A mixture of compound 3 (50 mg),
dimethyl sulfate (1 mL), and anhydrous potassium carbonate
(1 g) in dry acetone (10 mL) was refluxed for 1 h with stirring.
After removal of the inorganic compounds by filtration, the
filtrate was concentrated to dryness. The residue was applied
to a column of silica gel eluting with toluene-acetone (5:1) to
give the methyl derivative (3a) (39 mg):