Boron tribromide → Apigenin dimethylether

Participants

Reaction Steps

The desired 6-iodinated species was prepared in excellent yield with almost exclusive regioselectivity by exploiting the ortho-directing capabilities of thallium(I) salts in the iodination of phenols14 (Scheme 2). Selective demethylation of apigenin trimethyl ether (5b) in the 5-position may be accomplished with boron tribromide in an amount generally ranging between about 0.9 equiv and about 3.0 equiv, preferably about 1.1 equiv boron tribromide, to afford apigenin 7,4′-dimethyl ether (6). The reaction is generally carried out a temperature ranging between about −40° C. and about 50° C., preferably about 25° C., for a time period ranging between about 0.5 h and about 24 h, preferably about 5 h, in the presence of a suitable solvent such as methylene chloride, chloroform, benzene and toluene, preferably methylene chloride, until a thick precipitate is formed. The precipitate is then collected and recrystallized in a suitable solvent such as ethanol, methanol, ethyl acetate/hexane, preferably ethanol. If desired, other boron compounds such as boron trichloride may be used in place of boron tribromide29.

Provenance & Evidence