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

L-vinylglycine has also been synthesized using methodology previously
developed in our laboratory. Cbz-Ser(a1d)-OB0 ester 3.37 was reacted with
methylenetriphenyl-phosphorane to give the vinylglycine intermediate 3.m which was
then deprotected to give vinylglycine in 5 1% yield over 3 steps albeit in 77% ee?
Scheme 3.19
3.1.4 Synthesis of Labelled fl ~Unsaturated Amino Acids
A major disadvantage of many of the methods developed for the synthesis of
vinylglycines, especially those that rely on degradation of an amino acid, is that isotopic
label incorporation is mcult.
Two synthesis leading to racemic products have been described. Chang and
Walsh1= prepared (z)-[~-~H], (E)-[~-~H] and (~)-[3,4-~~~]vin~l~l~cine for mechanistic
studies by catalytic hydrogenation of aikyne intermediates. Sa~ada~~ and CO-workers
prepared (~)-[3,4-~~z], (2)-13 ,4-2~t], (E)-[~-~H] and [3 ,4,4-2~3]-vinylg~ycine via a
stereoselective reduction of phenyl-2-(TMS)-ethynyl sulfone.
3.2 Results and Discussion: Synthesis of Vinylglycine from Serine
Given that the synthesis of vinylglycine 3.7 via Wittig olefination of Fmoc-
Ser(a1d)OBO resulted in partial racernization (77% ee) and very poor yields due to Fmoc
cleavage,' we hypothesized that the basic ylide was responsible for both the racernization