Prins Cyclization in Ionic Liquid Hydrogen Fluoride Salts: Facile and Highly Efficient Synthesis of 4-Fluorinated Tetrahydropyrans, Thiacyclohexanes, and Piperidines

Prins cyclization of homoallylic alcohols with various aldehydes was investigated in ionic liquid hydrogen fluoride (HF) salts, which played roles as a reaction medium, a catalyst, and a fluorine source. The reaction afforded the corresponding 4‐fluorinated tetrahydropyrans in excellent yields with a high stereoselectivity (cis form exclusively). When benzaldehydes having a strongly electron‐donating group at the para position were used, the stereoselectivity was lost in this system. In order to apply this method to the synthesis of other 4‐fluorinated heterocyclic compounds, we also carried out thia‐Prins and aza‐Prins cyclization in HF salt and successfully obtained 4‐fluorinated thiacyclohexanes and piperidines, respectively.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) Prins cyclization of homoallylic alcohols with various aldehydes was investigated in ionic liquid hydrogen fluoride (HF) salts, which played roles as a reaction medium, a catalyst, and a fluorine source. The reaction afforded the corresponding 4‐fluorinated tetrahydropyrans in excellent yields with a high stereoselectivity (cis form exclusively). Thia‐ and aza‐Prins cyclization were also studied.