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H
MeO 2C
•
H
H
NHBoc
2-tert-Butoxycarbonylaminohexa-3,4-dienoic acid methyl ester (71). To a solution of 70 (120
mg, 0.383 mmol), in THF (5 mL) was added TBAF (0.5 mL of a 1.0 M solution in THF, 0.5
mmol) premixed with phosphate buffer solution (pH = 7.3) (0.3 mL) at rt. The reaction was
stirred for 30 min when TBAF (0.3 mL of a 1M solution in THF, 0.3 mmol) was added. The
reaction was allowed to continue until TLC indicated absence of starting material (~ 3 h). Water
was added, and the aqueous layer was extracted with Et2O (3 x 20 mL). The organic layers were
combined, washed with sat’d aq. NH4Cl, dried over MgSO4 and concentrated under vacuum.
Purification by flash chromatography (hexanes-EtOAc, 9 : 1, v/v) afforded 71 (72 mg, 79%).
1 H NMR (300 MHz, CDCl3): δ 5.39-5.36 (m, 1H), 5.29 (bs, 1H), 5.13 (m, 1H), 4.78 (br s, 1H),
3.76 (s, 3H), 1.69 (dd, J = 7.0, 3.1 Hz, 3H), 1.45 (s, 9H).
Me
H
MeO 2C
H
72
71
H
NHBoc
2-tert-Butoxycarbonylaminohexa-2,4-dienoic acid methyl ester (72). To a solution of 70 (41
mg, 0.13 mmol) in THF (2 mL) was added TBAF (157 µL of a 2.0 M solution in THF, 0.314
mmol) at rt. After 10 min. water was added, and the aqueous layer was extracted with Et2O. The
organic layers were combined, washed with sat’d aq NH4Cl, dried over MgSO4 and concentrated
under vacuum to afford 72 (35 mg, >95%). Diene 72 was obtained as a mixture of isomers that
were separated by semipreparative HPLC (hexanes-EtOAc 9 : 1, v/v).
Spectral data of mixture: IR (thin film): ν 3358, 2980, 1727, 1159 cm -1 ; MS m/z (%) 241 (26),
166