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TMS
MeO 2C
•
70
H
H
NHBoc
2-tert-Butoxycarbonylamino-3-trimethylsilylhexa-3,4-dienoic acid methyl ester (70). Ester-
enolate Claisen rearrangement. To a freshly prepared solution of LDA ii (20.4 mmol) in THF (10
mL) was added 69 (1.6 g, 5.1 mmol) in THF (10 mL) at -78 °C. After 10 min of stirring ZnCl2
(20 mL of a 0.5 M solution in THF, 9.8 mmol) was added at -78 °C. The reaction mixture was
allowed to warm to rt over 12 h. Upon completion the reaction mixture was diluted with Et2O
(100 mL), and treated with 1M HCl (100 mL). The aqueous layer was extracted with Et2O (2 x
100 mL), and the organic layers were combined and washed with brine. Concentration under
vacuum afforded a oily residue which was dissolved in DMF (10 mL). Pulverized KHCO3 (1.08
g, 1.07 mmol) was added at rt, followed by MeI (530 µL, 8.24 mmol). The reaction mixture was
left stirring for 2h at rt, and then diluted with water (100 mL). The cloudy mixture was extracted
(benzene-EtOAc, 1 : 9, v/v) (3 x 75 mL), the organic layers were combined, washed with brine
(2 x 50 mL) and concentrated in vacuo. Purification by flash chromatography (gradient elution:
hexanes-EtOAc, 32 : 1 to 19 : 1, v/v), afforded 70 (740 mg, 44%).
1 H NMR (300 MHz, CDCl3): δ 5.10-5.03 (m, 2H), 4.75 (d, J = 8.3 Hz, 1H), 3.72 (s, 3H), 1.63 (d,
J = 7.1 Hz, 1H), 1.45 (s, 9H), 0.15 (s, 9H); 13 C NMR (75 MHz, CDCl3): δ 207.2, 172.1, 155.0,
95.4, 84.5, 80.0, 53.4, 52.1, 28.4, 13.2, -1.1; IR (thin film): ν 3373, 2955, 1943, 1749, 1720 cm -1 ;
MS m/z (%) 314 (6), 257 (43), 242 (45), 198 (100); EI-HRMS calcd for C11H19NO4Si m/z [M-
56] + 257.1083; found 257.1080.
ii LDA was prepared as outlined in general procedure E.
165