THE HYDROGEN FLUORIDE METHOD 157» The Hydrogen Fluoride MethodIn 1939 it was observed16that anhydrous liquid hydrogen fluorideis an excellent agent for effecting intramolecular acylation of aryl sub-stituted aliphatic acids. Since that time a number of cyclizations havebeen performed with this reagent (Table VIII).The general procedure consists in allowing a solution of the acid inhydrogen fluoride to stand for a few hours in an open vessel at roomtemperature. The excellence of.the method lies in the simplicity ofmanipulation and in the consistently good yields. The examples inTable VIII include the cyclizations of a 'number of different types ofarylbutyric and arylpropionic acids. Of the twenty-nine ring closures,nineteen were accomplished in yields of 87% or better. The uniquecyclizing power of hydrogen fluoride is shown with 7-(4-methoxy-3-biphenyl)-butyric acid (formula XXXVII). Ring closure was realized,if only to the extent of 16%, whereas other methods, including theFriedel-Crafts, were unsuccessful.
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