305 ml (4.1 mmol) TFA was added to a vigorously stirred suspension of bis-
potassium salt 3 (1.86 mmol) in 10 ml TFAA at 220 uC. After a few
minutes, a solution was formed. The reaction mixture was allowed to warm
to 0 uC and stirred for a further 2 h. The reaction was monitored by TLC,
and the starting material had been completely consumed after 2 h. The
excess TFA/TFAA were removed by distillation under normal pressure.
The remaining traces of TFA could be removed azeotropically with
toluene. The residue was poured into 50 ml of vigorously stirred ice-water,
the pyrone precipitating immediately. To complete the precipitation, the
crude product was stored at 4 uC for 12 h. The precipitate was filtered,
washed with water and dried. For 4a–d, the aqueous solution was saturated
with sodium chloride and extracted four times with CH2Cl2. The combined
organic phases were washed with water and dried over MgSO4. The
volatiles were removed in vacuo and the residue submitted to flash
chromatography on silica gel.
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4734 | Chem. Commun., 2006, 4732–4734
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