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

*
mp 69-7 1 OC; [a]2057s = -24.5 (c = 1.04, EtOAc ); TLC (1: 1, Et0Ac:Hex) Rf = 0.2 1; 'H
Nl4.R (-13, 250 MHz) 6 5.40 (br d, 1 H, J = 9.2W NH), 4.54 (d, 2H, J = 6.1 Hz,
oxetane Cm), 4.50 (d, 2H, J d.1 Hz, oxetane Cm), 4.44 (d, 1 H, J = 6.1 Hz,
CozCHH), 4.42 (d, 1H, J= 6.1Hz, C02CHN), 4.40-4-35 (m, lH, p-CH), 4.30 (br d, lH, I
= 9.2Hz, a-CH), 2.60 (br s, lH, OH), 1.36 (s, 9H, (CH&C), 1.27, (s, 3H, oxetane CC&),
1.21 (d, 3H, J = 6.3& p-CH3); 13c NMR (CDC13, 63 MHz) G 171.6 (C=O), 155.8
GQNH), 80.1 ((CH3)Q 79.5 (oxetane GHzO), 68.6 (COKH*), 68.0 @-CH), 59.0 (a-
- CH), 39.7 (oxetane -&), 28.3 (cH3)3C), 20.8 (oxetane CCH3), 20.0 ($-CH3); IR (cast
in CH2C12) 3386, 2975, 1694, 15 15, 1367, 1 164, 1055; HRMS (FAB) calculated for (M +
8.31; N, 4.62. Found: C, 55.74; H, 8.51; N, 4.68.
2.4.6 (1s)-1-[la-(tert-Butoxy~~~bon y1)-(lS)-1-amin0-2-hydr0xyethy1]4-methyl-
2,6,7-trioxabicyclo[2.2.2]0ctane, Boc-Laer-OB0 ester, 2.44.
Boc-Ser oxetane ester 2.42 (5.0 g, 0.0183 mol) was dissolved in dry CH2C12 (250 mL)
and cooled to O°C under Ar. BF3- EtzO (0.1 1 mL, 0.93 mmol) was diluted in CHIC12 (5.0
mL) and added to the reaction flask. The reaction was allowed to warm to room
temperature and checked by TLC. After 18 hours, Et3N (0.51 mL, 3.66 rnmol) was added
and the reaction stirred for an additional 30 minutes before being concentrated to a thick
oil. The crude product was redissolved in EtOAc (200 mL) and washed with 3% -1
(2 x 100 mL), 10% NaHC03 (100 mL), saturated NaCl (100 mL), dried (MgS04), and