The Role Ionic Liquid [BMIM][PF₆] in One-Pot Synthesis of Tetrahydropyran Rings through Tandem Barbier-Prins Reaction

Tetrahydropyran (THP) rings are common in several natural products, therefore, various strategies are being developed to synthesize these rings. The present work described the study of a one-pot synthesis of 2,4,6-trisubstituted tetrahydropyran compounds promoted by the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF₆] through a Barbier-Prins reaction between allyl bromide and aldehydes. The use of [BMIM][PF₆] gave Prins products from several aldehydes in good yields and reaction times. We also found that the anion, PF₆-, accelerates the Barbier reaction when used alone, and the excess SnBr₂ from the reaction conditions of the Barbier reaction leads to the formation of the THP rings, thus acting as a catalyst for Prins cyclization. Additionally, we demonstrate that ionic liquid can be recovered and reused five times in the preparation of 4-bromo-tetrahydro-2,6-diphenyl-2H-pyran without significant yield loss.