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

5.4.5 ~Methyl-(2S)-2-[(be~loxy~ny1]amin011-(&metbyI-2,6,7-tri0~8bicycIo-
[2e2e2]~t-l-yl)pmpm~~ Cbz-L-Asp(0Me)OBO ester, 5.3û.
Cbz-L-Asp(0Me)-oxetane ester 5.35 (10.97 g, 30.0 mmol) was dissolved in dry CH2C12
(400 mL) while stimng under Ar. The mixture was cmled to 0°C then BF3 - Et20 (0.38
mL, 3.0 rnrnol) was added by syringe. The mixture was allowed to warm to rwm
temperature and stir for 4 hours after which a TLC indicated the reaction was complete.
Et3N (0.84 mL, 6.0 mmol) was then added and the mixture stirred an additional 30 min
before the solvent was removed in vocuo. The resulting oil was dissolved in EtOAc (600
m.) and extracted with 3% m C1 (2 x 100 mL), saturated NaHC03 (100 mL), brine
(100 mL) and dried over MgS04. The solvent was removed under reduced pressure to
reveal a light coloured oil w hich was purifileci by flash chromatograph y ( 1 : 1 Et0Ac:Hex)
to give a clear oil which recrystallized from Et0Ac:Hexanes to give 7.45 g (68%) of long
white crystals.
NMR (Cm13, 250 MHz) 6 7.39-7.26 (m, 5H, AM), 5.2 1-5.02 (m, 3H, NH, CbzC&),
4.33 (ddd, lH, J = 5.3, 7.9, 13.OHz, a-CH), 3.85 (s, 6H, OB0 ester Cm), 3.61 (s, 3H,
C02CH3). 2.68 (dd, 1H7 J= 5.3, 15.lHz, PCHH), 2.42 (dd, lH, J = 7.8, lS.lHz, B-CW
0.77 (s, 3H, OB0 ester CCH3); 13c C (ml3, 63 MHz) 6 171.4 CS), 155.9
(CONH), 136.6 (Cbz