Pyridinium poly-hydrogenfluoride

However, the ring expansion of 1-substituted tertiary cyclopropanemethanols to tertiary fluo-rocyclobutanes is efficiently brought about with pyridinium poly(hydrogenfluoride) in the presence of diisopropylamine and potassium fluoride-hydrogen fluoride.19 2(1 Under the strictly anhydrous conditions employed, the intermediate cyclobutyl cation is efficiently trapped. Illustrative is the rearrangement of methanesulfonate 9, the key step in a synthesis of ( )-2-fluorograndisol.21 Other examples are collected in Table 3. 

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