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

solution, enolization was previously dismissed as a source of racernization for Boc and
Fmoc protected Ser(ald)-OB0 esters 2.46 and 2*47 as described in chapter two.
Reducing the reaction time during Swem oxidation resulted in significant deterioration of
purity, a result of incomplete oxidation since HPLC andysis indicated serine or threonine
contamination after cleavage off the resin.
The stereochemistry of addition is identical to that observed in solution phase
chemistry, confirmed by the cornparison of HPLC prducts derived after cleavage off the
resin to those reported previously6 and in chapter two. HPLC analysis generaily showed
ail four diastereomers although the very minor erythro-D-isomer was usually dificult to
identify. MAS "C NMR of 6.37 also clearly showed two stereoisomers (Figure 6.2).
f3-Dialkyl-8-hydroxy-a-amino acid 655 was not isolated and is consistent with
the reported instability of this class of compounds.19