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

esin placed
hem
Figure 6.3: Experimental setup for Swern Oxidation. (a) Requires aIhg of the RBF
to trander the resh into the mixauP (b) resin is added to the mixture md any
remaiaing beads washed into solution.
The optirnized conditions for oxidation were determined by observing the
appearance of the aldehyde peak for 6.34 or the methyl ketone for 6.35 in the MAS 'H
NMR (Figure 6.2). The requirement of high purity in the find product necessitated
quantitative conversion to the ddehyde 6.34lketone 6.35 to prevent contamination with
remaining serine or threonine. Large excesses of reagents for the Swern oxidation were
required, (16-50 eq.) since performing the reaction on a scale comparable to that in
solution phase (1.6-5 eq.) failed to give any oxidation product. Eight hours at -78°C was
required for complete conversion to the aldehyde 6.34ketone 6.35 (Scheme 6.12). Other
methods of oxidation were also investigated including the Doering oxidation
(py~SOflMS0),~ Dess-Martin oxidation with 2.48:5 and TPAP (tetrapropyl
ammonium pemthenate)/NMe6 with littie success.