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ºC. After quenching the reaction by adding water (10 mL), the organic layer was washed with
diluted NH4Cl solution (2 x 100 mL) to remove the excess Et3N, dried over MgSO4, and
concentrated under vacuum. The crude residue was purifed by flash chromatography (hexanes-
EtOAc, 4 : 1 to 1 : 1, v/v) to afford the imino-oxazolidinone 155a (0.645 g, 85%). Note:
extended exposure of 155a to silica gel causes hydrolysis to 154a. All operations during the
aqueous workup procedure and chromatography were performed fast in order to minimize the
decomposition of 155a.
1 H NMR (300 MHz, CDCl3): δ 7.25-7.21 (m, 3H), 7.09-7.06 (m, 2H), 6.23 (s, 1H), 6.23 (q, J =
7.5 Hz, 1H), 6.19 (dd, J = 17.2, 10.8 Hz, 1H), 5.56 (dd, J = 17.2, 1.2 Hz, 1H), 5.32 (dd, J = 10.8,
1.2 Hz, 1H), 4.25 (s, 2H), 3.81 (s, 3H), 3.22 (d, J = 13.6 Hz, 1H), 3.11 (d, J = 13.6 Hz, 1H), 2.04
(d, J = 7.5 Hz, 3H); 13 C NMR (75 MHz, CDCl3): δ 169.5, 160.4, 153.3, 146.0, 144.7, 138.8,
132.7, 132.1, 131.0, 128.5, 128.0, 126.8, 119.6, 118.9, 66.6, 52.3, 49.2, 47.5, 16.3; IR (thin film):
ν 2953, 1840, 1744, 1272 cm -1 ; MS m/z (%) 380 (32), 336 (14), 245 (61), 217 (35), 91 (100); EI-
HRMS calcd for C21H20N2O5 m/z [M] + 380.1372; found 380.1370.
O
O
N
8a-Benzyl-6-ethylidene-1-isobutylimino-7-vinyl-6,8a-dihydro-1H-oxazolo[3,4-a]pyridine-
3,5-dione (155b). Prepared by following general procedure J, using 154b (0.170 g, 0.503 mmol),
Et3N (0.182 mL, 0.159 mmol) and phosgene (0.310 mL, 0.657 mmol of a 20 % solution in
toluene). Isolated yield 155b (0.125 g, 68%).
1 H NMR (300 MHz, CDCl3): δ 7.22-7.20 (m, 3H), 7.05-7.03 (m, 2H), 6.22-6.12 (m, 3H), 5.54
(dd, J = 17.4, 1.2 Hz, 1H), 5.29 (dd, J = 10.8, 1.2 Hz, 1H), 3.23 (d, J = 6.8 Hz, 2H), 3.12 (d, J =
N
Bn
O
155b
211