Alternative perfluoroacylation procedures

As mentioned earlier, acylated imidazoles have the advantage of reacting in a non-acidic environment, and are therefore useful for acylating acid-sensitive compounds such as those where dehydration might be caused by the use of anhydrides. Imidazole reagents can acylate alcoholic and primary and secondary amino groups. The 

Indoleamines and indole alcohols, typically acid-sensitive compound, may be smoothly acylated with HFB-imidazole. The indolic compounds (1—2 mg) are dissolved in HFB-imidazole (100—200 fil) and heated in a reaction vial with a PTFE-lined screw-cap at 80 °C for 2-3 hours. After cooling, the products are extracted into hexane (3x5 ml). The hexane extracts are combined and chilled to —14 C to precipitate residual reagent. The hexane layer is decanted, the precipitate is washed with hexane and the combined hexane solutions are concentrated to 500 1 for analysis by GC (113, 114]. Alternatively, excess reagent may be decomposed with water (1 ml) in the presence of toluene (2 ml), the aqueous layer being extracted with three further 2 ml portions of toluene. These extracts are washed once more with water, separated and filtered through filter paper before concentration for analysis [115,116). A novel way of separating the layers is to freeze the two layers in a solid COj/acetone bath and to withdraw the upper layer from the ice with a Pasteur pipette [117[. Phenolic acids, after esterification, may be acylated in a similar procedure, but here with 10% PFP-imidazole in ethyl acetate for 10 minutes at 70 °C [118]. 

Metoclopramide in plasma has been smoothly acylated with HFB-imidazole for gas chromatography with electron capture detection. The extract of the drug (from I ml of plasma) is heated with 20 /tl of HFB-imidazole for 90 minutes at 75 °C and then cooled. Bicarbonate-carbonate buffer of pH 10 (2 ml) is added, the derivative is extracted into 1 ml of hexane and the extract is evaporated to dryness. The residue is dissolved in 50 1 of hexane immediately prior to analysis [119]. A similar method was used for clebropride, although on a smaller scale, and using only 65 °C for 15 minutes [120]. 

A combined acylation and silylation can be done in a non-acidic environment using HFB- or TFA-imidazole 

The reagent MBTFA was used first by Donike for introducing the TFA group under mild conditions [124]. 

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Although amines do not have to be in the free-base form for acylation by perfluoroacyl anhydrides, and derivatize smoothly even as their salts, some workers have included bases for catalytic purposes, and also to remove acid formed during the reaction, in addition to a solvent. In one recipe, the amine is dissolved in benzene (500 y ) and trimethylamine is added (100 fi of a 0.05 M solution in benzene) followed by 10 1 of anhydride, and the reaction is allowed to go to completion at room temperature. With the low concentration of anhydride this may take some hours. Excess anhydride is removed by washing with 3M ammonium hydroxide and the benzene solution is dried before GC analysis (80, 81]. In a similar procedure, the rather uncommon chlorodi-fluoroacetic anhydride is used. Here the base, extracted from its biological matrix, is dissolved in chloroform (20 fi ) and derivatized with triethylamine (1.5 yl) and the anhydride (3 y ) at 50 °C for 30 min. Excess reagent is neutralized with alkali and the derivatives are concentrated to dryness and taken up in toluene for GC analysis [82]. Trichothecenes (250 mg) are treated with TFAA and about 10 mg of solid sodium bicarbonate for 30 minutes at 80 C [83]. Alternatively, 0.5 ml of 10% acetonitrile in toluene is added followed by 100 y of triethylamine and 100 ul of PFPA. The reaction is carried out at 60 C for 15 minutes, and the reaction solution is washed with two 0.5 ml portions of 5% aqueous ammonia and one 0.4 ml portion of water to remove the surplus reagents [84]. 

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