Hydrogen fluoride, reaction with cyclic

In contrast to other acids, anhydrous hydrogen fluoride does not cause hydroly SIS and decarboxylation of the malonic acid residues in these reactions [5]. It is a good reagent for the cyclization of a-benzamidoacetophenones to 2,5 diphenyl-oxazoles [6] (equation 7) The same reaction with concentrated sulfuric acid gives cyclic product with only a 12% yield [6]... 

The stereochemical outcome of the overall reaction is interpreted as the consequence of a stepwise fluorination. - The first fluorine comes in via an Sp,2-type substitution of the cyclic intermediated with inversion. The second fluorine is introduced with retention from the intermediate 7, which is formed by the action of hydrogen fluoride and sulfur tetrafluoride on intermediate fluorosulfitc 5 via 6. 

A number of carboxylic anhydrides have been converted into trifluoromethyl compounds by treatment with sulfur tetrafluoride, usually in the presence of hydrogen fluoride. As in the case of carboxylic esters, vigorous conditions are needed. Some examples have been presented in Houbcn-Weyl, Vol. 5/3, pp 92, 93. Depending on the reaction conditions, different compounds can be isolated. In some, cyclic di- or tetrafluorinated ethers predominate. 1.253.254 Upon reaction with sulfur tetrafluoride/hydrogen fluoride, pyromellitic dianhydride, as well as pyromellitic acid (benzcnc-l,2,4,5-tetracarboxylic acid), gives a mixture of 1,2,4,5-tet-rakis(trifluoromethyl)bcnzcne, 1,1,3,3-tetrafluoro-5,6-bis( trifluoromethyl)- ,3-dihydroisoben-zofuran, and the fully fluorinated tetrahydrobenzodifuran in an 85-88 10-13.5 1-2 ratio (see formation of compounds 13-15, vide supra)d ... 

Syntheses of 5,6- and Other Unsaturated Cyclic Compounds - A general approach to enol ethers, illustrated by conversion of nitro-alkene 37 into 38 by treatment with tetrabutylammonium hydrogen sulfate-potassium fluoride, has been described. The reaction was also found to be applicable to a wide range of acyclic sugars (see Section 4 below). 

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