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with brine and concentrated under vacuum to afford a dark-yellow oil.
Step 3. TMS removal: The dark yellow oil from step 2 was dissolved in THF (20 mL), and
phosphate buffer (pH = 7.0, 1 mL) was added, followed by dropwise addition of TBAF (8 mmol
of a 1.0 M solution in THF) at rt. The reaction mixture was stirred for 3 h at rt and then diluted
with water (100 mL) and the aqueous layer extracted with Et2O (3 x 100 mL). The organic layers
were combined, dried over MgSO4 and concentrated under vacuum. Purification of the crude
residue by flash chromatography (hexanes-EtOAc, 4 : 1, v/v) afforded the allenic amino-ester.
H
MeO 2C
•
65d
H
NHCbz
2-Benzyloxycarbonylamino-2-methylhexa-3,4-dienoic acid methyl ester (65d). Prepared by
the general procedure E, using:
Step 1: LDA (17 mmol), 64d (2.36 g, 6.80 mmol), ZnCl2 (16.3 mL of a 0.5 M solution in THF,
8.16 mmol). Step 2: KHCO3 (1.37 g, 13.6 mmol), MeI (0.67 mL, 11 mmol). Step 3: TBAF (6
mL of a 1.0 M solution in THF, 6 mmol). Yield 65d (970 mg, 49%, 3 steps).
65d : 1 H NMR (300 MHz, CDCl3): δ 7.36-7.32 (m, 5H), 5.50 (bs, 1H), 5.44-5.36 (m, 2H), 5.13
(d, J = 12.5 Hz, 1H), 5.08 (d, J = 12.3 Hz, 1H), 3.74 (bs, 3H), 1.69-1.66 (m, 6H); 13 C NMR (75
MHz, CDCl3): δ 202.9, 173.2, 154.9, 136.5, 128.6, 128.3, 94.4, 91.9, 66.8, 58.6, 53.0, 23.6, 14.1;
IR (thin film): ν 3348, 2951, 1966, 1724, 1267 cm -1 ; MS m/z (%) 289 (21), 274 (10), 230 (42),
91 (100); EI-HRMS calcd for C16H19NO4 m/z [M] + 289.1314; found 289.1316.
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