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

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

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

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5.21<br />

5.00<br />

16.63<br />

Figure 4.5: GCMS <strong>of</strong> 2S,4S-Cbz-L-G1u(yCH3)(OMe)OB0 ester 4-73 (top) <strong>and</strong><br />

epimerized 2S,4RS-Cbz-L-Glu(yC&)(OMe)OBO ester 4.7314.74 (bottom). The<br />

isoeyanate (rt 16.5 min) is a result <strong>of</strong> pyroiysis <strong>of</strong> the carbamate on the columo.<br />

Extensive attempts to punfy the 2S,4S-Cbz-Glu(y-Me)(OMe)OBO ester 4.73 by<br />

both flash chromatography <strong>and</strong> HPLC failed to remove the unknown doublet at 61 .O5<br />

ppm. In order to dismiss the possibility that racemization had axumd at the a-carbon,<br />

Cbz-L-Glu(0Me)OBO ester 4.69 was treated with excess LiHMDS for 6 hours then<br />

161<br />

CbzH<br />

m . . . - , . . . . , . ,<br />

1 O. 00 1 5.00 20.00<br />

1<br />

. . r i .<br />

25.00

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