Isomerizations fluorosulfuric acid

Fluorosulfuric acid [7789-21-17, HSO F, is a colodess-to-light yellow liquid that fumes strongly in moist air and has a sharp odor. It may be regarded as a mixed anhydride of sulfuric and hydrofluoric acids. Fluorosulfuric acid was first identified and characterized in 1892 (1). It is a strong acid and is employed as a catalyst and chemical reagent in a number of chemical processes, such as alkylation (qv), acylation, polymerization, sulfonation, isomerization, and production of organic fluorosulfates (see Friedel-CRAFTSreactions). 

Hydrogen Fluoride-Fluorosulfuric Acid. Fluorosulfuric acid containing up to 5% of HF acting as a protic coacid was found to be very efficient for isomerization of ra-butane to isobutane at room temperature, whereas pure HSO3F did not show this activity. Moreover, at higher HF concentration the activity diminishes substantially.68... 

Fluorosulfuric acid is a widely used, highly effective reagent808 in electrophilic polyene cyclizations to synthesize fully cyclized compounds in a selective and stereospecific way. Furthermore, an internal nucleophile, most often oxygen, allows the construction of polycyclic skeletons with a heteroatom (vide supra). Snowden and co-workers809 transformed trienone 202 (a mixture of 4 diastereomers) into three isomeric irone derivatives [Eq. (5.293)]. Other acids gave inferior results. 

Structurally related dienols and acyclic trienols, when reacted in fluorosulfuric acid, give tricyclic ether derivatives in kinetically controlled cyclization.810,811 The stereospecific product formation is rationalized by synchronous internal anti-addition via chair-like conformations of the protonated cyclohexene ring, resulting in ring closure with equatorial C-C bond formation and concomitant internal nucleophilic termination by anti-addition of the OH group [Eq. (5.294)]. Z/E isomerization may be competitive with cyclization. 

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