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

The main byproduct of elimination was the a#-unsaturated isomer 3.27. The
protected vinylglcine 3.26 was found to be particuiarly base sensitive as prolonged
stimng with triethylarnine or N-methylmorpholine in organic solvent or in aqueous 0.5 N
LiOH gave quantitative conversion to 3.27. However, vinylglycine is stable to the acid
hydrolysis conditions required for protecting group removal(6N HCl, I hour at 100°C).'
An equally effective method was described in 1984 by Hanessian and Sahoo in
which L-vinylglycine was prepared fiom glutamic acid 3-28 by a decarboxylative
elimination of N-Cbz-L-glutamic acid monomethyl ester 3.29 (Scheme 3.1 l)?' Acid
hydrolysis gave L-vinylglycine in 49% yield from the protected glutamic acid 3.29,
which in tum was obtained in 88% yield from Cbz-Glu. An improved version of the
Hanessian decarboxylation/elimination procedure was reported in 1991 after the authors
found that the synthesis often led to substantial racemization (40-90% ee) when either the
N-Boc benzhydryl ester, N-Cbz benzyl ester or N-Cbz methyl ester was prepared."
Enantiomerically pure N-Cbz benzyl ester could be obtained by crystallization (in 28%
yield from N-Cbz-L-glutarnic acid).
Two other routes to enantiomencally enriched vinylglycine were reported in 1985
(Scheme 3.12), both proceeding through photochernical rearrangements and selenium
oxidative eliminations giving comparable overall yields of approximately 45% of
vinylglycine 3.7.
Scheme 3.11