Secondary amines selective acetylations, acetic anhydride

Selective trifluoroacetylation of primary amines in the presence of secondary amines can be accomplished by reaction with a stoichiometric amount of ethyl trifluoroacetate (bp 60-62 °C) in THF, acetonitrile or dioxane at 0 °C. The product is simply isolated by evaporation of the solvent and liberated ethanol,45-47 Perhaps the most common method entails acylation of the amine with trifluoro-acetic anhydride in the presence of a suitable base such as trie thy lamine or pyridine in dichloromethane [Scheme 8.26],40 New reagents for the N-trifluoro-acetylation of amines include Af-ftrifluoroacetyOsuccinimide,48 a solid and storable reagent, W-(trifluoroacetoxy)succinimide,49 which must be stored in a frozen benzene matrix, and l-(trifluoroacetyl)-l,2,3-benzotriazole.50 The latter reagent is a stable, crystalline reagent (mp 89-91 °C) prepared in quantitative... 

A unique method for selective acylation of secondary amines in the presence of primary amines involves the use of 18-Crown-6 with acetic anhydride and triethylamine. It is believed that the 18-crown-6 complexes primary alkylammonium salts more tightly than the secondary salts, allowing selective acetylation. 

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