Protected propargylic acetals. Nicholas–Prins cyclization leading to functionalized 2-alkynyl-tetrahydropyrans. Intramolecular trapping by allenes

Dicobalt hexacarbonyl complexes derived from propargylic acetals undergo Lewis acid-mediated Nicholas–Prins cyclization in the presence of homoallylic alcohols. The trapping of the resulting cyclic carbocation enables the synthesis of a series of functionalized tetrahydropyrans. The complexation of the triple bond contributes to the suppression of the side reactions and very significantly increases both the yield and the diastereoselectivity of the reaction. Homoallenic alcohols lead to dienic compounds through proton elimination.