desired olefin 338 in 7696 yield over two steps (Swem and
olefination). Critical to the success of the reaction was use of freshly purified zinc5' and
rigorous anhydrous conditions to prevent Lewis acid cataiyzed opening of the OB0 ester.
Subsequent acid hydrolysis and ion exchange purification gave L-vinylglycine in 39%
yield over five steps and 86% ee determined by derivatization and HPLC anaiysis as
described in section 2.4.15a. The cause of racemization is unclear since the optical purity
of the aldehyde 3.37 was known to be >97% ee and vinylgîycine has been shown to be
stable to the acid hydrolysis conditions ~quired to remove the protecting groups."
Scheme 3.20
3.2.2 Peterson Olefination of Cbz-L-Ser(ald)-OB0 ester
The racemization diff~culties occurring with the Takai reaction led us to consider
other olefination techniques. At that time, a report appeared in the literature detailing the
use of the Peterson reaction to synthesize (S)-2-amino-(2)-3,s-hexadienoic acid, an
isolate from the beetle Leptinotarsa decemheata (Scheme 3.2 1 ).52
Scheme 3.21