General procedure of Friedel-Crafts acylation of phenols in TfOH

Phenol (0.28 mmol) and acyl chloride (0.28 mmol) were dissolved

in TfOH (3 ml) at 0C. The reaction mixture was warmed to

room temperature for appropriate time in Table 2, then poured into

cold water and ethyl acetate. The organic layer was washed with

1 M HCl, saturated NaHCO3, and saturated NaCl, and dried over

MgSO4, then filtrated. The filtrate was concentrated and the residue

was subjected silica column chromatography to afford acylated

products. All spectral data were identical with the literatures.

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Unfortunately the hydroxyl group is a strong activator and an ortho-, para- director. It is unlikely that you will be able to selectively obtain only the desired para- product. A suggestion would be to protect the hydroxyl with a bulky protecting group prior to the Friedel Crafts acylation, which will help maximize para addition by virtue of steric hindrance of the ortho positions. Furthermore protection of the phenol will reduce its activating ability and you will be less likely to get multiple additions of the acyl group.

Take look at this patent 

https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/509f32ae80fd851f2c18/EP0167286A1.pdf

According to Darren's suggestion, one possibility is that you use pivaloyl chloride to esterify the phenol before performing the acylation reaction.