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

Procedure C: 2-Methyl-2-(toluenesulfony1oxymethyl)oxee 2.38 (25.0 g, 97.53
mmol) and NaBr (50.18 g, 0.49 mol, 5.0 equiv) were suspended in dry acetone (250 rnL)
under dry nitrogen and refiuxed for 30 hours. The solution was first filtered and
decolonzing charcoal was added. The solution was filtered and the acetone was
evaporated in vacuo to give a colorless thick oil (14.8 g, 92%) which was used without
further purification.
2.4.4 (2-MethyC2-oxetanyI)methyl (ZS)-2-[(te~-b~toxy~bonyl)amino]-3-hydroxy-
propanoate, Boc-Ser oxetane ester, 2.42.
Boc-L-Ser ZAO (5.0 g, 0.024 mol) and Cs2C03 (4.69 g, 0.014 mol. 0.6 eq) were
combined and dissolved in &O (50 mL). The water was then removed in vacuo and the
resulting oil lyophilized for 12 hours to give a white foam. To this foam was added
oxetane tosylate 2.38 (6.46 g, 0.025 mol) and Nd (0.72 g, 4.8 mmol, 0.2 eq) which was
then taken up in DMF (250 mL) and allowed to stir under Ar for 48 hours. The DMF
was then removed in vacuo and the resulting solid dissolved in EtOAc (300 mL) and H20
(100 mL) and extracted with 10% NaHC03 (2 x 50 d), saturated NaCl (1 x 50 rnL) and
dned over MgSQ. The solvent was removed under reduced pressure to yield a yellow
oil which was purificd by flash chromatography (1: 1, Et0Ac:Hex) to yield the ester 2.42
in 66% yield (4.32 g) as a paie oil.
[a~*~~~~=
- 15.4 (c=1.02, EtOAc); TU3 (1 : 1, Et0Ac:Hex) Rf = 0.33; 'H NMR (CDC13,
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