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

In order to confimi the 'H-NMR assignments, the B-carbon of 5.41 was
epimerized with excess LDA to give a 1:l mixture of the 2$3S:2S,3R 8-methyl
derivatives 5.41 and 5.42 respectively (Figure 5.2) that were separable after labourious
flash chromatography. The 2S,3S diastereomer had a lower Rf than 2S,3R-5.41.
Although the a- and fl-protons of 5.41 and 5.42 were distinguishable at 300 MHz. higher
fields (500 MHz) were necessary to clearly differentiate between the 3s and 3R p-methyl
protons in order to assign diastereoselectivities for addition. ' H-NMR in solven ts other
than acetone-4 did not particularly influence the chemicai shifts of the resolved peaks
except that a rotamer becarne apparent in toluene-de.
Figure 5.2 30 MHz 'LI-NMR of ZSJS 5.41 (top) pnd S,3R Sm42 (bottom) Cbz-L-
Asp(p-Me)(OMe)OBO ester.