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

of thiol trityl led to racemic cysteine (Scheme 1.25)? On closer examination, the S-trityl
anion was shown to cause the elimination of the tosyl group to give dehydroalanine 1.64,
followed by subsequent addition of thiol tri@ to the dehydrodanine adduct.
Despite this tendency of activated serine derivatives to eliminate, a number of
displacement reactions have been successfÛlly accomplished. For example, both sodium
benzyl selenoatea and thioacetate' displace the tosylate from O-tosyl, N-Boc or N-Cbz
senne methyl or benzyl ester to give the enantiomerically pure product.
The Cbz-Ser(0Ts)-OBn serine derivative can be converted to the P-iododanine
compound by Finkelstein reaction with NaI. Viallefont and coworkers have successfuliy
used organocuprates for displacement reactions with O-tosyl serine 1.62 and iodoalanine
1.63 with 2-70% formation of dehydroalanine 1.64 (Scheme 1.25).=
1.62 X=OTs Nal.
1.63 X= l acetone
Scheme 1.25
P = Boc, Ts
R = Me, Et, nPr, Mu, Bu, Ph, vinyl
Aziridines are another source of fbcation equivalents described by Okawa and
coworkers. Azindines are generated from serine in much the same manner as used to
form 1.62 (P = Cbz) except a N-trityl protecting group is used (Scheme 1.26)."
Scheme 1.26