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

vs. 1.40- 1.55 ppm and 2.00-2.10 ppm) and ?protons (2.36-2.50 ppm vs. 2.50-2-6 1 ppm)
were clearly distinguishable allowing for the assignment of diastereoselectivity in the
alkylation (Figure 4.4). It is interesting to note that 2S94R-Cbz-Glu(yMe)(OMe)OBO
ester 4.74 does not have the corresponding minor y-methyl doublet. GC-MS studies aiso
clearly indicated that methylation occumd stereospecifically (Figure 4.5). The two
diastereoisomers were also separable by TLC, although 2S,4R-Cbz-Glu(y-
Me)(OMe)OBO ester 4.74 had an identical Rf to that of the Cbz-L-Glu(0Me)OBO ester
4.69 synthon.
2 . - - 1 - i 1 I I 1 1 L
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0-5 ppm
F i 4.4: 300 MHz 'H-NMR of 2S,4R 4.74 (top) and 2S,4S 4.73 (bottom) Cbz-L-