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A
mixture of (2-amino-6-methoxycarbonylbenzyl)-
triphenylphoshonium bromide (4a) (1 mmol, 506 mg),
acetic anhydride (10 mmol, 0.94 mL) and triethylamine
(3.6 mmol, 0.5 mL) was heated at 120°C for 2 h. After
dilution with H2O, the mixture was neutralized with
NaHCO3 and extracted with ethyl acetate. The combined
organic layer was washed with H2O, dil. HCl and brine,
and dried over Na2SO4. After removal of solvent in
vacuo, purification by column chromatography (n-hex-
ane/ethyl acetate, 5/1) afforded methyl 1-acetyl-2-
methylindole-4-carboxylate (6a) (182 mg, 79%) as a pale
yellow powder.
7. The reaction was examined at 120°C employing 3a and
base (3.6 equiv.), varying the amount of acetic anhydride.
Representative data from this study: 1.5 equiv. of Ac2O
and i-Pr2EtN, 12 h, 60%; 1.5 equiv. of Ac2O and Et3N,
12 h, 28%; 2.5 equiv. of Ac2O and i-Pr2EtN, 2 h, 64%.
8. Typical procedure of acyl choride-2,6-lutidine system:
Acetyl chloride (3 mmol, 0.21 mL) was added at 0°C to
a
suspension of (2-amino-6-methoxycarbonylbenzyl)-
triphenylphoshonium bromide (4a) (1 mmol, 506 mg) and
2,6-lutidine (1 mL), and the mixture was heated at 120°C
for 12 h. After dilution with H2O, the mixture was
neutralized with NaHCO3 and extracted with ethyl ace-
tate. The combined organic layer was washed with H2O,
dil. HCl and brine, and dried over Na2SO4. After
removal of solvent in vacuo, purification by column
chromatography (n-hexane/ethyl acetate, 5/1) afforded
methyl 1-acetyl-2-methylindole-4-carboxylate (6a) (144
mg, 62%) as a pale yellow powder.
9. We failed to isolate pure 10, because this compound was
extremely moisture sensitive and readily decomposed.
1
The ratio of 10 and 11 was determined by H NMR.
3. (a) Heron, B. M. Heterocycles 1995, 41, 2357–2386 and
references cited therein; (b) Murphy, P. J.; Lee, S. E. J.
10. The ratios of two products were determined by 1H NMR.