Imidazolides acid halide synthesis

Staab introduced azolides as versatile reagents in organic synthesis. The reaction of carbonyldiimid-azolide with carboxylic acids produces imidazolides under mild conditions which can be converted by the action of hydrogen halides to acid halides in high yield (equation 20). The method does not even require the isolation of the imidazolides, but can be carried out as a one-pot synthesis. The possibility of carrying out this reaction at low temperatures allows the preparation of temperature sensitive compounds. Formyl chloride, which decomposes at -40 °C, has been prepared in this way. 

This synthesis also gives a small glimpse at the chemistry of heterocyclic compounds. Most active compounds in today s pharmaceuticals or agrochemicals include heterocycles, as well as most vitamins and natural products. The chemistry of heterocycles is thus very important and lectures or textbooks should be consulted.6 Formation of amide bonds also plays a large role in this problem. It was demonstrated that the strong amide bond can be formed from an amine and a carboxylic acid only after the acid has been activated. This can be done by transformation into the carboxylic halide or imidazolide or by application of an activating agent developed for peptide synthesis. 

Instead of alkylating reagents, the use of acyl or aroyl halides in fhis reaction conveniently leads to the synthesis of selenoesters as useful acylating reagents in organic synthesis. A mild and convenient method is reaction of trimethylsilyl selenides with acyl halides [27] or reaction of selenols with imidazolide (or triazolide) of carboxylic acids (Scheme 15.8) [28]. 

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