Pyryliumverbindungen. 37. Arylbenzene aus 2,4,6‐Triaryl‐pyryliumsalzen und Carbonsäureanhydriden

Pyrylium Compounds. 37. Arylbenzenes from 2,4,6‐Triarylpyrylium Salts and Carboxylic Acid Anhydrides Refluxing 2,4,6‐triarylpyrylium salts 1 with excess carboxylic acid anhydrides (RCH2CO)2O (2a: R = H, 2b: R = Me) in the presence of condensing agents like sodium or potassium acetate, sodium carbonate or methoxide, triethylamine or pyridine results in 1,3,5‐triarylbenzenes 3 (R = H, Me). Under similar conditions, phenylacetic acid anhydride (2c), generated in situ from sodium phenylacetate and excess 2a or 2b, yields 1,2,3,5‐tetraarylbenzenes 3 (R = Ph). Thus, the reaction 1 + 2 → 3 represents a new and simple method for replacing the pyrylium heteroatom =O⊕– by the =CR– moiety (R = H, Me, Ph). The structure of the arylbenzenes 3 was proved by spectroscopic methods, by comparison with literature data or by independent synthesis. As by‐products 2‐acyloxy‐benzophenones 10 are formed. Reaction of 3,5‐dimethyl‐2,4,6‐triphenyl‐pyrylium perchlorate (11) with acetic acid anhydride/sodium acetate (or sodium phenylacetate) takes another course leading to 2‐acetoxy‐3,5‐dimethyl‐4,6‐diphenyl‐benzophenone (15), whereas treatment of 11 with propionic acid anhydride/sodium acetate follows the reaction scheme 1 + 2 → 3 giving 1,3,5‐trimethyl‐2,4,6‐triphenylbenzene (16). The mechanisms of the different pyrylium ring transformations are discussed.