Carboxylic acid anhydrides: aliphatic aromatic, synthesis

An acyl transfer agent which can be used for the synthesis of acid anhydrides is obtained from the reaction of an acid chloride with 4-benzylpyridine (equation 24). In this way benzoic acid anhydride and cinnamic acid anhydride were obtained in 72% and 57% yields, respectively. As the intermediate, 1-acyl-4-benzylidene-l,4-dihydropyridines, can be isolated, Ais procedure should be well suited for the preparation of mixed anhydrides. Mixed aromatic and aliphatic anhydrides can be prepared with 2-ben-zoylthio-l-methylpyridinium chloride and salts of carboxylic acids. These reactions are carried out in aqueous solution. Iliey make use of the high reactivity of esters of thiocarboxylic esters towards nucleophiles. The mixed anhydrides of benzoic acid with 3-phenylpropanoic acid, phenoxyacetic acid, isobu-tyric acid, p-toluic acid and cinnamic acid were formed in 82, 79,61,91 and 66% yields, respectively. 

Related condensations involving the replacement of the anhydride component with succinic acid (Fit-tig synthesis)27 or with malonic acid (Knoevenagel condensation, see Volume 2, Chapter 1.11) provide complementary preparative methods which are useful for both aromatic and aliphatic systems. The Fittig extension of the Peikin transformation involves heating an aliphatic or aromatic aldehyde with sodium succinate and acetic anhydride to give the condensation products (17), which lose carbon dioxide to afford p.y-unsaturated carboxylic acids (18), accompanied by small amounts of the corresponding y-buty-rolactones (19 Scheme 7).27... 

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