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1.93 (d, J = 1.6 Hz, 3H), 1.85 (d, J = 6.9 Hz, 3H); 13 C NMR (75 MHz, CDCl3): δ 172.7, 166.8,
149.6, 135.4, 134.2, 131.4, 130.3, 129.0, 128.6, 128.3, 126.8, 116.9, 90.5, 35.9, 13.9, 10.7; IR
(thin film): ν 3344, 2924, 1758, 1639 cm -1 ; MS m/z (%) 297 (81), 252 (38), 192 (49), 174 (84),
105 (100); EI-HRMS calcd for C18H19NO3 m/z [M] + 297.1365; found 297.1379.
BzHN
O
179
O
PPh 3
CCl 4
Et 3N
MeCN
N O
Ph
•
O
H
1M HCl
70 o C
>95%
BzHN
HOOC
180 181
2-(Benzamido)-2-methylhexa-3,4-dienoic acid (181). Propargylic ester 179 (1.05g, 4.29 mmol)
was dissolved in acetonitrile (10 mL) at rt. Then Et3N (1.67 mL, 12.0 mmol), CCl4 (0.99 mL,
10.3 mmol) and PPh3 (2.36 g, 9.0 mmol) were added in consecutive order. The reaction mixture
was stirred for 2 h at rt while gradually turning brown color. The volatiles were removed under
vacuum (using a rotary evaporator heated to ~40 °C) and the crude brown residue was dissolved
in acetone (5 mL) and poured in an Erlenmeyer flask. Under vigorous stirring, Et2O (50 mL) was
added followed by hexanes (20 mL) which led to formation of a viscous precipitate (Ph3PO). The
mixture was filtered on a fritted funnel over a plug of silica gel (~ 2 cm height) eluting with
hexanes-EtOAc (1 : 1). The collected filtrate was concentrated under vacuum to afford allenyl
oxazolone 180 (718 mg, 74%).
Oxazolone 180 (105 mg, 0.415 mmol) was dissolved in 1,4-dioxane (3 mL) in a 25 mL round-
bottom flask and 1M HCl (3 mL) was added. The flask was sealed with a rubber septum and
heated to 70 °C for 10 min. The reaction solution was then diluted with water and extracted with
Et2O. The organics were combined, washed with brine, dried over MgSO4 and concentrated
under vacuum to afford acid 181 (100 mg, >95%) which was used in the next step without
additional purification.
224
•
H