Prins fluorination cyclisations: Preparation of 4-fluoro-pyran and -piperidine heterocycles

The Prins reaction was investigated using BF(3)·OEt(2) as a Lewis acid. It has been recently demonstrated, that if BF(3)·OEt(2) is used in stoichiometric amounts then these reactions generate fluorinated products where the BF(3)·OEt(2) contributes fluoride ion to quench the intermediate carbocations. In this study oxa- and aza-Prins reactions for the synthesis of 4-fluoro-pyrans and -piperidines were investigated. The products were obtained in good yields, but only with moderate diastereoselectivity. These Prins fluorination reactions can be accelerated under microwave conditions. The study extends the Prins fluorination methodology for the generation of the C–F bond in heterocycles.